Morphology of the intraslab seismic zone and devolatilization phase equilibria of the subducting slab peridotite

The dehydration-induced earthquake hypothesis for intermediate-depth earthquakes in a subduction zone was examined semi-quantitatively in the light of multi-component phase equilibria of the mantle. Based on the expected dehydration equilibria in subducting peridotite, the authors found a further organized structure in the hypocentral distribution observed in northeastern Japan to the double seismic zone (DSZ). The structure of the seismic zone is possibly controlled by a chemical rather than a mechanical process. First, a phase diagram was constructed in the model system using thermodynamic calculations. Then, possible hypocenter distributions in the subducting mantle were semi-quantitatively predicted on the assumption that: 1) any dehydration induces earthquakes; 2) a subducting slab is more or less hydrated; 3) a hydrated mantle is approximated by the model system; and 4) dehydration proceeds near equilibrium. The predicted topology of dehydration-induced seismic zones reproduces the double seismic zone; also predicted are multiple seismic zones, multiple convergences of the seismic planes, and thermal structure dependence of the convergence depth of the DSZ. These predictions are compared to seismic observations in northeastern Japan and the world's subduction zones.