Fluid‐Induced Earthquake Nucleation on Aging Rate‐and‐State Faults: Influence of Hydraulic Diffusivity and Injection Rate Under Different Nucleation Regimes

Abstract Earthquake nucleation length, a critical parameter characterizing the transition from quasi‐static propagation to dynamic rupture in the nucleation zone, has been observed to decrease with elevated shear stress loading rate. Recent laboratory experiments suggested that injection can also act as a loading condition, with the nucleation length shortening under high‐rate injection. In this study, we perform numerical simulations to investigate how hydraulic diffusivity and injection rate affect the nucleation length of injection‐induced seismicity on (aging) rate‐and‐state faults. Similar to tectonic earthquakes, the nucleation process of injection‐induced seismicity falls into two distinct nucleation regimes—no‐healing and constant‐weakening—defined by the ratio of the weakening to healing rates at the center of the nucleation zone (). The nucleation length is generally much larger in the constant‐weakening regime. Interestingly, we find that the effects of injection rate and hydraulic diffusivity on nucleation length depend on the nucleation regime. In the no‐healing regime ( ≫ 1), the nucleation length decreases with increasing injection rate or decreasing hydraulic diffusivity. In contrast, within the constant‐weakening regime (), the nucleation length displays an opposite trend in most cases. The contrasting behavior can be attributed to differences in the weakening processes within each regime and the timing of the transition from no‐healing to constant‐weakening. We discuss underlying mechanisms driving local fault response to changes in hydraulic diffusivity and injection rate, as well as the implications of these findings for field‐ and lab‐scale injection‐induced ruptures.