The ability to forecast earthquake timing would represent the most significant breakthrough in geohazard mitigation for our societies. Recent large-scale seismological observations in Baja California have shed light on the occurrence of strain localization and foreshock migration towards the epicenter of large earthquakes, years before the actual rupture. However, this process that could serve as an early warning is still largely unknown. To this day, rock mechanics experiments have focused on strain localization on intact samples and friction experiments mostly overlooked volumetric processes that could occur in the rock surrounding the fault. In this study, we present state of the art friction experiments conducted in an oil-confined biaxial shear apparatus. We recorded rate-and-state friction parameters and off-fault deformation in bare surfaces of Carrara marble subjected to increasing load point velocity (from 10-6 to 10-2 m/s) under low and high stresses (Pc = 15 and 50 MPa). Our findings indicate that there is a direct link between stick-slip nucleation and off-fault deformation at low load point velocity when rock deformed inelastically, provided that the fault is conditionally unstable (a-b < 0). We suggest that off-fault inelastic deformation may trigger unstable slip by decreasing the stiffness of the surrounding rock volume. Our study suggests that inelastic off-fault deformation favors earthquake nucleation, and the localized to ductile transition may partially control the minimum depth of the seismogenic zone. Furthermore, we also present the first laboratory observation of unequivocal precursory strain localization around a fault during stick-slip cycles.
