Rupture and Afterslip Controlled by Spontaneous Local Fluid Flow in Crustal Rock

Shear rupture and fault slip in crystalline rocks like granite produce large dilation, impacting the spatiotemporal evolution of fluid pressure in the crust during the seismic cycle. To explore how fluid pressure variations are coupled to rock deformation and fault slip, we conducted laboratory experiments under upper crustal conditions while monitoring acoustic emissions and in situ fluid pressure. Our results show two separate faulting stages: initial rupture propagation, associated with large dilatancy and stabilized by local fluid pressure drops, followed by sliding on the newly formed fault, promoted by local fluid pressure recharge from the fault walls. This latter stage had not been previously recognised and can be understood as fluid-induced afterslip, co-located with the main rupture patch. Upscaling our laboratory results to the natural scale, we expect that spontaneous fault zone recharge could be responsible for early afterslip in locally dilating regions of major crustal faults, independently from large-scale fluid flow patterns. Faults in rock form during what is known as rupture; rupture is followed by sliding along the newly formed fault. If this occurs quickly, we speak of an earthquake. When creating the fault during rupture, small cracks are formed in the intact rock that grow and link up. This creates local void spaces in the nascent fault zone. Pore fluids, which typically reside at pressure everywhere within the Earth's crust, will expand in this newfound space so that the pore pressure drops in the rupture zone. Such a drop may slow down the rupture and fault slip-an earthquake may be postponed. Here, we measure this so-called dilatancy effect in unprecedented detail: We observe how the rupture is controlled by the dilatancy effect, and we discover that after rupture fluids from further away flow into the void spaces in the newly formed fault, thereby increasing pressure again and driving fault slip. This pore fluid control on fault behavior is a fundamental mechanism that can explain why faults continue slipping just after an earthquake. We observe two stages during faulting in the presence of fluids: Dilatancy-dominated shear rupture and slip accommodated by fluid rechargeFault formation by shear rupture evolves non-linearly with stress drop and may be stabilized by dilatancy-induced local pore pressure dropsSlip accommodated by local fluid recharge might be responsible for early afterslip following coseismic slip observed in nature