Shear-wave Polarization Anisotropy and Upwelling Flow in the Mantle Wedge beneath Hokkaido and Tohoku, Japan

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  • SHIMIZU Jumpei
    Research Center for Prediction of Earthquake and Volcanic Eruptions, Graduate School of Science, Tohoku University
  • NAKAJIMA Junichi
    Research Center for Prediction of Earthquake and Volcanic Eruptions, Graduate School of Science, Tohoku University
  • HASEGAWA Akira
    Research Center for Prediction of Earthquake and Volcanic Eruptions, Graduate School of Science, Tohoku University
  • OBARA Kazushige
    National Research Institute for Earth Science and Disaster Prevention

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Other Title
  • 北海道および東北日本沈み込み帯におけるS波偏向異方性とマントルウェッジ内の二次対流
  • ホッカイドウ オヨビ トウホク ニホン シズミ コミ タイ ニ オケル Sハ ヘンコウ イホウセイ ト マントルウェッジ ナイ ノ 2ジ タイリュウ

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We analyzed shear-wave splitting beneath Hokkaido and Tohoku, Japan. Waveforms of 912 intermediate-depth earthquakes recorded at 239 seismic stations were used, and 2276 splitting parameters, the leading shear-wave polarization direction (fast direction) and delay time between two split waves, were observed. Obtained results show that most of fast directions observed at stations in the back-arc side are perpendicular to the trench-axis, whereas those at stations in the fore-arc side are sub-parallel to it. Average delay times observed at stations in the back-arc side are 0.1-0.5 s, and those at stations in the fore-arc side are smaller (0.05-0.1s). This systematic spatial variation is observed for the whole study area, from the southern part of Tohoku to the eastern part of Hokkaido. We infer that the anisotropy caused by lattice-preferred orientation of olivine, which is probably attributable to the upwelling flow portion of the mechanically-induced convection in the mantle wedge, is a likely candidate for the shear-wave splitting in the back-arc mantle wedge. If this is the case, the present observations for the east of the arc-arc junction may indicate that the upwelling flow direction is sub-parallel not to the relative plate motion direction but to themaximum dip-direction of the subductiog slab, in contrast with that in the south of the arc-arc junction where the upwelling flow sub-parallel to the relative plate motion is expected. The anisotropy structure revealed in this study plays a crucial role on understanding ongoing mantle dynamics in subduction zones.

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