11:00 AM - 11:15 AM
[SSS08-02] Time variation of the S-wave polarization anisotropy in the 2024 northern Iwate seismic activity.
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-wave 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 advantages of S-wave splitting for the estimation methods for stress field and crustal structure are that it can be obtained from a single pair of hypocenter and station, which is effective for regions with a small number of stations and microearthquakes, and that the higher time resolution of the interval of earthquake occurrence, which is ideal for tracking the temporal variation of anisotropy. In addition, the S-wave splitting parameters are known to show a frequency dependence caused by the width of microfractures or the size of the anisotropic medium in the crust.
In the northern part of Iwate Prefecture, a Mw 4.0 earthquake on July 19, 2024, and a Mw 4.2 earthquake on July 28 occurred in the upper crust, followed by a seismic activity with nearly 100 M1-2 earthquakes detected as aftershocks or seismic activity in the vicinity. In this analysis, 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. We also included small M1-2 earthquakes in the vicinity of the two M4 earthquakes to examine detailed temporal changes of parameters.
According to the frequency dependence validation, in the low-frequency band, the oscillation direction of the S-wave, φ(direction of anisotropy), was homogeneous, and the difference in arrival times of the two waves, δt (magnitude of anisotropy, the density of microcracks) was long, whereas φ was heterogeneous and δt was short in the high-frequency band. This was due to the difference in wavelength, which appeared to reflect the average or local anisotropy on the ray path, respectively. The time variation of anisotropy was particularly observed in the high-frequency band. Compared with the aftershocks that occurred immediately after the July 19 Mw 4.0 earthquake, the ratio of observed anisotropy parallel to the SHmax direction decreased, and the ratio of anisotropy parallel to the fault increased about 28 July, when the Mw 4.2 earthquake occurred. Especially for the fault-parallel anisotropy, the increase of δt was also remarkable.
Stress release and the development of fault-parallel fractures had been detected with seismic activity (e.g., for the 1995 Hyogo-ken Nanbu Earthquake; Tadokoro et al., 1999), and the temporal changes in anisotropy observed in this study were consistent with such changes in the stress field and crustal structure. By examining the detailed time variation of the S-wave splitting parameters with frequency dependence, we expected that the local stress field and fault rupture process could be inferred with higher time resolution than before.
In the northern part of Iwate Prefecture, a Mw 4.0 earthquake on July 19, 2024, and a Mw 4.2 earthquake on July 28 occurred in the upper crust, followed by a seismic activity with nearly 100 M1-2 earthquakes detected as aftershocks or seismic activity in the vicinity. In this analysis, 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. We also included small M1-2 earthquakes in the vicinity of the two M4 earthquakes to examine detailed temporal changes of parameters.
According to the frequency dependence validation, in the low-frequency band, the oscillation direction of the S-wave, φ(direction of anisotropy), was homogeneous, and the difference in arrival times of the two waves, δt (magnitude of anisotropy, the density of microcracks) was long, whereas φ was heterogeneous and δt was short in the high-frequency band. This was due to the difference in wavelength, which appeared to reflect the average or local anisotropy on the ray path, respectively. The time variation of anisotropy was particularly observed in the high-frequency band. Compared with the aftershocks that occurred immediately after the July 19 Mw 4.0 earthquake, the ratio of observed anisotropy parallel to the SHmax direction decreased, and the ratio of anisotropy parallel to the fault increased about 28 July, when the Mw 4.2 earthquake occurred. Especially for the fault-parallel anisotropy, the increase of δt was also remarkable.
Stress release and the development of fault-parallel fractures had been detected with seismic activity (e.g., for the 1995 Hyogo-ken Nanbu Earthquake; Tadokoro et al., 1999), and the temporal changes in anisotropy observed in this study were consistent with such changes in the stress field and crustal structure. By examining the detailed time variation of the S-wave splitting parameters with frequency dependence, we expected that the local stress field and fault rupture process could be inferred with higher time resolution than before.