Opposite Variations for Pore Pressure on and off the Fault During Simulated Earthquakes in the Laboratory.

We measured the spatiotemporal evolution of pore pressure on‐ and off‐fault during failure and slip in initially intact Westerly granite under triaxial conditions. The pore pressure perturbations in the fault zone and the surrounding bulk presented opposite signs upon shear failure, resulting in large pore pressure gradients over small distances (up to 10 MPa/cm). The on‐fault pore pressure dropped due to localized fault dilation associated with fracture coalescence and fault slip, and the off‐fault pore pressure increased due to bulk compaction resulting from the closure of dilatant microcracks mostly parallel to the maximum compression axis. We show that a reduction in bulk porosity and relatively undrained conditions during failure are necessary for the presence of the off‐fault pore pressure elevation. Considering this phenomenon as a consequence of a main shock, we further show that off‐fault pore pressure increase has the potential to trigger neighboring fault instabilities. In nature, we expect the phenomenon of off‐fault pore pressure increase to be most relevant for misoriented faults, where the pre‐rupture stresses can be large enough to reach the dilatancy threshold in the wall rocks. Plain Language Summary: The interplay between fluid flow and rock deformation plays a pivotal role in understanding the mechanics of geological processes and engineering activities, particularly in the context of seismic events. The slip of tectonic faults during earthquakes is expected to produce non‐uniform variations of fluid pressure, notably along the fault plane and in its vicinity. These variations may have a substantial impact on the dynamics of fault slip, an aspect that has thus far eluded precise quantification. We analyzed the evolution of pore pressure during simulated earthquakes in the laboratory, focusing on the contrast in pore pressure dynamics on and off the fault within a cm‐sized sample of granite, a rock representative of the continental crust. Opposing trends were observed upon rock failure: The on‐fault pore pressure dropped, which can be attributed to dilation within the fault zone (a well documented phenomenon) while the off‐fault pressure increased, which was a surprise. The off‐fault pressure increase can be explained by the rapid closure of small cracks upon unloading of the rock mass as the fault is formed. During earthquakes in nature, we expect off‐fault fluid pressure to increase in regions where initial stresses are elevated, for example, where fault geometry is complex. Key Points: Opposite variations of pore pressure occur on and off the fault upon shear failureOn‐fault pore pressure drop results from fault dilatancyOff‐fault pore pressure elevation requires bulk porosity reduction and undrained conditions during failure [ABSTRACT FROM AUTHOR]