Slow Stick-Slip Failure in Halite Gouge Caused by Brittle-Plastic Fault Heterogeneity

Slow earthquakes, including low-frequency earthquakes and tremor, occur in the brittle-ductile (plastic) transition zone on plate boundary faults. To understand how transitions in deformation mechanisms can influence seismic behavior, we conducted stick-slip experiments on halite gouge at normal stresses (σ_n) of 8 to 113 MPa. In the brittle regime (σ_n ≤ 18 MPa), the halite gouge showed fast and regular stick-slip associated with velocity-weakening behavior. In contrast, increasing the normal stress within the semibrittle regime (σ_n ≥ 27 MPa) led to a significant decrease in slip velocity and stress drop, which was associated with a transition from velocity-weakening to velocity-strengthening behavior. Local stress measurements along the simulated faults, made using strain gauges, revealed that the critical nucleation length (L_c) increased with increasing normal stress. Macrostructural and microstructural observations showed that the transition from fast dynamic slip to slow quasi-static slip could be attributed to an increasing contribution from semibrittle/plastic deformation occurring within domains that are heterogeneously distributed within the gouge layer. Our findings suggest that brittle-plastic fault heterogeneity, which may be generated by spatial variations in pore fluid pressure and/or lithology, contributes to the emergence of slow earthquakes.