Determination of parameters characteristic of dynamic weakening mechanisms during coseismic slip

While sliding at seismic slip-rates of ca. 1 m/s, a natural fault undergoes an abrupt decrease of its strength called enhanced dynamic weakening. Asperity-scale (<< mm) processes related to flash heating & weakening and meso-scale (mm-cm) processes involving shear across the bulk slipping zone related to frictional melting or viscous flow of minerals, have been invoked to explain pronounced velocity-dependent weakening. Here we present a compilation of ca. 100 experiments performed with two rotary shear apparatuses, i.e. SHIVA installed in INGV (Rome, Italy) and HVR installed, at the time of experiments, in Kyoto University (Japan). Cohesive rock cylinders of basalt, gabbro, tonalite, granite and calcitic marble were sheared under a range of effective normal stresses (s_n=5-40 MPa), target slip-rates (V_t=0.1-6.5 m/s) and fluid pressures (from room humidity conditions RH or P_f=0, to P_f =15 MPa). We fit the measured shear stress evolution with slip with two dynamic weakening mechanisms models, which include, depending on rock type: (1) flash heating and bulk melting (granitoid, gabbro and basalt), (2) flash heating and diffusion creep (calcitic marble), (3) flash heating and dislocation creep (calcitic marble). We provide a set of optimized parameters, specific for each mechanism, that control the dynamic weakening.

Lastly, the modelling procedure allow us to estimate the slip-switch distance d₀, i.e. the slip necessary for the complete transition from the asperity-scale to bulk slipping zone dynamic weakening mechanism. Our analysis shows that (1) the d₀ decreases with increasing effective normal stress acting on the fault and, (2) for the same type of transition between dynamic weakening mechanisms (e.g., from flash heating to bulk melt lubrication) the d₀ is a function of rock composition. The decrease of d₀ with normal stress indicates that during earthquakes, bulk mechanisms dominate over asperity scale weakening mechanisms with increasing crustal depths. This study provides constitutive law parameters to be included in physically- and geologically-based dynamic earthquake rupture simulations.