Along-fault pore-pressure evolution during a slow-slip event in Guerrero, Mexico

Abstract

Slow earthquakes are important constituents of the seismic cycle and are involved in the stress transfer between the viscously slipping portion of the plate interface and the seismogenic zone. Their occurrence is likely associated with the near-lithostatic pore pressure in the slow earthquake source region, where fluids might modify fault friction and whose presence is indicated by high ratios of compressional (P)–wave velocity to shear (S)–wave velocity observed at the interface between the subducting plate and the overlying crust. Here we compare two slow earthquake phenomena observed in the Guerrero region of the Mexican subduction zone: low-frequency earthquakes (LFEs) and a slow-slip event (SSE) recorded by GPS. We observe variations of the LFE occurrence rates over month-long time scales during a large SSE that we interpret as a manifestation of transient changes in the fault shear strength. We argue that these transient changes are caused by a pore pressure fluctuation that migrates updip along the subduction interface. This mechanism suggests that fluids do not only passively weaken the plate interface but also play an active role in slow earthquake source regions.