Tsunamigenic earthquake at the Sunda trench promoted by aseismic slip after a previous megathrust event

The shallow portion of megathrusts can exhibit various modes of failure: interseismic creep, triggered afterslip, or rupture during a tsunami(genic) earthquake, resulting in a potential hidden danger to onshore communities. However, determining which mode is likely for a given portion remains challenging due to a lack of near-trench observations and limited resolution in inversion models. Here, we present a new two-step hybrid postseismic modelling approach that integrates the advantages of conventional forward and inverse methods, greatly improving our resolution of afterslip on the fault, viscous strain in the mantle, and the associated mechanical properties of both regions. We apply this approach to image the fault properties and slip evolution of the region surrounding the 2007 Mw 8.4 Bengkulu earthquake, and show how it promoted the subsequent 2010 Mw 7.8 Mentawai tsunami earthquake. Our results indicate that the deeper portion of the 2010 coseismic rupture area is velocity-strengthening, and slipped aseismically following the 2007 event. Upward propagation of this afterslip then loaded the shallow velocity-weakening area of the 2010 rupture, which generated a large tsunami in the Pagai Islands. Our approach affords a powerful tool for assessing the future possibility of tsunami(genic) and other triggered earthquakes in seismic gaps near recent large events. The 2007 Mw 8.4 Bengkulu earthquake promoted the subsequent 2010 Mw 7.8 Mentawai earthquake by loading the shallow, velocity-weakening area that ruptured in the latter, according to hybrid postseismic modelling that integrates forward and inverse methods