5:15 PM - 6:45 PM
[U15-P53] Variations of the leading shear-wave polarization directions caused by the 2024 Noto Peninsula Earthquake
Keywords:S-wave anisotropy, directional statistics
The large earthquake of Mj7.6 occurred in the northern Noto Peninsula on 1 Jan 2024, and caused serious seismic damage around the source area. In the northeastern part of the Noto Peninsula, seismic swarms have occurred for more than three years prior to this earthquake, and the seismic activity in this area remains high. We have studied the shear-wave anisotropy in the northeastern Noto Peninsula by shear-wave splitting analysis using this earthquake swarm (Murakami, 2022; 2023) to develop a method to estimate the migration path of crustal fluids. We performed shear-wave splitting analysis using seismic waveforms from the permanent seismic stations in the Noto Peninsula to investigate whether the shear-wave anisotropy was changed by the Mj7.6 earthquake.
We used the seismic waveform records of the events that occurred in our study area for approximately 20 years from 1 Apr 2004 to 31 Jan 2024 whose magnitudes are larger than Mj1.0 (JMA unified catalog). In the shear-wave splitting analysis, it is necessary to select source-station pairs within a shear-wave window (e.g., Gao and Crampin, 2008), less than 35 degrees of incidence angle, because of avoiding S-to-P conversion at the free surface. In order to calculate the incident angles, we interpolated the seismic velocity structure model of Matsubara et al. (2019) to each seismic station. We used the semi-automatic analysis method by Saiga et al. (2013), which based on the covariance matrix method of Silver and Chan (1990). After applying the covariance matrix method, we further limited only the direct S-waves of the source-station pairs for which the correlation coefficient between the two split S-waves is greater than 0.90. Although the number of available earthquakes at each station differs due to the incident angle limitation, the shear-wave splitting parameters (phi and dt) were estimated for 6569 source-station pairs at the station with the most available earthquakes (N.SUZH).
The leading shear-wave polarization directions at the stations just above the source area of the 2024 Noto Peninsula earthquake (Mj7.6) (N.WMZH, N.TGIH, and N.SUZH) were in a northeast-southwest direction, while those at the eastern station (SUZU) in the northeast part of the Noto Peninsula were in a northwest-southeast direction. Comparing these leading shear-wave polarization directions before and after the Mj7.6 earthquake, there was no significant change in the direction at most of the stations. However, at the station located just above the hypocenters of swarm earthquakes before the Mj7.6 earthquake (N.SUZH), the leading shear-wave polarization directions before the Mj7.6 earthquake were in the northeast-southwest direction, while after the Mj7.6 earthquake, the directions were predominantly in the east-west direction. The cause of this difference remains unclear. However, the seismic activity is high even before the large earthquake in the areas around this station where we observed the variations of the leading shear-wave polarization direction, and some studies suggested the influence of crustal fluid to this swarm earthquakes (e.g., Nishimura et al., 2023; Kato et al., 2024). Therefore, it is considered that crustal fluids were abundant in the cracks parallel to the strike of the active fault just below this station before the Mj7.6 event. After Mj7.6 event, the crustal fluid in the cracks parallel to the fault may have been drained by the mainshock and aftershocks, and the drained fluid penetrated the cracks oriented in various directions, which may have changed the predominant direction of the opening cracks.
Acknowledgments: This study was funded by the Ministry of Economy, Trade and Industry (METI), Japan, as part of its R and D supporting program entitled "Establishment of Technology for Comprehensive Evaluation of the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste (Fiscal Year 2023)" (Grant Number: JPJ007597).
We used the seismic waveform records of the events that occurred in our study area for approximately 20 years from 1 Apr 2004 to 31 Jan 2024 whose magnitudes are larger than Mj1.0 (JMA unified catalog). In the shear-wave splitting analysis, it is necessary to select source-station pairs within a shear-wave window (e.g., Gao and Crampin, 2008), less than 35 degrees of incidence angle, because of avoiding S-to-P conversion at the free surface. In order to calculate the incident angles, we interpolated the seismic velocity structure model of Matsubara et al. (2019) to each seismic station. We used the semi-automatic analysis method by Saiga et al. (2013), which based on the covariance matrix method of Silver and Chan (1990). After applying the covariance matrix method, we further limited only the direct S-waves of the source-station pairs for which the correlation coefficient between the two split S-waves is greater than 0.90. Although the number of available earthquakes at each station differs due to the incident angle limitation, the shear-wave splitting parameters (phi and dt) were estimated for 6569 source-station pairs at the station with the most available earthquakes (N.SUZH).
The leading shear-wave polarization directions at the stations just above the source area of the 2024 Noto Peninsula earthquake (Mj7.6) (N.WMZH, N.TGIH, and N.SUZH) were in a northeast-southwest direction, while those at the eastern station (SUZU) in the northeast part of the Noto Peninsula were in a northwest-southeast direction. Comparing these leading shear-wave polarization directions before and after the Mj7.6 earthquake, there was no significant change in the direction at most of the stations. However, at the station located just above the hypocenters of swarm earthquakes before the Mj7.6 earthquake (N.SUZH), the leading shear-wave polarization directions before the Mj7.6 earthquake were in the northeast-southwest direction, while after the Mj7.6 earthquake, the directions were predominantly in the east-west direction. The cause of this difference remains unclear. However, the seismic activity is high even before the large earthquake in the areas around this station where we observed the variations of the leading shear-wave polarization direction, and some studies suggested the influence of crustal fluid to this swarm earthquakes (e.g., Nishimura et al., 2023; Kato et al., 2024). Therefore, it is considered that crustal fluids were abundant in the cracks parallel to the strike of the active fault just below this station before the Mj7.6 event. After Mj7.6 event, the crustal fluid in the cracks parallel to the fault may have been drained by the mainshock and aftershocks, and the drained fluid penetrated the cracks oriented in various directions, which may have changed the predominant direction of the opening cracks.
Acknowledgments: This study was funded by the Ministry of Economy, Trade and Industry (METI), Japan, as part of its R and D supporting program entitled "Establishment of Technology for Comprehensive Evaluation of the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste (Fiscal Year 2023)" (Grant Number: JPJ007597).