Stratigraphic Control of Frontal Decollement Level and Structural Vergence and Implications for Tsunamigenic Earthquake Hazard in Sumatra, Indonesia

Propagation of fault rupture to the seafloor is a likely cause of enhanced tsunami generation during megathrust earthquakes. New, high-resolution seismic reflection profiles and swath bathymetry collected across the northern limit of the M-w 7.8, 25 October 2010 Mentawai tsunami earthquake rupture reveal significant and systematic lateral variations in both the stratigraphic level of the frontal Sunda megathrust and the vergence of its frontal ramp faults. Where ramp faults are uniformly seaward vergent, the decollement resides on top of a strong reflector marking the inferred top of pelagic sediments. Where ramp faults are bivergent (both landward and seaward), the decollement is localized within the subducting clastic sequence, above a xseismically transparent unit inferred to be distal fan muds. Where ramp faults are uniformly landward vergent, the decollement is directly on top of the oceanic crust of the subducting Investigator Fracture Zone. Enhanced surface uplift and tsunamigenesis during the 2010 tsunamigenic earthquake appear to have coincided with propagation of rupture into frontal areas with well-developed structural bivergence. Frontal bivergence is a geological signal of low basal traction during accrual of slip, and offshore of Sumatra this structural style may mark areas of enhanced tsunami hazard posed by small-magnitude, shallow megathrust ruptures that propagate into the incoming terrigenous sequence at near-trench levels. Plain Language Summary Very large magnitude subduction earthquakes commonly produce huge tsunamis that devastate shorelines great distances away from the source. These tsunamis are generally preceded by strong ground shaking that warns coastal residents near the rupture area to evacuate. Over the last several decades, the subduction zone south of Sumatra and Java has produced three deadly tsunamis that were caused by much smaller-magnitude earthquakes. The high casualty count of these events, called tsunamigenic earthquakes, is largely due to the lack of warning from major ground shaking before arrival of the tsunami waves. The cause of these earthquakes remains poorly understood and few studies have directly examined the fault systems that produce the excessively large waves. We used seismic reflection and bathymetry data to study an area of the Sumatran subduction zone that produced a tsunamigenic earthquake in 2010 and killed more than 500 people. We focused on understanding the different kinds of rock the shallowest megathrust fault cuts through, which might have different frictional properties. By comparing the stratigraphic position of the fault with the geological structure of the overlying wedge, we show that excessive slip occurred in a specific geological context that indicates low friction on the fault surface. This is a significant step in better understanding how tsunamigenic earthquakes happen in this region.