Evidence of Frontal Rupturing in the Mentawai Gap, SW Sumatra, Newly Acquired High-Resolution Seismic Reflection and Bathymetry Data

Abstract:

We present here the first results of the Mentawai Gap Tsunami Earthquake Research Assessment (MEGATERA) survey, conducted onboard R/V Falkor of the Schmidt Ocean Institute from 23 May-29 July 2015. The Mentawai Gap, located along the Sunda subduction zone offshore SW Sumatra, has been inferred to have the potential to produce a devastating great earthquake in the near future. Along this segment of the megathrust, only limited/small patches ruptured on 12 September 2007, during a twin earthquake of Mw = 8.5 and 7.9, and on October 25th, 2010, during the Mw=7.8 shallow earthquake that generated a large tsunami with run‐up of up to ~8 m on Pagai islands. To better understand the mechanism of tsunamigenesis in general and better assess this hazard in the Mentawai locked zone in particular, we acquired a total of 1725 km high-resolution seismic reflection data across the subduction front along with more than 10000 km² of multibeam bathymetry data. Along the Mentawai trench, we imaged the 2010 tsunami earthquake rupture zone, the Mentawai locked zone, the region impinged by the Investigator Fracture zone as well as the bending-related normal faults dissecting the incoming plate.

We find that the morphology of the frontal accretionary wedge section within both the region ruptured in 2010 and the locked segment consists of multiple folds bounded by double-vergent active faults, which when activated during a megathrust event could generate a large tsunami. We also find evidence of mass wasting along thrust escarpments, which could be erosional features or more likely landslides scars triggered by earthquake shaking. The density and distribution of these geologic features along strike could give clues on the seismic history of the segments. These new data will allow us to elucidate the tsunamigenic potential of the Mentawai locked zone.