16:15 〜 16:30
[S22-17] Site response changes caused by strong ground motions of the 2024 Noto Peninsula earthquake
On 1 January 2024, a devastating MJMA 7.6 earthquake occurred on the Noto Peninsula in Japan. Seismic waves were generated on tectonic faults beneath the Noto Peninsula and the Sea of Japan [1] and caused widespread damage to civil infrastructure on the ground surface. In general, the near-surface soil and rock layers cause amplification of seismic waves, which is included in the design of earthquake-resistant buildings by considering site-specific geological conditions. Nevertheless, when strong ground motions impinge, the near-surface soils exhibit nonlinear behavior and may undergo some structural changes driven by liquefaction or elastic softening phenomena. The structural changes can cause variations in the shear-wave velocity [2], which cause a change in the site response in the short- and long-term (during the strong ground shaking and after it, respectively). The site response changes influence the characteristics of the destructive ground shaking itself, and hence, their evaluation is important for engineering purposes.
In this study, we perform an analysis of Horizontal-to-Vertical Spectral Ratios (HVSR) from regional earthquake waveforms [3] recorded at 160 sites of the Noto Peninsula and its surrounding region. The waveform data originate from the K-NET and KiK-net [4], Shindo-kei networks of the Japan Meteorological Agency (JMA) and local governments, stations operated by the Disaster Prevention Research Institute (DPRI) of Kyoto University, and stations of the Port and Airport Research Institute (PARI). We identified significant HVSR peaks and their directionality characteristics separately for periods before, during, and after the devastating MJMA 7.6 earthquake. This allows us to measure the temporal variations in the predominant site-specific resonance frequency during and after the strong ground shaking.
The long-lasting site response change is characterized by a predominant frequency drop and it is correlated with the peak ground velocity (PGV) measured during the MJMA 7.6 earthquake. This proves it is a phenomenon related to site-specific near-surface geology. The short-lasting site response change is evaluated by using the HVSR in short-time windows from the mainshock waveforms, and it reveals strongly nonstationary behavior (Figure 1). Frequencies of HVSR peaks decrease simultaneously and omnidirectionally with the strong shaking and logarithmically recover long after it. The observed extreme predominant frequency drops show relative change values exceeding -70%. These results indicate that strong ground motions with PGV > 0.2 m/s may cause a drop in the site-specific resonance frequency. This behavior is physically related to temporal variations in near-surface shear-wave velocity. The introduced phenomenon may have a significant impact on a broad scale of research topics, from the dynamic soil characteristics to the input ground motions for building collapse.
Acknowledgments: We acknowledge the NIED, JMA, DPRI, PARI, and prefectural governments for operating monitoring stations used in this study. Miroslav Hallo was supported by the Japan Society for the Promotion of Science (JSPS) through Fellowship number P23070 and Grant-in-Aid for JSPS Fellows number 23KF0149.
References:
[1] Asano K & Iwata T, Source rupture process of the 2024 Noto Hanto earthquake by waveform inversion of strong motion records, Poster U15-P20, JpGU2024, Chiba, Japan, 2024.
[2] Sawazaki K, Sato H, Nakahara H, & Nishimura T, Temporal change in site response caused by earthquake strong motion as revealed from coda spectral ratio measurement, Geophys Res Lett 33, L21303, 2006.
[3] Lermo J & Chávez-García FJ, Site effect evaluation using spectral ratios with only one station, Bull Seismol Soc Am 83, 1574–1594, 1993.
[4] NIED, NIED K-NET, KiK-net, National Research Institute for Earth Science and Disaster Resilience, 2019. DOI: 10.17598/NIED.0004
In this study, we perform an analysis of Horizontal-to-Vertical Spectral Ratios (HVSR) from regional earthquake waveforms [3] recorded at 160 sites of the Noto Peninsula and its surrounding region. The waveform data originate from the K-NET and KiK-net [4], Shindo-kei networks of the Japan Meteorological Agency (JMA) and local governments, stations operated by the Disaster Prevention Research Institute (DPRI) of Kyoto University, and stations of the Port and Airport Research Institute (PARI). We identified significant HVSR peaks and their directionality characteristics separately for periods before, during, and after the devastating MJMA 7.6 earthquake. This allows us to measure the temporal variations in the predominant site-specific resonance frequency during and after the strong ground shaking.
The long-lasting site response change is characterized by a predominant frequency drop and it is correlated with the peak ground velocity (PGV) measured during the MJMA 7.6 earthquake. This proves it is a phenomenon related to site-specific near-surface geology. The short-lasting site response change is evaluated by using the HVSR in short-time windows from the mainshock waveforms, and it reveals strongly nonstationary behavior (Figure 1). Frequencies of HVSR peaks decrease simultaneously and omnidirectionally with the strong shaking and logarithmically recover long after it. The observed extreme predominant frequency drops show relative change values exceeding -70%. These results indicate that strong ground motions with PGV > 0.2 m/s may cause a drop in the site-specific resonance frequency. This behavior is physically related to temporal variations in near-surface shear-wave velocity. The introduced phenomenon may have a significant impact on a broad scale of research topics, from the dynamic soil characteristics to the input ground motions for building collapse.
Acknowledgments: We acknowledge the NIED, JMA, DPRI, PARI, and prefectural governments for operating monitoring stations used in this study. Miroslav Hallo was supported by the Japan Society for the Promotion of Science (JSPS) through Fellowship number P23070 and Grant-in-Aid for JSPS Fellows number 23KF0149.
References:
[1] Asano K & Iwata T, Source rupture process of the 2024 Noto Hanto earthquake by waveform inversion of strong motion records, Poster U15-P20, JpGU2024, Chiba, Japan, 2024.
[2] Sawazaki K, Sato H, Nakahara H, & Nishimura T, Temporal change in site response caused by earthquake strong motion as revealed from coda spectral ratio measurement, Geophys Res Lett 33, L21303, 2006.
[3] Lermo J & Chávez-García FJ, Site effect evaluation using spectral ratios with only one station, Bull Seismol Soc Am 83, 1574–1594, 1993.
[4] NIED, NIED K-NET, KiK-net, National Research Institute for Earth Science and Disaster Resilience, 2019. DOI: 10.17598/NIED.0004