High‐Rate Fluid Injection Reduces the Nucleation Length of Laboratory Earthquakes on Critically Stressed Faults in Granite

We conducted fluid injection experiments on cylindrical low‐permeability granite samples with a critically stressed sawcut fault at local injection rates of 0.2 and 0.8 mL/min and confining pressures of 31 and 61 MPa. A local array of six strain gauges attached close to the faults allows us to estimate the nucleation length of each injection‐induced dynamic slip event (i.e., laboratory earthquake). We find nucleation lengths decrease from approximately 90% to <15% of the fault length with higher injection rate and increased effective normal stress. Injection‐induced laboratory earthquakes with smaller nucleation lengths show generally higher peak slip rates and larger fault slip displacements, signifying an intensified seismic hazard. Our results also indicate that initially stable fault patches may be reactivated to slip seismically by increasing injection rates. This study systematically demonstrates that higher injection rates constitute dynamic loading, which increase the seismic hazard by shrinking the earthquake nucleation length.