Nucleation and Kinematic Rupture of the 2017 Mw 8.2 Tehuantepec Earthquake

Integrated observations from the 2017 Mw 8.2 Tehuantepec, Mexico, earthquake probe one of the largest normal-faulting events inside a subducting slab. In this study, we utilize a template matching approach to detect possible missing earthquakes within a 2-month period before the mainshock. The seismicity rate shows an abrupt increase in the last day around the mainshock hypocenter. The large distance between most of the foreshocks and the mainshock is not consistent with static stress triggering but suggests alternative mechanisms such as delayed dynamic triggering or aseismic transients. Back-projection using the USArray network reveals that the rupture propagated northwestward unilaterally at a speed of 3.6 km/s and terminated north of the Tehuantepec Ridge. Towards the end of the rupture, a wide step-over occurred onto an adjacent fault parallel to the main fault plane. The mainshock is likely a reactivation of subducted outer-rise faults, supported by the similarity of fault-strike angles. The surprisingly large magnitude is consistent with exceedingly large dimensions of outer-rise faulting in this segment of the central Mexican trench.