The dynamic strength of seismogenic faults has a critical effect on earthquake slip instability and seismic energy release. High velocity friction experiments on simulated faults in serpentinite at earthquake slip conditions show a decrease in friction coefficient from 0.6 to 0.15 as the slip velocity reaches 1.1 m/s at normal stresses up to 24.5 MPa. The extraordinary reduction in fault strength is attributed to flash heating at asperity contacts of gouge particles formed during sliding. The rapid heating at asperities causes serpentine dehydration. In impermeable fault zones in nature, serpentine dehydration and subsequent fluid pressurization due to coseismic frictional heating may promote further weakening. This dynamic fault-weakening mechanism may explain the lack of pronounced heat flow in major crustal faults such as the San Andreas.