Tsunami Risk from a Mentawai Islands Megathrust Earthquake: 2006AGUFM.S31C..07M

Interaction stresses from recent great earthquakes on the Sunda Trench subduction zone have made another earthquake, this time under the Mentawai Islands, more likely. The megathrust under Siberut Island has not ruptured since the great 1797 earthquake and may be ripe for triggered failure. Paleogeodetic studies support a range of possible events, the smallest of which would rupture only that part of the megathrust which rupture in 1797 but not in 1833, the largest could propagate southward, breaking the Enganno Island barrier. The practical consequences of these events would depend crucially on their tsunami potential, which in turn depends on details of slip distribution on the future earthquake. Here we employ a Monte Carlo approach to the forward problem and model about 1000 possible earthquakes, calculating the seafloor displacements and tsunami wave height distributions resulting from the most likely 100 or so. Results forecast generally smaller tsunamis compared to those generated by the Sumatra-Andaman earthquake. We show that the Mentawai Islands, on the fore arc high, play a cruical role in moderating the tsunami hazard, reducing the impact of high slip on at intermediate depths on the megathrust and dissipating energy from tsunami waves generated over the trench. These results will help to constrain preparedness strategies around the Indian Ocean and in particular on western Sumatra.