Megathrust earthquakes can nucleate in the forearc mantle: Evidence from the 2004 Sumatra event
- J.-X. Dessa1,
- F. Klingelhoefer2,
- D. Graindorge3,
- C. André3,
- H. Permana4,
- M.-A. Gutscher3,
- A. Chauhan5,
- S. C. Singh5 and
- the SUMATRA-OBS Scieic Team
- 1GéoAzur, Université Pierre et Marie Curie, CNRS-IRD-UNS, 2 quai de la Darse, BP 48, F-06235 Villefranche-sur-Mer, France
- 2IFREMER, BP 70, F-29280 Plouzané, France
- 3Institut Universitaire Européen de la Mer (UBO), Technopôle Brest-Iroise, Place Nicolas Copernic, F-29280 Plouzané, France
- 4Research Center for Geotechnology-LIPI, Bandung, 40135, Indonesia
- 5Institut de Physique du Globe de Paris, 4 place Jussieu, F-75252, Paris, France
Abstract
Current models predict that the seismogenic zone along subduction thrusts, where the largest earthquakes nucleate and propagate, does not extend to the forearc mantle below the crust of the upper plate. Stable sliding conditions have been shown to prevail there, particularly along several circum-Pacific margins that underwent great megathrust earthquakes (Mw > 8.5) during the twentieth century. Based on geophysical investigation, we show that the great 2004 Sumatra-Andaman earthquake (Mw = 9–9.3) contradicts these models: not only did it propagate downdip along the interface between the forearc mantle and the subducting plate, but it actually nucleated along this reportedly aseismic part of the interplate contact. Petrological models can therefore underestimate the downdip extent of rupture zones to be expected in megathrust earthquakes, and need to be revised to account for this observation, albeit unusual.
Footnotes
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- Received 7 November 2008.
- Revision received 6 February 2009.
- Accepted 5 March 2009.
- © 2009 Geological Society of America
