Morphology of the Intraslab Seismic Zone and Devolatilization Phase Equilibria of the Subducting Slab Peridotite
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概要
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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, we found a further organized structure in the hypocentral distribution observed in northeast Japan to the double seismic zone (DSZ). The structure of the seismic zone is possibly controlled by a chemical rather than a mechanical process. We first constructed a phase diagram in the model system MgO-Al2O3-SiO2-H2O (MASH) using thermodynamic calculations. Then possible hypocenter distributions in 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) hydrated mantle is approximated by the MASH model system; and, 4) dehydration proceeds near equilibrium. The predicted topology of dehydrationinduced seismic zones reproduces the double seismic zone (DSZ); 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 NE Japan and the world's subduction zones. In NE Japan, the geometry of the double seismic zone and the hypocenter clusters between the double seismic planes compare well to the predicted dehydrationinduced multiple seismic zones and multiple convergences, assuming a simple prograde P-T path for the coldest thermal center of the slab. The correlation of the age of the subducting plate and the depth of the convergence of the world's DSZ is also consistent with the prediction. The assumption of a hydrated mantle is examined by seismic tomography beneath the Kanto area. The Poisson's ratio calculated between the two seismic planes at a depth ca. 50-80 km suggests the existence of serpentinized peridotite in the subducting mantle at those depths, even though the degree of hydration is not so strong (ca. 50-30%) and is heterogeneous. Transform faults, oceanic fracture zones, and faults at the trench are all considered to be possible hydration sites of the oceanic mantle. Using the dehydration induced-earthquake model, the link between the dehydration and the seismic zone provides us with the thermal structure in the subducting slab. The estimated thermal structure exhibits a higher temperature than previous numerical simulations, particularly within the slab. Our estimate suggests viscous heating has been underestimated in previous numerical simulations and heat transfer by a fluid produced by the dehydration of the slab mantle must be included in the model. The dehydration model is also applicable to deep seismicity in the mantle transition zone.
- 2002-03-25
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