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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: [email protected]
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
c© 2010 Department of Geological Engineering, Gadjah Mada
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HUSEIN et al.
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.
106 c© 2010 Department of Geological Engineering, Gadjah Mada
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PALEOSTRESS ANALYSIS TO INTERPRET THE LENDSLIDE MECHANISM
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.
c© 2010 Department of Geological Engineering, Gadjah Mada
University 107
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HUSEIN et al.
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).
108 c© 2010 Department of Geological Engineering, Gadjah Mada
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PALEOSTRESS ANALYSIS TO INTERPRET THE LENDSLIDE MECHANISM
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c© 2010 Department of Geological Engineering, Gadjah Mada
University 109
IntroductionThe Parangtritis Paleo-landslidePaleostress Analysis
on Field DataInterpretation on Landslide MechanismConclusions