1:45 PM - 3:15 PM
[SSS09-P11] Radiation Characteristics of S-wave of 2021 and 2022 off Fukushima Earthquakes
Keywords:Off Fukushima Earthquakes, S-wave radiation pattern
On February 13, 2021(Mw 7.1, depth 55km) and March 16, 2022(Mw 7.4, depth 57km),M7-class earthquakes occurred with almost the same epicenter off Fukushima. The 2022 earthquake caused building damage, railway viaduct damage and derailment accident on the bullet train line. S-waves are responsible for such destruction of engineered structures, so it is important to detect the spatial distribution of the S-wave amplitudes. Currently, most empirical ground motion prediction models are developed using the RMS amplitude of horizontal motion without taking into account any differences in direction. About inland crustal earthquakes of strike-slip fault source, several previous studies (e.g., Takemura et al. 2009, Tanaka 2020) have indicated that S-waves show a four-quadrant radiation pattern in the low-frequency range as the theory. In this study, we analyzed the observed S-wave horizontal motion of the 2021 and 2022 off Fukushima earthquakes which are deep intraslab earthquakes and investigated the radiation characteristics of S-wave.
The two horizontal motion components of the observed acceleration records were transformed into Radial (R) and Transverse (T) components, and S-wave acceleration amplitude spectra were calculated for 20.48 s of S-wave part. For FKS003, the peak acceleration of the R component was 295 cm/s*s, and that of the T component was 272 cm/s*s, which was almost the same. On the contrary, the peak velocity of the R component was 28 cm/s, and the T component was 14 cm/s. The R component was about twice as large as the T component. The Fourier spectrum of the R component at the same station was two to three times larger than that of the T component in the low frequency range (< 1 Hz), while the amplitude in the high frequency range (> 10 Hz) was comparable for both components.
To confirm the differences in amplitude between the R and T components of each observed record, the amplitude ratio of the T component to horizontal motion (At: Takemura et al. 2009) was calculated for each frequency between 0.1 and 20 Hz. These observed Ats were compared to the theoretical Ats (Aki & Richards 2002). The spatial distribution of observed Ats is biased in the low frequency range (< 2 Hz) but not in the high frequency range (> 3 Hz). In order to evaluate the theoretical and observed Ats quantitatively, the correlation coefficients were calculated for each frequency and distance. The correlations decreased with increasing frequency and were low in the high frequency range over 6 Hz within 200 km of the epicentral distance of the 2022 earthquake, indicating that the radiation pattern of the S-wave collapsed in the high frequency range. The correlation for the 2021 earthquake is lower than that for the 2022 earthquake, and few change with frequency. The 2021 earthquake has two separate SMGAs (Shiba 2022); the cause of this may be affected by their radiation characteristics. The correlation coefficients for each hypocentral distance band at 0.1 Hz showed that the correlation coefficients for the 2022 earthquake decrease with increasing hypocentral distance and are uncorrelated above 300 km. The radiation pattern of the S-wave also collapsed in a long-distance range.
The radiation pattern's collapse is caused by seismic wave scattering on the short-wavelength heterogeneous structures in the crust and the heterogeneity of the fault rupture (Takemura et al. 2009). In the case of deep intraslab earthquakes near oceanic trenches, it is necessary to consider heterogeneous structures around the subduction zone. We will analyze the other off Fukushima earthquakes and confirm the radiation characteristics by three-dimensional simulation.
[Acknowledgments] We used strong-motion records from K-NET and KiK-net, the strong-motion observation networks of the NIED.
The two horizontal motion components of the observed acceleration records were transformed into Radial (R) and Transverse (T) components, and S-wave acceleration amplitude spectra were calculated for 20.48 s of S-wave part. For FKS003, the peak acceleration of the R component was 295 cm/s*s, and that of the T component was 272 cm/s*s, which was almost the same. On the contrary, the peak velocity of the R component was 28 cm/s, and the T component was 14 cm/s. The R component was about twice as large as the T component. The Fourier spectrum of the R component at the same station was two to three times larger than that of the T component in the low frequency range (< 1 Hz), while the amplitude in the high frequency range (> 10 Hz) was comparable for both components.
To confirm the differences in amplitude between the R and T components of each observed record, the amplitude ratio of the T component to horizontal motion (At: Takemura et al. 2009) was calculated for each frequency between 0.1 and 20 Hz. These observed Ats were compared to the theoretical Ats (Aki & Richards 2002). The spatial distribution of observed Ats is biased in the low frequency range (< 2 Hz) but not in the high frequency range (> 3 Hz). In order to evaluate the theoretical and observed Ats quantitatively, the correlation coefficients were calculated for each frequency and distance. The correlations decreased with increasing frequency and were low in the high frequency range over 6 Hz within 200 km of the epicentral distance of the 2022 earthquake, indicating that the radiation pattern of the S-wave collapsed in the high frequency range. The correlation for the 2021 earthquake is lower than that for the 2022 earthquake, and few change with frequency. The 2021 earthquake has two separate SMGAs (Shiba 2022); the cause of this may be affected by their radiation characteristics. The correlation coefficients for each hypocentral distance band at 0.1 Hz showed that the correlation coefficients for the 2022 earthquake decrease with increasing hypocentral distance and are uncorrelated above 300 km. The radiation pattern of the S-wave also collapsed in a long-distance range.
The radiation pattern's collapse is caused by seismic wave scattering on the short-wavelength heterogeneous structures in the crust and the heterogeneity of the fault rupture (Takemura et al. 2009). In the case of deep intraslab earthquakes near oceanic trenches, it is necessary to consider heterogeneous structures around the subduction zone. We will analyze the other off Fukushima earthquakes and confirm the radiation characteristics by three-dimensional simulation.
[Acknowledgments] We used strong-motion records from K-NET and KiK-net, the strong-motion observation networks of the NIED.