Updating megathrust coupling models for the Mentawai Seismic Gap and surrounding regions, Sumatra

Following the 2004 Mw 9.2 Sumatra-Andaman event, a series of earthquakes occurred along the Sunda megathrust of the Sumatran subduction zone, extending from the southern terminus of the 2004 rupture to Bengkulu. A notable exception is the Mentawai seismic gap, spanning from just south of the Batu Islands to Sipora for ~200 km in length. Historical records of regional seismicity from paleogeodetic measurements (i.e. coral microatolls) indicate that the last major event that ruptured the current seismic gap occurred in 1797. An adjacent patch ruptured in 1833, broadly coincident with the 2007 Bengkulu rupture. More recent M≥7 events surrounding the Mentawai seismic gap have occurred in 2007, 2008, 2010, and 2023. However, slip distributions of these events show limited slip propagation into the gap and a significant slip deficit remains. Thus, a potential earthquake in the region poses a threat to local communities from both ground shaking, as well as a potential tsunami. Previous geodetic estimates of coupling in the region indicate low coupling at shallow depths on the megathrust. However, these estimates lack near-trench observations and ignore the influence of stress shadows originating from frictionally locked asperities downdip, and thus may underestimate the tsunami hazard, especially in light of the 2010 Mentawai tsunami earthquake that ruptured to the trench at depths of <6 km. Additionally, new estimates of long-term slip rates on the Sumatran Fault indicate the forearc sliver is deforming as rigid block and substantial oblique convergence is taken up within the oceanic plate. By correcting published geodetic velocities to remove the motion of the forearc sliver, we place updated constraints on subduction obliquity. Combining these observations with paleogeodetic uplift and subsidence rates, we invert for a coupling distribution on the Sunda megathrust, accounting for the effect of stress shadows, and constraining the coupling direction based on earthquake slip vectors. We find, in contrast to previous estimates, that the megathrust appears coupled to the trench. This coupled region extends from just north of Siberut south to the Pagai Islands and includes the region of the 2007 Bengkulu rupture. While the risk for large earthquakes in this region is relatively well known, our results indicate that the Mentawai seismic gap contains a strongly coupled patch that extends to the trench, suggesting that the tsunami hazard is significantly higher than inferred from previous coupling estimates. Additionally, this updated coupling model allows us to place new constraints on the influence of tectonics on regional sea level projections.