The nucleation of injection-induced earthquakes on low-permeability strike-slip faults

The injection of fluids into underground formations may induce damaging earthquakes and increase the sensitivity of injection sites to remote triggering. If the fault constitutive behavior and geomechanical conditions permit the development of a frictional instability, slip may eventually accelerate and trigger a coseismic slip event. We investigate the frictional and hydromechanical mechanisms that control the slip instability preceding an induced earthquake, the nucleation phase. Understanding fault reactivation and the transition from quasi-static aseismic slip to dynamic rupture is an important objective, as the nucleation phase may provide the key to detect preseismic signals and estimate the magnitude of the resulting earthquake. Our simulations show that poroelasticity coupling delays the onset of slip and dynamic rupture and creates asymmetric slip and pressure distributions on the fault. Our results indicate that pressure-driven nucleation patterns, while qualitatively similar to those of tectonic earthquakes in elastic media, are controlled by flow processes and poroelastic couplings that favor nucleation-zone expansion. Our numerical results suggest that nucleation lengths $L$ for induced events scale proportional to the classical scaling $L_{\infty }=\frac{b}{{(b-a)}^2}\frac{G^{\prime }{D}_c}{{\sigma }_n^{{}^{\prime }}}$ and time to nucleation with $L^2$. Moreover, since ${\tau }_{nuc}\sim L^2$ and $L\sim L_{\infty }$, then ${\tau }_{nuc}\sim L_{\infty }^2$. A longer nucleation phase leads to higher pore pressures and a weaker fault at the onset of dynamic rupture.