Frictional properties of amphibolite from the Anninghe Fault Zone under hydrothermal conditions and implications for earthquake nucleation

The Anninghe Fault Zone, located along the southeastern margin of the Tibetan Plateau, represents a significant active tectonic belt with a documented history of major seismic events. To explore frictional properties and seismic nucleation conditions, this study performed frictional experiments using the representative fault rock sample, amphibolite, collected from the Mianning fault outcrop. The experiments were conducted under hydrothermal conditions with temperatures ranging from 100 ℃ to 500 ℃, confining pressures of 150 MPa and 200 MPa, and fluid pressures of 50 MPa and 100 MPa. Results indicate that the friction coefficient $ \mu $ of amphibolite varies between 0.67 ~ 0.72 with a positive temperature dependence emerging above 200 ℃. The velocity-dependent parameter of friction coefficient (a−b) exhibits a temperature-dependent transition: velocity strengthening (a−b > 0) at low temperatures (100 ~ 200 ℃), velocity weakening (a−b < 0) at intermediate temperatures (300 ~ 400 ℃), and a reversion to velocity strengthening at higher temperature (500 ℃). By analyzing and fitting the experimental data, the frictional properties were extrapolated to natural in-situ fault conditions, enabling the determination of the seismic nucleation depth (4 ~ 12 km) and fault strength profile for the Mianning segment of the Anninghe Fault Zone. This study enhances our understanding of regional seismicity patterns and fault mechanics, providing fundamental insights for future earthquake hazard assessments in this tectonically active region.