The theoretical nucleation length (Lc) using the rate-and-state friction law (23) is defined by Eq. 1, where G is the rock shear modulus defined by E/[2(1 + ν)]: E is the Young’s modulus of the rock surrounding the fault surface (E = 20 GPa), ν is the Poisson’s ratio (ν = 0.33), dc is the critical slip distance (dc = 10 μm) over which the state parameter evolves, σn is the normal stress (σn = 4.25 MPa), p is the fluid pressure (p = 0 to 3.5 MPa), and a and b are the rate-and-state parameters for steady-state velocity [a-b = −0.0026 on average at low slip velocity (v < 10 μm/s) and a-b = −0.001 on average at high slip velocity (v > 10 μm/s)]. Slipping patches with radii greater than the critical nucleation length L > Lc are susceptible to trigger seismicity, whereas those with radii L < Lc are not.

From the analysis of seismic events recorded during 11 injection tests performed in different sections of the same fault zone at similar pressures tested in the present study (11), we inferred an estimated source radius ranging from 0.19 to 0.57 m and moment magnitudes between −4.2 and −3.14. Seismic events were located between 1 and 12 m from injections, with some clusters that are independent of fluid pressure diffusion and, rather, related to stress perturbation. In this series of experiments, the accuracy of the earthquake locations is estimated to be 1.5 m (11). Here, the size of the nucleation zone below the low slip velocity limit (~1.325 to 8 m) is generally consistent with theoretical estimates, but the source radius of the seismic events (0.19 to 0.57 m) observed in (11) is an order of magnitude smaller than the theoretical minimum length over which earthquake nucleation can occur. Our assumption is that the stability of fault and the transition between seismic and aseismic slip are likely controlled, at least, by local rapid stressing rate and friction weakening over short distances on small earthquake-prone fault areas that may radiate seismic waves. This is consistent with previous laboratory experiments on meter-sized rocks that investigated interactions between aseismic slip, stress changes, and seismicity on a critically stressed fault during the nucleation of stick-slip instability (45).

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