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Published February 27, 2007 | Supplemental Material + Published
Journal Article Open

Interseismic deformation above the Sunda Megathrust recorded in coral microatolls of the Mentawai islands, West Sumatra


The geomorphology and internal stratigraphy of modern coral microatolls show that all the outer arc Mentawai islands of West Sumatra have been subsiding over the past several decades. These same islands rose as much as 3 m during the giant megathrust earthquakes of 1797 and 1833, and the current subsidence probably reflects strain accumulation that will lead to future large earthquakes. Average subsidence rates over the past half century vary from 2 to 14 mm yr^(−1) and increase southwestward, toward the subduction trench. The pattern is consistent with rates of subsidence measured by a sparse network of continuously recording Global Positioning System (cGPS) stations and with locking of a 400-km-long section of the underlying subduction megathrust, between about 1°S and 4°S. This record of subsidence and tilting, extending nearly a century into the past, implies that the region is advancing toward the occurrence of another giant earthquake. However, evidence of episodic rather than steady subsidence reflects a behavior that is more complex than simple elastic strain accumulation and relief. Most prominent of these episodes is an extensive emergence/subsidence couplet in about 1962, which may be the result of rapid, aseismic slip on the megathrust, between the islands and the trench. Lower subsidence rates recorded by the corals since about 1985 may reflect failure on many small patches within the locked section of the megathrust.

Additional Information

© 2007 American Geophysical Union. Received 15 April 2006; revised 28 August 2006; accepted 9 October 2006; published 27 February 2007. Many friends, colleagues, and family members provided critical support of our field efforts in 1997, 2000, 2002, and 2003. Most central to the success of the project were the dedicated efforts of Imam Suprihanto, Susi Mardia, Dudi Prayudi, Robert Taylor, and our boat crews. The hospitality, friendship, and interest of the people of the Mentawai islands have been essential to this work. Most of this research was supported by NSF grants EAR-9628301, EAR-9804732, EAR- 9903301, and EAR-0208508 (to K.S.) and EAR-9628716, EAR- 9903443, and EAR-0207686 (to R.L.E.), administered through the NSF tectonics and geophysics programs. A RUTI IV grant from Indonesia's Ministry of Research and Technology provided support for the final phase of data analysis in 2005. This is contribution 30 of Caltech's Tectonics Observatory.

Attached Files

Published - 2006JB004450.pdf

Supplemental Material - 2006jb004450-fs01.eps

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