Pore Pressure Drop During Dynamic Rupture and Conditions for Dilatancy Hardening

Abstract

AbstractPore pressure drop brought about by fault dilatancy during accelerating slip may suppress nucleation of earthquakes. Yet, direct measurements of pore pressure during dynamic slip are challenging to produce. We present results of a physics‐based model simulating onset of slip in saturated granular layers coupled to a constant fluid pressure reservoir. Grain rearrangements required for slip to commence induce incipient rapid dilatation during which the maximum pore pressure drop is generated. We find that up to a critical slip rate the pore pressure drop is consistent with a prediction derived for an incompressible fluid flow. In this “drained” regime, excess pore pressure is efficiently relaxed and has little effect on slip stability. Above the critical slip rate, marking the onset of undrained conditions, the pore pressure drop decays slowly, inhibits dilatation rate, and significantly increases strength of the layer, stabilizing the rupture growth. The magnitude of the pore pressure drop increases monotonically with the drainage number given as the ratio of the dilatation rate to a characteristic fluid infiltration rate. The pore pressure drop in the undrained regime also depends on a second non‐dimensional parameter, , where β is storage capacity, and is the effective normal stress. Low values of this parameter enhance localization of strain near the drained boundaries of the layer, promoting fluid flow into the layer. Our results can be used to better constrain drainage conditions associated with changes in slip rate, the magnitude of the generated pore pressure and the corresponding fault strengthening.