Kinetic Shear Resistance, Fluid Pressures and Radiation Efficiency During Seismic Faulting

Summary

During earthquake faulting, radiation efficiency and the degree of stress relief are critically dependent on the kinetic shear resistance. This is often assumed to stay constant during slip, but geological evidence suggests that for moderate or large shallow earthquakes it may decrease dramatically to near-zero values once slip is initiated, either by melt formation or by transient increases in fluid pressure on the fault plane. The latter, probably more common process may arise partly through an interaction between temperature and water pressure, and partly through dilatancy recovery as shear stress is relieved. If the fault remains undrained, stress relief should be absolute with seismic efficiency reaching high values, so that stress drops give a measure of the level of tectonic shear stress in fault zones. Supporting evidence comes from the observation that apparent stress is generally about half the stress drop.