Deep Embrittlement and Complete Rupture of the Lithosphere During the M8.2 Tehuantepec, Mexico Earthquake

Date: 4/26/2019

Time: 08:45 AM

Room: Cascade II

Subduction zones, where two tectonic plates converge, are generally dominated by large thrust earthquakes. Nonetheless, normal faulting from extensional stresses can occur as well. Rare large events of this kind in the instrumental record have typically nucleated in and ruptured the top half of old and cold lithosphere that is in a state of extension driven by flexure from plate bending. Such earthquakes are limited to regions of the subducting slab cooler than ~650°C and can be highly tsunamigenic, producing tsunamis similar in amplitude to those observed during large megathrust events. Here we show from analyses of regional geophysical observations that normal faulting during the 2017 M8.2 Tehuantepec, Mexico earthquake ruptured the entire Cocos slab beneath the megathrust region. We find that the faulting most likely reactivated a bend fault fabric and ruptured to a depth well below the predicted brittle-ductile transition for the Cocos slab, including regions where temperature is expected exceed 1000°C. Our findings suggest that young oceanic lithosphere is brittle to greater depths than previously assumed and that rupture is facilitated by wholesale deviatoric tension within the subducted slab possibly due to fluid infiltration or other unknown processes. We conclude that in this region lithosphere can sustain brittle behavior and fail in a large earthquake at greater P-T conditions than previously considered. Finally we show that should such intraslab events be possible oceanward of the trench there is significant cause for concern. The modest (3m) tsunami generated during the 2017 earthquake revealed a host of resonant phenomena associated with the long and flat Tehuantepec shelf which significantly protracts the duration of hazardous tsunami waves.

Presenting Author: Diego Melgar

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