15:45 〜 16:00
[SCG45-08] Monitoring of Seismic Wave Attenuation Using Cross-correlation and its Relationship to Historical Seismic Events near the Japan Trench
キーワード：減衰、地震、地震波干渉法、日本海溝、S-net
In surface wave analysis using ambient noise, we typically use phase information, while amplitude information (i.e., wave attenuation) is rarely used due to its difficulty of measurements. This study aims to spatiotemporally understand the subsurface structures by utilizing amplitude information, specifically the attenuation coefficient or Q value. While previous studies have rarely documented consistent changes in attenuation caused by external factors such as earthquakes, we extend the spectral-ratio method to monitor spatiotemporal variations in attenuation. Specifically, we propose a method that applies the transfer function derived from seismic interferometry to the spectral-ratio method calculation. Here, we used the data from S-net, a network of seismometers extensively deployed on the seafloor near the Japan Trench, in order to characterize and monitor the plate convergent margin. We analyzed data from 150 S-net stations collected between May 2017 and December 2018. We calculated cross-correlation function (CCF) for the vertical component and produced transfer function by stacking 20 days of data. Our results revealed that, before and after relatively large earthquakes (approximately Mw 6 or higher), Q values in specific regions exhibited more significantly changes compared to seismic velocity variations. To identify regions where Q values were prone to change, we calculated the standard deviation of Q values. These findings indicate that Q values tend to vary in areas where historical seismic events have occurred. We interpret that Q values of surface wave (Scholte wave) are more sensitive to localized heterogeneous structures such as faults and cracks, which are created or influenced by historical megaquakes (e.g., 2011 Tohoku-oki earthquake), compared to velocity changes. This sensitivity difference can also be attributed to the distinct response of Q values and velocity changes to pressure fluctuations. Additionally, in the incoming Pacific plate, we identified regions with relatively large standard deviation in both Q values and velocity changes. This observation may result from factors such as earthquakes increasing pore pressure near petit-spot volcanic regions on the seafloor, which in turn leads to enhancement of both seismic wave attenuation and velocity reduction. The study demonstrates that monitoring Q values can enhance the understanding of subsurface structures in subduction zones, help detect anomalies, and pinpoint locations with a higher likelihood of large earthquake occurrences.