Summary

The observation that serpentinite becomes weak and brittle at its dehydra-tion temperature provides an experimental basis for the hypothesis that mantle earthquakes are caused by shear fracturing where interstitial fluid pressure effectively reduces the frictional resistance to a low or negligible value. If convection currents in the mantle turn down at the foredeeps of the Pacific island arcs, dehydration of an oceanic crust of serpentinite and down-ward convection of the dehydration products beneath the island arc would give rise to the anomalously low shallow mantle velocities and heat-flows in the region between the trenches and island arcs. Seismic activity would be confined to the zone along which the weak and brittle dehydration pro-ducts move downward against the immobile continental mantle and would be concentrated at the leading edge of the focal zone due to release of stored elastic strain upon the initiation of weakening. Pore fluid pressures giving rise to faulting in the deepest earthquakes are considered to be due to interstitial magma