J. SE Asian Appl. Geol., May–Aug 2010, Vol. 2(2), pp. 104-109

PALEOSTRESS ANALYSIS TO INTERPRET THELANDSLIDE MECHANISM: A CASE STUDY INPARANGTRITIS, YOGYAKARTA

Salahuddin Husein∗1, Ignatius Sudarno1, Subagyo Pramumijoyo1, and DwikoritaKarnawati1

1Department of Geological Engineering, Faculty of Engineering, Universitas Gadjah Mada, Indonesia

Abstract

Paleostress analysis on the landslide boundaryfaults is able to explain the sliding mechanism.This method is particularly useful to study a pa-leolandslide. About 30 striated fault planes fromthe Parangtritis paleo-landslide, located in the Yo-gyakarta coastline, were analyzed to define theirprinciple stress axes. The eastern boundary fault,named as the Girijati Fault, was the main faultresponsible for the mass movement and leaving aconsiderable steep cliff. It moved normal in a left lat-eral sense with ENE – WSW extension and draggedthe rockmass southward, creating a NNW – SSWextension along the Parangtritis Fault and turnit into the western boundary fault. The rockmassslided along the stratigraphic contact between theunderlying Nglanggran Formation and the over-lying Wonosari Formation, created a semi-circularcrown cliff as the northern boundary and producedsome isolated topographic highs of the thrust blocknear the toe.Keywords: Paleostress, landslide boundary, fault,paleolandslide.

1 Introduction

An occurrence of a landslide commonly con-trolled by geological structures, i.e. beddingand fault planes (Lutton et al., 1979). Thus, iden-

∗Corresponding author: SALAHUDDIN HUSEN, De-partment of Geological Engineering, Faculty of Engineer-ing, Universitas Gadjah Mada, Jl. Grafika 2, Yogyakarta55281. E-mail: shddin@gmail.com

tification the controlling geological structuresand their kinematics on a rockslide is principalin studying the occurrence of a landslide. Whenthe controlling faults appear as striated minorplanes in the outcrop, they might be used to es-timate the orientations of principal stress axes(Angelier, 1990). This method is known as pa-leostress analysis and the result can be appliedfor interpretation of the sliding mechanism.

This paper attempt to present an example ofpaleostress analysis to interpret the landslidemechanism. A case study in Parangtritis Beach,Yogyakarta, was chosen to demonstrate theirbeneficial approaches to study a paleolandslide.

2 The Parangtritis Paleo-landslide

The Parangtritis Beach in Yogyakarta has beenwell known as a tourist area (Figure 1). It is fea-tured by interesting landscapes. Instead of itsrare-to-find tropical sand dunes that are well-developed along the coastal belt, an imposingsteep cliff of the Southern Mountains can beseen to the north and to the east.

The southern part of the cliff suggests apaleo-landslide morphological characteristic(Figure 2). It has a moderate slope gradientwith some small hills scattered to the south andis bounded by circular cliff to the north. A steepcliff of northsouth trend is found at the easternboundary, separating typical karst topographyto the east. Srijono and Untung (1981) con-ducted a geomorphological mapping based onaerial photograph analysis and identified the

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PALEOSTRESS ANALYSIS TO INTERPRET THE LENDSLIDE MECHANISM

Figure 1: Location for the study area, yellow box.

moderate slope as pseudokarst morphologicalunit associated with a landslide. They also in-ferred some faults which acted as boundariesfor the landslide: two north-south faults for theeastern and western boundaries, as well as oneeast-west fault for the northern boundary. Theeastern boundary fault was named as GirijatiFault, while the western boundary fault wasnamed as Parangtritis Fault (Sudarno, 1997).

Geophysical investigation on the area withmagnetotelluric methods suggests that thebasal plane for the paleo-landslide occurredin a depth of 400 m, along the stratigraphicboundary between the underlying NglanggranFormation andesitic breccia and the overlyingWonosari Formation limestone (Husein et al.,2007). Both formation have angular uncon-formity contact, with the Nglanggran Forma-tion dips about 25◦ southeastward and is LateOligocene to Early Miocene in age (Salahud-din, 1995), while the Wonosari Formation dipsgently 10◦ southeastward and is Late Mioceneto Late Pliocene in age (Salahuddin, 1995). Itwas estimated that the landslide dimension in-

volved a length of 2700 m and a width of 1500m, approximately the sliding mass volume wasa number of 810 million m3 (Husein et al., 2007).

3 Paleostress Analysis on Field Data

Kinematic data on the boundary faults wererequired to interpret the landslide mechanism.Thorough observation on numerous minorfaults along the boundary faults, particularlythe Girijati and Parangtritis faults, were col-lected and analyzed (Sudarno, 1997) (Figure3). Paleostress analysis on those data thenre-analyzed according to inversion method(Angelier, 1990) and a new perspective was ap-plied to the result in order to explain the slidingmechanisms (Figure 4).

This paleostress method calculates the stresstensor by solving equations whose param-eters are computed using the orientation offault planes and slip vectors (Figure 5a). Thismethod is based on the assumption that, al-though fault orientation may be arbitrary ifinherited faults are present, the direction and

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Figure 2: Satellite images on the study area, highlighting the Parangtritis paleo-landslide withboundary faults (white dashed lines in b).

Figure 3: (a) Girijati fault zone, camera facing eastward. (b) Striated fault plane of the Girijati faultzone, their location in the figure 5a is indicated by the red box.

Figure 4: Paleostress analysis on Girijati (a) and Parangtritis (b) faults.

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sense of each slip vector should correspond toa single common stress tensor (Angelier, 1990).Let θ (theta) be the angle between two vec-tors in the fault plane: the observed striae andthe theoretical direction of displacement basedon the orientations of the calculated principalstress axes. The orientations of the principalstress axes are those that minimize the sum ofthe n values of θ (where n is the number offaults included in the analysis). This methodalso provides stress ellipsoid analysis (Φ ratio)which indicates their movement origin (Figure5b).

About 24 striations from numerous minorfaults along the Girijati Fault zone indicate ENE– WSW extension and their stress ellipsoidssuggest normal with slight strike-slip origin (Φratio ~ 0.31). On the other side, about 6 stri-ations from the Parangtritis Fault zone indicateNNW – SSE extension and their stress ellipsoidssuggest normal origin (Φ ratio ~ 0.15).

4 Interpretation on Landslide Mecha-nism

In Parangtritis, morphological evidences thatindicate the presence of a considerably 250 mheight, steep cliff, of the Girijati Fault suggestthat faulting seems to be the primary cause ofthe landslide. The normal with sinistral move-ment with ENE – WSW extension of the Giri-jati Fault once was active and dragged the rock-mass southward, creating a NNW – SSW exten-sion along the western boundary fault and acti-vated the Parangtritis Fault as a normal fault.The resulted mass movement thus broke thelimestone bedding in the northern part and cre-ated a semi-circular crown cliff as the northernboundary.

As the rockmass moved southward along thestratigraphic contact as triggered by the Giri-jati faulting event, the basal plane concavelycurved head-ward and toe-ward and cut thelimestone bedding planes. Head-ward, theconcavity accommodated the southward nor-mal faulting as the rockmass moved down-ward. Toe-ward, the concavity accommodatedthe northward thrust faulting as the rockmasspushed downward, thus produced some iso-

lated topographic highs of the thrust block. Fur-thermore, the landslide event created more frac-tures and tilted the limestone blocks steeperthan the surrounding area. This conditionprohibited karst topography to develop in thelandslide area.

Interpretation on timing of the sliding eventwas mainly based on the stratigraphic and mor-phologic information. As the landslide in-volved the Wonosari limestone as the youngestrock unit, the event had to be occurred after itsdeposition, i.e. post Late Pliocene. The eventalso had to take place during the uplifting ofthe area as the main mechanism was normalfaulting with strike-slip sense. It is assumedthat the landslide might occurred on the latestSouthern Mountain uplifting event during LatePleistocene as supposed by Husein and Srijono(2007). That regional-scale event were countedfor creating the Wonosari depression as well asfor commencement of the karst topography de-velopment in the southern part of the SouthernMountain, which today is known as GunungSewu. A long period of geological time sincethe landslide event has given the high energywaves and coastal processes to erode the land-slide toe, straightened the coastline and coveredthe toe with the Holocene sand dunes.

5 Conclusions

Paleostress analysis based on striated fault datais able to explain the landslide mechanism,particularly for ancient events. The interpretedmechanisms of the Parangtritis paleolandslidewere sequential events which triggered bythe activation of the normal-sinistral GirijatiFault and simultaneously coupled by acti-vation of the normal Parangtritis Fault. Therockmass moved along the basal plane of thestratigraphic contact between the Wonosariand Nglanggran formations, created a seriesof semi-circular normal faults in the headwardand a series of topographic highs in the toe-ward as a result of thrust upward movement.Morphological and stratigraphical data suggestthat the landslide event occurred during the lat-est uplifting episode of the Southern Mountain,possibly during Late Pleistocene.

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Figure 5: (a) Components of fault slip (Angelier, 1994). D: total displacement (net separation); S:displacement along slope; T: transverse horizontal component of displacement; V: vertical offset; L:lateral horizontal component of displacement; F: fault plane; s: slickenside lineation; p: fault dip;io: pitch of slickenside lineations, from 0 to 90◦. Sense of arrows (D, S, T, V and L) refer to relativemovement of downthrown block. (b) Stress ellipsoid indicates principal axes of the stress, withσ1>σ2>σ3 (Angelier, 1994). Their shape was indicated by Φ ratio ((σ2− σ3)/(σ1− σ3)) which isranging from 0 to 1 and reflects the magnitude of the intermediate principal stress (σ2) relative tothe extreme principal stresses (σ1 and σ3).

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References

Angelier, J. (1990). Inversion of Field Data in FaultTectonics to Obtain the Regional Stress. III. ANew Rapid Direct Inversion Method by Analyt-ical Means. Geophysical Journal International,103, pp. 363-376.

Angelier, J. (1994). Fault Slip Analysis and Pale-ostress Reconstruction. in: P.L. Hancock (ed.),Continental Deformation, Pergamon Press, Ox-ford, pp. 53- 100.

Husein, S. and Srijono (2007). Tinjauan Geomor-fologi Pegunungan Selatan DIY/Jawa Tengah:telaah peran faktor endogenik dan eksogenikdalam proses pembentukan pegunungan. Sem-inar Potensi Geologi Pegunungan Selatan dalamPengembangan Wilayah, Pusat Survei Geologi,Yogyakarta, 27-28 November 2007, 10 pp.

Husein, S., Ign. Sudarno, and A. Nugraha (2007).Megascale Paleo-landslide at Parangtritis, as de-duced from Geological and Geophysical Data.Proceeding of Joint Convention Bali 2007, the32nd HAGI, the 36th IAGI and the 29th IATMIAnnual Convention, JCB2007-083, Bali, 8 pp.

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Salahuddin (1995) Geologi Daerah Terong –Muntuk, Kecamatan Dlingo, Kabupaten Bantul,Daerah Istimewa Yogyakarta. Laporan PemetaanKuliah Lapangan, Jurusan Teknik Geologi Fakul-tas Teknik Universitas Gadjah Mada, Yogyakarta,41 pp. (unpublished).

Srijono and S. Untung (1981) Perpetaan Geomor-fologi Metode ITC – dengan conto studi daerahParangtritis, Proceedings PIT X IAGI, Bandung,pp. 64-73.

Sudarno, I. (1997) Kendali Tektonik terhadap Pem-bentukan Struktur pada Batuan Paleogen danNeogen di Pegunungan Selatan, Daerah IstimewaYogyakarta dan Sekitarnya, M.Sc. Thesis at Pro-gram Studi Geologi Institut Teknologi Bandung,Bandung, 167 pp. (unpublished)

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IntroductionThe Parangtritis Paleo-landslidePaleostress Analysis on Field DataInterpretation on Landslide MechanismConclusions