Inferring earthquake source properties from laboratory observations and the scope of lab contributions to source physics

Abstract

I summarize implications of lab-measured high slip speed fault strength for earthquake source properties as could be inferred from radiated displacements. Source physics influence the radiated field through the magnitude of dissipative processes that reduce the energy available to be radiated. Based on the energy budget I conclude, as others have previously, that the static stress drop and a measure of efficiency, the ratio of apparent stress to static stress drop, are particularly useful for classifying source physics. For the limited laboratory experiments on known mechanisms of high speed strength loss, low temperature friction, flash weakening, bulk melting, and unexpected weakening associated with silica gel formation, I estimate the implied stress drop and efficiency, with reference to typical (MPa stress drops, 20% efficiency). Conventional friction produces typical stress drops and typical efficiencies, thus it can naturally explain source properties of typical earthquakes. Unexpected weakening produces large stress drops and typical efficiencies. This is not a likely mechanism to produce typical earthquakes but could be involved in exceptional events. Flash weakening produces high efficiency and is not a likely mechanism for producing typical earthquake source properties. Dynamic stress drops would be very large and on-fault effective shear fracture energies are negligible. Similar to flash weakening, bulk melting will produce large dynamic stress drops. Not enough is known at present to estimate efficiency; the on-fault effective shear fracture energy is relatively small.