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Plate-boundary deformation associated with the great Sumatra–Andaman earthquake


The Sumatra–Andaman earthquake of 26 December 2004 is the first giant earthquake (moment magnitude Mw > 9.0) to have occurred since the advent of modern space-based geodesy and broadband seismology. It therefore provides an unprecedented opportunity to investigate the characteristics of one of these enormous and rare events. Here we report estimates of the ground displacement associated with this event, using near-field Global Positioning System (GPS) surveys in northwestern Sumatra combined with in situ and remote observations of the vertical motion of coral reefs. These data show that the earthquake was generated by rupture of the Sunda subduction megathrust over a distance of >1,500 kilometres and a width of <150 kilometres. Megathrust slip exceeded 20 metres offshore northern Sumatra, mostly at depths shallower than 30 kilometres. Comparison of the geodetically and seismically inferred slip distribution indicates that 30 per cent additional fault slip accrued in the 1.5 months following the 500-second-long seismic rupture. Both seismic and aseismic slip before our re-occupation of GPS sites occurred on the shallow portion of the megathrust, where the large Aceh tsunami originated. Slip tapers off abruptly along strike beneath Simeulue Island at the southeastern edge of the rupture, where the earthquake nucleated and where an Mw = 7.2 earthquake occurred in late 2002. This edge also abuts the northern limit of slip in the 28 March 2005 Mw = 8.7 Nias–Simeulue earthquake.

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Figure 1: Tectonic setting and ruptures of major interplate earthquakes along the Sunda megathrust.
Figure 2: Comparison of near-field geodetic measurements (black arrows) with predictions (green arrows) of the seismic model III of ref.14.
Figure 3: Fault slip distribution determined from the geodetic data.
Figure 4: Latitudinal variations of scalar moments as determined from seismic waveforms (model III of ref.14) and from geodetic data.


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We thank the team from BAKOSURTANAL who collected the GPS field data in Sumatra under difficult conditions (J. Efendi, A. Indrajit, M. Nyamadi, C. Bagandi, D. Sudharmono, A. Suryono, M. Achmad, U. Santoso, C. Yuniarsa, Endang, A. Pujobuntoro, H. Dradjat, B. Susilo and B. Parjanto). We are grateful to C. Vigny for comments and suggestions. This work has benefited from discussions with C. Ji. The Gordon and Betty Moore Foundation, the US National Science Foundation, the Southern California Earthquake Center, and BAKOSURTANAL supported this research. We thank R. W. Matindas and J. McRaney for their support. This is Caltech Tectonics Observatory contribution number 31.

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Correspondence to Jean-Philippe Avouac.

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Supplementary information

Supplementary Figure 1

Slip model derived from the inversion of body waves and surface waves with predicted horizontal and vertical displacements. (PDF 684 kb)

Supplementary Figure 2

Observed and predicted displacements from slip model obtained from the inversion of horizontal and vertical measurements. (PDF 901 kb)

Supplementary Figure 3

Latitudinal variations of scalar moments as determined from seismic model and from the slip models A, A2 and A3 listed in Table 2. (PDF 205 kb)

Supplementary Figure 4

Checker board resolution test. (PDF 1856 kb)

Supplementary Data

This file contains Supplementary Tables 1–8 and additional references. (DOC 70 kb)

Supplementary Methods

This section contains additional explanations of the data handling and the modelling. (DOC 113 kb)

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Subarya, C., Chlieh, M., Prawirodirdjo, L. et al. Plate-boundary deformation associated with the great Sumatra–Andaman earthquake. Nature 440, 46–51 (2006).

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