Seismic images of the megathrust rupture during the 25th October 2010 Pagai earthquake, SW Sumatra: Frontal rupture and large tsunami

International audienceThe Mentawai segment of the Sumatra subduction zone is locked and likely to produce a large earthquake in the near future. A part of this locked zone ruptured on 12 September 2007 producing twin earthquakes of Mw = 8.5 and 7.9. Recently, a third earthquake of Mw = 7.8 occurred on the 25th October 2010, SW of Pagai Island, Sumatra. The earthquake generated an unexpected very large tsunami on Pagai Islands with run-up height of up to 8 m. Here we present seismic reflection and bathymetry images from the 2010 epicentral region acquired before the earthquake. We find that the frontal thrust is the main active fault in this region and might have ruptured up to the seafloor at 6 km water depth uplifting the water column and producing a large tsunami. Furthermore, finite fault models indicate that this earthquake ruptured the frontal section of the subduction zone, which is generally believed to slip aseismically and be incapable of producing large earthquakes. The presence of aftershocks near the subduction front further confirms that the frontal section of the subduction zone is not aseismic. If the rest of the Mentawai locked zone ruptures the frontal section of the subduction zone during a megathrust, then the resulting tsunami in the Indian Ocean could be devastating