framework for earthquake hazard assessment

FRAMEWORK FOR EARTHQUAKE HAZARD ASSESSMENT

SEISMIC HAZARD RISK ASSESSMENT OF BAGO, TAUNGOO

AND SAGAING CITIES, MYANMAR

Myanmar Geosciences Society (MGS)

16 July 2013

Contents

Background ............................................................................................................................................. 1

Objectives and Scope of the Project ....................................................................................................... 2

Methodology of the Project .......................................................................................................................... 5

Seismic Hazard Assessment .................................................................................................................... 5

Task 1. Seismic sources identification and characterization ............................................................. 5

Task 2. Estimating the temporal occurrence of earthquake and maximum magnitude ................... 6

Task 3. Attenuation relationship selection ........................................................................................ 6

Task 4. Calculation of the seismic hazards ........................................................................................ 6

Generation of Microzonation Map (Amplification Map or Map of Amplification Factor)...................... 6

1 | F r a m e w o r k f o r S e i s m i c H a z a r d A s s e s s m e n t

Seismic Hazard Assessment of Bago, Taungoo and Sagaing, Myanmar

Background

Situating in one of the two major earthquake belts, Myanmar is prone to earthquakes. Numerous

historical earthquakes recorded in damage of religious buildings since fifth century C.E. exhibit seismic

nature of almost all land of Myanmar. Since the 18th century, at least 15 major earthquakes including two

historical events in 1762 (northern Rakhine Coast) and 1839 (Innwa ancient capital) occurred in Myanmar

territory. Some recent earthquakes with smaller magnitude (more than 6.5 R.M.) occurred as strong

earthquakes in Central Myanmar i.e. 1975 Bagan Earthquake (M=6.5), 2003 Taungdwingyi Earthquake

(M=6.8) and in Eastern Myanmar like 2011 Tarlay Earthquake (M=6.8). Being its location at convergence

zone between Indian and Eurasian tectonic plates, western part of Myanmar lies on Burma Plate of which

Bengal subduction zone in the west and Sagaing transform fault in the east. As the tectonics is still active,

underthrusting, transcurrent and collisional structures overwhelmed the whole country and devastating

earthquakes have been subjected to the people and infrastructure of Myanmar.

Recent studies on earthquakes in Myanmar have been started since the event of 2003

Taungdwingyi Earthquake. Geologists from the Myanmar universities were later organized in Myanmar

Earthquake Committee and paleoseismological research began after 2004 Sumatra Earthquake and

Tsunami and the work was emphasized on marine terraces along the northern Rakhine Coast where

occurred 1762 Bengal Earthquake (M7). From early of 2007, studies on active faults in Myanmar became

a joint project among scientists of Myanmar, and its international partners like AIST (Geological Survey of

Japan), Kyoto and Hiroshima Universities (Japan), California Institute of Technology (USA), National

Taiwan University (NTU) and Earth Observatory of Singapore (Nan Yang Technological University,

Singapore). Works emphasized on interpretation of SRTM (Shuttle Radar Topographic Mission) imagery

and aerial photographs of Kabaw Fault, Sagaing Fault, Western Rakhine Coast, and Northern Yangon areas

and subsequent field observations. Activity of faults were determined through tectonic geomorphology

of Kyaukkyan Fault (Shan Plateau of eastern Myanmar) and along the Sagaing Fault (Central Myanmar).

On Sagaing Fault, paleoseismological trenching have been conducted in two southern segments like Bago

Segment at west of Payagyi Town and Phyu Segment at Taungthonlon (west of Phyu). Paleoseismic data

indicates that recurrence of surface rupturing earthquake is approximately 100 years in southern Sagaing

Fault.

Bago and Taungoo cities are the highly populated places of Bago region and the population is

above 300,000.They have being continually developed with the growth of population, various sorts of

builds and the infrastructures, etc. The deterministic seismic hazard map of Bago - Oakthar Myothit area

was developed by San Shwe and Maung Thein in 2007, and that of Taungoo by Aye Aye Myint in 2008 and

are the first seismic hazard maps of these cities and the seismic hazards were expressed in term of peak

ground acceleration (PGA). However, no systematic research on seismic hazard and risk assessment has

done. Therefore systematic seismic hazard and risk assessments are tended to carry out.

Sagaing City, populated and also important for its accommodation of hundreds of monasteries of

Buddhist monks, is prone to large earthquake and is well-known for being reference place to Sagaing


Fault. There has been no systematic research on seismic hazard and present study intends to fill the gap

in present day development work of the country.

Objectives and Scope of the Project

The fundamentals to evaluate the seismic hazard is for the invaluable information for

development work of the urban areas and for the background information in seismic risk assessment the

organization working cooperatively with Myanmar Engineering Society (MES).

i. To identify the most possible seismic sources for Bago, Taungoo and Sagaing Cities

ii. To assess the geological and seismological information related to the seismic sources

iii. To evaluate the ground motion parameters of the seismic hazards

iv. To appraise the site condition that influence the amplification of the ground motions

Specifically, the seismic hazard assessment will include

i. The seismic sources identification and characterization

ii. The seismicity or temporal distribution of earthquake recurrence characterization

iii. The site characterization

iv. The seismic hazard calculation


Figure 1. Tectonic Map of Myanmar (Soe Thura Tun and Maung Thein 2012)


Figure 2. Hypothetical earthquake segments of active faults threatening Sagaing City and central

Myanmar region.


Figure 3. Map showing the relation of earthquake and surface fault trace of the part of Central

Myanmar.

Methodology of the Project

Since the project include the seismic hazard analysis, the methodology related with each portion

is discussed separately in brief. The seismic hazard assessment will be carried out by using the classical

probabilistic seismic hazard analysis of Cornell (1968). This method comprises of four tasks accounted by

Reiter (1990) and Kramer (1996).

Seismic Hazard Assessment

Task 1. Seismic sources identification and characterization

In the seismic source identification, the geometry and spatial distribution of seismic sources of

future seismicity (the earthquake potentials) in and around the proposed urban area (Bago, Taungoo &

Sagaing) area will be determined by reviewing the available literatures and unpublished documents


related with the previous historical events and by using the image and air photo interpretation, especially

for the active fault sources.

Task 2. Estimating the temporal occurrence of earthquake and maximum magnitude

The paleoseismological investigation such as trenching are tended to carry out to obtain the

reasonable geological data related to those such as fault slip rate, fault geometry, and activity rate,

especially for Sagaing Fault.

Instrumental earthquake catalog and historical records of the previous earthquakes will be

analyzed to estimate the seismicity parameters such as estimation of a- and b- values for each seismic

source. The estimation of occurrence rate of the earthquake with the function of a magnitude for each

seismic source, and the maximum magnitude of the earthquake potential for each seismic source is

required to carry out with the cooperation of their uncertainty.

Task 3. Attenuation relationship selection

The attenuation relationships have been developed for various region and different sort of

tectonic setting. In Myanmar, there are no enough strong ground motion records to develop and even to

validate which one should be suitable for a certain region. Therefore, we will make comparative study on

several attenuation relations of the same tectonic setting by utilizing the seismic sources and site

parameters for Bago, Taungoo and Sagaing areas. Normally the attenuation relations of Boore et al. (1997)

and Takahashi et al. (2000) were applied in seismic hazard assessment for other areas or regions (even

national seismic hazard assessment of Myanmar).

Task 4. Calculation of the seismic hazards

The seismic hazards will be calculated by applying the EZFRISK computer program for firm rock

condition. The results will be represented in terms of peak ground acceleration (PGA) in g, peak ground

velocity (PGV) in cms-1 and spectral acceleration (SA) in g at the periods of 0.2s, 0.3s and 1.0s for 10%,

39%, 63%, 86% and 99% of probabilities of exceedance of 50 years.

Generation of Microzonation Map (Amplification Map or Map of Amplification Factor)

It is also need to account the effect of local site condition on the seismic hazards since the

amplification characteristics (site effect) of local soil has a great influence on the damage intensity during

an earthquake. To understand the amplification characteristics of underlying soil deposit, linear or non-

linear response analysis has to be conducted. 1D dynamic response analysis which is based on equivalent

linear method will be performed in this project by using DYNEQ program coded by Nozomu Yoshida, 2004.

A first level seismic microzonation map of three cities have been produced with a GIS platform using the

themes, viz, Peak Ground Acceleration (PGA), Shear wave velocity at 30 m, Geology, Ground water

fluctuation and bed rock depth. For that purpose, the following measurements and data collection will be

carried out.

Geotechnical site investigation: Drilling at least 30m depth will be conducted in three cities together with

Standard Penetration Test (SPT) and samplings. The number of proposed boreholes was presented in


figures 5 to 7 and Appendix 1. 2-D soil profile of each borehole will be constructed based on drilled log,

SPT values and laboratory results. Shear wave velocity structure of each borehole will be estimated based

on SPT data. The delineation of bedrock profile, the mapping of subsoil strata, and the associated soil

structure, fluctuations of ground water table, determination of soil properties, and understanding of site-

effects represent routine elements of the microzoning exercise. Soil-foundation-structure interaction,

which turns crucial during an earthquake, depends in a big way on the quality of soil characterization.

Microtremors survey: It will be conducted for a number of sites (100 or more) including drilling sites to

determine horizontal to vertical spectral ratio (H/V ratio) which is mainly related to sediment thickness

and average shear wave velocity at upper 30m (vs30). Earthquake effects are usually quantified on the

basis of degree of damage and the recorded ground motions at a site. Beside the building quality and

desing, the local soil conditions dominate the damage and loss of life in earthquakes. Therefore, it is

needed to investigate the site condition of the three cities. In this project, we conducted the microtremor

survey before borehole logging. Microtremor survey method is rapid, cost effective and efficient method

for delineating the site condition by estimating the average shear wave velocity of the soil layers occurred

at the site, especially for the upper 30m, Vs30.

Construction of Subsurface soil Model and S-wave velocity structure: Based on S-wave velocity

structures from SPT results, and microtremors survey, final S-wave velocity structure of each site (drilled

site, microtremors site) will be constructed. Moreover, 2-D soil profiles of each site (drilled site,

microtremors site) will be constructed based on S-wave velocity structure, soil profiles of boreholes,

laboratory results, and H/V ratios from microtremors survey.

Generation of Synthetic Bedrock Motion: The required bedrock motion will be generated based on

earthquake source parameters for a point source (historical earthquake) or a line source (Sagaing fault

segment).

1D Equivalent Linear Seismic Response Analysis: Based on 2-D soil profile, engineering properties and S-

wave velocity of each soil layer, and synthetic bedrock motion, response analysis will be performed for all

investigated sites.

Amplification map, PGA map, PGV map, maps of fundamental frequency and predominant period, and

map of sediment thickness above engineering bedrock (vs = 500 m/s) will be final outcomes.


Figure 4. The map of proposed Borehole locations for Bago city (across those locations

microtremor survey to be carried out).

Figure 5. The map of proposed Borehole locations for Taungoo city (across those locations

microtremor survey to be carried out).


Figure 6. The map of proposed Borehole locations for Sagaing city (across those locations

microtremor survey to be carried out)

Table 1. Activity for Seismic Hazard Assessment at 3 Cities in Central Myanmar

Sr. Activity Sub-Activity Month

1 2 3 4 5 6 7 8 9

1 Seismic Source Identification

Air Photo Purchase

Image/ Photo Interpretation

Field Survey

Trenching

Data Analysis and Reporting

2 Probabilistic Seismic Hazard Assessment

Area Source Classification

Line Source Classification

Seismic Hazard Analysis

Spectral Analysis

PSHA Map Output



3 Site Amplification Assessment

55 mm Drilling up to 30m

Microtremor Survey


framework for earthquake hazard assessment

Documents

myanmar earthquake committee

scientists of myanmar

central myanmar

myanmar territory

land of myanmar

eastern myanmar

myanmar universities

infrastructure of myanmar