Abstract
The Mw 7.8 October 2010 Mentawai, Indonesia, earthquake was a “tsunami earthquake,” a rare type of earthquake that generates a tsunami much larger than expected based on the seismic magnitude. It produced a locally devastating tsunami, with runup commonly in excess of 6 m. We examine this event using a combination of high-rate GPS data, from instruments located on the nearby islands, and a tsunami field survey. The GPS displacement time series are deficient in high-frequency energy, and show small coseismic displacements (<22 cm horizontal="" and=""><4 cm subsidence).="" the="" field="" survey="" shows="" that="" maximum="" tsunami="" runup="" was="">16 m. Our modeling results show that the combination of the small GPS displacements and large tsunami can only be explained by high fault slip at very shallow depths, far from the islands and close to the oceanic trench. Inelastic uplift of trench sediments likely contributed to the size of the tsunami. Recent results for the 2011 Mw 9.0 Tohoko-Oki earthquake have also shown shallow fault slip, but the results from our study, which involves a smaller earthquake, provide much stronger constraints on how shallow the rupture can be, with the majority of slip for the Mentawai earthquake occurring at depths of <6 km. this="" result="" challenges="" the="" conventional="" wisdom="" that="" the="" shallow="" tips="" of="" subduction="" megathrusts="" are="" aseismic,="" and="" therefore="" raises="" important="" questions="" both="" about="" the="" mechanical="" properties="" of="" the="" shallow="" fault="" zone="" and="" the="" potential="" seismic="" and="" tsunami="" hazard="" of="" this="" shallow="">