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

Abstract

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 Mw 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 décollement resides on top of a strong reflector marking the inferred top of pelagic sediments. Where ramp faults are bivergent (both landward and seaward), the décollement 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 décollement 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.

Earthquake Cycles in Fault‐Bend Folds

“…Fault structures with significant fault bends may be more susceptible to shallow ruptures. These findings provide a theoretical context to understand the emergence of shallow ruptures at continental fold‐and‐thrust belts like Nepal and Taiwan and at subduction zones around the world (Bradley et al, ; Chen et al, ; Hubbard et al, , ; Lavé & Avouac, ; Seeber et al, ; Yue et al, ).…”

“…The root cause for shallow frictional instabilities for these shallow ruptures have been attributed to frictional contrasts at the smectite-illite transitions (e.g., Bilek & Lay, 2002;Saffer & Marone, 2003;Ikari et al, 2013;Ikari & Kopf, 2017). However, efficient seafloor uplift could be the result of slip on ramps and other fault bends (Behrmann & Kopf, 2001;Bradley et al, 2019;Geersen, 2019;Hubbard et al, 2015;Kopp, 2013). The nature of down-dip segmentation between shallow earthquakes and seismogenic zone earthquakes and the type of earthquake patterns it produces is still poorly understood (Ammon et al, 2006;Kozdon & Dunham, 2013;Lay et al, 2013;Martin et al, 2019;Sladen & Trevisan, 2018;Scholz, 2014).…”

Long-lived shallow slow-slip events on the Sunda megathrust

“…While this model is plausible, such behavior is not ubiquitousit has mainly been seen in halite and certain types of serpentinite gouges 47 . The shallow velocity-strengthening section of the Sunda megathrust may have localized within layer silicates (within the subducting sedimentary package) 48 . The frictional properties of these rocks have not been tested at the slip rates relevant to this study; however, they have been noted to be purely velocity-strengthening over the ranges (0.1-10 µm/s) testable with experimental setups 47,49 .…”

Heterogeneous Sediment Input at the Nankai Trough Subduction Zone: Implications for Shallow Slow Earthquake Localization

“…Lithological heterogeneity along the plate boundary may be an important condition for stable sliding to SSEs in other subduction systems (Barnes et al, 2020;Saffer & Wallace, 2015). In contrast to some margins (e.g., Bradley et al, 2019), the Nankai décollement propagates within a comparatively homogeneous mud-dominant lithofacies. Therefore, it is more likely that pore pressure in the subducting sediment plays a more important role in controlling plate boundary variations.…”