Abstract
Seismological observations highlighted that earthquakes are often followed by changes in elastic properties around the fault zone. Here, we studied the origin of these variations using stick-slip experiments on saw-cut granite samples presenting different degrees of bulk damage (i.e., microcracks). Stick-slip events were induced under triaxial compression configuration with continuous active ultrasonic measurements at confining pressures representative of upper crustal conditions (15-120 MPa). Both the P-wave velocity ( VP ) and amplitude ( AP ) showed drops, concurrently with stress drops, and had a non-monotonic dependence toward the fault's stress state. Our experimental results suggest that co-seismic changes in VP were mostly controlled by the elastic re-opening of microcracks in the bulk, rather than by co-seismic damage or the formation of fault gouge. Co-seismic changes in AP were controlled by a combination of elastic re-opening of microcracks in the bulk and inelastic processes (i.e., co-seismic damage and gouge formation and dilation).