Conditions for Triggering Seismic Ruptures and/or Slow Slip Events in the Framework of a Poro‐Plastic Fault Zone Model

Active fault zones are not simple frictional interfaces. They are complex, evolving 3‐D structures that contain pore fluids in drained or undrained conditions. They show stable or unstable slip, dilatant or contractant elastic or plastic local deformations in response to far‐field extensive or compressive displacements. Pore fluid pressure variations induce effective stress and fluid compressibility changes, possibly up to degassing. For these reasons, we choose to use an analytical model in which the fault zone is schematized by a mesoscale poroplastic interface surrounded by two symmetrical elastic pads to investigate the conditions that can lead to slow slip vs. seismic slip. The chosen end‐cap plastic failure criterion and associated flow rule account for hardening/softening regimes. The model computes stress paths, normal/tangential displacement, and fluid pressure variations of the fault zone with time. Depending on the initial stress, pore pressure conditions, far‐field loading rate, and fluid compressibility, scenarios emerge that link together stable plastic deformation regimes (months to years, small displacements) and unstable regimes (seconds, metric displacements). While some scenarios show the occurrence of instability preceded or not by slow slip events, others show slow events without instability (mainshock). Also investigated are the effect of a switch from undrained to drained condition and the role of the change in fluid compressibility due to dilatancy. Finally, we discuss the geodetic and geophysical data necessary to calibrate the parameters of the empirical relationships. Slow slip event areas are therefore not necessarily “safe” and might produce earthquakes if conditions change.