10th Asian Regional Conference of IAEG (2015)

Geomorphological and geological features of the collapsing landslides induced by the2009 Padang earthquake
Maho Nakano (1), Masahiro Chigira (2), ChounSian Lim(3), Sumaryono gajam(4)(1) CTI Engineering Co., Ltd
E-mail: mh-nakano@ctie.co.jp(2) Disaster Prevention Research Institute, Kyoto University, Japan
E-mail:chigira@slope.dpri.kyoto-u.ac.jp(3) University of Kebangsaan, Malaysia(4) Center for Volcanology and Geological Hazard Mitigation, Indonesia
Abstract
The Mw7.6 Padang earthquake in 2009 attacked the northwest of Sumatra, Indonesia,
and triggered many landslides. We made satellite image interpretations, field investigations,and laboratory tests to identify the geomorphological and geological features of theselandslides. As a result, we found that the number of landslides was 159, slid materials werepumice fall deposits, and their sliding surface was made in the base of the pumice layer,
where pumice grains were mixed with the soil beneath the pumice bed. These landslides
had the following characteristics: 1) they occurred in the areas with pumice fall deposits(Qhpt: Quaternary hornblende-hypersthene pumice tuff) with >3 m thickness, which wascontrolled by the distance from their source; 2) the pumice fall deposits had a slope-parallel
layering, which had been cut at the foots of slopes; and 3) the mixed layer at the base of thepumice fall deposits was heavily weathered to be clayey materials with abundant halloysite.
Interpretations of stereoscopic satellite images and field surveys showed that there are four
terraces along the Magung River, and Qhpt covers widely distributed higher terraces (Lh)and middle terraces (Lm) but are cut by lower terraces (Ll1 and Ll2) as well as small nearby
tributary gullies. This undercutting likely reduced the support of Qhpt beds from downslope.
The mixed layers, in which sliding surfaces were formed, were heavily weathered and very
weak. XRD analyses showed that pumice grains of the main part of Qhpt scarcely had
halloysite but pumice grains and weathered debris flow deposits in the mixed layers wererich in halloysite. The geological history, which is volcanic eruption, weathering, andundercutting by river incision as stated above, is typical in tropical volcanic areas. Thatmeans we can make a hazard map of such a catastrophic landslide induced by earthquakes
on the basis of geological development.
Keywords: 2009 Padang earthquake, Halloysite, Collapsing landslide, Pumice fall deposits,
weathering
1.

Introduction
The Mw7.6 Padang earthquake on 30
September, 2009 triggered many landslides, whichwere very rapid and highly destructive. Those
landslides attacked valley bottom villages andkilled at least 130 people (Fig.1). The landslides
occurred during heavy rain, of which amounts were
not recorded. We made satellite image
interpretation and field survey to clarify landslide
distribution, characteristics, and the relationshipbetween landslides and the geological orgeomorphological features.

Central Sumatra of Indonesia (Fig. 2). Landslides
occurred in the Tandikat area at an epicentral
distance of 45-55 km (Fig. 2).

2.

Fig. 1 Landslides triggered by the 2009 Padang
earthquake (AP Photo/Dita Alangkara date：Oct 3.
2009).

Location
The epicenter of the 2009 Padang earthquake
was located at 60 km offshore from Padang city;1

10th Asian Regional Conference of IAEG (2015)

Fig. 2 Location of the epicenter of the 2009
Padang earthquake (USGS, 2009) and thelandslide concentration area (red square).

Fig. 3 Distribution of landslides (black) and riverterrace.
small channels.
We identified 159 landslides on the Spot 5
image within 64 km2.

3. Methods
3.1 Mapping of the collapsing landslide
To identify the distribution of landslides, weinterpreted a Spot-5 Image with 10-m resolution
taken 13 Oct, 2009. During the field surveys, wechecked 14 landslides.
3.2 Classification of the topography
To classify the landscape, particularly riverterraces and to clarify the landscape development,
we made stereoscopic examination on ALOS
PRISM images with a resolution of 2.5 m taken on
11 February 2012.
3.3 Field investigation
We made geological mapping, investigation on
landslides with cone penetration tests (3kg weight50 cm falling) and sampling for laboratory tests.
Our field observations in and around Tandikat area
were on 8/31 – 9/9, 2012, and 8/20 – 8/25, 2013.
3.4 Soil test and analysisWe made mineral analysis with X-ray
diffraction, scanning microscopic observation(SEM), simple shear tests under the conditions ofconstant normal stress or constant volume, andmeasurements on physical properties (permeability,consistency limits, water contents, degree ofsaturation).

4.2 Geology
The affected area is widely covered with pumice
fall deposits just below the ground surface. This
pumice fall deposits has been called as Qhpt andbelieved to be from Maninjau Caldera (Tjia andMuhammad, 2008), but the isopach map strongly
suggests that Qhpt was from Tandikat Volcano (Fig.
4). The age of Qhpt is not specified but is younger
than 0.08 Ma, which is the age of pyroclastic flow
deposits older than Qhpt (Nishimura, 1980). The
distribution of the pumice layer and the landslides
indicates that the landslides concentrate in the areas
where the pumice layer was 3.5-5.5 m thick. All of
the 14 landslides we studied had a pumice layer
with slope-parallel layering. Below the pumicelayers are heavily weathered gravels or volcanic
soils, the latter of which are distributed away fromthe terraces.
4.3 Sliding surface of landslides
Sliding surfaces were made within the base of
the pumice layer, where pumice grains were mixed
with soil, which is heavily weathered debris flow
deposits (Fig. 5). Grains of pumice and soil in themixed layer were very weak and easily smeared byfinger and rich in hydrated 1.0 nm halloysite, while
the pumice grains in the main part of pumice layer
were hard and scarcely had halloysite. Pumicegrains in the mixed layer had no remaining
volcanic glass but consisted of halloysite, which
had precipitated on the previous bubble walls toform a very fragile framework. According to thecone penetration tests performed on 2 landslide
sites, mixed layer had the lowest penetration
resistance. Permeability of the mixed layer and thedebris flow deposits were about two orders smaller
than the permeability of the pumice layer. Simple
shear tests for the mixed layer under constant

4. Results
4.1 Topography and landslide distribution
We found four terrace groups (high terrace,middle terrace, low terrace 1, and low terrace 2 inFig. 3) and that landslides occurred on the valley
slopes which were made by river incision of thehigh terraces. The slopes incised on the highterraces were recognized to be cut by the scarps of
middle terrace and low terrace 1 on the PRISM
images. Along the Maggung River, we observedthat landslide slopes had been cut by the scarps oflow terrace 1. Landslide slopes on nearby tributarycatchments can be assumed to have been cut by2

10th Asian Regional Conference of IAEG (2015)

Fig. 4 Isopach map of the Qhpt and the distribution of landslides.

Fig. 5 Mixed layer and overlying pumice layer
and underlying debris flow deposits.

volume yielded 18.2 kPa of cohesion and 15° ofinternal friction angle. Water contents were 95 %and higher than the liquid limit of 84%. Heavily
weathered debris flow deposits below the mixed
layer commonly contained gibbsite.

Fig. 6 SEM images of pumice grains. Upper:
Pumice grains in the main part of the pumice layer.
Volcanic glass (yellow) is covered by amorphous
precipitate. Lower: Pumice grain in the mixed
layer. No volcanic glass but small spherical
halloysite.

310th Asian Regional Conference of IAEG (2015)

5. Discussion
5.1 Mechanism of the weathering of the mixedlayer
The sliding surfaces of the landslides weinvestigated were all within the mixed layer which
is rich in halloysite, at the base of Qhpt layer.
Halloysite is contained in the mixed layer and alsosoil of weathered debris flow deposits just below it,suggesting that it is made after the deposition of
Qhpt. Soil much below the mixed layer was lack in
halloysite but had gibbsite, which is the residue ofintensive leaching. These occurrences suggests thatonce gibbsite is made by leaching, pumice grainsdeposited, and then water percolated from theground surface, got SiO2 from the pumice, thenreached to the base of the Qhpt and reacted withthe soil and pumice to form halloysite. The
silica-rich water reacts with gibbsite-rich soil,
forming halloysite, Al2Si205(OH)4・2H2O (Kleber,Schwendenmann et al., 2007). Similar reaction has
been proposed for the halloysite formation in apaleosol where sliding surfaces were made duringthe 1978 Izu-Oshima-Kinkai earthquake (Chigira,1982). Pumice grains in the mixed layer isdissolved to the water and halloysite is presumed tohave formed on their surfaces.

of PRISM images strongly suggest that major
geological causes of the landslides were slopeparallel layering of the pumice fall deposits,undercutting by river incision, and the weatheringof the mixed layer at the base of the pumice falldeposits. This undercutting reduced the support ofpumice fall deposits from downslope. These factors
can be considered in the contexts of landscapedevelopment (Fig. 8), which in sequence was thedeposition of debris flow, weathering, volcanic
eruption and the deposition of pumice fall deposit,
the interaction between percolating water and thematerials in the mixed layer and beneath, andundercutting by river incision. Such a history could
commonly occur in tropical volcanic areas.
6.

Conclusions
The 2009 Padang earthquake triggered manylandslides. We have investigated the factors oflandslides based on field survey and laboratory
tests.
As a result, we found that these landslides had
the following characteristics: 1) they occurred in
the areas with pumice fall deposits with >3 m
thickness, which was controlled by the distancefrom their source; 2) the pumice fall deposits had aslope-parallel layering, which had been cut at thefoots of slopes; 3) pumice and soil grains in themixed layers at the base of the pumice fall deposits
were heavily weathered to be clayey materials with
abundant halloysite; and 4) sliding surfaces weremade in the weak mixed layers.
The geological history, which is volcanic
eruption, weathering, and undercutting by river
incision, is common in tropical volcanic areas. Thatmeans we can make a hazard map of such acatastrophic landslide induced by earthquakes on
the basis of geological development.

5.2 Factor of the landslides occurrence from theviewpoints of geotechnics and landscapedevelopment
Infinite slope stability analysis has been donewith parameters of slope angles and thickness ofbeds that slid. The strength parameters used for thesliding surface were 18.2 kPa of cohesion and 15°of internal friction angle. One example with ahorizontal seismic acceleration of 250 gals is shownin Fig. 7, in which 9 of 10 landslides: all landslides
we measured slope angle in field survey, plotted
had safety factors less than 1. This acceleration isconsistent with that estimated in the affected areaby the US Geological Survey.
Our field survey and stereoscopic examination

Fig. 8 Bird eye’s view of the schematic of
the topographic development.

Fig. 7 Safety factors obtained frominfinite slope stability analysis withparameters of slope angles and thedepth of a sliding surface.

410th Asian Regional Conference of IAEG (2015)

Pumice fall deposits
Mixed layer
Terrace deposits
Debris flow deposits

Fig. 9 Conceptual image of the geological
profile of the slope.

Acknowledgements
The authors are grateful to prof. Tajul
Jamalludin, prof. and Ibrahim Komoo of theUniversity of Kebangsaan, Malaysia, and Ms. Sri
Hidayati, Mr. Yunara dasa triana, Mr. Amalfy
omang, and Mr. Gasid of the Center for
Volcanology and Geological Hazard Mitigation ofIndonesia for the discussion in the field. Yuki
Matsushi of DPRI Kyoto University is
acknowledged for his help in the simple shear tests
in the laboratory.
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