healthy holiday recipe ebook

Review and investigation of slope instability and active fault features in the Mt Lyford Village area, North Canterbury

G T Hancox R Langridge D Townsend GNS Science Consultancy Report 2006/26 12 June 2006

GNS Science Consultancy Report 2006/26 12 June 2006 Project Number: 430W1200

CONFIDENTIAL

This report has been prepared by the Institute of Geological and Nuclear Sciences Limited (GNS Science) exclusively for and under contract to Hurunui District Council. Unless otherwise agreed in writing, all liability of GNS Science to any other party other than Hurunui District Council in respect of the report is expressly excluded.

The data presented in this Report are

available to GNS Science for other use from 30 June 2006

© Institute of Geological and Nuclear Sciences Limited 2006

Confidential 2006

GNS Science Consultancy Report 2006/26 12 June 2006 i

CONTENTS

SUMMARY.............................................................................................................................. III

1.0 INTRODUCTION ..........................................................................................................1

1.1 Background.......................................................................................................1

1.2 Scope of the review ..........................................................................................1

1.3 Information used in report.................................................................................2

2.0 SITE DESCRIPTION ....................................................................................................3

2.1 Location and topography ..................................................................................3

2.2 Geology ............................................................................................................3

3.0 REVIEW OF ACTIVE FAULT AND SLOPE FEATURES ............................................4

3.1 Active Faults .....................................................................................................5

3.2 Slope rents........................................................................................................7

3.3 Other features...................................................................................................9

3.3.1 Areas of slope extension and partial collapse....................................9

3.3.2 Uphill limits of slumping and stream incision......................................9

3.3.3 Large old landslides ...........................................................................9

3.4 Classification of ‘slope rents’ ..........................................................................11

3.5 URS Peer Review...........................................................................................12

3.6 Suggested investigation and analysis of ‘slope rents’.....................................13

4.0 GNS ASSESSMENT OF ‘SLOPE RENTS’ AND ACTIVE FAULTS..........................14

4.1 GNS geomorphic mapping .............................................................................14

4.2 GNS Investigation of active faults at Mt Lyford...............................................18

4.2.1 Introduction ......................................................................................18

4.2.2 Neotectonic background ..................................................................18

4.2.3 Geologic age control at Mt Lyford ....................................................19

4.2.4 Trenches across fault features.........................................................21

4.2.5 Summary of fault data in trenches ...................................................29

4.2.6 Structural model of the Mt Lyford piedmont .....................................31

4.2.7 Implications of active fault studies....................................................32

4.3 Revised active fault and slope features map and Land Stability Classes.......37

5.0 DISCUSSION .............................................................................................................38

5.1 Collection and peer review of data .................................................................38

5.2 Use of geological hazard data at Mt Lyford ....................................................41

Confidential 2006

GNS Science Consultancy Report 2006/26 12 June 2006 ii

6.0 CONCLUSIONS .........................................................................................................42

7.0 ACKNOWLEDGEMENTS ..........................................................................................45

8.0 REFERENCES ...........................................................................................................45

FIGURES

Figure 1. Topographic map of the Mt Lyford Village area and Cross Section (A-B) ......47

Figure 2. 1966 aerial photo of Mt Lyford Village area (5007/4 - 1966). ........................ 48

Figure 3. Map of the Hope Fault at Mt Lyford (after Eusden et al.. 2000)..................... 49

Figure 4. Geological map of Mt Lyford area.................................................................. 50

Figure 5/5a Integrated active faults and slope features map of the Mt Lyford area (after ECan & Yetton, 2005). ................................................................... 51/52

Figure 6. Geomorphic map of the Mt Lyford area. ....................................................... 53

Figures 7–14. Oblique aerial photos of Mt Lyford area showing features discussed...... 54-61

Figure 15. Logs of trenches across drainage channels and prehistoric landslide.......... 62

Figure 16 (a-f). Logs of trenches across active fault traces at Mt Lyford.......................... 63-68

Figure 17. Geological model for the Hope Fault zone at Mt Lyford. .............................. 69

Figure 18. GNS modified active fault and slope features map of Mt Lyford area. ......... 70

Figure 19. Revised map of active faults, slope features, Land Stability Classes........... 71

APPENDIX 1: Fault classification used in the Environment Canterbury Active Fault Database. ......................................................................................... 72

APPENDIX 2: Extracts from Ministry for Environment Active Fault Guidelines................. 73

APPENDIX 3: Definition of Landslides and Slope Movement Features ............................ 83

APPENDIX 4: Landslide and environmental criteria in the MM Intensity Scale . .............. 85

Confidential 2006

GNS Science Consultancy Report 2006/26 12 June 2006 iii

SUMMARY

This report presents results of a review and investigation by GNS Science (GNS) for Hurunui District Council (HDC) of active fault traces and slope features mapped at Mt Lyford Village, North Canterbury, in 2005 by Geotech Consulting Ltd on behalf of Environment Canterbury (ECan). The new hazard data became an issue of public concern after HDC advised Mt Lyford land owners of their intention to refer to the active fault and land instability hazard features when issuing LIM (Land Information Memorandum) and PIM (Project Information Memorandum) reports, and ECan would refer to the information when preparing Land Information Reports.

The review and detailed investigations by GNS of the Mt Lyford active fault and ‘slope rent’ features data provided to ECan, has shown that while many of the mapped geomorphic features have been confirmed, in the opinion of GNS a number of those in the centre of the village are not considered to be either active fault traces or slope instability features. This principal finding of the review significantly reduces the size of the area within the village that is potentially at risk from active faulting or land instability hazards. The assessment is based on a review of the active fault and slope features data provided to ECan, previous geological mapping and geotechnical work in the area, and recent geotechnical investigations at Mt Lyford by GNS. The latter included aerial photography, detailed geomorphic mapping, site inspections, and trenching of slope features and active fault traces within the Hope Fault zone. These investigations have enabled GNS to assess the origin and hazard implications of geomorphic features in the Mt Lyford area with greater certainty than was possible from the initial reconnaissance mapping carried out for Environment Canterbury by Geotech Consulting Ltd.

Other information and conclusions presented in the report relate to: (a) the criteria used for assessing the origin and significance of the slope features (which are identified mainly as old slump scarps or drainage channels); (b) the unusual classification of the ‘slope rent’ features as “potentially Class 1 active features”; (c) the limited effects that future large earthquakes are expected to have on old slump scarps in the area (these scarps are thought to have survived many earthquakes on the Hope Fault in the last 10,000 years); and (d) the adequacy of the initial mapping and peer review of the active fault and slope features data, and its eventual use and application by Environment Canterbury and Hurunui District Council. One of the important findings that emerged from the GNS review was the need for geotechnical consultants and reviewers to investigate the origin and significance of geomorphic features before they are classified and presented to Regional and District councils as confirmed hazard data.

Recent paleoseismic studies (trenching and geomorphic dating) of active faults by GNS at Mt Lyford have shown that the subsidiary traces of the Hope Fault have lower rates of activity (probably in the order of 0.05 – 0.2 mm/yr) and longer recurrence intervals (� 6000 years) than the main fault. The main trace of the Hope Fault has a much higher slip rate (c.23 ± 4 mm/yr) and shorter recurrence interval (c. 200-400 years), and so presents a greater risk than the subsidiary traces. Subsidiary faults in the Mt Lyford Village area have a relatively low level of geologic activity and probably fall into RI Class IV (Recurrence Interval Class of the Ministry for the Environment active fault guidelines). Conservatively, because of uncertainty in the geomorphic ages used in our assessment, we have placed the subsidiary faults into RI class III. This still amounts to a relatively low level of risk from fault rupture hazard on the subsidiary fault features in the Mt Lyford Village area. Evidence for the possibility of Holocene faulting (<12,000 years) on subsidiary faults at Mt Lyford Village is only seen in one trench (T-3).

Confidential 2006

GNS Science Consultancy Report 2006/26 12 June 2006 iv

The identification and classification of active faults has important implications for planning and building development in New Zealand. GNS recommends that Regional and District councils follow the Ministry for the Environment (MfE) guidelines for the development of land on or close to active faults. The recurrence interval-based classification within the MfE guidelines can be more easily related to Building Importance Categories (BIC) and accepted levels of risk in the current Building Code, and provides a consistent treatment of both short-recurrence interval (higher risk) and longer recurrence interval (lower risk) active faults across New Zealand.

The MfE guidelines allow significant flexibility in the consent process for building on or adjacent to low-activity faults or distributed zones of faulting. In this report, the subsidiary faults can be thought of as being part of a broad, distributed zone of deformation related to the Hope Fault. This would usually make consenting more restrictive; however, as we have undertaken significant analysis of these faults, we have determined information about these faults individually that puts them in a recurrence interval class (� III) that is reasonably unrestrictive to most regular BIC structures (BIC 2a and 2b - one and two story timber-framed houses as at Mt Lyford) at already subdivided sites such as the current Mt Lyford village.

If the MfE Guidelines are followed, construction of BIC 1 (accessory and farm buildings) and 2a structures (single-storey timber-framed dwellings) is a permitted activity at both previously developed and ‘greenfield’ sites within and outside Mt Lyford Village, even within the designated Fault Avoidance Zones (� 40m wide) related to these faults. Construction of a BIC 2b structure (one or two storey timber-framed dwellings) is a permitted activity, even within the ±20 m buffer area of these RI Class III faults in the already subdivided or developed parts of the village. For Greenfield sites, construction of BIC 2b structures is a discretionary activity (within a distributed deformation zone), and thus a resource consent would have to be applied for. In general, at the level of a Class III fault (RI 3500-5000 years), the risk of surface rupture during the expected life of those structures at Mt Lyford is deemed to be acceptable under the MfE guidelines.

From the investigations carried out by GNS during this review, three Land Stability Classes are proposed to show and classify the areas where slope instability features have been identified. Hazard implications within the classes are also discussed. Most of Mt Lyford Village is zoned as Land Stability Class 1, in which no significant slope instability features are present. Land Stability Class 2 affects areas on or immediately adjacent to old slump scarps. The possible presence of weak materials associated with these old scarps may cause local slope instability or foundation problems for buildings, and excavations across them. This would need to be determined by a geotechnical report for future building or site development within 10 m of these scarps. Land Stability Class 3 covers areas of minor superficial instability on the steeper parts of properties below Tinline Terrace, Foggy Lookout, and the sides of Lulus Creek. Building sites originally specified in those areas have generally avoided the steeper, less stable slopes. Applications for future building within 5 m of Class 3 areas should be accompanied by a geotechnical report, at which stage building designs and safe setback distances should be determined. The existing vegetation cover should be maintained on this land in the future to reduce potential slope instability in the area.

GNS considers that within the area studied by GNS the only active faults and slope instability hazards features at Mt Lyford that may be relevant to LIM and PIM reports, or other statutory purposes, are those shown along with the Land Stability Classes on the following map.

GN

S S

cien

ce C

onsu

ltanc

y R

epor

t 200

6/26

12

Jun

e 20

06v

R

evis

ed m

ap o

f act

ive

faul

t tra

ces,

slo

pe in

stab

ility

feat

ures

and

Lan

d S

tabi

lity

Cla

sses

at M

t Lyf

ord

Vill

age.

Confidential 2006

GNS Science Consultancy Report 2006/26 12 June 2006 1

1.0 INTRODUCTION

1.1 Background This report presents the results of a review and investigation by GNS Science (GNS) of geological hazard information and associated documentation provided by Environment Canterbury (ECan) to Hurunui District Council (HDC) in November and December 2005. The information provided to HDC included active fault and slope instability hazard data at Mt Lyford Village, North Canterbury (Figure 1). The new hazard data were based on aerial photo mapping and limited ground checking carried out for ECan by Dr Mark Yetton of Geotech Consulting Ltd (GCL) in October 2005. The mapping was commissioned because ECan was updating its active faults database, and recommended to HDC that larger (better) scale mapping be done around three towns in Hurunui District, including Mt Lyford Village (Letter ECan to HDC, Sept 2004). Although data on slope instability was not requested by ECan, many landforms were mapped as slope instability hazard features, and included with the data provided to ECan, who passed it on to HDC. The new hazard data became an issue of public concern after HDC advised Mt Lyford land owners of their intention to refer to the active fault and land instability hazard features when issuing LIM (Land Information Memorandum) and PIM (Project Information Memorandum) reports, and ECan would refer to the information when preparing Land Information Reports.

GNS has been asked by HDC to review and investigate the validity of the active fault and slope features, and their hazard implications for building and development planning at Mt Lyford. The review was commissioned by Mr Brent Pizzey (Senior Policy Planner) on behalf of Hurunui District Council (HDC) in his email of 9 February 2006.

1.2 Scope of the review

The scope of work agreed between GNS (principal author G Hancox) and HDC has included: (a) Review of a map prepared by GCL showing active faults and slope features at

Mt Lyford; two letters by GCL describing the mapping and recommendations; a peer review of GCL’s work by URS New Zealand Ltd. (URS) on behalf of ECan; GCL’s (email) proposal to HDC for further geological investigations of active faults and slope features at Mt Lyford; and any other relevant letters and reports.

(b) A site visit to Mt Lyford Village was carried out by G Hancox from 2–4 March 2006. Work undertaken included oblique aerial photography of active fault traces and slope features in the area, ground inspections and trenching of three ‘slope rents’ and a large old landslide southeast of the village, and collection of old wood samples for C14 dating.

(c) Trenching and assessment of active fault traces in the area (by Dr Rob Langridge and Dr Dougal Townsend, with visits from 10–13 and 26–28 April 2006).

(d) Aerial photo analysis and geomorphic mapping of fault traces and slope instability features in the Mt Lyford Village area.

(e) Preparation of a report that: (a) comments on the validity of GCL’s mapping of the active faults and ‘slope rents’, and conclusions about the origin and significance of the latter and their classification as Class 1 active features; (b) presents the results of the authors’ investigation of the ‘slope rents’ and active faults at Mt Lyford.

Conclusions reached in this report supersede those presented in an interim letter report by GNS Science to HDC on this issue dated 16 March 2006, before the review and investigations had been completed 18. There are, however, no significant differences between the interim conclusions and those reached in this report.

Confidential 2006


1.3 Information used in report

The information used for this report includes material provided by HDC and relevant maps and scientific papers, as follows: (1) Integrated active faults and slope features map (1:15,000 at A4 size) prepared by GCL

showing active faults and slope features ('slope rents') in the Mt Lyford village area (AFSF map) 1. This map (dated 25 November 2005) was posted on the ECan website (in late January 2006), and was circulated by ECan and HDC to Mt Lyford property owners in January 2006 27.

(2) GCL's letters to ECan dated 27 October 2 and 18 November 2005 3, discussing the 'slope rent' features in the Mt Lyford area, which were regarded by GCL as “potentially Class 1 active features”.

(3) A peer review letter to ECan dated 23 November 2005 on GCL’s work at Mt Lyford by Tim McMorran and Don Macfarlane of URS New Zealand Limited (URS) 4.

(4) Letters dated 8 September 2004 5, 14 November 2005 5a and 12 December 2005 5b from ECan to HDC regarding active faults and slope features at Mt Lyford Village.

(5) Email messages from GCL to HDC relating to proposed major geological investigations at Mt Lyford (dated 11 and 16 January 2006) 6.

(6) A letter dated 8 February 2006 by David Bell (Senior Lecturer in Engineering Geology, Canterbury University) to Davis, Ogilvie & Partners (consultant working for a Mt Lyford land developer) in relation to GCL’s AFSF map and planning implications at Mt Lyford 7.

(7) Evidence dated 22 January 1987 8 and a report (including a site plan) dated 22 September 1988 by David Bell presenting engineering geological information for development of the Mt Lyford subdivision 9

(8) Field observations, oblique aerial photos, and subsurface geological data and C14 dating of wood and charcoal samples from trenches dug across ‘slope rents’ and faults.

Other material used in the review has included: the 1:50,000 topographic map NZMS 260-N32 (Hanmer, 2003), on which Figure 1 is based; the most recent regional geological map 12 of the area; relevant papers on the Hope Fault 13, 14; active fault guidelines for planning in New Zealand 15; earthquake-induced landsliding in New Zealand 16, 17, active fault earthquake sources in the Canterbury Region 19; and criteria used for identifying and determining the significance of landslides and associated landforms 21, 22, 23.

Stereo pairs of vertical aerial photos taken in 1966 (Run 5007, Photos 3, 4, and 5) were used to examine and map (independently) the active fault and ‘slope rent’ features, and to determine what geomorphic changes had occurred in the last 40 years. These older photos were compared with a rectified orthophoto of the Mt Lyford Village area taken in 2001 and provided by ECan, on which property boundaries and 20 m contours had been superimposed. Aerial oblique digital photos of the site taken on 2 March 2006, and observations made during the site visit 2–4 March were also used for the study. Some of these photos are included in this report to illustrate the geomorphic features that are discussed (Figures 7 to 12).

GNS has not seen the brief for the mapping done by GCL for ECan, so this aspect and any constraints (such as timing) related to GCL’s work have not been considered in this review.

_______________________________________________________________________Note: Superscript numbers (1–27) used throughout the text identify the documentation and technical references that have

been referred to in this report and are listed in Section 8.

Confidential 2006


2.0 SITE DESCRIPTION

2.1 Location and topography

Mt Lyford Village is located on a broad spur between the Wandle and Mason rivers on the southeast flanks of the Amuri Range. It lies about 20 km northeast of Waiau at an altitude of 500-730 m above sea level, and is accessed via the Inland Kaikoura Road and Mt Lyford Forest Drive (Figure 1). The village area is bounded by deeply (50–100 m) incised streams which drain the highly dissected piedmont fan flanking the base of the Amuri Range – to the northeast by Lulus Creek, and southwest by an unnamed stream (Figure 7). The spur of which Mt Lyford village is located and the spur northeast of Lulus Creek are broad-crested and slope gently (5–15°) to the south and east. These spurs are crossed by a number of the ‘slope rents’ and active faults* mapped by GCL1, the origin and hazard potential of which are the focus of this review. Some of these landforms and geological features in the area are shown on an annotated 1966 aerial photo (Figure 2). It should be noted, however, that the ‘slope rent’ features have been identified and mapped previously in this area by Eusden et al.. 13, who referred to them as ‘ridge rents’ (Figure 3).

The steeper (25–35°) valley sides of Lulus Creek, Whales Back Stream, and the unnamed stream west of the village show extensive slumping**. Several deep-seated landslides have been identified south and west of the area, some of which are actively creeping (Figures 2 and 3). The largest landslide in the area lies immediately southeast of Mt Lyford Village and is crossed by Mt Lyford Forest Drive. This is an inactive prehistoric feature, with large mounds of old debris (Figures 7). A large old rock avalanche** scar and deposit is present northwest of the village and is crossed by the road to the ski field (Figures 2 and 7). These landslide features are discussed later.

2.2 Geology

The Mt Lyford area is underlain by soft siltstone and sandstone of the Greta Formation, which is of upper Tertiary age (~2–5 million years), and which overlies calcareous mudstone and conglomerate of mid Tertiary age (~5–24 million years). The area northwest of the village is composed of upper Jurassic to lower Cretaceous age Torlesse Terrane greywacke sandstone and argillite, which are separated from the Tertiary rocks by the active Hope Fault, the major range-bounding fault of the Amuri Range (Figure 4) 12.

The Hope Fault is a major northeast-striking, reverse dextral strike-slip fault. The fault zone is up to 2 km wide in the Mt Lyford Village area, and contains several active fault traces and secondary fault scarps (Figures 3 and 4) 12, 13. The main trace at the foot of the Amuri Range is relatively straight, and is defined by the bedrock contact between the Jurassic-Cretaceous rocks to the northwest and the Tertiary rocks to the southeast (Figure 4). Some rivers and streams crossing the fault are clearly displaced in a dextral (right-lateral) sense, with uplift on the northwest side13 (Figure 7). The last movement of this (Conway River) segment of the Hope Fault is thought to have occurred in about 1780 AD. The average recurrence interval of this segment of the fault is estimated to be 180–390 years 14. This suggests that there is a relatively high probability of an earthquake occurring on the Hope Fault during the expected life of residential buildings in Mt Lyford Village (say the next 100 years).

* Active faults are defined in Appendix 1 and Appendix 2. **See Appendix 3 for definitions of landslide types and terms.

Confidential 2006


3.0 REVIEW OF ACTIVE FAULT AND SLOPE FEATURES

This section of the report provides a review of the Mt Lyford Integrated Active Faults and Slope Features Map (AFSF map) prepared by GCL and posted on the ECan website. Figure 5 shows the Mt Lyford AFSF map as provided by ECan1. In order to show the geomorphic relationships of features that were mapped, GNS added streams, 20 m contours, and a scale to the AFSF map (Figure 5a). The active faults and slope features shown on the AFSF Map are discussed, as well as GCL’s explanation 2, 3 and the URS peer review 4 of the mapping. Comments are then made by GNS on the mapped hazard features and associated issues, and also the URS peer review. These comments are based primarily on an examination of available data and investigations carried out by GNS during this study.

The Mt Lyford AFSF map (Figure 5/5a)* covers the Mt Lyford Village area, which lies within the deformation zone of the Hope Fault. It shows a number of features which have been mapped based on topographic lineaments on aerial photos with limited ground checking 1, 2, 3. These features include:

(a) Active faults (definite, approximate, or inferred – Activity Class 1 – see Appendix 1).

(b) Slope rents (definite or inferred – potentially Class 1 active features).

(c) Areas inferred to have undergone significant past slope extension and partial collapse.

(d) The uphill limits of obvious slumping and stream incision (inferred).

(e) Scarp of possible ancient large landslide (inferred) south of the village.

(f) Scarp and debris of past rock slide (approximate) northwest of village. The origin and significance of these geomorphic features, and their assessed hazard potential and implications for planning and development at Mt Lyford by GCL and ECan are the main focus of this review, along with advice given by ECan’s independent peer reviewer (Tim McMorran of URS NZ Ltd) 4. Geotechnical information provided by David Bell based on his recent investigations at Mt Lyford 7 and previous work in the 1980s 8, 9 has also been taken into consideration.

Besides reviewing the material provided on active faults and slope features at Mt Lyford (Section 1.3), GNS undertook extensive investigations in order to independently determine the nature of these features and their hazard potential, including ground inspections, trenching, and geomorphic mapping using vertical and oblique aerial photos. In this process, the locations and characteristics of all active fault traces and ‘slope rent’ features shown on Figure 5 were carefully examined. These geomorphic features (lineaments) were identified and plotted on a 2001 orthophoto of the area supplied by HDC (from ECan). Ground inspections and trenching allowed their nature and probable origin to be determined using commonly accepted criteria. Geomorphic features identified during these investigations are presented on a geomorphic map of the Mt Lyford area (Figure 6), and are illustrated by selected oblique aerial photos (Figures 7 to 14). Logs of trenches dug across ‘slope rents’ and active faults are shown in Figures 15 and 16. Active fault traces and slope features that could be substantiated by this work are shown on a GNS modified AFSF Map (Figure 18). The results of the GNS investigations, and the suggested origin and age of the geomorphic features, and their probable hazard potential and implications are discussed in Section 4.

* Further references to Figure 5 also relate to Figure 5a unless stated otherwise.

Confidential 2006


3.1 Active Faults*

The dominant features on the AFSF map (Figure 5) are described in the legend as HurunuiActive Faults (the main and subsidiary traces of the Hope Fault, Figure 3). These traces are shown by lines of different thickness and colour to define definite, approximate and inferredsections. Mapping of these faults was based on identification of topographic lineaments on aerial photos with ‘limited ground checking’ (what this involved, and which photos were used are not described by GCL). The difference between the narrow and wide lines representing fault features is not explained on Figure 5. Features shown with thicker (c. 50 m) lines were apparently derived from the GNS Active Fault Data (1:50,000 and 1:250,000 scale data) imported into the ECan Active Fault database (pers. comm. ECan, 2006, and reference 5a).

The thinner (20 m wide) fault features shown on Figure 5 were mapped by GCL as part of an “…extensive system of as Class 1 active fault scarps… ”, apparently because they are within the Hope Fault zone 2, 3. However, the characteristics of these features and the criteria used to distinguish them as ‘Class 1 active’ faults (as in Appendix 1, pers. comm. ECan 2006) are not described. Although the limitations of the positional accuracy of the active fault features are indicated because of the reconnaissance scale (1:10,000) of the mapping (Figure 5), no uncertainty is indicated about their faulting origin or Class 1 Active classification 2, 3.

Comments: (1) Integration of active fault data from different sources, and at different scales, results in

‘overprinting’ and plotting of features in slightly different positions on Figure 5. This is potentially confusing and is not explained on the AFSF map (but it is in the supporting documentation provided to HDC 5a, 5b). Some of the older (1:250,000 scale) GNS fault lines on Figure 5 are of lower accuracy and reliability. For example, the thick fault trace southeast of the village is shown as passing through the old large landslide southeast of Mt Lyford Village (Figure 2). However, in the earlier mapping by Eusden et al.. 13 (Figure 3), and more recent mapping by GNS 12 (Figure 4), this trace does not extend southwest of Lulus Creek, or across debris of the old landslide (Figures 3 and 4). Investigations carried out by the authors have confirmed that assessment (Figure 6).

(2) Previous mapping by Eusden et al..13 defined the main and subsidiary traces of the Hope Fault (Figure 3). The main traces mapped by Eusden et al.. are generally consistent with those mapped by GCL (Figure 5). However, the basis for classing all of the features mapped as faults as “Class 1 active scarps” 3 is not stated. There seems to have been no consideration of the amount or sense of ground surface fault displacement by those features, or the age of displaced surfaces 2, 3. Figures 3 and 4 show that surfaces cut by the faults shown on Figure 5 are of pre Holocene age (>12,000-50,000 years).

(3) There is also no explanation as to why the many small and discontinuous traces southeast of Foggy Lookout (Figure 5) are interpreted as being of fault origin 2, 3. Most of these features are discontinuous and poorly defined, and are unconvincing as active fault scarps. Some of these features, especially the rounded, subtle, down-hill facing scarps within ~150 m of the head scarp of the large prehistoric landslide, are considered more likely to be old slump scarps associated with incipient slope collapse (Figure 6).

_________________________________________________________________________ * Active faults may be generally described as faults that have moved in the last c.125,000 years, and are likely to move again in the foreseeable future causing a large earthquake (~ � M 7) and possibly ground surface rupture. The active fault classifications referred to in this report are those of Pettinga et al.. (1998) 19, as used by ECan 5a, 5b and defined in Appendix 1, and Kerr et al.. (2003) 15, and defined in Appendix 2.

Confidential 2006


(4) Other features on Figure 5 that do not appear to be active faults include several short (c. 200-400 m) traces near Mt Lyford Avenue that do not show topographic displacements. These are thought to be shallow drainage gullies draining into Lulus Creek (Figure 6). The three closely spaced short traces mapped on the steep slope on the northeast side of Lulus Creek are also unlikely to be fault traces. On the 1966 aerial photos theses features appear to be small erosion gullies rather than faults (Figure 2).

(5) GNS also has doubts about some of the active faults mapped by GCL on the wide terrace surface northeast of Lulus Creek (below the 600 m contour - Figure 5a). Some of these features are very subtle and do not show clear topographic offsets. Therefore, only some of the active faults shown on Figure 5 were able to be confirmed as probable fault traces during geomorphic mapping (Figure 6).

(6) From the documents seen by GNS, we infer that the mapped fault in the Mt Lyford Village area are described by GCL as “Class 1 active fault scarps” because they are within the Hope Fault zone 2, 3. We understand that the fault classification used was that proposed by Pettinga et al.. (1998) 19 and has been adopted by ECan for its Active Fault Database (pers. comm. ECan, 2006). In that classification Activity Class 1 faults are faults that displace Holocene surfaces (<10,000 years old – see Appendix 1). The Pettinga et al.. (1998) scheme differs significantly from the Ministry for the Environment (MFE) Active Faulting Guidelines 15, which are now widely accepted for assessing active faults in New Zealand (Appendix 2). In the MFE guidelines a Class 1 active fault (or more correctly Recurrence Interval Class I) is the highest class of activity assigned to faults with recurrence intervals of 2000 years or less, and so present the greatest risk of fault rupture. This classification system is considered by GNS to be more appropriate for the assessment of active faulting hazard and risk as it more correctly reflects the likelihood of future fault movements, and takes account of faults with longer recurrence intervals (>10,000 – 20,000 years) that have displaced surfaces of Holocene age (less than c. 10,000 years). Active faults with longer recurrence intervals (i.e. 3,500-10,000 years – RI classes III and IV), even those displacing Holocene surfaces, are considered to present a lower risk of fault rupture in the near future than faults with shorter recurrence intervals (< 3,500 years – RI classes I and II).

Further discussion on active faulting issues at Mt Lyford Village is included in Section 4.2. This includes further discussion of the geological setting and significance of the Hope Fault, and presentation of the results of trenching investigations across four subsidiary fault traces and an unusual lineament (which proved to be a secondary fault) at the northern end of the village.

The implications of the results of GNS paleoseismic studies in relation to residential buildings at Mt Lyford Village, and recommendations for use of the MFE Active Fault Guidelines are discussed in section 4.2.6.

Confidential 2006


3.2 Slope rents

The AFSF map (Figure 5) shows geomorphic features called ‘slope rents’, which are inferred to be part of a “network of slope collapse features” 3. Some of these features were mapped as ‘ridge rents’ by Eusden et al. 13, who described them as ‘topographic lineations along the head scarps of major slump blocks and areas of active* slumping’ (Figure 3). Eusden et al.. 13 attributed this active slumping and ridge renting to the combined effects of change in river base level during faulting events on the Hope Fault.

The ‘slope rents’ mapped by GCL are shown on the AFSF map by 20 m-wide lines, and most are mapped as ‘definite’ features. They are inferred to “represent the first stages of slope collapse and the possible subsequent landslide failure of elevated areas of soil and bedrock, most likely during extreme earthquake shaking… The presence of ‘slope rents’, in conjunction with the intensive pattern of fault traces, raises significant issues with respect to existing and future residential development at Mt Lyford” 3. Because of the association of the ‘slope rents’ and active faults in the area, GCL recommended that the ‘slope rents’ should be classified as Class 1 active features in the ECan database 3. On the AFSF Map they are referred to as ‘potentially Class 1 active features’.

Although many ‘slope rents’ are shown on the AFSF map (Figure 5), the exact nature and characteristics of these geomorphic features are not described. In describing the ‘slope rents’ GCL 2, 3 states that “...those features associated with slope collapse can not always be distinguished from active fault traces, except where their orientation at a high angle to the fault zone, and/or parallel to the incising creeks, strongly suggests that they are slope related…”; “…the ‘slope rents’ represent the first stages of landslide failure of higher areas of soil and rock, most likely during extreme earthquake shaking... which raises significant issues with respect to existing and future residential development at Mt Lyford…”. And also that “…their activity level and the potential magnitudes of future movement are highly unpredictable. Some rents may now be largely inactive, while others may be about to undergo more serious or even complete failure…” 2, 3.

Comments: (1) The locations and extent of the ‘slope rents’ are clearly shown (Figure 5), but the

characteristics of these features are not described in detail, as would normally be expected for significant slope instability features. For example, the morphology of the ‘slope rents’– the nature and height of scarps, up-hill or down-hill facing; whether troughs are closed, drained, or vertically offset – are not described. These characteristics are important in determining the origin and hazard potential of such features.

(2) Some of the ‘slope rents’ were apparently identified by GCL as slope instability-related because of their ‘high angle orientation’ to the Hope Fault zone, and alignment parallel to deeply incised creeks in the area. However, no specific characteristics or criteria are described to show that the ‘slope rents’ are in fact slope instability related. Other possible explanations for their origin are not specifically considered in the supporting documentation by GCL 2, 3, or the URS peer review 4.

__________________________________________________________________________ * In landsliding and slope instability, ‘active’ refers to slope failures, or evidence of slope movements in the form of fresh slump scarps, tension

cracks, deformation ridges, subsidence and uplift features, tilting of trees and poles, and damage to buildings, roads, or underground services.

Confidential 2006


(3) The ‘slope rents’ were regarded as potentially serious land instability features by GCL, who advised a map polygon width of 20 m for the ‘slope rents’, with a 20 m ‘buffer zone’ on either side, giving an ‘avoidance zone’ 60 m wide along ‘slope rents’ mapped through Mt Lyford Village 2, 3. That treatment is consistent with recommendations for some active faults in the MFE guidelines 15 and the ECan Active Fault Database 5, 5a, 5b. However, GNS considers this to be inappropriate for slope instability features, which are generally assessed on the basis of slope conditions and site specific criteria.

(4) GCL’s recommendation that ‘slope rents’ be classed as ‘Class 1 active features’’ in the ECan active faults database for Mt Lyford 3 was therefore unusual. In New Zealand and overseas, active fault classifications 15, 19 are not intended to be applied to landslides or slope collapse features. Landsliding is controlled more by geology and the prevailing geomorphic and hydrological conditions in slopes, not by the recurrence interval of possible triggering events, such as strong earthquake shaking, or whether or not the displaced surfaces are less than 10,000 years old (in fact no surfaces of this age are mapped in the Mt Lyford Village area – see Figures 3 and 4). These aspects are discussed further in Section 3.4.

(5) In our view, the main problem with the geomorphic features identified as ‘slope rents’ at Mt Lyford arises because the features do not have the same characteristics, so it is likely that these features do not have a common origin or hazard and risk potential.

Most of the ‘slope rents’ around the head scarp of the prehistoric landslide southeast of the village, and along the sides of streams, appear to be typical old ‘slump scarps’ (rounded, down-hill facing 5-10 m high scarps) that appear to be related to partial failure of the area adjacent to the old landslide (Figures 7 and 8).

However, the trough or channel-like features on the broad interfluve surface through Mt Lyford Village, and between Lulus Creek and Whales Back Stream do not appear to be typical slope instability landforms (Figure 9). Most of these channel-like features are wet areas which carry storm water runoff rather than permanent streams, and have no obvious vertical offset across them. These features are therefore more likely to be drainage channels on the old fan surface, rather than slope instability features. This possibility was investigated during the GNS ground inspections, during which trenches were dug across three of the main features (Figure 5) to determine their origin and likely significance. Logs of these trenches are shown in Figure 15 and the results of these investigations are discussed in Section 4.

Confidential 2006


3.3 Other features

3.3.1 Areas of slope extension and partial collapse

The AFSF map provided by ECan (Figure 5) also shows areas that are inferred to have undergone significant previous slope extension and partial collapse. The mapped extent of these areas is based on the assumption that all ‘slope rents’ are related to slope collapse. Although some ‘slope rents’ are accepted by GNS to be old slump scarps, many are considered (based mainly on their locations and geomorphic characteristics) to be drainage channels and are unrelated to land instability. While it is accepted that areas of extension and partial slope collapse exist at Mt Lyford, GNS believes that the affected areas are much smaller than those shown on Figure 5. This has significant implications for land instability in the area. The basis for that assessment and the hazard implications will be discussed later.

3.3.2 Uphill limits of slumping and stream incision

The uphill limits of obvious slumping and stream incision shown on Figure 5 is for the most part accurately plotted. On steeper slopes (>15°) below this line a few recently-active, shallow (400–500 mm) slump scarps can be seen at the top of the steep scrub-covered slope below Tinline Terrace. Similar features are present on the sides of Lulus Creek and Whales Back Stream (Figure 6).

Many of these shallow slump features were identified in the initial investigations for Mt Lyford Village in 1987 and 1988 by David Bell 8, 9, who described them as follows 8: “…A number of relatively small landslide features have been identified on the flanks of Lulus Creek and the unnamed stream to its southwest, and these have developed as a consequence of valley incision through the gravel veneer and into the underlying weak mudstones. The rate of scarp retreat is, however, likely to be slow as not to constitute a hazard to log cabin construction in the area provided that due precautions are taken in siting and effluent etc disposal...” GNS agrees with this assessment, and notes that the steeper areas have generally been avoided for building sites. This earlier work by David Bell was not seen by GCL during the mapping for ECan (pers. comm. Mark Yetton, 2006).

3.3.3 Large old landslides

Large prehistoric landslide

The AFSF map (Figure 5) shows the scarp of a possible (inferred) ancient large landslide southeast of the village, but is not discussed by GCL in the supporting documentation 2, 3. This feature is mapped on the draft GNS 1:250,000 geological map 12 of the area (Figure 4) as a prehistoric landslide deposit of late Quaternary age (>12,000 years old). The landslide has a prominent, arcuate head scarp (see Figure 7) and extensive ‘hummocky’ debris (c. 0.25 km2) at least 50 m deep in places, with an estimated volume of about 10 million m3. The slide debris includes large 'blocks' of mudstone which form rounded ‘knobs’ and hollows, and has a thin cover of sandy, silty soil, which is probably loess (Figure 15). The complex old landslide was noted by David Bell during his initial investigations at Mt Lyford in 1987 8, but was not considered to be relevant as it was outside area of development at that time.

Confidential 2006


The geomorphology and geology of the area suggests that this very large landslide is a complex block slide, with failure possibly occurring along bedding near the ‘core’ of a gentle syncline (down-fold) in underlying Tertiary siltstone (Figure 4). Slope failure was probably caused by undercutting of the interfluve by the unnamed stream southwest of Mt Lyford Village, and may have been triggered by an earthquake on the Hope Fault. The rounded and subdued morphology (Figure 7) and loess cover of the landslide debris suggests that failure probably occurred more than 12,000 years ago, after deep stream incision had occurred.

The distribution of slide debris does not suggest there have been multiple slope failures in this area. Although there may have been more than one event when failure first occurred, there is no evidence of recent failures. The landslide mass and the surrounding head scarp area have probably been subjected to many Hope Fault earthquakes in the last few thousand years without further failures occurring. Apart from minor slumping into the stream at the toe (Figure 6), the landslide now appears to have little potential for further movement, and is not regarded as an active geological hazard feature. Old rock slide or avalanche

A large old rock slide or avalanche is located northwest of Mt Lyford Village (Figures 5). Blocky greywacke landslide debris is shown as extending to the bottom of the Mt Lyford ski field road and almost reaching the Hope Fault trace. Although this feature was mapped by GCL, it is not discussed in detail in letters by GCL 2, 3 or the URS peer review 4. GNS agrees that this feature is an old rock slide/avalanche scar and deposit, which show up clearly on the 1966 aerial photos (Figure 2) and recent oblique aerial photos (Figure 7). The failure scar and bush-covered debris on the lower slopes show that it is not a recent landslide. Given the close proximity of this feature to the Hope Fault, it may have occurred during an earthquake on that fault, possibly within the last few hundred years. Historical records 16, 17 and mature beech forest growing on the slide debris suggest it was formed more than 150 years ago. This old landslide was noted by David Bell during his investigations in 1987 8. Although the old landslide debris extended onto the fan inside the area of possible future subdivision development, it was not considered to be a problem as it was ‘no longer active’ 8.

GNS also believes that, because a source area no longer exists at the top of the slope above the rock avalanche scar (now a scree slope, Figure 7) this feature does not represent a slope instability hazard to the existing village area. However, there is potential for similar rock slides or avalanches, and possibly debris flows in the future from the high rocky peak at the head of the Wandle River (Figure 7). Such events may present a potential risk that should be taken into account in the development of a residential subdivision being planned in the area to the northwest of the present village (Figure 13). Although GNS does not think it is appropriate to class these possible future events as ‘Activity Class 1 features’ (hazards), as suggested in the URS peer review report 4, it is important that the risk they present be considered in future residential development of the My Lyford area.

Both of the old large landslides in the Mt Lyford area may have been triggered by very strong earthquake shaking associated with past earthquakes on the Hope Fault. The rock avalanche north of the village is typical of the type of landslides triggered by large earthquakes in New Zealand 16, 17. Similar landslide events can be expected during future large earthquakes in mountainous areas where very steep and high slopes are present. These conditions do not currently exist at the top of the old rock avalanche scar, but they do on the high rocky peak at the head of the Wandle River (Figure 7).

Confidential 2006


3.4 Classification of ‘slope rents’

Because of the association of the ‘slope rents’ and active fault traces on the AFSF map (Figure 5) they are referred to as ‘potentially Class 1 active features’. As already discussed, this was an unusual recommendation, because this activity or recurrence interval -based classification 15, 19 is not normally used for landslides or slope collapse features. The potential for slope instability or complete slope failure depends more on the site conditions (slope angle, height, geology, and groundwater) than the recurrence interval of possible triggering events, such as strong earthquakes or heavy rainfall. In the context of Mt Lyford, the classification of old slump scarps (‘slope rents’) based either on the age of a displaced surface, or the recurrence interval of earthquakes on the Hope Fault, does take into account the present-day slope conditions, which generally control the likelihood of slope failure.

GNS believes it is inappropriate to classify features related to slope failure using active fault recurrence interval/last movement criteria, or because of an inferred association with active faults in the area. Slopes and slope instability features should always be assessed on their own characteristics. Some of the ‘slope rents’ at Mt Lyford are almost certainly old slump scarps related to incipient (partial) slope collapse in the past, as can be recognised along the sides of deeply incised streams in the area, and around the head scarp of the large old landslide. The nature, age, and both past and recent activity of those features must be considered when assessing their hazard potential.

GNS investigations during this review have shown that most of the old slump scarps are subtle, rounded features with a thick coating of loess. They also affect surfaces that are probably at least 25,000–50,000 years old or greater (see Section 4.3), as can also be inferred from Eusden et al.. in their studies of the Hope Fault 13. While it is not always possible to distinguish the smoothed, obscure nature (and the hence age) of these landforms on vertical aerial photos, this is readily apparent on low-level oblique photos (Figures 8 and Figure 9) and on the ground. However, this aspect does not appear to have been considered by GCL or URS in their assessments of the slope features.

Based on the form of the old slump scarps, it is reasonable to infer that they have survived many (possibly 20 to 30) large earthquakes in the last 10,000 years without further slope failure occurring. The suggestion that they should be classified the same as Class 1 active faults is inappropriate and cannot be supported. If it could be shown that the ‘scarps’ have moved as frequently as the nearby active faults, such a classification might be understandable, despite being somewhat irregular from a geotechnical point of view. However, this is not the case at Mt Lyford, and it is therefore recommended that the Class 1 active status assigned to all ‘slope rents’ in that area be removed from the ECan database, and also from future LIM and PIM reports issued by HDC.

Although the old slump scarps might be regarded as potential land instability hazards, because of their inferred age the current and future risk from these features is probably very low under present slope conditions. Those slope rents that are clearly old drainage channels are geomorphically quite different from old slump scarps – they are ephemeral stream channels that carry storm runoff, and they should therefore not be classified as land instability hazard features.

Confidential 2006


3.5 URS Peer Review Environment Canterbury commissioned URS New Zealand Ltd. to carry out a peer review of GCL’s mapping of active faults and ‘slope rent’ features at Mt Lyford Village. The review included a brief site inspection by Tim McMorran (with Dr Yetton) of some of these features on the ground and on aerial photos. The peer review letter signed by Tim McMorran and Don Macfarlane of URS 4 endorsed Dr Yetton’s mapping and recommendations, and noted that “…the ‘slope rent’ features are not strictly active faults, but we consider that they are likely to undergo displacement during a Hope Fault rupture, so it would be appropriate to treat them as Class 1 active faults also. The ‘slope rent’ features appear to be an indication of incipient slope failure, which is a serious risk to residential development because of the potential large deformations that could result from future slope movement…”.

Comments:

(1) URS appears to have accepted the GCL active fault and ‘slope rent’ data, and how it was interpreted and used, without a detailed or critical examination of the evidence it was based on, or suggesting that the evidence be tested and proved. No alternative explanations appear to have been considered or suggested by URS 4 to explain the origin and significance of the ‘slope rents’. Other aspects that were not explored include the nature and age of the active faults and ‘slope rents’, and the criteria used to identify them as being related to active faulting, slope instability, or some other process. These aspects are critical in establishing the hazard potential of such landforms.

(2) This apparent oversight by URS may have arisen because some ‘slope rents’ are obvious scarp features, which clearly appear to be related to slumping on the sides of gullies and around the head of the large old landslide southeast of the village (Figure 7). However, the long ‘slope rents’ through the middle of Mt Lyford Village and the broad interfluve between Lulus Creek and Whales Back Stream are not typical slope instability landforms. These extensive, shallow, trough-like wet areas currently act as storm drainage channels through the village area. Had these landforms been closely observed during the site visit, the possibility might have been considered that they are old stream channels on the ancient fan surface, and are not related to slope instability.

(3) If the origin and significance of some ‘slope features’ had been questioned by URS, further investigations could have been recommended before the data was processed by ECan and passed on to HDC. In a letter to ECan on 18 November 2005, GCL acknowledges that “…the proper evaluation of such (‘slope rent’) features requires detailed engineering geological mapping and subsurface investigations (trenching, auger holes)…” 3. However, such investigations were not specifically recommended by GCL 3

before presenting it to ECan for inclusion in their database and use by HDC. Although the need for such investigations to establish the significance of the ‘slope rent’ features was apparently suggested to Dr Yetton by Tim McMorran during his site visit 3, this was not mentioned in the URS peer review report 4. GNS therefore believes that it would have been more appropriate to propose further investigations of the ‘slope rents’ and the active fault features before making a recommendation to ECan about how they should be classified and treated for planning purposes 4.

Confidential 2006


3.6 Suggested investigation and analysis of ‘slope rents’

An email dated 16 January 2006 from GCL to HDC 6 presents a proposal to investigate the active fault and ‘slope rents’ that GCL identified at Mt Lyford. The proposed investigations included: detailed mapping of active faults, slope rents, shallow and deeper landslides; mapping of soils and rocks; surface seeps and springs; digger test pits across slope rents in ‘key areas’ (for evidence of past movements); one or two drill holes; abney level slope cross sections; and slope stability (sensitivity) analysis using a range of variables. GNS considers that, although the suggested studies were generally sound, it was unlikely that computer-based slope stability analyses would show anything conclusive about slope performance during strong earthquakes. The reasons for this view are as follows:

(a) The slope stability analysis procedure would have involved estimating slope conditions and material properties when the initial (partial) failure occurred, forming the old slump scarps. The parameters needed for the analyses would include: slope cross section, failure surface location and shear strength, groundwater levels at time of failure, and the conditions that triggered the failure, such as high groundwater levels, or very strong seismic shaking (c. 0.7–1.0 g), that could be expected for an earthquake on the Hope Fault. These parameters would have to be estimated. The difficulty with this approach is that, although it is possible (by sensitivity analysis) to duplicate failure in an analytical slope model (using computer software), application of the results is less certain. The assumed parameters at failure have to be analysed using present-day conditions to prove the validity of the results. This is the difficult and generally inconclusive part, because it is often very difficult to locate a potential failure surface and determine its shear strength. It is also very difficult to carry out a dynamic (seismic) analysis of the slope, as an appropriate earthquake accelerogram also needs to be obtained.

(b) To avoid this generally inconclusive procedure it is usually better to assess earthquake-induced slope failure potential using precedent evidence of slope performance in the area during past large earthquakes. In applying this to Mt Lyford, if it can be shown that the slump scarps in the area are old features (this is generally agreed) that were probably formed during or shortly after stream incision, it is reasonable to conclude that those partially failed areas below the old scarps have survived many Hope Fault earthquakes in the last 10,000 years. Therefore, under present day conditions, with little stream incision or toe erosion occurring, the probability of slope failure during a Hope Fault earthquake in the next 50-100 years can be regarded as low. It should remain low providing slope conditions change little over time. The implications of this assessment for planning and development at Mt Lyford Village are discussed later.

The precedent-based approach for assessing the stability of slopes under strong earthquake shaking described above is a widely recognised analytical method. It was used effectively in the assessment of landslides in the Cromwell Gorge 20 (numerical analysis of slope stability during earthquakes was rejected for the reasons outlined above). This approach is now supported by the more extensive studies of earthquake-induced landsliding in New Zealand that have been carried out by GNS since 1997 16, 17. It was used for the GNS assessment of land instability hazards at Mt Lyford Village described in Section 4.

Confidential 2006


4.0 GNS ASSESSMENT OF ‘SLOPE RENTS’ AND ACTIVE FAULTS 4.1 GNS geomorphic mapping

Following the ground inspection, aerial photography and trenching of ‘slope rents’ at Mt Lyford Village in early March it became apparent that some of the ‘slope rents’ were clearly old slump scarps (Figure 8), while others appeared to be drainage channels on the old fan surface. One of these features is slightly deflected by an active fault trace crossing the terrace surface northeast of Lulus Creek (near trench site T1, Figure 9).

In order to determine which of these geomorphic features were old slump scarps and which were drainage channels, and to accurately locate them in relation to properties at Mt Lyford, detailed geomorphic mapping of the area was carried out. All of the ‘slope rents’ shown on the AFSF map (Figure 5) were carefully examined using approximately 60 oblique aerial photos, and classified either as slump scarps, fault traces, or drainage channels based on standard geomorphic criteria and supported by subsurface data revealed in trenches. The criteria used for assessing the origin, and ultimately the significance and hazard potential of each geomorphic lineament shown on Figures 5 and 6 are summarised in Table 1. These geomorphic features, along with landslides and areas of recently active shallow slumping, were plotted on to an orthophoto base map provided by ECan. This allowed plotting to be done with a resolution of ~± 5 m (the size of small trees). The resulting geomorphic map (Figure 6) can be printed out at sizes ranging from A4 to A1 (scales of ~1:16,000 to 1:6000), which allows mapped features to be related accurately to property boundaries in the area.

A number of long ‘slope rents’ features were mapped by GCL on the flat tops of the low spurs between the unnamed stream west of the village, Lulus Creek, and Whales Back Stream (Figure 5a). Geomorphic mapping by GNS (Figure 6) has identified many of these as shallow drainage channels, which form part of a dendritic network of runoff channels on the old fan surface, with most draining into Lulus Creek. These channels are currently wet, swampy areas, which carry storm runoff but not permanent streams (see Figures 9 to 12). The main geomorphic criteria that identify these features as drainage channels (rather than slope instability or fault features) include: (a) sinuous, shallow, trough-like form, (b) their dendritic, interconnecting pattern developed on the broad, flat-topped interfluves of the dissected old fan surface, and (c) lack of bounding scarps or vertical offsets across them, as would normally be expected for gravitational failures on slopes, or dip-slip faulting.

Trenches B, C, and D (Figure 6) were dug across three of the longer and more contentious “slope rents’, which did not appear to be typical (down-hill facing) slump scarps and strongly resembled drainage channels based on criteria listed above and in Table 1. The locations and nature of these features, and trench positions in relation to the terrain, are more clearly shown in Figures 11 and 12. Logs of trenches B, C, and D are shown in Figure 15. None of these trenches showed any signs of disturbance or offsets of soil deposits typically found in ridge-crest troughs and scarps associated with ridge rents or incipient slope collapse 22. Instead, their geomorphic features strongly suggest that all of the longer ‘slope rents’ on the top of the broad interfluves are old drainage channels that have been eroded into the surficial colluvial deposits (gravely, clayey silts) by water draining off the old fan surface. Radiocarbon ages for wood samples from three channels (Figure 15) of 1463 ±35 yrs BP (trench B); 1019 ±30 yrs BP (trench C); and 247 ±30 yrs BP (trench D) were inconclusive.

Confidential 2006


Table 1. Criteria used to determine the origin of geomorphic features at Mt Lyford Village.

Geomorphic Features

Typical distinguishing geomorphic features and criteria

Active fault traces

� Linear scarp-like feature displacing the ground surface, with topographic offsets (vertical and/or horizontal) of late Quaternary age deposits or surfaces (less than c. 125,000 years old); and tilting and warping of these geologically-young surfaces. Trenching usually necessary to obtain geological offset data needed to determine faulting history, slip rates, and recurrence intervals.

� Offset stream and river channels; disruption and ponding of drainage. � Vertical slope offsets are inconsistent with slope direction (non gravitational).

Fault scarps may be down-hill or up-hill facing (antislope), and independent of topography and slope direction; often cut flat alluvial terrace surfaces.

Active faults and old faults

� Topographic lineaments formed by linear courses of streams, rivers, and gullies eroded along weak fault zones. Offset or uplifted spur ends; and alignment of saddles, or sharp changes in gradient on spurs and ridges.

� Fault-line scarps - erosional or geological (features of old and active faults).

Landslide 21, features

� Shorter, slightly arcuate, generally down-hill facing scarps (also up-hill facing) on slopes and sides of valleys. Head scarp ponds, grabens, and hummocky slide debris may or may not be present (depends on age and erosion history).

� Fresh (bare) scarps, ground cracking, and slide debris indicate recent or ongoing slope movements (activity). Rounded scarps covered with soils or slope deposits generally indicate older, incipient (partial) slope failure.

� Down-hill facing (slump) scarps aligned normal to slope direction, with clear potential for downslope movement (generally normal to scarps).

� Evidence of ongoing landslide activity or slope instability in area (fresh scars, cracked and undulating ground, suggesting shallow soil slipping or creep).

Ridge rents 21, 22, 23 (sackungen)

� Ridge-top scarps in mountainous and hilly areas; typically with antislope scarps, but also down-hill-facing scarps, crestal troughs, and closed depressions.

� Scarps may be gravitational collapse features formed by slow aseismic creep and /or strong earthquake shaking (coseismic events).

� Longer, more linear ridge rents may resemble tectonic fault scarps; gravity scarps are commonly much shorter for a given height.

� Sediments within ridge-top troughs typically exhibit extension, offsets, and deformation features that can indicate whether formation was slow and gradual (by slope creep - sackung), or was episodic (i.e. during earthquakes) 22, 23.

Drainage channels

� Shallow troughs or drainage paths on slopes with little or no vertical offset from one side to another. No geological offsets. Several channels may feed into a deeper and longer main channel, which drains into a nearby stream.

� Sinuous wet areas that carry storm runoff but not permanent streams. Generally these features run downslope with a consequent or dendritic drainage pattern, especially on broad fan-shaped surfaces (dendritic drainage is typical of uniform gravely clay (solifluction) deposits where faulting or jointing is insignificant).

� Show no evidence of deep seated slope instability, although scouring and erosion may occur during extreme rainfall events.

� Drainage channels are not land instability hazard features, but they may have weak, wet ground conditions and be affected by periodic flooding.

Confidential 2006


Because of location constraints at two of the trench sites (trees and cables at trench B; road fill at trench C), only one trench D was able to be excavated across the entire feature to prove there were no offsets (fault or gravitational) of near surface soil deposits at the channel sides. However, soil deposits within all trenches showed no evidence of disturbance or offsets, and the two main features trenched (T-B, and T-D) did not have bounding scarps or vertical offsets across the channels. Based on the weight of geomorphic evidence, and supported by subsurface data, the longer slope rent features mapped through Mt Lyford Village (Figure 5) are thought to be drainage channels rather than slope collapse features.

The shorter ‘slope rent’ feature investigated by trench C was more equivocal as the area had been modified by the construction of Mt Lyford Avenue (road fill) and the nearby ‘duck pond’ (Figure 11). This is a relatively short, trough-like feature located below road level; it does not continue southeast of the pond. The channel which was dammed to form the pond drains south into the unnamed stream (Figure 6). If this feature is not a drainage channel it could be another old slump scarp on the side of the unnamed stream, but may be a short subsidiary fault scarp (it has a similar trend to the fault in trench T5 –see Figures 6 and 16e). In the absence of conclusive subsurface data, for planning purposes it may be better to regard this feature as a possible subsidiary active fault scarp (as in Figures 6, 18 and 19) and treat it in the same way as other active fault features in the area.

A number of slope features in the area were identified by as old slump scarps. These are mainly located at the top of steeper slopes along the sides of the deeply incised streams, and around the head scarp of the large prehistoric landslide (see Figures 6 and 9). These features were not trenched as they show a clear relationship to slopes, and appeared to be typical old slump scarps, as shown on Figure 5. Based on geomorphic criteria (Table 1) they are considered to be slope instability features related to incipient slope collapse.

No features that would typically be called ridge rents (slope rents by GCL) were identified in the Mt Lyford Village area. However, the slump scarps on the narrow ridge southwest of the unnamed stream (Figure 6) are characteristic of ridge rents or sackungen 21, 22, 23, both in form and location. These features are quite different from the shallow drainage channels on the broad interfluve surfaces. Sackungen features typically include ridge-top cracks, fissures, and closed depressions or troughs (grabens) with antislope (uphill-facing) scarps (Table 1) that are related to incipient slope failure or collapse of a large area of bedrock.

Because slope features identified as drainage channels (Figure 6) show no evidence of past ground surface or subsurface movements, and are located on broad interfluve surfaces, it is concluded that they do not represent a landslide or incipient slope collapse feature. Drainage channels are not slope or land instability hazard features, but they may be affected by periodic flooding during severe rainstorms (although GNS has seen no erosional scouring or other evidence of this at Mt Lyford), and offer poor foundation conditions because of the soft, wet ground. These features have generally been avoided for building sites in Mt Lyford Village, and should continue to be avoided because of the soft and frequently wet ground conditions, and potential for concentrated surface runoff. Any potential foundation or flooding problems that specific drainage channels may present for future building in the village should be assessed at the site development stage, as would streams and gullies on properties.

Confidential 2006


The old slump scarps are slope instability hazard features. However, their probable age (>12,000 years) suggests that those features have survived many rainstorms and earthquakes on the Hope Fault in the last 10,000 years, and are most unlikely to be reactivated by earthquakes or heavy rainfall under present-day conditions. However, soft ground conditions may cause local instability and foundation problems for buildings and excavations near or across the old scarps. This would need to be determined by a geotechnical report with respect to future building or site development within 10 m of the scarps. Few properties in Mt Lyford Village would be affected by this requirement.

Figure 6 shows the upslope limit of shallow slumping at the top of steeper slopes (~15-25°) along sides of incised streams, and small, shallow (~1-2 m) slides below Tinline Terrace on the side of the unnamed stream, Lulus Creek, and Whales Back Stream. As previously mentioned, these features were described by David Bell during investigations for the subdivision in 1987 8 and 1988 9. From a comparison of the slump scars shown on David Bell’s 1988 map with recent oblique photos, the scars do not seem to have changed much since 1988, and appear to be less active now than they have been in the past. The slump scars are certainly less obvious now because of significant scrub growth in the area (this can be seen by comparing Figures 2 and 6 with Figures 7 to 14).

The geomorphic map (Figure 6) was used to prepare the GNS modified Mt Lyford AFSF map shown in Figure 18. This map distinguishes between the old slump scarps and drainage channels, which were previously shown as ‘slope rents’ on the original map (Figure 5). Confirmed active fault traces are also shown, but those that could not be substantiated as faults, mainly around the head of the old landslide, are shown either as old slump scarps or small drainage channels. A few geomorphic features that could not be substantiated as fault traces have been deleted from the map (they are thought to be small erosion gullies - see Section 3.1). An attempt has also been made to rationalise the plotting of active fault traces north of Mt Lyford Village, with the main change being removal of the thick line 200–300 m southeast of the main trace of the Hope Fault (from the old GNS database), and accepting the mapping of GCL, which agrees more closely with the recent geological and geomorphic mapping by GNS, and also the latest work done by David Bell (pers comm. 2006).

Another change that has been made on Figure 18 is a considerable reduction in the size of areas affected by inferred previous slope extension and partial collapse (compare extent of dotted areas with those on Figure 5). Such areas are still recognised on the steeper slopes of the unnamed stream, areas affected by old slump scarps adjacent to Lulus Creek and Whales Back Stream, and around the head of the large prehistoric landslide. The main residential areas of Mt Lyford Village and most of the interfluve surface northeast of Lulus Creek are not mapped as areas of ‘previous slope extension and partial collapse’, because most of the slope features in those areas are drainage channels, not slump scarps. This change and other interpretative amendments to the slope features shown on the AFSF Map considerably reduce the extent of areas affected by potential land instability hazards and implications for planning and development at Mt Lyford Village.

Confidential 2006


4.2 GNS Investigation of active faults at Mt Lyford 4.2.1 Introduction

Six “paleoseismic” trenches were excavated across five geomorphic features mapped as active fault traces in the area of Mt Lyford Village. These trenches were required in order to answer a number of questions about the mapped fault trace features (Figure 5) in light of ongoing concerns over active faulting and land instability hazards in the village area.

In each case the trenches were excavated to consider:

1. Is the mapped feature an active fault?

2. Based on the age of the faulted surface/deposits and their displacement, what is the geologic slip rate of the feature trenched?

3. How long ago did the last surface deformation (fault movement) occur?

4. What is the recurrence interval (time between) of such surface deformation events?

By answering these questions, GNS believed it would be possible to identify and classify the five mapped active faults, and to apply this scheme of understanding to other mapped features in the Mt Lyford area.

The paleoseismic (earthquake fault) trenches were opened and logged by Robert Langridge on 11 and 12 April 2006. A field review was undertaken by Dougal Townsend of GNS Science on 13 April, at which time some additional deepening was done at each site. Two further trenches were excavated and logged by D. Townsend on 27 and 28 of April 2006.

4.2.2 Neotectonic background

As discussed previously, the Mt Lyford subdivision is built on the lower flanks of the Amuri Range adjacent to Mt Lyford and Mt Terako (3.5 and 4 km north of the village, Figure 7). The active dextral-slip Hope Fault occurs at the edge of the mountain rangefront, above properties in the Mt Lyford village. The Hope Fault can be traced from the West Coast, where it joins the Alpine Fault, to north of Kaikoura, where it extends offshore 24. It has a high slip rate (c. 23 ± 4 mm/yr), short recurrence interval (c. 200-400 yr), and multi-metre displacements during large, surface-rupturing earthquake events 14, 24.

At Mt Lyford, a number of subsidiary faults and other geologic structures, including ridge (slope) rents and landslides have been mapped across the surfaces below the main traces of the Hope Fault through the village and surrounding farmland (Figures 2 and 5). Some of these features have previously been well-mapped and described by Eusden et al.. 13 (Figure 3), who developed a structural model of tectonic deformation across the Hope Fault and Terako fan surfaces (Figure 17). The village area and adjacent farmland cover a number of generally apron to fan shaped geologic surfaces, known as the Terako surfaces, between Highway 70 and the main Hope Fault (Figures 1, 3, and 7).

Given the high slip rate and activity of the main trace of the Hope Fault, it is reasonable to presume that subsidiary (secondary) faults displacing the Terako surfaces and surfaces higher in the landscape might be active and pose a fault rupture hazard in the Mt Lyford Village area.

Confidential 2006


Recent mapping by consulting geologist Dr Mark Yetton (Figure 5) has suggested that these and other structural features are ‘potentially active’ and need to be avoided. The best approach to dealing with these mapped features that we consider to be active faults is to follow procedures outlined according to the Ministry for the Environment (MFE) guidelines concerning active surface-rupturing faults (Kerr et al.. 2003 15, see Appendix 2).

4.2.3 Geologic age control at Mt Lyford

A broad piedmont surface consisting of several fan to apron-shaped geomorphic units occurs downslope of the Amuri Range and Hope Fault rangefront (Figures 3 and 7). Aerial photograph analysis, mapping and field checking undertaken by GNS Science for this review and investigation, and as part of the GNS QMap geological mapping project 12, have identified a number of surfaces that relate to different periods of warmer or cooler climate during the last c. 130,000 years.

Designation and correlating of distinct geomorphic surfaces across the site has allowed GNS to establish the activity of faults and features that traverse the village area. Put simply, the topographically highest, gently sloping, but dissected surfaces southeast of the Hope Fault trace (Figure 7) are probably quite old. Corresponding surfaces have been mapped east and west of Mt Lyford Village (Figure 4) 12. The age assigned to these highest Terako surfaces is c. 130,000 years, as part of a cold climate known as Stage 6 12, 13. [Note: The stages (strictly Oxygen Isotope Stages) relate to globally-correlated late Pleistocene climatic episodes, and are used on the draft QMap of the area, a small part of which is shown in Figure 4. Stage 6 relates to an approximate age range of 186,000–128,000 years] 12 .

Moderately high and dissected surfaces are inset into older Stage 6 surfaces and have been assigned to Stage 4 (c. 71,000–59,000 years 12) and Stage 3 (c. 59,000–24,000 years 12). Small inset fans related to Stage 2 (the last glacial period, c. 24,000–12,000 years BP 12) have also been mapped locally. Stage 1 (the Holocene period, <12,000 years BP 12,

[<12 ka]) covers the current warmer climatic period since the end of the last glacial period.

Terrace surfaces along a section of the Hope Fault including Mt Lyford Village (Figure 3) were mapped by Eusden et al.. (2000). Ages were assigned to the terraces based on a study by Tonkin & Almond (1998) 26 at Charwell River, 15 km to the NE of Mt Lyford Village. The “lower” of the Lottery surfaces in the Mt Lyford area was inferred to be a similar age to the Flax Hills terrace at Charwell River (43-49 ka). However, based on surface morphology, degree of dissection of the remnants and thickness of covering silt (loess), they are probably older, and may be equivalent to Stage 4 (59-71 ka). These older terraces are collectively mapped by Rattenbury et al.. (2006)12 as undifferentiated Quaternary deposits.

The highest of the Terako terraces (Figure 3) was inferred by Eusden et al.. (2000) to be the same age as the Stone Jug terrace (<26 ka) at Charwell River. The upper Terako terrace is mildly dissected and mantled by up to 2 m of stony silt (loess?), a characteristic that is more diagnostic of terraces that are older than Stage 2. This surface was correlated with Stage 3 (24-59 ka) by Rattenbury et al.. (2006) 12, a correlation that we have retained. The lower of the Terako terraces were assigned ages >14 ka and are mapped as Stage 2 (12-24 ka).

Confidential 2006


Thus there is a degree of uncertainty involved with assigning ages to the terrace deposits at Mt Lyford Village. The assigned ages for the older terraces could be in error by as much as one Stage, which will have implications for any slip rate or fault recurrence interval calculated. However, we believe our age assignments and using the scheme used in the QMap program for Oxygen Isotope Stages are sound and we apply this framework to an understanding of the activity of the subsidiary faults. However, because our ages are correlated using a regional geological scheme based on Oxygen Isotope Stages, we believe that they are sound, and we therefore apply this framework to understand the activity of the subsidiary faults within the Hope Fault zone at Mt Lyford.

At Mt Lyford the Holocene period is characterised by erosion and deep stream incision into the old fan surfaces. Some local redeposition of older material has occurred in small channels and in ponded areas, particularly along the fault scarps. Eusden et al. (2000) 13

show a limited occurrence of Holocene deposits (terraces) in the Mt Lyford area (Figure 3). This is in general agreement with Rattenbury et al. (2006) 12. In addition, this scheme of landscape identification and dating has been used at the Charwell River to the east of Mt Lyford (Bull, 1991) 25. At the Charwell River, flights of fluvial terraces can be mapped adjacent to the current river and they correspond directly to climate-driven alluvial periods, e.g. 20 ka Stone Jug surface. At Mt Lyford, the surfaces have a piedmont (alluvial fan) character, but must still be influenced by fluctuating climate-driven process. Note: throughout this report we use the term Holocene to refer to follow the chronology used in the QMap program where the Pleistocene-Holocene boundary is placed at 12,000 years BP (12 ka).

This technique can also be used to date landslide features (slump scarps, ridge rents) that displace those surfaces. This is important here as we found only four small samples of charcoal suitable for radiocarbon dating, which might provide absolute age control on the fault features that were investigated by trenching. [Note: only 1 of these samples has been analysed in time for this report]. If it is considered appropriate to have a better absolute age technique to verify the geomorphic model of surface ages that we have developed here, then we recommend that Optically Stimulated Luminescence (OSL) dating be attempted. This technique relies on dating fine-grained sediments (silts/ sands) rather than organic-based samples, as in radiocarbon dating.

Confidential 2006


4.2.4 Trenches across fault features

(1) Trench T-1

Terako-1 trench was excavated across a linear geomorphic feature which is continuous over about 500 m across the interfluve surface adjacent to the SE end of the village near Doug Simpson’s homestead (Mt Lyford Station; grid ref. N32/ 241571; Figure 6). The trench was c. 12 m long and up to 3 m deep, and was excavated across a c. 1.4 m high, uphill-facing scarp, i.e. the scarp was up-to-the-southeast, causing the deflection of stream drainages and ponding of water against the scarp (Figure 6). The broad, relatively undissected alluvial interfluve surface (which is correlated to Stage 3 and therefore inferred to be c. 24-59,000 years old) is cut and displaced by the fault scarp (Figure 9). Terako-1 trench is located between two apparently deflected streams (drainage channels) on the interfluve; the stream to the southwest flows along the scarp to join Lulus Creek while the stream to the northeast of Terako-1 cuts through the scarp and the uplifted terrace.

The trench was excavated near a pond, with the expectation that in situ organic deposits occurred or were preserved adjacent to the scarp. The trench has a trend of c. 154° and was excavated roughly normal to the trend of the scarp (Figure 16a).

A series of tightly packed, oxidised gravels occur on the upslope side of the trench and correspond to the materials that make up the surface designated as Stage 3 in age (coloured green on Figure 16a). A series of oxidised fine sands and silts abut against these gravels on the uphill side of the trench. Their exact stratigraphic position and relative age is uncertain, but we suspect that these fine-grained units that are approximately co-temporal with the Stage 3 gravels. On the downslope side of the trench, gravels were exposed at the base of the trench, underlying a thick, conformable silt unit. We suspect that the silt represents late Stage 2 deposition (conservatively 12-24 ka, where ka = 1000 years). Near its top, the silt becomes gleyed, i.e. reduced by high groundwater conditions. A soil of likely Holocene age covers the entire length of the trench.

A clear zone of faulting occurs approximately in the middle of the trench, separating units on the upthrown side from the downthrown (northern) side of the scarp. A number of moderately to steeply north-dipping faults separate compact, stiff, oxidised angular gravel on the upthrown side of the fault, from generally massive fine-grained silt (possibly weathered loess) on the downthrown side. The sense of dip-slip separation on these faults is normal. During review procedures on 13 April, the trench was deepened to expose the coarse angular gravels on the downthrown side of the fault corresponding to those observed on the upthrown side of the fault.

Displacement across the top of the coarse angular gravels is c. 2 m vertically, or 3 m in a dip-slip sense, i.e. movement in the plane of the faults. If we infer that these gravels correspond in age to the mapped Stage 3 surface (24-59 ka) then the estimated slip rates for this structure are 0.03-0.08 mm/yr (vertical) or c. 0.05-0.13 mm/yr (dip-slip). Distinct earthquake event horizons were difficult to distinguish. We suspect that one or more mapped stone lines in the thick silt unit (weathered loess?) on the downthrown side of the fault could correspond to surface-rupturing events. There is no clear thickening of the soil profile adjacent to the downthrown side of the fault zone.

Confidential 2006


There are no clear indications of lateral movement in association with the faulting in the trench or in the surrounding landscape. The two deflected streams described above are not dextrally (right-laterally) deflected. In particular, we have considered the stream northeast of Terako-1 trench. Though it meanders near the fault, the main channel axis does not appear to show a dextral step across the fault zone. Therefore, we conclude that this subsidiary fault is a normal, dip-slip fault.

The lack of evidence for many small events suggests that displacements are relatively large (possibly about metre-sized). It is therefore reasonable to infer that the current scarp height (c. 1.4 m) was formed by two events (c. 0.7 m vertical each), and that a third event accounts for the total of c. 2 m vertical separation seen across the trench. This implies a recurrence interval of greater than c. 8,000 years between faulting events. Relying on our argument for geomorphic age control of surfaces and without any radiocarbon material to date, this is probably the best available data for the activity of the structure.

(2) Trench T-2

Trench T-2 (Terako-2) was a 12 m long excavation across a poorly defined geomorphic lineament c. 400 m northwest of trench Terako-1 (grid ref. N32/ 238575; Figure 6). The lineament corresponded to an uphill-facing scarp that created a hillock across an inferred Stage 4 Lottery surface (c. 59,000-71,000 years). This site was chosen because it included a subtle linear trace and is close to anomalous (scarp?) topography near swampy ground. The trench log is shown as Figure 16b.

The trench crossed a c. 1 m high topographic change at the base of the hillock. This part of the slope is underlain by Tertiary age Greta Formation sandstone/siltstone (local bedrock), exposed in the floor of the trench at the uphill end of the trench.

The overlying stratigraphy across the length of the trench was almost entirely composed of fine-grained silt to silty clay with occasional greywacke and Tertiary clasts. These fine-grained units have a moderately-developed soil above them in the subsurface.

Upon review on 13 April, the trench floor was extended and deepened to ascertain the nature of faulting, if any. It was extremely difficult to positively identify faulting as defined by displaced stratigraphy in trench T-2. However, zones of apparent shear fabric seen in the trench (particularly between metres 5-7) were subtle but convincing. In addition, the slope and apparent termination of the Tertiary rock near metre-5, was deemed to be additional evidence for deformation by faulting.

Though we have probably confirmed the presence of a fault at Terako-2 there are many unknowns about its level of activity. These include:

(i) No event horizon or dating information from the exposed stratigraphy.

(ii) No indication of single-event displacement data.

(iii) Limited continuity of the fault trace to the east or west (> 200 m) from the trench site.

Confidential 2006


We suspect that the thick silt sequence exposed on the downslope end of Terako-2 and draping the hillslope at the upslope end of the trench is late Stage 2 (conservatively 12-24 ka) in age. Although we see evidence for shear fabric in this material, no buried scarp had been formed and no displaced horizons, or earthquake-generated colluvial units exist in the trench. This seems to imply that there has been no recognisable faulting event in Terako-2 during the Holocene period (last 12,000 years) and that earthquake recurrence for this feature is relatively long (> 12 ka).

An estimate of the slip rate for the fault could be developed by comparing the height of the hillock and its scarp with the age of the gravels it is composed of. Based on its geomorphic expression, we consider that this fault must have a similar or lower rate of slip to that described from Terako-1 trench, i.e. c. <0.05-0.1 mm/yr. There was no evidence of horizontal movement in the trench or the nearby landscape features to suspect that there was a significant strike-slip component related to this fault.

(3) Trench T-3

Trench T-3 (Terako-3) was excavated across a well-defined uphill-facing scarp feature on the ridge southwest of Mt Lyford Ave in open ground (N32/ 231573; Figure 6). The scarp is continuous over a distance of c. 150 metres. The trench was opened near a man-made pond, where the scarp is smaller and the possibility might exist for preserved, datable organic material in the trench. The trend of the trench is 165º.

In T-3, Tertiary bedrock was exposed at the southeast, upthrown end of the trench, with c. 1m of weathered bedrock above it near the ground surface (the trench log is shown as Figure 16c). The bedrock progressively steps-down to the northwest on a series of faults with apparent normal dip sense (the same as in T-1) to below the trench floor.

On the downthrown (northwest) side of the fault and at the northern end of the trench, rounded to sub-angular, boulder to cobble gravel was exposed. This end of the trench was deepened by the excavation of a pit during the review process, showing that the gravel is at least 1.8 m thick. The gravel unit dips down to the southeast (towards the fault scarp) and is overlain by a poorly-sorted, massive unit consisting of greywacke and Tertiary clasts floating in a silty matrix. This fine-grained (colluvial) unit may have formed as a result of solifluction during a cold climatic period (inferred age Stage 2 – 12,000 to 24,000 years BP). While it was difficult to distinguish between this unit and the weathered silty bedrock (mainly done by the presence or absence of greywacke clasts), it is believed, at least in its lower part, to be faulted against Tertiary bedrock at the southeast end of the trench.

The central part of the trench on the downthrown side was also deepened during the review process, so that the top of the gravel unit was excavated close to the zone of faulting. While the gravel unit has been eroded from the upthrown side of the fault, it is exposed not far away in a track cutting, thereby constraining the total dip separation on the base of the gravel unit to at least 6 metres (c.4 m offset + >2 m thickness).

Age control: A morphologically higher exposure of gravels crops out further up the ridge to the northwest along Tinline Terrace. These gravels appear weathered and oxidised. Inset below these gravels are those that underlie the terrace at the location of Trench 3. Gravels exposed in an old track cutting on the upthrown side of the fault appear moderately weathered. They consist of rounded –to sub-angular boulders and cobbles of greywacke sandstone similar to those excavated in the trench, and are inferred to correlate with Stage 4 (c. 59-71,000 yrs).

Confidential 2006


A small piece of charcoal was found within the fine-grained colluvial unit (Sample Ter 3-1). This is the currently the best opportunity to obtain an absolute date from any of the trenches excavated (at the time of writing this report). Sample Ter 3-1 yielded a radiocarbon age of 76 ± 30 yr BP. This age is calendar calibrated to c. AD 1700-1950. This is obviously a very young result. In the context of this study, we suspect that this age comes from charred material from the most recent forest that grew on the piedmont of Mt Lyford and dates to clearance of that forest. We think it unlikely that the date gives a true reflection of the age of the deposit from which it was sampled and its subsequent faulting. Consequently the origin of this charcoal must be from a burned root fragment of a tree that grew within the silt unit.

Rate of vertical movement: The >6 m offset of the gravels over a time period of c. 59-71 ka yields a minimum vertical slip rate of c. 0.08-0.1 mm/yr (about the same rate as the fault found in Trench 1). An alternative estimate of slip rate can be calculated by considering the geomorphic relations at the trench. As described the coarse angular deposit is not present along the length of scarp where we trenched, but is present further up slope along the fault trace. If we assume that the scarp was eroded down to the Tertiary bedrock during Stage 3 (24-59 ka) and that the coarse Stage 4 gravel was removed, then c. 4 metres of displacement has accrued on this scarp since Stage 4. This produces a slip rate of 0.07-0.17 mm/yr, which is consistent with that described above. There was no evidence of horizontal movement in the trench or the nearby landscape features to suspect that there was a significant strike-slip component related to this fault.

Displacement events/recurrence: This trench shows evidence of multiple displacement events. Supporting evidence for this statement include: (i) the pervasively sheared nature of the Tertiary bedrock, including zones of mottled and sheared rock; (ii) generation of colluviated Tertiary material from the scarp into vertically-sliced zones; (iii) juxtaposition of late Pleistocene or Holocene deposits against faults and fault zones; (iv) tilting of late Pleistocene units toward the fault zone; (v) large vertical separations across deposits in the trench; and (vi) the generation of stonelines within the thick silt unit on the downthrown side of the fault. However, considering the age of the juxtaposed units in the trench that there have been several events since the deposition and/or erosion of the Stage 4 gravels, and a few events since the deposition of the Stage 2 or Holocene silt unit and cover deposits. We observe evidence for the generation of a scarp-derived colluvium in relation to Holocene (< 12 ka) units and perhaps one stoneline in the younger part of the Stage 2 silt sequence. We also observe a possible fault near metre-7 in the trench, that does not extend up into the Holocene soil (a pebbly, sandy grit unit). A preliminary recurrence interval based on these observations is c. >6000 years.

Confidential 2006


(4) Trench T-4

Trench 4 (Terako-4) was excavated across the apparent continuation of the “Tinline Fault” to the east of Mt Lyford village (grid ref. N32/ 227582). Terako-4 was sited in a geomorphically low area to the north of Mt Lyford Ave along a trend of 157º (Figures 6 and 14). This trench was dug across a prominent geomorphic lineament that significantly displaces two ridges, to the south and north of Mt Lyford village that are capped by probable Stage 6 gravels (to the southwest this feature is a subsidiary fault scarp which has been trenched and confirmed by David Bell, pers. comm. 2006). Inset below the ridges, but still higher than the trench site, Stage 4 fans are apparently displaced by a lesser amount. Lower still, near the trench site, a modified (draped) Stage 3? surface has only a small c. 1 m high uphill-facing ‘scarp’. At the trench site itself, there was no real scarp evident – just the back edge of the draped Stage 3 surface which merges to the northwest with a steeper fan of probable Holocene age. As we were aware that the Tinline Fault had been trenched farther to the west by David Bell and was shown to have distinct displacement events, the goal of this trench was to estimate the age of the youngest displacements (if any?) so that its recurrence interval could be assessed, and the elapsed time since the last displacement could be investigated.

No evidence of ground surface fault rupture was found in T-4 (Figure 16d). The oldest units exposed in the trench are gravels that probably belong to Stage 3 (24-59,000 years BP; units 3) is a poorly-sorted, partly oxidised sandy gravel. This gravel unit drapes down towards the northwest where it inter fingers with green-grey silty sand. Overlying the gravel/sand package is a massive, grey-brown gritty silt unit, which thickens towards the northwest. This unit is probably of Last Glacial age (inferred age Stage 2- c. 12,000 to 24,000 years BP). Above this, the modern soil shows evidence of gleying (due to perched water table) also towards the northwest. Several variably-oriented seams of silt and clay were found cutting the massive silt unit. These may be tectonic in origin, resulting from the injection of fine-grained clay mobilised by ground shaking in proximity to the main trace of the Hope Fault.

While no direct evidence of faulting was found in Terako-4, the change from gravel to sand at the northwest end of the trench indicates a change of depositional environment and/or age. Stage 3 gravels mimic the shape of the scarp as seen near the trench, but have probably been truncated by erosion within the trench. The overlying silt unit above thickens towards the northwest in the same place that the transition from gravel to sand takes place, possibly indicating a component of tilting in this direction.

We expected to observe a buried fault at this location. Its absence at this site means that either (a) our trench was poorly located with respect to the fault trace, or (b) that no faulting has occurred within these deposits since they were laid down. However, if the latter is the case, then the implication for this trench is that we observe no deformation in Holocene or Stage 2 deposits (< c. 24,000 yr BP).

It is perhaps more likely that we did not span the zone of faulting with this trench. Other exposures of the Tinline Fault (T-6 and Bell trenches) show it to have Stage 2 faulting activity.

Confidential 2006


(5) Trench T-6

Trench 6 (Terako-6) was sited at the top junction of Tinline Terrace with Mt Lyford Avenue and it was excavated to the southwest of Trench 4 across the same subsidiary trace of the Hope Fault that we call the Tinline Fault (grid ref. N32/ 225580; Figures 6 and 14). Its purpose was to accurately locate the fault where it crosses Mt Lyford Avenue, and to obtain potentially datable materials that would lead to a fault-rupture history.

A branching fault plane striking 236º and dipping 78º northwest was revealed in the trench (Figure 16e), in a position coincident with the ground surface scarp (see Figure 13). Densely-packed, oxidised, sub-angular gravel was exposed on the upthrown (southeast) side of the fault. These gravels underlie the morphologically highest terrace/fan surface that comprises the ridge crest and are inferred to have been deposited during Stage 4 (59,000–71,000 years).

On the downthrown (northwest) side, 2–3 metres of massive to poorly-bedded, pale brown stony silt are offset against the fault, with a 5–10 cm thick layer of pebbly clay (fault gouge?) adhering to the fault. Above the silt, a layer of pale yellow, stony clay appears to overly one bifurcation of the fault and may abut the scarp formed by the fault in trench T-5. This layer appears to be un-faulted and should therefore post-date the last fault movement.

Before Trench 6 was filled in it was deepened a further 1.5 m on the downthrown side with a view to finding the top of the correlative gravels on the upthrown side. These gravels were not encountered. However, at about 3.5 m depth, blue-grey sandy silt resembling Tertiary bedrock was excavated. The lack of gravels could mean that:

(a) there is an over-thickened sequence of colluvial silt and/or reworked bedrock on the downthrown (NW) side; or

(b) there is strike-slip movement on this fault strand that has juxtaposed an area which had been stripped of the Stage 4 gravels and eroded down to bedrock prior to the deposition of the colluvial silt.

We favour the latter option above as strike-slip/oblique-slip motion agrees well with the steepness of the fault plane (c. 80º) encountered in Trench 6. In addition, there is a further 2-3 metres of elevation from the trench to the top of the terrace on Tinline Rd. This means that there is a large apparent vertical separation across the fault. However, there has been little or no generation of gravelly colluvium across the fault as a consequence of vertical movements, suggesting perhaps that strike-slip motion may be important on the Tinline Fault.

Dating potential:

Two samples of charcoal were found in Trench 6, one from the older stony silt (TER6-1) and one from the younger stony clay (TER6-2) (Figure 16e). These samples should bracket (define) the last movement on this fault strand. These samples have been submitted to the Rafter Radiocarbon lab for age analysis. However, the results of those two dates will not be available at the time of the report, and we again need to consider the relative age of units recognised in the trench with respect to our geomorphic model.

Confidential 2006


Slip Rate and Displacement Events:

There is c. 3.2 metres (minimum) of vertical separation across the fault zone in Terako-6 trench, and an additional 3 metres of vertical separation to the top of the offset terrace/fan surface. If we accept that the gravels that underlie this terrace were deposited during Stage 4 (59-71 ka BP), then the associated vertical slip rate across the whole terrace is on the order of 0.09-0.11 mm/yr (probably a minimum). We cannot assess the amount of strike-slip displacement for the Tinline Fault, or its rate of strike-slip displacement.

(6) Trench T-5

Trench T-5 was excavated to the northeast of Mt Lyford Rd across a c.1.4 m high scarp that has trend of 160º (grid ref. N32/ 227581; Figures 6 and 14). This trend is unusual with respect to the generally northeast-striking faults of the area. The aim of trenching here was to determine whether the linear geomorphic feature was an old drainage channel or a fault scarp, and if it proved to be the latter to possibly assess the recurrence interval and age of the last fault movement.

An initial trench was excavated close to the uphill end of a small swamp (area of water ponding) on the lower (southwest) side of the lineament. This excavation, however, soon began to fill with water jetting in from the uphill (southeast) wall and was abandoned for safety reasons. Upon cursory inspection from the top, it was noted that the water was entering the trench in line with a prominent sub-vertical plane (fault) that denoted a change in lithology and lined up exactly with the surface lineament.

Trench 5 proper was excavated c. 50 m further to the southeast (closer to Mt Lyford Avenue) along the same lineament, which turned out to be a fault (Figure 16f). On the upthrown side of the fault, weathered Tertiary siltstone bedrock is overlain by oxidised, sub-rounded pebble to cobble gravels. Some of these gravel clasts lie exposed on the upthrown fan surface to the northeast of the scarp. The stratigraphy of the downthrown side is somewhat equivocal, but consists of massive to poorly-bedded, probably colluvial, gravely silt to silty gravel (solifluction deposit?). A massive gravel unit adjacent to the fault appears to be an infilled fissure comprised of reworked clasts from the gravel on the upthrown side of the fault.

The overall vertical displacement across the trench is at least 2 m (measured from the top of the Tertiary bedrock/base of the gravels to the base of the trench on the downthrown side). This displacement must involve multiple fault-rupture events because the base of the gravels is offset more than the faulted ground surface (c. 1.4 m).

Several steeply-dipping clay seams were seen in the trench. As noted in the nearby Trench T-4 (Figure 6), these clay seams may have resulted from severe ground-shaking brought about by a Hope Fault rupture event. Other steeply-dipping lithological contacts in the trench may also be minor faults, but this was not possible to ascertain in the field due to the nature of the materials being offset (e.g. gravels against gravels).

The origin of this fault is uncertain. With a strike of 160º this “oblique fault” (Figure 16f) is essentially perpendicular to the main trace of the Hope Fault (c. 068º) and the other subsidiary faults on the piedmont surface. There is no evidence for strike-slip movement along this scarp, thus, it is a normal, dip-slip fault, and in some way must act as a tear fault or subsidiary bedrock fault. Possibly in the context of this part of the study, its origin is less important than its activity and recurrence interval.

Confidential 2006


Dating potential:

A pod of gravely silt with abundant fragments of charcoal (Ter5-1) was noted on the downthrown side of the fault. This sample has been submitted to the Rafter Radiocarbon Lab, but the result of this date will not be available at the time of this report. However, we believe that sample Ter5-1 should be older than the most recent event and therefore give some constraint on the timing and recurrence interval of this fault.

Slip Rate and Displacement Events:

Without the absolute age control from radiocarbon dates, we can still use the information from geomorphic relationships in the trench and its surroundings to consider the slip rate of this fault. The likely age of these gravels is inferred to be Stage 3 (24-59 ka BP). The base of the iron-stained gravels corresponding to Stage 3 are offset c. 2 metres vertically. This yields a minimum vertical slip rate range of 0.03-0.08 mm/yr. There is no evidence of lateral movement on this feature in the trench or in the landscape geomorphology.

There is evidence for multiple events in Terako-5 trench. It is clear that the youngest sedimentary unit (silty gravel) below the modern soil is faulted. In other trenches we have correlated the fine-grained upper unit to Last Glacial time (Stage 2 – 12-24 ka BP). This unit appears to be displaced by 20-40 cm based on the stratigraphy present. The Holocene soil is draped over the fault scarp, but is not displaced by the fault. Additional displacement may have occurred near sample Ter5-1, though the relationships are equivocal.

Confidential 2006


4.2.5 Summary of fault data in trenches

The six trenches excavated across five separate structures near Mt Lyford village have allowed us to sample the nature and activity of faulting on subsidiary structures outboard of the main zone of the Hope Fault. Below is a brief summary of the results from trenches Terako-1 to -6. The geomorphic model of surface ages allows us to compare the results at each trench, as shown in Table 2.

Table 2. Summary of seismic hazard parameters for the Terako trenches.

Terako trench number

Estimated minimum vertical slip rate

(mm/yr)

Estimated single-event displacement

(m)

Recurrence interval (yr)

T-1 0.03-0.08 C. 0.7 > 8000

T-2 <0.05-0.10 ? � 12,000

T-3 0.07-0.17 ? c. > 6000

T-4 (Tinline Fault) ? - � 12,000

T-6 (Tinline Fault) >0.09-0.11 1.0 ? c. 10,000

T-5 0.03-0.08 >0.2-0.4 > 2500-13,300?

These results have major caveats as they remain very incomplete. It is clear that the time (t) parameter that goes into these calculations is inferred from our geomorphic model of landscape ages. In addition, we have reasonably poor control on the amount of single-event displacement.

However, in summary, we consider that the data for vertical slip rate shows that as a set of faults, the five subsidiary faults that we studied have low rates of slip - probably all under 0.2 mm/yr (vertical slip rate). In addition, aside from trench T-6 across the Tinline Fault where there is an equivocal stratigraphic relationship, we see no evidence for strike-slip (horizontal) movement in relation to any of the trenches or faults studied. Therefore, we presume that the values of vertical slip rate that are calculated, though generally minimum values, are meaningful assessments of the fault slip rates on these subsidiary faults.

It may be that the Tinline Fault, being the most continuous subsidiary fault and in close proximity to the main trace of the Hope Fault, also carries some component of strike slip movement. As a test of this we looked at the potential offset of topography along the Tinline Fault in two places: (i) on the spur between Lulus and Whales Back Stream, and (ii) on the spur adjacent to T-6 at the top of Tinline Tce. Both spurs are capped by piedmont deposits downslope of the Tinline Fault.

Confidential 2006


The former spur (i) has deposits correlated to Stage 6 (130-186,000 yr). This spur does not appear to be significantly displaced from its upslope topographic equivalent (ridgeline). The same can be said for the spur at Tinline Tce (ii), which is inferred to be capped by Stage 4 deposits. We conclude therefore, that there is effectively no strike-slip on this fault or the other faults, and that true strain partitioning occurs on the subsidiary faults, i.e. they are pure dip-slip (vertical-slip) structures.

In most trenches it was difficult to assess the magnitude of a single event displacement. In those cases where we were able to, the values are based on assumptions about how many event horizons were recognised to account for the scarp height or vertical displacement. It appears that there is generally not an abundance of stone lines, colluvial wedges or buried soils that help determine at what stratigraphic level the faulting events occurred. Therefore, the assumption has generally been made that there are few displacement events, and that the magnitude of displacements may be on the order of 0.7-1.0 m per event.

An alternative to this assessment is that these subsidiary faults rupture frequently with the main trace of the Hope Fault, i.e. c. every 200-400 yr. If this is so we expect that there would be evidence for this type of behaviour, such as a thickening of the Holocene soil or Holocene sediments against the fault scarp on the downthrown side, as seen in Terako-3. This is not the case. We typically see a soil of relatively even thickness formed (presumably) throughout the Holocene across the whole scarp in each case. In several cases the faulting penetrates to the base of the soil and has not continually faulted it (every hundreds or thousands of yr).

As an example, let us consider the displacement in T-5 as if it occurred progressively each time the main trace of the Hope Fault ruptured. There is at least 2 m of displacement there that is considered to have accrued on Stage 3 (24-59 ka) deposits. If this fault ruptured every 200-400 yr, then each of 61-328 displacement events on the Hope fault would have caused 6-33 mm of movement on the fault observed in T-5. This would be: (i) very hard to observe due to its small size; (ii) hard to date; and (iii) only a minor hazard to a built structure if it occurred. Because we do not observe progressively thickening soils (up to 33 mm/ event), we prefer our single event displacement model of 0.7-1.0 m/event. Our assessment of the displacement in that trench is that some events may be 0.2-0.4 m in size. This would yield a recurrence interval of 2500-13,333 years (Table 2). This result is difficult to understand in relation to the other 5 trenches that yield low slip rates and long recurrence times, particularly when the feature is almost normal to the main Hope Fault and other subsidiary faults and should thus be of lower activity. The 0.2 m single event displacement is probably too small, and the recurrence interval should be at least 5000 years.

Similarly, for recurrence interval, data is based on assumptions about how many potential event horizons are recognised in each trench, compared with the relative age of the faulted surfaces and gravels that are exposed. We believe we have calculated relatively conservative recurrence interval results that are probably � 6000 years in each trench. The only evidence for possibly Holocene faulting (<12,000 years) in any of the trenches at Mt Lyford is seen in Terako-3 trench.

We accept that our age assessment of geomorphic surfaces and single event displacement could be flawed in some respects. At face value our results put all of the faults into recurrence interval class IV of the MfE guidelines (Kerr et al. 2003), i.e. with RI 5000-10,000 yr. Based on this uncertainty, if we were to build in an extra level of conservatism into our approach, we could accept that it is possible that these faults could belong in recurrence interval class III, i.e., with a recurrence interval of 3500-5000 years. This also perhaps covers the level of uncertainty that the results from Trench-5 bring to bear on this study.

Confidential 2006


4.2.6 Structural model of the Mt Lyford piedmont

An amended version of Figure 6 of Eusden et al. (2000) 13 is included here as Figure 17. This figure provides a useful structural model of the Mt Lyford piedmont area and the nature of the zone of subsidiary faulting in the footwall of the main Hope Fault. For correlation of the structure to the trench excavations we have included approximate locations of all 6 “paleoseismic” trenches in the Mt Lyford Village area. We note that in all six trenches the faults have normal separations (they are normal faults). We did not trench any reverse-slip features. In this respect the cross-section B-B’ on Figure 17 is more appropriate to the structure observed in trenches, than is A-A’. This model also agrees with our prognosis of little to no strike-slip movement in the zone of subsidiary faulting.

Let us consider the application of this model to the MfE Guidelines. The Guidelines describe zones of “distributed” faulting. This is the case where not one single sharply defined fault defines the mapping of a structure, but where deformation is partitioned across a zone or a number of active fault traces. Mt Lyford is a rather special case, but we consider that the subsidiary faulting here is akin to “distributed” faulting described in the Guidelines, even though it occurs across a width of 1-1.5 km from the main trace of the Hope Fault.

Another question that has arisen is whether these faults operate with the Hope Fault or whether they constitute their own seismogenic sources. Faults that form their own seismogenic source, i.e. faults that rupture due to large earthquakes, generally are named features on geological maps and extend to the base of the seismogenic zone in the crust (generally 10-12 km). We have generally not named the subsidiary faults here for this reason, i.e. we do not expect them to be individual sources, except for the Tinline Fault, that we trenched in two places and has also been trenched by D. Bell and J. Campbell to the west of the main village. Whether the subsidiary faults themselves are seismogenic sources remains in question. The structural model of Eusden et al. (2000) shows how these faults might inter-connect with each other along strike, and at depth to the main Hope Fault (Figure 17). The faults shown extend to depths of up to 2.7 km in the fault plane and lengths of up to a few km. We consider it unlikely that they constitute separate rupture sources from the main Hope Fault, but instead believe it more likely that the subsidiary faults act together with the main Hope Fault, in sympathetic fault ruptures, even if they do not rupture every time the main trace does.

Confidential 2006


4.2.7 Implications of active fault studies

The first part of our discussion concerning the activity of faults in the Mt Lyford Village area has involved presenting the results of fault mapping (Figures 6), geomorphic assessment of landscape features and their relative ages, and documentation of faulting in six paleoseismic trench excavations (Figs 16a-f). This has been necessary to both attempt to quantify the activity of the faults and to help clear up significant confusion that exists about the treatment of faults in the village by different organisations and the application by ECan and HDC of regulations with respect to these faults. The following discussion is concerned with active faults, and not other potential natural hazards in the village such as landsliding.

To help convey the arguments and recommendations by GNS Science, it is useful for us to pose and answer a series of targeted questions on the issues regarding active faulting. These are:

A. Who establishes the Active Fault Guidelines and who is responsible for implementing them?

The Active Fault Guidelines (strictly, “Planning for Development of Land on or Close to Active Faults” see Appendix 2) (Kerr et al.., 2003) were developed by the Ministry for the Environment (MfE) to establish recommendations concerning building practice related to active faults. These guidelines were commissioned by the Parliamentary Commissioner for the Environment following cases brought up by the Geological Society of New Zealand (GSNZ). The MfE guidelines were researched by a multi-party team including experts in the field of geology and earthquake engineering from MfE, GNS, GSNZ, the NZEE Society, and BRANZ. According to the Resource Management Act (RMA) the responsibility for dealing with natural hazards and implementing such practice sits at the District Council level and should be included in the district plan.

B. Are we (GNS Science or HDC) required to follow the MFE Guidelines or those established by Environment Canterbury?

GNS Science recommends that the MfE Active Fault Guidelines (Appendix 2) be used by all regions and districts in New Zealand as a national standard in dealing with hazards related to surface faulting. These guidelines effectively supersede the guidelines produced by ECan and we hope that ECan will take up the MfE guidelines for use with all of its districts.

The ECan policy directives with respect to active faulting, based on Pettinga et al.. (1998) 19 differ significantly to the MfE Guidelines, outlined in Kerr et al.. (2003) with regard to the (recurrence interval) classification of faults and the dimensions of Fault Avoidance Zones. This has created some confusion as elements of both policies/guidelines have been combined and submitted to Hurunui District council (see ECan letter to HDC dated 8 September 2004) 5.

The MfE guidelines describe fault activity in terms of the general recurrence time (see Table 7.1, Appendix 2) for most active faults in New Zealand, and how to deal with fault complexity. The recurrence interval classification is designed to be “wedded with” the Building Importance Categories (BIC –Table 9.2) to produce consent guidelines, and gives recommendations on Fault Avoidance Zones. Importantly, different risk criteria are established for low versus high-hazard faults and low versus high importance buildings. In addition, a comprehensive compilation of active faults in New Zealand (the GNS Active Faults database) can be viewed online at http://data.gns.cri.nz/af/. We consider this database as the national reference for initial information on active faults in New Zealand.

Confidential 2006


The MfE guidelines, as a combined recurrence interval and building-based classification can be more easily related to accepted levels of risk in the current Building Code. GNS believes this is important so that there is consistent treatment of both short recurrence interval (higher risk) and longer recurrence interval (lower risk) active faults throughout New Zealand.

If it is accepted that the MfE guidelines provide the national point of view for application of Active Fault concerns at District Council level, we can consider and answer the following questions concerning the activity of faulting in and adjacent to Mt Lyford Village and comment on the implications of the faults for current and future land use. These questions and answers are as follows:

(1.) How active are the faults in Mt Lyford Village?

Our trench excavations have shown that all of the features investigated, at our current level of understanding, are features of low activity. The MfE guidelines 15 define fault activity with a classification based on recurrence interval (RI) classes. These are: Class I (� 2000 yrs); Class II (2000 – � 3500 yrs); Class III (>3500 – � 5000 yrs); Class IV (>5000 – � 10,000 yrs); Class V (>10,000 – � 20,000 yrs); Class VI (>20,000 – � 125,000 yrs) as defined in Table 7.1 of Appendix 2. Trenching and geomorphic assessment strongly suggests that all of the trenched faults, and likely all subsidiary faults, at Mt Lyford have recurrence intervals of �6000 years. This places these faults in recurrence interval class IV (or >IV). Conservatively, due to a lack of certainty, we will assume that these faults could be placed in recurrence interval class III (3500-5000 yr). The MfE guidelines (Appendix 2) also show the relationships between the faulting recurrence interval classes and Building Importance Categories (BIC) (Table 9.2), and the application of active fault data for resource consent purposes in ‘Greenfield’ sites (Table 11.1), and for developed and already subdivided sites (Table 11.2).

(2.) What is the likelihood that these faults will move and when?

For faults with recurrence interval class III (or greater) the risk of fault rupture in the next 100 years or so is thought to be relatively low, and should be evaluated as such, as it is for other extreme (long return period) hazard events. Although it is possible that such a fault could be close to failure, i.e. be at or beyond its recurrence time, this is either unlikely, or we do not currently have enough knowledge to predict the exact timing of such an event. Therefore, GNS believes that the risk they pose, at a societal level, is relatively low.

(3.) Are the faults connected to the Hope Fault and will they rupture together?

Geologic studies along the main trace of the Hope Fault show that it has a recurrence interval of c. 200-400 years in this area (Langridge et al. 2003) 14. Eusden et al.. (2000) 13 show that the subsidiary faults that traverse Mt Lyford village probably do merge with the Hope Fault at some depth beneath the surface (Figure 17). Our trenching studies imply much longer recurrence intervals for the subsidiary faults compared with the main Hope Fault. While it is likely that rupture of the main trace of the Hope Fault causes co-temporal rupture on some subsidiary faults, it must happen rather infrequently compared with the recurrence of rupture on the main trace.

Confidential 2006


(4.) Should faults in the area be regarded as part of a “distributed’ Hope Fault zone, or as individual subsidiary faults?

The subsidiary faults at Mt Lyford should be considered as a broad km-wide zone of active faulting associated with the Hope Fault. This is a complex issue and the zone of deformation expressed in the piedmont surface at Mt Lyford was probably not considered when the MfE Guidelines were established, i.e. structurally it is rather unique. Most distributed zones of deformation around faults in NZ may be tens to 100 m in width. However, the zone of piedmont faults as proposed by Eusden et al. (2000) 13 is up to 1.5 km wide from the main trace to the edge of the zone of deformation (Figure 3). Under the MfE guidelines this would place all faults in the village in RI Class I (i.e. <2000 yr recurrence interval), through association to the main trace of the Hope Fault. This would require more restrictive consent activity status be applied to these faults. However, the MfE guidelines also stipulate that if detailed geological work is undertaken on the faults and an assessment of the activity is achieved, then it is possible to treat these structures in their own right. Our detailed mapping, trenching and geomorphic assessment has yielded specific information on these faults that places them in recurrence interval Class IV (5000-10,000 years; conservatively as low as RI class III) rather than Class I.

(5.) Do these faults need to have Fault Avoidance Zones attached to them?

The subsidiary faults at Mt Lyford Village are mapped and recognised as active faults in a distributed zone of deformation. In this regard, they need to be identified for the purposes of a District Plan. Our geological studies have placed this set of subsidiary faults in a recurrence interval class with interval 5000-10,000 years (RI class IV), though we have chosen to conservatively place the faults in the 3500-5000 years (RI class III), allowing for the possibility that the assignment of geomorphic surfaces that we have developed is too simplified. If the MfE guidelines are followed – and these faults are within RI class III ( or IV) then there is no need for a fault avoidance zone about these faults at present, i.e. for BIC 1, 2a and 2b structures in the village.

The MfE guidelines are slightly more restrictive for “greenfield” sites, i.e., those that are neither developed or subdivided. HDC will need to consider this variant when dealing with the new planned (west) portion of Mt Lyford village.

(6.) In comparison, how would the main trace of the Hope Fault be zoned?

The main trace, with a recurrence interval of <400 yr, is a Recurrence Interval Class I structure. A minimum 40 metres wide (± 20m about the edges of the fault trace), but generally wider, Fault Avoidance Zone would be mapped about the fault. Usual practice is to create a buffer zone of 20 m width about both the base and top of the fault scarp, or about the zone of deformation. Without appropriate geological investigations of the fault, only BIC 1 buildings, e.g. sheds, could be permitted in this zone.

Confidential 2006


(7.) What type of activity (building) can take place around the subsidiary faults?

If we conservatively consider the faults as RI Class III faults, then the construction of Building Importance Category 1 (accessory and farm buildings ) and 2a (single-storey timber-framed dwellings) is permitted and the construction of 2b (two-storey timber-framed) structures is discretionary at “Greenfield” sites, e.g. the new part of Mt Lyford Village, where the faulting is distributed. At already developed or subdivided sites, which includes the current Mt Lyford Village, BIC 1, 2a and 2b structures are permitted activities on or adjacent to the faults. In addition, BIC 3 buildings, e.g. structures where people and communities gather is a discretionary activity at already developed sites. This type of development is probably not possible in the current Mt Lyford Village due to the nature of the subdivision.

(8.) Can I build within the 20 m buffer area of these RI Class III faults?

If the MfE Guidelines are followed, construction of BIC 1 (accessory and farm buildings), 2a and 2b buildings (one and two-storey timber-framed dwellings) in the current Mt Lyford village should be permitted (see Tables 11.1 and 11.2 in Appendix 2). At the level of a Class III fault, the risk of surface rupture during the expected life of those structures at Mt Lyford is deemed to be acceptable under the MfE guidelines.

9. My property has a Fault Avoidance Zone crossing it, will this show up on a PIM or LIM report?

The question of how natural hazard information such as this appears in property information is up to the council involved, e.g. HDC. MFE Active Fault Guidelines provide a useful basis for decision making, which ultimately resides at the District Council level.

10. My house seems to be within 20 m of a fault scarp, what action do I need to take?

Many of the houses in Mt Lyford Village fit into BIC 2b, i.e. 1 or 2 story log-house dwellings. With respect to the Class III faults through the village, this activity is permitted in an ‘already subdivided site (see Table 11.2 in Appendix 2). Therefore, no action is required by the landowner. It may be that such information appears in a PIM or LIM report. However, the council would need to explain the level of natural hazard (risk) that is associated with the feature in its LIM report.

Confidential 2006


To summarise this section, it is clear that the subsidiary faults at Mt Lyford Village have a relatively low level of geologic activity (c. <0.2 mm/yr) and have recurrence intervals of greater than a few thousand years (RI Class III or greater). This amounts to a relatively low level of risk from this natural hazard. The MfE guideline accordingly allows significant flexibility in the consent process for building on or adjacent to such low activity features. Within the village, which has already been subdivided, resource consent categories recommended by the MfE guidelines for BIC 1, 2a and 2b structures would be permitted, even for those buildings already within a designated Fault Avoidance Zones (>40m wide) related to these faults.

The decision on how this information is relayed through PIM and LIM reports is ultimately the responsibility of Hurunui District Council. GNS recommends that the information be presented openly and accurately. It is not the function of the MfE Guidelines to defeat development or to penalise landowners who have built near faults unknowingly. The main purpose is to provide criteria for avoiding the most active faults in New Zealand and providing sensible planning advice with respect to active faults as one of many natural hazards that this country possesses. In many cases, for slow-moving (long recurrence interval) faults it should be possible to plan and/or build in the vicinity of these features as they constitute a relatively low risk to society.

Confidential 2006


4.3 Revised active fault and slope features map and Land Stability Classes

GNS has prepared a revised version of the AFSF map to help HDC differentiate between areas of different geological hazard at Mt Lyford Village (Figure 19). This map shows active faults and slope instability features, with land areas separated into three Land StabilityClasses based on their geomorphic characteristics and land instability potential. Previously contentious slope features which are now mapped as drainage channels are not shown on this map as they are not classed as hazards. The Land Stability Classes are proposed to clearly identify areas that have or do not have landforms indicating past or present land instability in relation to properties within the village. The criteria used to define the Land Stability Classes and their land instability hazard implications are as follows:

Land Stability Class 1: Areas with gentle to moderate slopes (<15°) without any obvious or significant slope instability features or constraints. Some localised slope undulations and old drainage channels are present, but these are not classed as land instability features. Hazard Implications: There are no slope instability hazards.

Land Stability Class 2: Gentle to moderately steep (10-20°) areas below or adjacent to rounded, inactive old slump scarps, indicating incipient or partial slope failure in the past. Hazard Implications: The ‘old’ slump scarps are thought to be at least several thousand years old. They do not show signs of recent activity and they are not regarded as active land instability hazard features. Precedent analysis suggests they are unlikely to move during a Hope Fault earthquake, or during heavy rainfall under present slope conditions. However, the possible presence of weak materials associated with inactive old slump scarps may cause local slope instability or foundation problems for buildings, and excavations across them. This would need to be determined by a geotechnical report with respect to future building or site development within 10 m of these scarps.

Land Stability Class 3: Areas with steep slopes (15-25°) on the sides of deeply incised streams, and evidence of shallow slump scars present in places (see Figure 6). Recent growth of scrub and exotic forest cover limits the extent and severity of this instability. There are few signs of recent slope failures except on the steep sides of streams (see Figures 6 to 14). Hazard Implications: Within this area a minimum set-back distance of 5 m from areas of active shallow slumping, and pole foundations required on steeper slopes, with a minimum depth of 500 mm into bedrock has previously been recommended by David Bell 9. These recommendations are endorsed by GNS, and we believe have been followed by HDC. Applications for future building within 5 m of Class 3 areas should be accompanied by a geotechnical report, at which stage building designs and safe setback distances should be determined. Where possible, the existing vegetation cover should be maintained on this land in the future to reduce potential slope instability in the area.

Confidential 2006


5.0 DISCUSSION

The remaining issues to be discussed relate to the collection of active fault and slope instability hazard data at Mt Lyford Village, and its use by ECan and HDC in relation to future LIM and PIM reports on Mt Lyford properties.

5.1 Collection and peer review of data The review presented in Section 3 of this report has highlighted potential quality control issues that can arise in the collection of geological hazard data by or on behalf of regional councils, who pass the data on to city and district councils for use in planning documents, and in LIM and PIM reports. The main points and issues of concern that have arisen over the provision and use of the Mt Lyford active fault and land instability hazard data by ECan and HDC are as follows:

(a) The collection and provision of active fault and land instability data at the District Plan level, or for statutory purposes, needs to be carried out to the highest possible standards, using the most up-to-date data available, and generally accepted geological mapping and classification criteria. The data presented should be supported by a report which describes how the mapping was done, the reference material (aerial photos, maps, technical references) and methods used for data collection, details of any field checking carried out, and the criteria used for the interpretation and assessment of mapped landforms.

The reconnaissance scale mapping by GCL of active faults and slope features at Mt Lyford was a valuable contribution to the collection of data on features that were potentially related to active faulting and slope instability in the area. However, because there was uncertainty about the locations, origin and hazard significance of the mapped features, HDC arranged for further geotechnical investigations to be carried out, and notified Mt Lyford residents accordingly 27.

(b) The classification of active faults and land instability features for inclusion in Regional and District Council databases should always be based on generally and professionally accepted criteria and usage. Informal or ad hoc classification of land instability features, as was done at Mt Lyford Village, can cause confusion and uncertainty for councils over the use and application of data. Where uncertainty exists about the origin and significance of geomorphic slope features, it would be better to recommend further investigation of those features before classifying and entering them in regional council databases, which ideally should contain only confirmed geological hazard data.

(c) In planning for the development of land on or close to active faults, or collecting and classifying data for that purpose, GNS believes it would be better for Regional and District Councils to use the guidelines published by the Ministry for the Environment (Kerr et al.., 2003) 15. These are the most recent guidelines for mapping, classifying, and assessing the risk from active faults to buildings in New Zealand.

The active fault classification included in the MfE guidelines (see Appendix 2) is based on faulting recurrence interval, which can be related to accepted levels of risk in the current New Zealand Building Code. The Faulting Recurrence Interval Classes (I-VI) are used in concert with Fault Complexity and the Building Importance Category (BIC) of buildings to assess the relative risk posed by active faults. This defines whether different types of buildings on or close to active faults are either: permitted; qualified permitted (may be controlled or discretionary); or non-complying.

Confidential 2006


A minimum ±20 m-wide Fault Avoidance Zone about the fault trace would normally be recommended for non-complying, and possibly controlled or discretionary situations, but not for permitted buildings. This more recent approach to managing risk from active faults is preferred by GNS to the Pettinga et al. (1998) 19 methodology currently used by Environment Canterbury (see Appendix 1).

(d) The ECan (Pettinga et al..1998) 19 and MfE (Kerr et al.. 2003) 15 systems for classifying active faults and slope features at Mt Lyford are summarised and compared in Table 3. This highlights the main differences between the ECan/GCL and GNS interpretations of these features, and the implications that these classification systems have for existing and future buildings at Mt Lyford Village, as discussed above. In particular it emphasises the fact that there is little evidence of Holocene faulting (<12,000 years) in any trenches at Mt Lyford (the exception is Terako-3), and also that the subsidiary faults in the area are within a distributed deformation zone, and have much longer recurrence intervals and thus present a lower level of risk than main Hope Fault trace.

(e) Classification of active faults and land instability features for inclusion in Regional and District Council databases should be based on generally accepted criteria and usage. Informal or non-standard classification of land instability features, as occurred with the Mt Lyford data, can cause uncertainty for councils in the use and application of data. Ideally, GNS believes that only confirmed geological hazard data should be used for planning documents or for statutory purposes. Where uncertainty exists about the locations, origin and hazard significance of geomorphic features, we believe that it would generally be better to recommend further investigation of those features before classifying and entering them into regional council databases.

(f) Peer review is often an important part of the collection and interpretation of active fault and slope instability hazard data. For this process to be effective, it is essential that peer reviews critically examine how the data were collected, interpreted and reported, and possible implications of the conclusions and recommendations that are presented. It was unusual that comments about the need for investigations to establish the significance of the ‘slope rents’ at Mt Lyford, which were reported to have been made by the reviewer in the field 4, were not included in the URS report.

Having been provided with the Mt Lyford active fault and slope features data collected by GCL, and then having it peer reviewed by URS, it was reasonable for ECan to accept that data and to enter it into the ECan database and pass it on to the HDC. Dr Yetton expressed concern to HDC about the accuracy of his reconnaissance scale mapping of the active faults and slope rent (instability) features, and pointed out that it “…should not be taken as a reliable guide to future risk in relation to individual house sites or lots” 3. These concerns were addressed by HDC by initiating further investigations of those potential hazard features, which GNS has now completed.

GNS therefore believes that ECan and Hurunui District Council* were correct to inform Mt Lyford land owners of their intention to refer (in a general rather than site-specific sense) to the new active fault and land instability hazard data at Mt Lyford (Figure 5) when issuing LIM and PIM reports and considering future resource consent applications.

� Joint letter from Ecan and HDC to Mt Lyford property owners dated 27 January 2006 27.

Confidential 2006


Table 3. Comparison of ECan/GCL and GNS classifications of active faults and ‘slope rent’ features, recommended avoidance zones, and implications for buildings at Mt Lyford Village.

CLASSIFICATION OF ACTIVE FAULT AND SLOPE FEATURES AT MT LYFORD VILLAGE Active Fault / Geomorphic Feature

ECan (Pettinga et al.. 1998) GNS (MFE Guidelines, Kerr et al.. 2003)

Main Hope Fault Trace Activity Class 1 (<10,000 years) Recurrence Interval (RI) Class 1 (� 2000 years)

The Main Hope Fault trace is well defined, and has a very short (200-400 year) recurrence interval. Presents a high risk of fault rupture to buildings close to or across the fault.

- Fault Avoidance Zone 60 m - 20 m on both sides of fault, plus fault zone width of ~20 m at Mt Lyford (line width on map)

20 m on both sides of fault, plus width of fault zone at Mt Lyford (~20 m line width on Figure 6).

- Implications for buildings No existing or proposed buildings at Mt Lyford Village close to Main Hope Fault Trace

Subsidiary traces within the Hope Fault zone

Activity Class 1 (<10,000 years) (~18 mapped, 10 in subdivided part of village)

RI Class III (>3500 -� 5000 years) –conservatively (trenching of fault traces suggests >c.6000 years)(~9 mapped, 4 in subdivided part of village). There is minimal evidence of Holocene faulting (<12,000 years) in any trenches at Mt Lyford.

- Fault Avoidance Zone Recommended width 60 m (20 m on both sides of fault, plus fault zone width of ~20 m at Mt Lyford -line width on map)

Minimum width of c.50 m (20 m on both sides of active faults mapped in Figure 6 of GNS Report (line width of faults on map c.10 m).

- Implications for buildings For faults with RI Class <10,000 years: Residential/small commercial/industrial buildings: � Restricted or discretionary within fault avoidance zone (60 m)

Subsidiary (secondary) faults have much longer recurrence intervals, and are within a distributed deformation zone (southeast of Main Hope Fault). Present a lower level of risk than main fault. � Within existing Mt Lyford Village: BIC structures1, 2a, 2b (residential housing) permitted (without restriction) within fault avoidance zones on or adjacent to faults. � At ‘greenfield’ (new) sites outside the village area BIC structures 1, 2a, 2b is discretionary within fault avoidance zones.

Old slump scarps (‘slope rents’ on ECan map)

Potentially Class 1 active (slope collapse) feature (same as active faults within Hope Fault Zone)(~20 mapped in subdivided part of village)

Old slump scarps not classified with Active Fault Classification. � MfE Active Fault Guidelines do not apply.

- Avoidance Zone 60 m (20 m buffer either side of ‘slope rents’, plus ~20 m line width on map – as for faults).

Weak materials associated with scarps map cause local stability or foundation problems for buildings or excavations across or within c.10 m of these old (inactive) features.

- Implications for buildings � Building not permitted in ‘avoidance zone’ -unless site specific investigations show buildings can be located closer to ‘slope feature’ (applies to all slope rents, even drainage channel features).

� Geotechnical report required for future building or site development within 10 m of old scarps. - Only 5 old slump scarps in existing village where this applies.

Drainage channels (‘slope rents’ on ECan map)

Potentially Class 1 active feature – as above (not recognized as drainage channels)

� MfE Active Fault Guidelines do not apply.

- Avoidance Zone 60 m (20 m buffer either side of ‘slope rents’, plus ~20 m line width on map – as for faults).

� Drainage channels are not land/slope stability hazard features – no avoidance zone specified.

- Implications for buildings � Building not permitted in ‘avoidance zone’ -unless site specific investigations show buildings can be located closer to ‘slope feature’.

Drainage channels may have soft ground and be subject to periodic flooding, and so should be treated in the same way as other streams and gullies on properties (i.e. avoided for building purposes, as has been the case within Mt Lyford Village).

Confidential 2006


5.2 Use of geological hazard data at Mt Lyford

Taking into account the preceding GNS review and assessment of active faults and land instability features at Mt Lyford Village, the following comments are made in relation to the use of such hazard data for statutory purposes, which include issuing LIM and PIM reports.

The AFSF map of Mt Lyford Village (Figure 5) has been shown to contain a combination of accurate and unsubstantiated data on active faults and land instability features. Many of the geomorphic features are correctly interpreted as active faults or slope instability features, but there are a number that are, in GNS’s opinion, incorrectly interpreted as active faults or land instability hazard features. These differences are shown on Figure 18.

Features that are accepted by GNS as relating to active faulting or slope instability at Mt Lyford are shown on a revised AFSF map (Figure 19). This map also shows Land Stability Classes, for which hazard avoidance measures are suggested for future development and building in the area. It is suggested that this map, or for planning purposes a map derived from it by HDC, could be used as the basis for identifying geological hazard information for properties in Mt Lyford Village.

Based on investigations carried out for this review, most of Mt Lyford Village is zoned as Land Stability Class 1, in which no significant slope instability features are recognised. LandStability Class 2 affects small areas which can easily be avoided as future building sites (Figure 19). Minor superficial instability affects steeper parts (Land Stability Class 3) of properties below Tinline Terrace, Foggy Lookout, and on the sides of Lulus Creek. However, building sites originally specified in those areas 8 have generally avoided the steeper slopes. That information may need to be referred to on LIM and PIM reports relating to those properties. Substantiated active or potentially hazardous slope instability features, and some active fault traces shown on Figure 19 may have to be referred to on LIM and PIM reports on properties affected by them (see Section 4.2.6), as is the case for other property areas in the Hurunui District.

Confidential 2006


6.0 CONCLUSIONS

This review and reinvestigation by GNS of the Mt Lyford Village active fault and ‘slope rent’ features data provided to Environment Canterbury by Geotech Consulting Ltd, has shown that while the implied origin of many of these geomorphic features has been confirmed, most of the those in the centre of the village are not believed to be active fault traces or slope instability features. This principal finding significantly reduces the size of the area within the village that is potentially at risk from active faulting or land instability hazards.

This assessment is based on a review of the new hazard data provided to ECan, previous geological mapping and faulting studies in the area, and recent geotechnical investigations at Mt Lyford Village by GNS, which included aerial photography, detailed and more accurate geomorphic mapping, ground checking, and trenching of slope features and active fault traces in the village area. These investigations have enabled GNS to assess the origin and hazard implications of geomorphic features at Mt Lyford Village with greater certainty than was possible from the initial reconnaissance mapping carried out for Environment Canterbury by Geotech Consulting Ltd. The other main findings and conclusions from our review and investigations are as follows:

(1) Our investigations at Mt Lyford have shown that some of the ‘slope rent’ features around the head of a large prehistoric landslide southeast of the village and sides of deeply incised streams are old slump scarps. However, many of the mapped ‘slope rents’, especially the long sinuous features running through the village, have been shown by geomorphic criteria (dendritic pattern, lack of ground offsets), supported by trenching, to be old drainage channels rather than slope instability features. At least one, or possibly two, of the ‘rent’s are short, southeast-trending fault traces. A revised map has been prepared which distinguishes between the old slump scarps and drainage channels, previously shown as ‘slope rents’. The drainage channels are not regarded as slope instability hazard features. This results in a considerable reduction in the area of the village identified as having been affected by incipient slope collapse in the past, and hence the extent of areas that are now potentially exposed to land instability hazard. Drainage channels may be subject to flooding and offer poor foundation conditions.

(2) The ‘slope rent’ (incipient slope collapse) features were classed by GCL as “potentially Class 1 active features” in the ECan database. However, the classification used was intended to be applied to active faults in the Canterbury region, and was not meant to be used for landslides or slope collapse features. Informal or unusual classification of land instability features, as was done at Mt Lyford Village, can cause confusion and uncertainty for councils over the use of such data, and should be avoided. We therefore recommend that such a classification applied to any slope instability features within the ECan database should be removed.

(3) The possible effects of large earthquakes on old slump scarps and slopes at Mt Lyford Village were assessed from precedent evidence of slope performance during past large earthquakes. These scarps are old features (c. >12,000–24,000 years) that have survived possibly 20 or 30 Hope Fault earthquakes in the last 10,000 years. Therefore, under present day conditions, with very little stream incision or toe erosion occurring, the probability of slope failure during a Hope Fault earthquake is regarded as low. It should remain low so as long as the overall slope conditions in the area change little over time. Because of the possible presence of weak ground a geotechnical report should be required for future building or site development within 10 m of these old scarps.

Confidential 2006


(4) To distinguish between areas of different geological hazard, a revised map was prepared showing active faults and slope instability features at Mt Lyford, with land areas separated into three Land Stability Classes based on their geomorphic characteristics and potential land instability and hazard implications, as follows: � Land Stability Class 1: Areas with gentle to moderate slopes (<15°) without significant slope

instability features. Some local slope undulations and old drainage channels present. Hazard Implications: There are no slope instability hazards.

� Land Stability Class 2: Gentle to moderately steep (10-20°) areas below or adjacent to old slump scarps. Hazard Implications: The old slump scarps show no signs of recent activity and are not regarded as active instability features. Precedent analysis suggests they are unlikely to move during a Hope Fault earthquake, or during heavy rainfall under present slope conditions. The possible presence of weak materials associated with the old slump scarps may cause local slope instability or foundation problems for buildings, and excavations across them. This would need to be determined by a geotechnical report with respect to future building or site development within 10 m of these scarps.

� Land Stability Class 3: Areas with steep slopes (15-25°) on the sides of deeply incised streams, with evidence of shallow slump scars in places. Recent growth of scrub and exotic forest cover now limits the extent and severity of this instability, and there are few signs of any recent slope failures. Hazard Implications: Suggested measures include a minimum set-back distance of 5 m from areas of active shallow slumping, and pole foundations on steeper slopes, with a minimum founding depth of 500 mm into bedrock. Applications for future building within 5 m of Class 3 areas should be accompanied by a geotechnical report to determine building designs and safe setback distances. Where possible, the existing vegetation cover should be maintained on this land to reduce potential slope instability.

(5) Based on the investigations carried out for this review, most of Mt Lyford Village has been zoned as Land Stability Class 1, in which no significant slope instability features are present. Land Stability Class 2 affects small areas which can be avoided in future building. In Land Stability Class 3 there are areas of minor superficial instability on the steeper parts of properties below Tinline Terrace, Foggy Lookout, and on the sides of Lulus Creek. However, building sites originally specified in those areas have generally avoided the steeper, less stable slopes.

(6) Geomorphic features at Mt Lyford which were interpreted by GCL as active fault traces were re-evaluated in the light of recent GNS geological mapping, and geomorphic mapping and trenching carried out during this study. In our revised active fault and slope features map, many of the fault traces mapped by GCL have been confirmed as active faults using accepted criteria. However, a number of small features previously mapped as faults are considered more likely to be old slump scarps, drainage channels, or erosion gullies. These changes reduce the size of the area and number of properties that are potentially exposed to active faulting hazard within Mt Lyford Village.

(7) Our recent paleoseismic studies (trenching and geomorphic dating) of active faults by GNS at My Lyford has shown that the subsidiary traces of the Hope Fault have lower rates of activity (c 0.05 – 0.1 mm/yr) and longer recurrence intervals (c 10,000 years) than the main fault. The main trace of the Hope Fault has a much higher slip rate (c.23 ± 4 mm/yr) and shorter short recurrence interval (c. 300-500 years), and so presents a greater risk than the subsidiary traces.

Confidential 2006


Subsidiary faults in the Mt Lyford Village area have a relatively low level of geologic activity (c. <0.2 mm/yr) with recurrence intervals of �6000 years (RI Class IV). Conservatively, due to anomalous results and the possibility of mis-interpretation of geomorphic ages, we have placed the subsidiary faults into RI class III. This still amounts to a relatively low level of risk from fault rupture hazard on the subsidiary fault features. The main trace of the Hope Fault has a much higher slip rate (c. 23 ± 4 mm/yr) and shorter short recurrence interval (c. 200-400 years), and so presents a greater risk than the subsidiary traces. No evidence for a strike-slip component of movement has been observed on any of the subsidiary faults. Evidence for the possibility of Holocene faulting (<12,000 years) on the subsidiary faults at Mt Lyford Village is only seen in one trench (Terako-3).

(8) The identification and classification of active faults has important implications for planning and building development in New Zealand. GNS recommends that Regional and District councils follow the Ministry for The Environment (MfE) guidelines for the development of land on or close to active faults. The recurrence interval-based classification within the MfE guidelines can be more easily related to Building Importance Categories (BIC) and accepted levels of risk in the current Building Code, and provides a consistent treatment of both short-recurrence interval (higher risk) and longer recurrence interval (lower risk) active faults across New Zealand.

(9) The MfE guideline allows significant flexibility in the consent process for building on or adjacent to low activity faults or distributed zones of faulting. In this case, the faults can be thought of as being part of a broad, distributed zone of deformation related to the Hope Fault. This would usually make consenting more restrictive, however, as we have undertaken significant analysis of these faults, we have determined information about these faults individually that puts them in a recurrence interval class (� III) that is reasonably unrestrictive to most regular BIC structures (BIC 1, 2a and 2b) at already subdivided sites such as the current Mt Lyford village.

(10) If the MfE Guidelines are followed, construction of BIC 1 (accessory and farm buildings) and 2a structures (single-storey timber-framed dwellings) is a permitted activity at both previously developed and ‘greenfield’ sites within and outside the current Mt Lyford Village, even within the designated Fault Avoidance Zones (� 40m wide) related to these faults. Construction of a BIC 2b structure (one or two storey timber-framed dwellings) is a permitted activity, even within the ±20 m buffer area of these RI Class III faults in the already subdivided or developed parts of the village. For Greenfield sites, construction of BIC 2b structures is a discretionary activity (within a distributed deformation zone), and thus a resource consent would have to be applied for. In general, at the level of a Class III fault (RI 3500-5000 yr), the risk of surface rupture during the expected life of those buildings at Mt Lyford is deemed to be acceptable under the MfE guidelines.

Confidential 2006


7.0 ACKNOWLEDGEMENTS

This report was prepared with the assistance and submissions from various GNS scientists and Graphics staff. In particular the authors wish to thank Grant Dellow, Phil Glassey, and Russ Van Dissen for reviews and comments on the draft report, Mauri McSaveney, Nick Perrin, and Stuart Read for helpful discussions during the study, and Associate Professor Jarg Pettinga (Canterbury University) for his peer review of the report. We also wish to thank David Bell, Tim McMorran, and Mark Yetton for their cooperation and discussions during the review process, Brent Pizzey of Hurunui District Council for coordinating the work done by GNS, Mike Powell for digging the trenches, and finally the many land owners at Mt Lyford for allowing access to their properties for trenching and site inspections. 8.0 REFERENCES

1. ECan, 2005. Mt Lyford Integrated Active Faults and Slope Features. 1:15,000 (A4) map posted on the ECan website in December 2005 (based on mapping by Mark Yetton, Geotech Consulting Ltd).

2. Yetton, M., 2005. Updating of Active Fault database–Hurunui District – Waikari and Mt Lyford areas. Letter to ECan by Mark Yetton, 27 October 2005.

3. Yetton, M., 2005. Updating of Active Fault database–Mt Lyford area. Letter to ECan by Mark Yetton, 18 November 2005.

4. URS, 2005. Mt Lyford Village Active Fault Hazard Assessment Review. Peer review. Letter to Environment Canterbury by Tim McMorran and Don Macfarlane, URS New Zealand Ltd, 23 November 2005.

5. ECan, 2004. Letter to Hurunui District Council with recommendations regarding active faults and avoidance zones for inclusion in the Hurunui District Plan, 8 September 2004.

5a. ECan, 2005. Letter to Hurunui District Council with recommendations regarding the ‘slope rents and zones of slope collapse at Mt Lyford Village, 14 November 2005.

5b. ECan, 2005. Letter to Hurunui District Council with further comments regarding the ‘slope rents and zones of slope collapse at Mt Lyford Village, 12 December 2005.

6. Yetton, M., 2006. Proposed investigation of slope rent features at Mt Lyford Village. Email message to Hurunui District Council (B Pizzey), 16 January 2006.

7. Bell, D.H., 2006. Letter to Davis, Ogilvie and Partners regarding Mt Lyford Integrated Active Faults and Slope Features Map published on ECan website, 8 February 2006.

8. Bell, D.H., 1987 Mt Lyford Log Village Residential / Recreation Zone. Evidence presented at Town and Country Planning Hearing, 22 January 1987.

9. Bell, D.H., 1988. Mt Lyford Log Village Subdivision. Investigation report prepared for Mt Lyford Joint Venture, 29 September 1988.

10. HDC, 2006. Example of Land Information Memorandum (LIM) notification sent to property owners regarding land instability features at Mt Lyford Village, January 2006.

11. HDC, 2006. Guidelines for Geotech and Active Reports at Mt Lyford, issued by Hurunui District Council, 2006.

12. Rattenbury, M.S., Townsend, D.B., Johnston, M.R. (compilers) 2006 (draft). Geology of the Kaikoura area. Institute of Geological Sciences 1:250,000 Geological Map 13 (Kaikoura). Draft map March 2006.

Confidential 2006


13. Eusden, J.D., Pettinga, J.R., Campbell, J.K., 2000. Structural evolution and landscape development of a collapsed transpressive duplex on the Hope Fault, North Canterbury, New Zealand. NZ Journal of Geology and Geophysics, 43: 391–404.

14. Langridge, R., Campbell, J., Hill, N., Pere, V., Pope, J., Pettinga, J., Estrada, B., Berryman, K., 2003. Paleoseismology and slip rate of the Conway Segment of the Hope Fault at Greenburn Stream, South Island, New Zealand. Annals of Geophysics, 46 (5):1119 – 1139.

15. Kerr, J., Nathan, S., Van Dissen, R., Webb, P., Brunsdon, D., King, A., 2003. Planning for Development of Land on or Close to Active Faults: A guideline to assist resource management planners in New Zealand GNS Client Report 2002.124, prepared for the Ministry for the Environment (ME Report 483).

16. Hancox, G.T., Perrin, N.D., and Dellow, G.D., 1997. Earthquake-induced landslides in New Zealand and implications for MM intensity and seismic hazard assessment. GNS Client Report 43601B, 10 Dec 1997.

17. Hancox, G.T., Perrin, N.D., and Dellow, G.D., 2002. Recent studies of historical earthquake-induced landsliding, ground damage, and MM intensity in New Zealand. Bulletin of the New Zealand Society for Earthquake Engineering, 35(2)59-95, June 2002.

18. Hancox, G.T., 2006. Interim report on ‘slope rent’ and active fault features at Mt Lyford Village. GNS Science Letter Report (GNS Project 430W1200), 16 March 2006.

19. Pettinga, J.R., Chamberlain, C.G., Yetton, M.D., Van Dissen, R., Downes, G., 1998. Earthquake hazard and risk assessment study: Stage 1 (Part A) –Earthquake source identification and characterisation. Canterbury Regional Council Publication U98/10.

20. Gillon, M.D., Hancox, G.T., 1992. Cromwell Gorge Landslides - a general overview. Proceedings of the Sixth International Symposium on Landslides, February 1992 Christchurch, New Zealand. Vol. 1, p 83–102.

21. Hutchinson, J.N., 1988. General Report: Morphological and geotechnical parameters of landslides in relation to geology and hydrology: 5th International Symposium on Landslides, Lausanne, Switzerland, Vol. 1, p 3–35.

22. McCalpin, J.P., 1999. Criteria for determining the seismic significance of sackungen and other scarplike landforms in mountainous regions. In Hanson, K.L., Kelson, K.I., Angell, M.A., Lettis, W.R., (eds.). Identifying faults and determining their origins: US Regulatory Commission (NRC), NUREG/CR-5503, Appendix A, p JM1–21.

23. McCalpin, J.P., Hart, E.W.., 2000. Ridgetop splitting, spreading, and shattering related to earthquakes in southern California: unpublished Final Technical Report submitted to U.S. Geological Survey by GEO-HAZ Consulting, Inc., Contract 99HQGR0042, April 6, 2000, 54p.

24. Freund, R., 1971. The Hope Fault: a strike-slip fault in New Zealand. DSIR N.Z Geological Survey Bulletin 86, p.49.

25. Bull, W.B., 1991. Geomorphic responses to climatic change. Oxford University Press, London. 326 p.

26. Tonkin, P., Almond, P., 1998. Using loess stratigraphy to reconstruct the late Quaternary history of piedmonts adjacent to large strike-slip faults. South Island. New Zealand. Geological Society of New Zealand Miscellaneous Publication 101A:227.

27. ECan and HDC, 2006. Joint letter from ECan and HDC to Mt Lyford property owners re Latest Active Fault Hazard Information at Mt Lyford Village (27 January 2006).

healthy holiday recipe ebook

Documents