Tectonic and thermal controls on megathrust earthquakes along the Sunda Arc

The Sumatra subduction zone has produced multiple great earthquakes (e.g., the 2004 Sumatra-Andaman event), whereas the neighboring Java-Lesser Sunda margin has experienced smaller megathrust ruptures. Previous studies attributed this contrast primarily to along-arc variations in mantle wedge corner (MWC) depth, assuming serpentinite downdip of the MWC is velocity-strengthening and impedes seismic slip. However, deep ruptures and locking offshore Sumatra challenge this view. Based on serpentine phase-transition studies and frictional experiments, a recently proposed hypothesis suggests a narrow lizardite-rich, velocity-strengthening barrier to seismic rupture immediately downdip of the MWC and antigorite-rich patches that exhibit unstable stick‐slip at elevated temperatures farther downdip. Yet, how this concept applies to the Sunda Arc remains unclear. Here we integrate two-dimensional thermal models, geophysical constraints, and laboratory friction data to evaluate how tectonic settings and temperature-dependent serpentine phase transitions modulate Sunda Arc’s seismic behavior. We show that the Sumatra megathrust is sufficiently warm to host a narrow lizardite-rich barrier immediately downdip of the MWC, followed farther downdip by antigorite-rich materials, allowing large ruptures to extend from crustal depths to the forearc mantle. In contrast, the colder Java megathrust develops a much wider lizardite-rich barrier beneath the forearc mantle that impedes downdip rupture and confines the seismogenic zone to shallow crustal depths. Sensitivity tests show that incoming-plate age and MWC depth are first-order controls on seismogenic-zone width, with additional modulation by megathrust dip, basal friction, convergence rate, and subduction obliquity. Our study reconciles Sunda Arc observations and also applies to the Mariana, Kermadec, Japan Trench, and Chile margins.