Evidence of active backthrusting at the NE Margin of Mentawai Islands, SW Sumatra

The Indo-Australian plate subducts obliquely beneath the Sunda plate leading to a slip partitioning into pure thrust and strike-slip motion. Just in the last 5 yr, three pure thrust earthquakes of M-w > 8.4 have occurred along this subduction interface. The Great Sumatra Fault, traversing the Sumatra continental block, takes up a significant part of the strike-slip motion, but the Mentawai Fault bounding the NE margin of Mentawai Islands has been suggested to accommodate a part of the strike-slip motion. Although the great Sumatra Fault is active, no seismicity has been observed along the Mentawai fault. Using a combination of high-resolution seismic reflection and bathymetry data, here we show that the Mentawai Fault seems to be characterized by active SW dipping backthrusts. The presence of recent steeply dipping thrust earthquakes suggests that these faults should be active. Combined with results from north in 2004 earthquake region and south of this study area, our results suggest that backthrusting should play an important role in forearc evolution SW of Sumatra. We also observed several mass wasting sites at NE margin of the Mentawai Islands, which could be erosional features or landslides triggered by earthquake activities. Localized uplift along the steeply dipping backthrusts at the NE margin of Mentawai Islands in the fully locked region could pose serious seismic and tsunami risks to the SW coast of Sumatra in the future.