The analysis of seismic moments of intermediate and deep-focus earthquakes in four island are regions supports the hypothesis that these shocks are the results of shear stresses cuased by phase transitions in the descending slab at depths near 150, 350, and 600 km. Cumulative seismic moments ?Mo for earthquakes listed by Gutenberg and Richter were found to exhibit four maxima when they were plotted as a function of depth: the maxima occur within the depth ranges 0--100, 100--250, 250--450, and 450--700 km. The uppermost range corresponds to the zone of underthrusting. The seismic moments of earthquakes in the three lower depth ranges are related to phase transitions in the slab by &Sgr;Mo=μLT‖ΔV/V‖vmuh, where μ is the modulus of rigidity, L is the length along strike of the subduction zone, T is the slab thickness, ΔV/V is the fractional volume change associated with a particular phase transition, and vsub is the subduction velocity. In the range 100--250 km the earthquakes result from volume decreases associated with the melting of the oceanic crust. In the depth range 250--450 km the volume change is due mostly to the transformation of olivine to spinel structure with a density increase of about 8%. In the lowest depth range the decrease in volume is due to the change of spinel to postspinel phases with a density increase of about 8%. For any island arc region, up to four independent estimates of vsub can be caluclated by using Brune's method for the zone of shallow underthrusting and the relationship between &Sgr;Mo and volume changes for the three deeper zones. As an example, the four estimates of vsub for the Tonga arc, in order of increasing depth, are 8.4, 7.9, 7.6, and 8.3 cm/yr.