[S-SS04] Seismological advances in the ocean

[SSS04-P02] Spatio-temporal variation of seismic velocity changes in the Nankai accretionary prism using body wave of ambient noise autocorrelation

*Yusuke Kakiuchi¹, Takeshi Tsuji¹, Takashi Tonegawa² (1.School of Engineering, The University of Tokyo, 2.Japan Agency for Marine-Earth Science and Technology )

Keywords:ambient noise, body wave, fluid, seismic velocity change

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.