The role of thermal pressurization in driving deep fault slip during the 2021 Mw 8.2 Chignik, Alaska megathrust earthquake

The 2021 Mw 8.2 Chignik earthquake ruptured a weakly coupled portion of the deep slab in the eastern Aleutian-Alaska subduction zone, with no significant shallow slip. The underlying physics driving such large earthquakes nucleating at large depth and their impact on seismic and tsunami hazards remain poorly understood. We perform 3D dynamic rupture simulations that couple thermal pressurization of pore fluids within a finite shear zone with geodetically derived slip deficit models, unraveling the potential mechanisms governing deep coseismic ruptures in a fluid-rich subduction environment. Our simulations account for 3D slab geometry, regional subsurface material properties, fault slip deficit models, fast velocity-weakening rate-and-state friction, and thermally activated weakening mechanisms. Array- and frequency-dependent back-projection analyses validate the key kinematic source characteristics in the preferred model, highlighting the role of fault shear zone heterogeneities in rupture initiation, propagation, and arrest. Our results reveal a smoothly expanding rupture, which initiates on the deep slab close to the brittle-ductile transition and dynamically propagates across multiple locked asperities, driven by rising temperature and pore fluids at increasing slip rates. Our study demonstrates that the enhanced weakening resulting from thermal pressurization of pore fluid could promote the rupture of a large, partially locked region of the fault interface. We find that along-strike variations in pore fluid evolution, frictional properties and long-term slip deficit patterns collectively influence rupture dynamics and its termination at shallower depths.These data-integrated models provide insight into the mechanical conditions in the Semidi gap with important implications for regional seismic and tsunami hazards.