Formulation of a fault governing law at high sliding speeds: Inferences from dynamic rupture models

Understanding the behavior of natural faults at cosesimic slip velocities (v ~ 1–10 m/s or more) has become a challenging achievement for experimentalists and modelers of earthquake instabilities. The rate– and state–dependent friction laws, originally obtained in slow slip rate conditions, have been widely adopted in dynamic rupture models by assuming their validity well above the experimental range of observations. In this paper we consider a modification at high speeds, in which the steady state friction becomes independent on v above a transitional value vT . Our results show that this modification has dramatic effects on the dynamic propagation; as long as vT decreases the breakdown stress drop decreases, as well as the slip–weakening distance and the fracture energy density. Moreover, we found that the subshear regime is favored as vT decreases; we found that for the strength parameter S greater than 1.482 the supershear rupture propagation is inhibited. Finally, we demonstrate that the exponential weakening, often observed in laboratory experiments, can be theoretically explained in the framework of the rate and state laws