5:15 PM - 7:15 PM
[SSS11-P02] Localized ground motion amplification during the 2024 Noto Peninsula earthquake
Keywords:2024 Noto Peninsula earthquake, microtremor array survey, site amplification
At 16:10 on January 1, 2024, a magnitude 7.6 earthquake occurred in the Noto Peninsula of Ishikawa Prefecture. A maximum seismic intensity of 7 was recorded during the main shock. In this study, we examined the records and amplification characteristics of K-NET ISK006 and surrounding stations in Togi, Shika Town, where a particularly large acceleration was observed during the earthquake and a JMA seismic intensity of 7 was recorded during the main shock.
Three stations (K-NET ISK006, KiK-net ISKH04, and JMA JMA914) are located in the Togi area. The peak ground acceleration (PGA) of the main shock was 2.6 × 10³gal, 452 gal, and 427 gal at ISK006, respectively. On the other hand, the difference of the peak ground velocities of 65 cm/s, 83 cm/s, and 68 cm/s, are small. Similarly, at 23:20 on 1/6 (M4.3), a large PGA of 1.2 x 10³ gal and JMA intensity of 6- were observed at ISK006, while ISKH04 had a PGA of 22 gal and an intensity of 3. These stations are only about 4 km apart at maximum.
Fourier spectra were obtained for seismic records of the 29 earthquakes that occurred around the Noto Peninsula, and the ratios of the Fourier spectra at ISK006 and JMA914 to those at ISKH04 were calculated. The resulted ratio for ISK006 showed a large amplitude ratio, up to about 280 times larger than that of ISKH04, at 4-7 Hz. It seemed that these significant spectral ratios were related to the direction of the epicenter. We obtained the logarithmic mean of the obtained spectral ratios at 4-7 Hz and organized by the back azimuth from ISK006. The geometrical spreading on amplitude due to the difference in hypocentral distances between the stations was corrected by assuming body waves. The resulting logarithmic mean spectral ratios of 4-7 Hz were about 2.2 times larger in the recordings of the eastern epicenter, while a significant amplification was observed at epicenters west of N58°W. In the western epicenter, the ratio in the 4-7 Hz range reached a maximum of 46 times, and the ratio was large in the EW component. This cannot be explained by a simple difference in the source distance between the stations.
To investigate the cause of this seismic amplification, a microtremor array survey was conducted at ISK006. Based on the obtained observation data, the surface part of the J-SHIS model was modified. As a result, it was found that ISK006 has a structure with Vs=278m/s in the surface layer on the bedrock of Vs > 1 km/s. Using the modified model for ISK006, we calculated the theoretical amplification of ISKH04 with respect to ISK006, assuming vertical S-wave incidence into the horizontally stratified structure. The log-average amplification at 4-7 Hz was 2.1 times larger than that of ISK006, which is consistent with the observed results for an earthquake with a source on the east. On the other hand, the significant amplification for earthquakes with western epicenters were not explained by the 1D velocity model. The cause of the significant amplification for earthquakes with western epicenters is a subject for future research, but it is expected to be due to azimuthal dependence caused by irregular structures in the vicinity of the observation points.
Acknowledgments: We thank Prof. Kimiyuki Asano (Kyoto University) for helping us with observation. This study was supported by the Core-to-Core Collaborative research program of the Earthquake Research Institute, The University of Tokyo and the Disaster Prevention Research Institute, Kyoto University.
Three stations (K-NET ISK006, KiK-net ISKH04, and JMA JMA914) are located in the Togi area. The peak ground acceleration (PGA) of the main shock was 2.6 × 10³gal, 452 gal, and 427 gal at ISK006, respectively. On the other hand, the difference of the peak ground velocities of 65 cm/s, 83 cm/s, and 68 cm/s, are small. Similarly, at 23:20 on 1/6 (M4.3), a large PGA of 1.2 x 10³ gal and JMA intensity of 6- were observed at ISK006, while ISKH04 had a PGA of 22 gal and an intensity of 3. These stations are only about 4 km apart at maximum.
Fourier spectra were obtained for seismic records of the 29 earthquakes that occurred around the Noto Peninsula, and the ratios of the Fourier spectra at ISK006 and JMA914 to those at ISKH04 were calculated. The resulted ratio for ISK006 showed a large amplitude ratio, up to about 280 times larger than that of ISKH04, at 4-7 Hz. It seemed that these significant spectral ratios were related to the direction of the epicenter. We obtained the logarithmic mean of the obtained spectral ratios at 4-7 Hz and organized by the back azimuth from ISK006. The geometrical spreading on amplitude due to the difference in hypocentral distances between the stations was corrected by assuming body waves. The resulting logarithmic mean spectral ratios of 4-7 Hz were about 2.2 times larger in the recordings of the eastern epicenter, while a significant amplification was observed at epicenters west of N58°W. In the western epicenter, the ratio in the 4-7 Hz range reached a maximum of 46 times, and the ratio was large in the EW component. This cannot be explained by a simple difference in the source distance between the stations.
To investigate the cause of this seismic amplification, a microtremor array survey was conducted at ISK006. Based on the obtained observation data, the surface part of the J-SHIS model was modified. As a result, it was found that ISK006 has a structure with Vs=278m/s in the surface layer on the bedrock of Vs > 1 km/s. Using the modified model for ISK006, we calculated the theoretical amplification of ISKH04 with respect to ISK006, assuming vertical S-wave incidence into the horizontally stratified structure. The log-average amplification at 4-7 Hz was 2.1 times larger than that of ISK006, which is consistent with the observed results for an earthquake with a source on the east. On the other hand, the significant amplification for earthquakes with western epicenters were not explained by the 1D velocity model. The cause of the significant amplification for earthquakes with western epicenters is a subject for future research, but it is expected to be due to azimuthal dependence caused by irregular structures in the vicinity of the observation points.
Acknowledgments: We thank Prof. Kimiyuki Asano (Kyoto University) for helping us with observation. This study was supported by the Core-to-Core Collaborative research program of the Earthquake Research Institute, The University of Tokyo and the Disaster Prevention Research Institute, Kyoto University.