Bridging the gap between subduction dynamics and the long-term strength of the Sunda megathrust.

The link between great earthquakes and subduction dynamics across a wide range of scales remains a crucial, yet elusive, tenet of the seismotectonics of convergent margins. Here, we show high-performance computational simulations of the three-dimensional Sunda subduction zone dynamics matching plate motions, tectonics and current deformation, thereby providing insights into the stress regime of the Java-Sumatra-Andaman margin. Testing various tectonic forces reveals the primary control of the Java slab pull and induced mantle flow on the whole margin tectonics, driving northward-increasing oblique convergence, Sumatran trench advance and increased tectonic coupling along this segment. The modelled deviatoric stresses reproduce the geodetically-constrained interseismic compression in Sumatra and megathrust stress orientations notably consistent with the seismic P-axes, with magnitude comparable to static stresses on seismogenic faults. We map the frictional strength along the Sumatra-Andaman megathrust, identifying critical thresholds at key seismogenic depths, which remarkably correlate with the location of great earthquakes. Our outcomes show how subduction dynamics may critically prime the conditions for seismicity along the Sunda margin.