The time recurrence of earthquakes is the result of the feedback between the tectonic loading and the evolution of fault strength during the seicmic cycle. This thesis aims to identify the chemical and physical processes in fault rocks from the modern seismogenic San Andreas Fault (California, USA) and the ancient seismogenic Gole Larghe Fault (Southern Alps, Italy). The San Andreas Fault was drilled to 2.7 km depth, and samples were extracted from the depth of nucleation of repeating microearthquakes. A cyclic recurrence of pressure-solution creep – hydrofracture - pressure solution creep supports the idea that isolated compartments of high fluid pressure might cause the nucleation of small to moderate size earthquakes, associated with the dominant creeping activity in this fault segment. The Gole Larghe Fault Zone was active 30 Ma ago at 9 – 11 km depth. The occurrence of pseudotachylytes witnesses its seismic behavior. Two topics were investigated: (i) The fabric evolution of cataclastic rocks with increasing deformation, to identify the processes potentially leading to the onset of unstable slip at the early stages of fault growth. (ii) The origin of fluids involved in seismic faulting and frictional melting. The formation of a cataclastic fault network allows the ingression of external hydrous fluids, probably of deep origin. The similar isotopic composition of natural pseudotachylytes and pseudotachylytes produced in dry conditions suggests that the fluid source is the dehydration of OH-bearing minerals in the wall rocks induced by coseismic frictional heating.
