[S-SS07] Seismic wave propagation: Theory and Application

[SSS07-P18] Monitoring fluid migration in the Nankai subduction zone using ambient noise autocorrelation

*Yusuke Kakiuchi¹, Takeshi Tsuji¹ (1.School of Engineering, The University of Tokyo)

Keywords:Fluid migration, Pore pressure

In the plate subduction zone, pore pressure plays an important role in earthquake mechanism. In such environment, high pore pressure develops mainly due to mineral dehydration, tectonic loadings. Recent studies have suggested that slow earthquakes are linked to high pore pressure. However, since borehole measurements are limited to a few km depths, the monitoring of pore pressure at certain depth is difficult. Instead of direct pore pressure estimation, we use temporal variation of P-wave velocity using ambient noise autocorrelation approach. We used seismometer data of vertical component acquired at DONET1 stations in the Nankai subduction zone from 2014 to 2019. After the data were divided into one hour segment, a bandpass filter with 2.0-4.0 Hz, one-bit normalization and spectral whitening were applied. Then, we calculated autocorrelation function for each segment and stacked for 100 days to obtain the stable autocorrelation waveforms. The reflection waves from the underthrust sediment were picked manually by comparing the result with the seismic reflection profiles in the Nankai subduction zone, and we calculated the two-way travel time of the reflection wave. As a result, we could estimate the P-wave velocity changes in the underthrust sediment, and the velocity decreased during the occurrence of SSE. The velocity change might be also related to tectonic tremors. This ambient noise monitoring technique could provide a pore pressure in the deeper part where borehole cannot reach.