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We combine multi-array relative back-projection of high-frequency P waves and finite-fault modeling of low-frequency P waves from the 8 September 2017 M-w 8.2 Chiapas, Mexico earthquake to image unilateral rupture on a southeast-northwest striking subvertical fault plane over depths of 10-70 km. Improved multi-array relative back-projection estimates of rupture speed and fault geometry provide key refinements to the slip model from finite-fault modeling. Compilation of prior seismicity and aftershocks reveals the earthquake initiated in the lower plane of a double Benioff zone (DBZ) and ruptured with large slip across the entire DBZ. The previously aseismic interior does not appear to be due to lack of stress as both planes show downdip extension stresses. Instead, tomographic imaging of DBZs find high Poisson's ratio with fast velocities in the DBZ interior, so we propose that highly hydrated harzburgite generates velocity-strengthening behavior that inhibits earthquake nucleation, yet it can slip seismically during dynamic weakening from a large throughgoing rupture.

Plain Language Summary We studied the detailed rupture properties of the damaging M-w 8.2 earthquake that occurred in Chiapas, Mexico on 8 September 2017, using a complimentary combination of techniques to refine our estimates of the rupture speed, depth, and geometry. We found that the rupture began near a depth of 60 km inside a subducting plate and rupture propagated upward across the entire plate, including a portion that is normally aseismic. Together, these observations suggest that the interior of the subducting plate cannot normally produce earthquakes, but the seismic threshold can be reached through processes such as dehydration of rocks being pushed to higher pressures and temperature or from the fault slip of nearby large earthquakes that dynamically weaken the rocks.