Seismicity on crustal faults is concentrated in the depth interval 1--3 to 12--15 km. Tse and Rice (1986) suggested that the lower bound on seismicity is due to a switch with increasing temperature from velocity weakening (destabilizing) to velocity strengthening (stabilizing) friction. They inferred this transition from friction data for dry granite; however, pore fluids exist at elevated temperatures throughout the crust, and may strongly influence strength and sliding behavior. We present new data from sliding experiments on granite at elevated T (23¿ to 600¿) thus elevated PH2O (100 MPa).

Our results show velocity strengthening at room temperature, but velocity weakening from 100¿ to 350 ¿C (except at 250¿). From 350¿ to 600¿ there are systematic trends from velocity weakening to strong velocity strengthening, and from high to low friction; neither trend was seen in tests on dry granite. The velocity dependence data imply the potential for unstable slip in the interval 100¿ to 350¿. Using a geotherm to map temperature to depth, this interval closely matches the observed earthquake distribution.