东北日本地震波速度、Vp／Vs和各向异性结构：对俯冲带水迁移过程的探讨

Seismic velocity structure,VP/VSratio and anisotropy beneath Northeast Japan:implications for water migration in subduction zones

本文利用区域地震初至波到时数据,通过地震层析成像研究获得了东北日本俯冲带上地幔(深至约150km)的P波速度(VP)、S波速度(VS)、VP/VS和P波各向异性结构.结果表明,低速及高VP/VS比异常体主要分布在火山下方的下地壳和地幔楔中,其与低频地震的分布吻合,该区域与俯冲板块脱水所释放的流体及其导致的部分熔融密切相关;俯冲的太平洋板块内可能由于脱水脆化导致的双层地震带区域则没有表现出整体的高VP/VS值,其可能与俯冲板块内部含水矿物含量有关;俯冲板块内双重地震带区域及上覆地幔楔薄层主要表现为与海沟平行的方位各向异性和正的径向各向异性,其可能是由于含水矿物的脱水使橄榄石晶格结构发生了从A型到B型的变化所引起的.我们研究表明,结合地震波速度和各向异性结构能够加深对俯冲带内水运移过程的认识.

We determined three-dimensional tomographic images of P-wave velocity （Vp）, S-wavevelocity （Vs）, Vp/Vs ratio, and P-wave anisotropy structures down to -150 km depth beneath Northeast Japan using a large number of first P- and S-wave arrival times of local shallow and intermediate-depth earthquakes. Our results show that zones of low-velocity and high Vp/Vs ratio exist beneath volcanoes in the lower crust and mantle wedge, which accords with the distribution of low frequency microearthquakes （M^2.5）. The low-velocity and high Vp/Vs anomalies in the lower crust and mantle wedge may be associated with fluids released from dehydration of the subducting Pacific slab and hot upwelling mehs related to arc magmatism. The double seismic zone in the subducting Pacific slab is possibly induced by dehydration embrittlement of hydrous minerals. High Vl,/Vs ratio anomalies are not dominant in the entire double seismic zone, which may be affected by the amounts of hydrous minerals in the subducting slab. Trench-parallel azimuthal anisotropy and positive radial anisotropy are dominant in the subducting slab and the overlying mantle wedge under the fore-arc, which are induced mainly by B-type olivine fabrics due to relatively low temperature and increasing water content from the slab dehydration. Our results indicate that the information of both seismic velocity and anisotropy is of great importance for understanding water migration in subducting zones.