Frontal Rupturing and Tsunamigenic Potential of the Mentawai Locked Zone from Seismic Full Waveform Inversion and Numerical Modelling

Abstract:

The Sumatra subduction zone is one of the most seismically active subduction zone on the Earth. Although there has been three Mw >8.4 earthquakes in the region, including the disastrous 2004 Mw=9.2 Sumatra-Andaman earthquake, a 500 km long patch around Mentawai Islands is still locked and could produce a large megathrust earthquake. If the rupture propagates to the subduction front, which was the case for the 2004 earthquake, it may lead to another devastating tsunami. We have access to a very high-quality industry deep seismic reflection profile from the locked zone, which shows seismic image of the subducting interface down to 20 km depth. These data also show bi-vergent 1-3 km wide anticlinal pop-up structures that extend up to 40 km from the subduction front.

Underneath these pop-up structures, we find sub-horizontal reflectors. Full waveform inversion of seismic data indicates that the base of these pop-up structures coincides with a sharp change in P-wave velocity where the velocity varies 3.55 - 3.79 km/s suggesting the presence of a décollement surface where the pop-up structures seem to be rooted. Using these results, we perform mechanical modelling to determine the mechanical process and frictional properties along the accretionary wedge. We find that the presence of a ramp with a lower effective friction (intermediate décollement) is required to develop bi-vergence (pop-up) structures. Furthermore, we model bi-vergence (pop-up) structures in the frontal part to estimate the frictional properties and assess the tsunamigenic potential. We find that the effective frictional values are low, suggesting that up dip part of the plate interface is seismogenic. We also estimate different scenario of frontal slip and possibly vertical uplift. We find that horizontal slip could be up to 8.8 m whereas the vertical slip could be up to 1.3 m, which at 6 km water depth could generate devastating tsunami in the region.