Giulio Di Toro

Seismic cycle in the presence of hot and pressurized fluids

Giulio Di Toro

Geophysical evidences 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” typical of geothermal systems (temperature and fluid pressure: 150°C < T 25 MPa). However, also because of technical difficulties in performing rock deformation experiments under such extreme conditions, the evolution of frictional properties of fault materials that contribute to the seismic or aseismic behavior of the fault/fracture network is poorly known.
Here, motivated by the study of fault zones exhumed from 4-8 km depth, we will discuss the results (mechanical data, microstructural-mineralogical-geochemical analysis, etc.) of experiments performed with new devices which combine rotary shear machines and hydrothermal vessels. These experimental devices can reproduce hydrothermal conditions in the laboratory by imposing on simulated faults aseismic (nm/s) to seismic (m/s) slip rates and slip distances up to several meters in the presence of hot (up to 500°C) and pressurized (up to 70 MPa) fluids. Based on the results obtained with these devices, we will discuss fluid-rock interaction, pressure and temperature of the fluid and stress loading in earthquake nucleation and healing of faults of the upper crust (Bolfin fault zone, Atacama Fault System, Chile; Krafla geothermal system, Iceland).
The main objective of this multidisciplinary scientific approach is to reveal (i) the main factors responsible for the seismic and aseismic behavior of natural faults, (ii) how fractures and faults “heal” during the seismic cycle, and (iii) the deformation mechanisms that operate under hydrothermal conditions. These results could indicate new methods for a safer exploitation of deep geothermal reservoirs.