Pore Pressure Drop During Dynamic Rupture and Conditions for Dilatancy Hardening

Pore 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,beta sigma '(n), where ss is storage capacity, and sigma '(n) n 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. Plain Language Summary Cores of geological faults are granular layers often filled with fluid, which tend to dilate on increase in slip rate, for example, during an earthquake nucleation. Dilation of the fault causes a drop in pore fluid pressure which in turn increases strength of the fault. This effect, known as dilatancy hardening, may suppress generation of earthquakes. However, the magnitude of fluid depressurization during dynamic slip is difficult to measure. We systematically study pore pressure drop following the onset of slip in fluid-saturated granular layers, using theory and numerical simulations. We find two regimes with distinct evolutions of pore pressure and dilatation. For slow slip rate, high permeability or high stress acting upon the layer (drained conditions), the pore pressure drop is quickly restored and its ability to suppress earthquake nucleation is weak. Faster slip rate, lower permeability or stress (undrained conditions) lead to an enduring pore pressure drop, which significantly increases strength of the layer. The growing magnitude of the pore pressure drop with slip rate promotes the stability of shear rupture. 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.

Automated summary

The pore pressure drop caused by fault dilatancy during accelerating slip may suppress earthquake nucleation. We conducted a physics-based model simulation of slip in saturated granular layers coupled to a fluid pressure reservoir. The grain rearrangements during slip induce rapid dilatation, generating the maximum pore pressure drop. The pore pressure drop is consistent with a prediction for incompressible fluid flow until the critical slip rate is reached, then it decays slowly, inhibits dilatation rate, and increases the layer's strength, stabilizing the rupture growth. The magnitude of the pore pressure drop depends on the drainage number and beta sigma '(n).