The Anninghe Fault (ANHF) is a major left‐lateral strike‐slip fault in southwestern China and one of the main seismogenic fault zones with a history of strong earthquakes. To understand the frictional properties of natural granitic gouges from the principal slip zone, we conducted hydrothermal friction experiments using both saw‐cut and ring shear methods. These experiments were performed at temperatures (T) of 25–600°C, pore pressures (P_f) of zero (dry), 30 and 100 MPa, sliding velocities (V) of 0.01–100 μm/s and effective normal stresses () of 68, 100, and 200 MPa. The (apparent) friction coefficient is low (μ < 0.5) at high T (600°C), high P_f (100 MPa) and low V ( 0.6) under all other T, P_f and V conditions. Under high P_f, the velocity dependence of friction, (a‐b), displays three regimes with increasing temperature, from positive below ∼100°C to negative at 100–300°C (at V = 1–3 μm/s) or else 100–450°C (at V = 30–100 μm/s), becoming positive again above 300–450°C. At low P_f, the negative (a‐b) expands to the range ∼300–600°C. Microstructural observations and microphysical interpretation imply that the frictional weakening and transitions in (a‐b) are related to competition between dilatant granular flow and deformation of the fine‐grained gouge by intergranular pressure solution accompanied by healing phenomena (leading to cavitation‐creep‐like behavior). Our results provide a possible explanation for the distribution of earthquakes at different depths in the continental crust, in particular for the depth range of the seismogenic zone between 4 and 24 km along the ANHF.

The deformation model of antigorite and its relationship to intermediate‐depth earthquakes are still controversial. Here, we conducted deformation experiments on foliated iron‐poor antigorite at confining pressures of 1,100–1,500 MPa, temperatures of 400–600°C, and axial displacement rates of 3.75 × 10⁻⁶–7.24 × 10⁻⁴ mm/s using a solid‐medium Griggs‐type apparatus. As the temperature and/or pressure increases, the stress exponent decreases, and all samples show stable fault slip. Microstructural observations show ductile behavior indicated by kinking and bending of the originally straight foliation, and brittle characteristics, such as Riedel shear, comminution, and microcracking. Therefore, antigorite deformation is controlled by a semibrittle regime. Antigorite decomposed to forsterite and enstatite in samples sheared at temperatures ≥550°C but not in samples triaxially compressed at 600°C. This observation, combined with phase boundary information, implies that the deformation geometry (e.g., triaxial compression or shear) may change the phase stability of antigorite. Following dehydration, samples still show stable fault slip, indicating that dehydration does not directly trigger earthquakes. In cold subduction zones, fine‐grained antigorite in the core of the shear zone likely induces seismic activity by producing water and talc (or a talc‐like phase) in the process of shear friction because such phases within the fault zone may reduce the rate dependence, thereby inducing unstable slip. The comparison of the semibrittle strengths of antigorite and dunite and the weak strain rate dependence of stress suggest that a thin antigorite layer immediately above the subducting slab is not sufficient to cause significant decoupling between the slab and the mantle wedge.