[S-CG60] 流体と沈み込み帯のダイナミクス

[SCG60-P03] 西南日本弧のP波とS波減衰トモグラフィー

Xin Liu¹、*趙 大鵬¹、西園 幸久²、稲倉 寛仁² (1.東北大学大学院理学研究科、2.西日本技術開発（株）調査解析部)

キーワード：西南日本, 沈み込み帯, 減衰構造, スラブ, 流体

We determined the first high-resolution P- and S-wave attenuation (Qp and Qs) tomography of the crust and upper mantle under the entire Nankai subduction zone from the Nankai Trough to the Japan Sea using a large number of high-quality t* data measured from P- and S-wave spectra of local earthquakes. The suboceanic earthquakes used in this study were relocated precisely using sP depth phases and ocean-bottom-seismometer data. The overall pattern of the obtained Q models is similar to that of velocity models of the study region. Our present results show that high-Q (i.e. weak attenuation) anomalies in the upper crust generally correspond to plutonic rocks widely exposed in the Nankai arc. Some of the low-Q (i.e. strong attenuation) anomalies in the upper crust along the Pacific coast are associated with the Cretaceous-Cenozoic accretionary wedge. Obvious low-Q anomalies exist in the crust under the active arc volcanoes. Most of the large inland crustal earthquakes are located in or around the low-Q zones in the crust. The subducting Philippine Sea slab is imaged clearly as a landward dipping high-Q zone. Prominent low-Q anomalies are revealed in the mantle wedge under the volcanic front and back-arc area, which reflect the source zone of arc magmatism caused by slab dehydration and corner flow in the mantle wedge. Significant low-Q anomalies exist in the fore-arc mantle wedge, which reflects a highly hydrated and serpentinized fore-arc mantle wedge due to abundant fluids released from dehydration of the young and warm Philippine Sea slab.