5:15 PM - 6:30 PM
[SSS09-P01] Temporal variation in seismic velocity at the shallow part of the overriding plate in the Japan Trench
Keywords:Seismic velocity, Temporal variation, Japan Trench
Ambient noise correlation technique is now a powerful tool for detecting temporal variation in seismic velocity. Because such studies require long-term continuous seismic records, they have been often used to land-based seismic records. However, permanent seismic observations have recently deployed offshore areas, particularly around Japan, this technique can be applied to temporal variation in seismic velocity structure at the subseafloor (e.g., Ikeda and Tsuji, 2018). In particular, Tonegawa et al. (2020, JpGU; AGU) has reported that seismic velocity and heterogeneity changes are induced by tectonic compression and fluid migration at the subseafloor structure in the Nankai subduction zone. In this study, we applied ambient noise correlation technique to the continuous records observed at S-net deployed around the Japan Trench, and investigated the relationships among temporal variation in seismic structure, seismic activities, and tectonic settings.
We calculated cross correlation functions (CCFs) at a frequency band of 0.5–1.0 Hz using ambient noise records in the vertical component. Here, the three-component seismograms were synthesized using a method of Takagi et al. (2019). The CCFs show ocean acoustically-coupled Rayleigh waves whose propagation velocity is slightly less than 1.5 km/s. The time length of the time series for calculating CCFs was 3600 s, but the time series was subtracted by those in the next 3600 s for removing instrumental noises, according to the method of Takagi et al. (2021). The calculated CCFs are stacked over 1 year for the reference CCF and 30 days for individual CCFs. We measured seismic velocity change (dv/v) by a stretching method (Wegler and Sens-Schönfelder 2007) and also seismic heterogeneity change (CC) by cross correlating the reference and individual CCFs (Obermann et al. 2013).
Our results show temporal changes in dv/v near the Japan Trench, while such changes cannot be observed in the region close to land. The maximum change of dv/v over 4 year was 0.3 % around the latitude of 39ºN. It is considered that the gradual change primarily corresponds to the recovery process of a velocity reduction associated with the 2011 Tohoku-oki earthquake. Another candidate is tectonic compression in the overriding plate near the trench at this region, which induces a seismic velocity increase due to fluid drainage at shallow depths to the sea water.
Our results also show abrupt reductions in CC, but the areas of the changes are wide compare with those observed in the Nankai area. Some of them correspond to the timing of relatively large earthquakes, indicating that seismic heterogeneity at shallow depths was varied with seafloor motion due to the earthquakes. However, it appears that other CC reductions do not correspond to the large earthquakes. It is necessary to investigate the CC reductions in more details.
Data citation
S-net: doi:10.17598/NIED.0007
We calculated cross correlation functions (CCFs) at a frequency band of 0.5–1.0 Hz using ambient noise records in the vertical component. Here, the three-component seismograms were synthesized using a method of Takagi et al. (2019). The CCFs show ocean acoustically-coupled Rayleigh waves whose propagation velocity is slightly less than 1.5 km/s. The time length of the time series for calculating CCFs was 3600 s, but the time series was subtracted by those in the next 3600 s for removing instrumental noises, according to the method of Takagi et al. (2021). The calculated CCFs are stacked over 1 year for the reference CCF and 30 days for individual CCFs. We measured seismic velocity change (dv/v) by a stretching method (Wegler and Sens-Schönfelder 2007) and also seismic heterogeneity change (CC) by cross correlating the reference and individual CCFs (Obermann et al. 2013).
Our results show temporal changes in dv/v near the Japan Trench, while such changes cannot be observed in the region close to land. The maximum change of dv/v over 4 year was 0.3 % around the latitude of 39ºN. It is considered that the gradual change primarily corresponds to the recovery process of a velocity reduction associated with the 2011 Tohoku-oki earthquake. Another candidate is tectonic compression in the overriding plate near the trench at this region, which induces a seismic velocity increase due to fluid drainage at shallow depths to the sea water.
Our results also show abrupt reductions in CC, but the areas of the changes are wide compare with those observed in the Nankai area. Some of them correspond to the timing of relatively large earthquakes, indicating that seismic heterogeneity at shallow depths was varied with seafloor motion due to the earthquakes. However, it appears that other CC reductions do not correspond to the large earthquakes. It is necessary to investigate the CC reductions in more details.
Data citation
S-net: doi:10.17598/NIED.0007