Seismic moments of earthquakes beneath island arcs, phase changes, and subduction velocities

Abstract

The analysis of seismic moments of intermediate and deep-focus earthquakes in four island arc regions supports the hypothesis that these shocks are the result of shear stresses caused by phase transitions in the descending slab at depth near 150, 350, and 600 km. Cumulative seismic moments summation M/sub 0/ 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 to 100, 100 to 250, 250 to 450, and 450 to 700 km. The uppermost range corresponds to the zone of underthrusting. The seismic moments of earthquakes in the three lower depth ranges ae related to phase transitions in the slab by summation M/sub 0/ = ..mu..LT vertical bar ..delta..V/V vertical bar v/sub sub/, where ..mu.. is the modulus of rigidity, L is the length along strike of the subduction zone, T is the slab thickness, ..delta..V/V is the fractional volume change associated with a particular phase transition, and v/sub sub/ is the subduction velocity. In the range 100 to 250 km the earthquakes result from volume decreases associated with the melting of the oceanic crust. In the depth range 250more » to 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 v/sub sub/ can be calculated by using Brune's method for the zone of shallow underthrusting and the relationship between summation M/sub 0/ and volume changes for the three deeper zones. As an example, the four estimates of v/sub sub/ for the Tonga arc, in order of increasing depth, are 8.4, 7.9, 7.6, and 8.3 cm/yr.« less