Seismic Tomography for the Crust and Upper Mantle behind the Japan Trench

Abstract:

The Pacific plate subducts WNW under the Eurasian plates with a ~30° angle of dip and a rate of ~8 cm/yr from the Japan–Kuril Trench. The Kuril-NE Japan arc of the uppermost mantle, overlying the subducting Pacific slab, is the locus of important processes, including serpentinization of the forearc mantle wedge, repeated genesis of megathrust earthquakes, slab dehydration, arc magmatism and interplate coupling. To improve our knowledge of crustal and upper mantle structures through tomographic imaging, we determined the three-dimensional (3-D) velocity (Vp, Vs) and Vp/Vs structures under the Kuril–NE Japan subduction zone. The Vp, Vs and Vp/Vs models provide compelling evidence for a highly hydrated and serpentinized forearc mantle and the fluids related to low-velocity and high-Vp/Vs anomalies associated with the slab dehydration. Significant slow anomalous Vp and Vs with a high-Vp/Vs ratio are clearly imaged along the volcanic front with an extended depth of ~100 km under the Kuril–NE Japan arc, showing good consistency with the results of previous studies. This is caused mainly by the fluids associated with the extensive dehydration of the subducting Pacific slab. Fluid-related anomalies under the Kuril–NE Japan arc system, attributed to various processes such as slab dehydration and serpentinization of the forearc mantle wedge, are contributed mainly by arc magmatism, interplate coupling and the repeated generation of megathrust earthquakes. The characteristic distribution of high and low Vp/Vs in the forearc continental crust along the trench-parallel direction may reflects the spatial heterogeneity of the amount of the subducted water which related to the difference of the sedimentary unit and seismic activity in the oceanic crust. Our study demonstrates that the directly optimization of Vp/Vs tomographic procedure provides more stable and reliable Vp/Vs image than previous method.