4:00 PM - 4:15 PM
[SCG61-09] The S-wave polarization anisotropy in the eastern Hokkaido, Japan.
Keywords:S-wave polarization anisotropy, S-wave splitting
S-wave splitting is a phenomenon in which S-waves splits in an anisotropic medium as two shear waves with different velocities and orthogonal oscillation directions to each other. In the upper crust, S-splitting is mainly caused by stress-induced anisotropy, and the oscillation direction of the fast wave is detected in parallel to the SHmax (Maximum horizontal compression axis) direction. However, in the vicinity of fault zones or volcanoes, anisotropy may be observed in a different direction from the regional stress field and is assumed to reflect fault-parallel fractures, local stress fields, or upwelling fluid paths (Boness and Zoback, 2006; Johnson et al., 2011). The S-wave splitting parameters can be obtained from a single pair of hypocenter and station and show a higher time resolution dependent on the interval of earthquake occurrence, so the method is ideal for tracking the temporal variation of anisotropy. The parameters are also known to show a frequency dependence caused by the size of microfractures or the spatial extent of the anisotropic medium in the crust.
The stress field with WNW-ESE compressional axis associated with the subduction of the Pacific Plate (Terakawa and Matsu’ra, 2010; Tagami et al., this meeting, 2025) is distributed widely in the eastern Hokkaido, Japan. Also, the volcanic front consisting of several active volcanoes and calderas (e.g., Atosanupuri and Kutcharo caldera) is distributed in this aera. Active faults with NE-SW strike are distributed along the volcanic front, and active reverse faults with N-S strike has been identified to the east of Lake Abashiri (AIST, 2024). Therefore, complex crustal structures are inferred in the eastern Hokkaido. However, few analyses using S-wave splitting have been conducted in this area. The objectives of this analysis are to estimate the stress field and crustal structure using S-wave anisotropy and to reveal the time evolution of them in the eastern Hokkaido. We used seismic waveform data based on the JMA (Japan Meteorological Agency) Unified catalog, and measured S-wave splitting parameters using MFAST (Multiple Filter Automatic Splitting Technique, Savage et al., 2010). We examined the frequency dependence by adjusting the filters and time windows.
According to the frequency dependence validation throughout the region, the oscillation direction of the S-wave, φ(direction of anisotropy), was mainly in the WNW-ESE direction, which was consistent with the SHmax direction in the eastern Hokkaido. In the low-frequency band, φ had regional differences, with a predominance of WNW-ESE anisotropy on the fore-arc side from the volcano front. In contrast, anisotropy was heterogeneously observed on the back-arc side. In the high-frequency band, small-scale variation in the anisotropic direction was observed. In the eastern part of Abashiri, we observed two peaks of φ, one WNW-ESE and the other N-S. They were parallel to the SHmax direction (WNW-ESE) and the reverse fault (strike N-S), respectively. A temporal change from the former to the latter was also observed with seismic activity in 2020. In the vicinity of Lake Kutcharo, the direction of anisotropy tended to radiate from the summit of Atosanupuri. Anisotropy in the circumferential direction was also observed at the outer rim of Kutcharo caldera, suggesting a caldera boundary fault or upwelling fluid paths. Crustal deformations near volcanoes have been occasionally observed in eastern Hokkaido, including the area around Lake Kutcharo, and we expect to detect temporal changes of anisotropy reflecting them.
The stress field with WNW-ESE compressional axis associated with the subduction of the Pacific Plate (Terakawa and Matsu’ra, 2010; Tagami et al., this meeting, 2025) is distributed widely in the eastern Hokkaido, Japan. Also, the volcanic front consisting of several active volcanoes and calderas (e.g., Atosanupuri and Kutcharo caldera) is distributed in this aera. Active faults with NE-SW strike are distributed along the volcanic front, and active reverse faults with N-S strike has been identified to the east of Lake Abashiri (AIST, 2024). Therefore, complex crustal structures are inferred in the eastern Hokkaido. However, few analyses using S-wave splitting have been conducted in this area. The objectives of this analysis are to estimate the stress field and crustal structure using S-wave anisotropy and to reveal the time evolution of them in the eastern Hokkaido. We used seismic waveform data based on the JMA (Japan Meteorological Agency) Unified catalog, and measured S-wave splitting parameters using MFAST (Multiple Filter Automatic Splitting Technique, Savage et al., 2010). We examined the frequency dependence by adjusting the filters and time windows.
According to the frequency dependence validation throughout the region, the oscillation direction of the S-wave, φ(direction of anisotropy), was mainly in the WNW-ESE direction, which was consistent with the SHmax direction in the eastern Hokkaido. In the low-frequency band, φ had regional differences, with a predominance of WNW-ESE anisotropy on the fore-arc side from the volcano front. In contrast, anisotropy was heterogeneously observed on the back-arc side. In the high-frequency band, small-scale variation in the anisotropic direction was observed. In the eastern part of Abashiri, we observed two peaks of φ, one WNW-ESE and the other N-S. They were parallel to the SHmax direction (WNW-ESE) and the reverse fault (strike N-S), respectively. A temporal change from the former to the latter was also observed with seismic activity in 2020. In the vicinity of Lake Kutcharo, the direction of anisotropy tended to radiate from the summit of Atosanupuri. Anisotropy in the circumferential direction was also observed at the outer rim of Kutcharo caldera, suggesting a caldera boundary fault or upwelling fluid paths. Crustal deformations near volcanoes have been occasionally observed in eastern Hokkaido, including the area around Lake Kutcharo, and we expect to detect temporal changes of anisotropy reflecting them.