[S-SS04] Seismological advances in the ocean

[SSS04-P02] 南海トラフ付加体における実体波を用いた地震波速度変化の時空間変動：雑微動の自己相関解析

*垣内 優亮¹、辻 健¹、利根川 貴志² (1.東京大学大学院工学系研究科、2.海洋研究開発機構)

キーワード：雑微動、実体波、流体、弾性波速度変化

Ambient noise interferometry provides information of dynamic changes in the subsurface structure. Coda of the ambient noise autocorrelation or cross-correlation is widely used to estimate seismic velocity changes in the crust which is sensitive to stress field, ocean tide, pore pressure, and volcanic activity. However, this method has difficulty in interpreting obtained results with rock properties since velocity changes using coda wave are sensitive to wide range of depth. In this study, we show that seismic velocity monitoring using reflected body waves is a useful method to estimate velocity changes at certain depth using long-term ambient noise records of the ocean bottom seismometers in the Nankai subduction zone. We observed the long-term velocity increase over the five years in the Nankai accretionary prism which is attributed to fluid release due to tectonic compaction. Furthermore, we observed repetitive decrease and increase of velocity changes at the frontal accretionary toe. We propose that intermittent fluid release in the shallow input sediment caused the repetitive decrease and increase of seismic velocity. This study suggests that body waves derived from ambient noise autocorrelation are capable of detecting interpretable changes of subsurface structure that cannot be captured by coda waves.