The Mw 7.6 Western Sumatra Earthquake of September 30, 2009
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EERI Special Earthquake Report — December 2009 Learning from Earthquakes The Mw 7.6 Western Sumatra Earthquake of September 30, 2009 From October 9th to 18th, a team Andrew Kizzee of Food for the people, including 900,000 in Pad- organized by the EERI investigated Hungry, Harkunti Rahayu, Syahril ang and 80,000 in Pariaman. Pad- the effects of the Western Sumatra Kusuma, and Anin Utami of the Insti- ang is the capital of West Sumatra, Earthquake. The team was led by tute of Technology Bandung, Kerry situated on the coast of the Indian Gregory Deierlein of Stanford Uni- Sieh of Nanyang Technical University, Ocean between the Sumatra fault versity, and included Nick Alexan- and Gina Sandoval, Richard Franco, and the Sunda Trench fault (Fig- der of Degenkolb Engineers, Veron- and Brian DiBarnaba of Degenkolb ure 1). ica Cedillos of GeoHazards Inter- Engineers. The earthquake caused 1,195 national, Louise Comfort of the The research, publication and distri- deaths and significant damage to University of Pittsburgh, Tim Hart bution of this report were funded by about 140,000 houses and 4,000 of Forell/Elsesser Engineers, Inc., the EERI Learning From Earthquakes other buildings (Satkorlak, 2009). Elizabeth Hausler of Build Change, project, under grant #CMMI-0758529 The casualties (383 deaths, 431 Scott Henderson and Kelly Wood of from the National Science Founda- serious injuries) in Padang were the Stanford Chapter of Engineers tion. Additional support was provided mostly due to building damage and for a Sustainable World, Sindhu by the Blume Earthquake Engineer- collapse. These numbers would Rudianto of Geo-Optima, Inc., and ing Center at Stanford University, likely have been higher had the Sugeng Wijanto of PT. Gistama the Pacific Earthquake Engineering earthquake struck earlier, when Intisemesta. Others who contrib- Research Center, and the host orga- schools and offices were in ses- uted to this report include Walter nizations of individuals named above. sion. Mooney and Art McGarr of the USGS, Carlos Cabrera of Risk Landslides in the outlying rural Management Solutions, Inc., Introduction mountain areas buried several vil- On Wednesday September 30, 2009, lages, damaged roads, and caused at 5:16 p.m., an Mw over 600 deaths. That the earth- 7.6 earthquake struck quake did little damage to roads the west coast of and bridges in and around Padang Sumatra, affecting facilitated the restoration of power, an area with a pop- communications and infrastructure ulation of about 1.2M to most regions within a week. Figure 1. Location of the September 30 and Octo- ber 1 earthquake epicenters on the Sunda thrust Figure 2. Padang and the Batang Arau River viewed from the fault and Sumatra strike-slip faults (Sieh 2009). mountains to the east. 1 EERI Special Earthquake Report — December 2009 Figure 4. Siti Nurbaya Bridge south approach ramp. code enforcement. The unusual. The thrust faulting source earthquake also high- mechanism indicates that it was lights the need for im- due to compression and internal proved emergency re- buckling of the oceanic lithosphere. sponse, tsunami evacua- A slip model developed by the tion strategies, and coor- USGS indicates a maximum slip d ination of relief efforts for within the rupture zone of about future disasters. 9 m. This, in conjunction with the seismic moment (about 2.6x1020 Seismicity and N-m), suggests a high maximum Figure 3. Ground acceleration and response Ground Motions slip rate, as well as strong radiated spectra (N-S component) and design earthquake energy. As with typical subcrustal spectra (BMKG/USGS, 2009). Earthquakes are abun- earthquakes (60-170 km depth), the abundant at the megathrust bound- earthquake produced only a few The low-lying coastal regions of ary between the subducting oceanic aftershocks, most soon after the Padang and the surrounding west Indo-Australian plate and the overrid- M7.6 main shock. On October 1, coast of Sumatra have one of the ing Sunda plate, which includes the the area experienced shaking from highest risks in the world from a island of Sumatra. The subduction a M6.6 earthquake, which was not tsunami, specifically one generated zone surrounding this event has not an aftershock, but instead origi- by a large earthquake on the Sunda had a megathrust earthquake since nated on the Sumatra fault about Trench, since it has a seismic gap a very large event (est. >M 8.5) in 215 km south east of Padang (see (Sieh, 2009). Padang is bordered 1797. However, the Mw7.6 quake was Figure 1). on the south by the Batang Arau neither a megathrust event nor did River and on the south and east by it generate a tsunami of any signifi- There is only one strong ground mountains; over time the city has cance. It was located at a depth of motion record from the region developed to the north along the about 80 km within the oceanic slab (BMKG/USGS 2009), which shows coast (Figure 2). When strong of the Indo-Australian plate, with its about 20 seconds of strong shak- ground shaking was felt on Sep- epicenter located offshore about 60 ing with a peak ground accelera- tember 30, most people in Padang km WNW of Padang. tion (PGA) of 0.3g (Figure 3). The spectral accelerations in the short attempted to evacuate inland to The rupture zone of the earthquake higher ground, but with limited period ranged from 0.5g-1.2g and is remarkably compact, with a nearly dropped off at longer periods. Since success due to traffic congestion. circular shape with a radius of only Fortunately, the earthquake was the instrument site was located at 15 km. In terms of its high-frequency the base of the mountains, about caused by a deep fault rupture that ground motion, this earthquake was did not trigger a tsunami. 12 km in from the coast and on similar to intra-slab earthquakes, at stiff soil, the ground motions in the The cities of Padang and Pariaman intermediate depth and comparable center of Padang, on softer deeper and the surrounding region face a magnitude, in other locations such soil deposits, are likely to have major building reconstruction effort. as the west coast of South America. been larger. Superimposed with the The widespread damage to build- However, its focal mechanism — measured spectral accelerations ings raises questions about the ef- thrust faulting on planes striking at are design earthquake spectra, fectiveness of design and construc- high angles to the trend of the sub- described later. Median PGA val- tion practices, as well as building duction zone off Sumatra — is quite ues from attenuation models for 2 EERI Special Earthquake Report — December 2009 Under a four-story public works building, located 100 m away from the river front, there was liquefac- tion that may have contributed to the building damage. The building configuration concentrated lateral deformations and residual drift in the first story (Figure 5). Adjacent to the building, fine beach sand spouted out of ground cracks, indi- cating that liquefaction may have caused foundation movement and increased the demands on the structure. There was ground defor- mation under several other smaller buildings along the river front and in other isolated parts of Padang. Landslides: At Lubuk Lawe, north- Figure 5. First-story column damage and residual drifts in public works east of Padang and Pariaman, ex- building. tensive landslides and mud flows buried hundreds of people and de- subduction earthquakes for M7.6, to be significant, except in few areas molished at least five villages (Fig- R=60km, and H=80km yield PGA of next to the Bantang Arau River, where ure 6). Observations of the spoil 0.4g to 0.6g for soil sites (Young et there was ground cracking up to sev- site showed that the pumiceous tuff al., 1997; Zhao et al., 2006; Atkin- eral hundred meters in from the river originating from late eruptions of son et al., 2003), which are consis- front. the Maninjau caldera was light and tent with the strong motion record- The Siti Nurbaya bridge (in Figure 2) porous and had little cohesion. ing. is one of the few bridges outside of Heavy rain over several days be- the mountain areas that suffered Geotechnical Aspects and fore the earthquake is likely to have earthquake damage. It is an impor- saturated the ground, increasing Landslides tant link between the low-lying old the driving force and weakening the Geotechnical conditions and town of Padang and high ground lo- soil resistances, causing the slope ground deformation: According cated to the south, and the bridge ap- to be marginally stable. The flat to the geologic maps of the Geo- proach had ground cracking due to lands below the hills at the toe of logical Survey of Indonesia (Kas- settlement and dynamic densification the hills, consisting mainly of loose towo et al., 1973), the coastal of fills within the reinforced earth silt, sand and gravel mixtures, may plains of Padang and Pariaman are ramp (Figure 4). have also lost lateral support and underlain by quaternary alluvium deposits, consisting of silt, sand, gravel and remnants of pumice tuff. Preliminary information from soil borings in Padang shows the sub- surface consisting of medium dense to dense silty sand and stiff to very stiff silt with relatively low ground water levels. In areas next to the Batang Arau River (Figure 2), the site conditions include undocu- mented fills consisting of loose, saturated fine sand, which were placed as part of the site develop- ment. Despite severe damage to many buildings, ground cracking and foundation damage did not appear Figure 6. Shallow slope failures and debris flows in rural areas northeast of Padang. 3 EERI Special Earthquake Report — December 2009 contributed to the massive land- In such cases, the infill walls tended Modeled after the New Zealand slides and debris and mud flows.