GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 The seismic cycle under hydrothermal conditions: experimental studies W. Feng 1 , L. Yao 2 , R. Gomila 1 , S. Ma 2 , G. Di Toro 1,3* 1 Dipartimento di Geoscienze, Università degli Studi di Padova, Padua, Italy 2 State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing, China 3 Sezione di Roma 1, Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy Geophysical and seismological observations and the investigation of exposed faults exhumed from upper crustal depths indicate that earthquakes nucleate and propagate often in the presence of fluids at “hydrothermal conditions” (150°C < T < 450°C and P > 25 MPa) typical of geothermal systems. However, the evolution and response of the friction coefficient of rocks to loading perturbations (e.g., slip rate, shear stress) is described by the Rate-and-State Friction Law (RSFL) which is based on experiments mainly performed at room temperature (20°C) and admitting low slip displacements (< 1 cm) and slip rates (< 1 cm/s). Because the RSFL is an empirical law and all deformation mechanisms are temperature-dependent, the results obtained at room temperature cannot be adequately extrapolated to natural conditions. Unfortunately, experiments that investigate the RSFL at hydrothermal conditions, including displacements larger than few mm or performed at seismic slip rates, remain scarce. The main reason is that shearing rocks at large (> 1 cm) slip distances and slip rates (e.g., up to seismic of ca. 1 m/s) under high confining pressures in the presence of hot fluids is an extraordinary technical challenge. Here we discuss experimental results obtained exploiting a new hydrothermal vessel (HYDROS) installed in 2021 on the Rotary Shear Apparatus ROSA at Padua University (Italy) and on a twin machine installed at the China Earthquake Administration laboratories (Beijing, China). HYDROS allowed us to investigate fault friction and healing in simulated faults made of gabbro and basalt under extreme deformation conditions of pore fluid pressure (up to 70 MPa), temperature (up to 500°C), effective stresses (up to 100 MPa), slip distance (μm to meters) and slip rates (sub-seismic to seismic, or from 10 nm/s to 2 m/s). Under these conditions water is under the vapor, the liquid and the supercritical state. When pressurized water is in the vapor state, rock friction decreases with increasing slip and slip rate, similarly as in room temperature experiments. But when pressurized water is in the liquid (T>200°C) or in the supercritical state, rock friction decreases with slip distance independently of the imposed slip rate. Based on high-resolution microanalytical investigations (FEG-SEM, micro-Raman spectroscopy, high-resolution XRD analysis, etc.), the mechanism (probably elasto-hydrodynamic lubrication) leading also to the dramatic weakening at sub-seismic slip rates,