Frictional Properties of Natural Granite Fault Gouge Under Hydrothermal Conditions: A Case Study of Strike‐Slip Fault From Anninghe Fault Zone, Southeastern Tibetan Plateau

Abstract 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 0.6) 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.