講演情報
[S04-01]近地と遠地地震スペクトルデータから推定した日本列島深部の３次元減衰構造
*趙 大鵬1、Wang Zewei2 (1. 東北大学大学院理学研究科、2. 中国中山大学)
So far many researchers have used waveform spectra data of local earthquakes to study the 3-D attenuation (Q) structure down to ~200 km depth beneath Japan, whereas the deeper Q structure is still unknown. In this work, we conduct the first joint inversion of local and teleseismic spectra data to determine 3-D Qp and Qs tomography of the Japan subduction zone down to 700 km depth. We develop a novel technique to reliably estimate the corner frequency and attenuation factor from the observed spectra of seismograms. Our results show the subducting Pacific and Philippine Sea (PHS) plates clearly as dipping high-Q zones. Beneath East Japan, significant low-Q and high-Q anomalies exist below the subducting Pacific slab, which may reflect subslab mantle convection probably caused by the overlying slab subduction and/or hot and wet upwelling flows from the deep mantle. The subslab anomalies may affect the generation of megathrust and intraslab earthquakes. Our Q tomography shows that the PHS slab has subducted aseismically down to ~400 km depth beneath SW Japan, supporting previous results of seismic velocity tomography. In addition, low-Q zones are revealed below the PHS slab and above the Pacific slab, which may reflect hot and wet upwelling flows associated with deep dehydration of the Pacific slab and corner flows in the big mantle wedge above the Pacific slab. The subslab hot upwelling flow may heat the overlying PHS slab, so the slab loses its brittleness in the shallow upper mantle, and its intraslab seismicity terminates at ~200 km depth. Partly due to the heating and buoyant force of the subslab hot upwelling flows, the PHS slab is strongly deformed and so exhibits a complex geometry in SW Japan.