An ancient shallow slip event on the Mentawai segment of the Sunda megathrust, Sumatra
The outer-arc islands of western Sumatra rise during great megathrust earthquakes, due to large slip on the underlying megathrust. In contrast, the islands subsided up to a few centimeters during the recent tsunamigenic earthquake of October 2010, due to slip far updip, near the trench. Coral microatolls on one of the islands recorded a much larger subsidence, at least 35 cm, during an event in approximately A.D. 1314. We calculate a suite of slip models, slightly deeper and/or larger than the 2010 event, that are consistent with this large amount of subsidence. Sea level records from older coral microatolls suggest that these events occur at least once every millennium, but likely far less frequently than their great downdip neighbors. The revelation that shallow slip events are important contributors to the seismic cycle of the Mentawai segment further complicates our understanding of this subduction megathrust and our assessment of the region's exposure to seismic and tsunami hazards.
Additional Information© 2012 by the American Geophysical Union. Received 7 December 2011; revised 15 March 2012; accepted 19 March 2012; published 1 May 2012. We thank Dudi Prayudi, Imam Suprihanto, John Galetzka, and all the crew members of the K.M. Andalas for field support, Ke (Coco) Lin for assistance with U-Th disequilibrium dating at the HISPEC Laboratory at the National Taiwan University, and Aron Meltzner, Jean-Philippe Avouac, and Mark Simons for helpful discussions. This paper was improved thanks to thoughtful reviews by Harvey Kelsey and an anonymous reviewer. This project was supported by National Science Foundation grants 0208508, 0530899, 0538333, and 0809223 to K.S.; Sumatran Paleoseismology grant M58B50074.706022; the Earth Observatory of Singapore; the Research Center for Geotechnology at the Indonesian Institute of Sciences (LIPI); the Caltech Tectonics Observatory, and the Gordon and Betty Moore Foundation. U-Th disequilibrium dating was supported by the Taiwan ROC National Science Council grants 99- 2611-M-002-006, 99-2628-M-002-012, and NSC100-3113-M-002-002 to C.-C.S., and by National Science Foundation grants EAR-0207686 and 0537973 to R.L.E. This is Earth Observatory of Singapore contribution 36 and Caltech Tectonics Observatory contribution 189.
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