09:45 〜 10:00
[SSS11-09] Nonstationary behavior of near-surface layers at instrumented sites during the 2024 MJ7.6 Noto Peninsula earthquake
キーワード：site effects, strong ground motion, nonstationary site response, liquefaction, elastic softening, 2024 Noto Peninsula earthquake
The MJ7.6 Noto Peninsula earthquake of January 1, 2024, caused extensive destruction and loss of many lives, highlighting the need for a better understanding of strong motion effects on near-surface soil and rock layers. Intense ground shaking can induce structural modifications in near-surface materials due to processes such as flow liquefaction, cyclic mobility, and elastic softening. These changes influence local site response characteristics by altering seismic velocities and shifting the predominant resonance frequency, both during and after the event (i.e., coseismic softening and postseismic healing, respectively). This study investigates the dynamic behavior of near-surface layers in response to the 2024 earthquake by analyzing temporal variations in the predominant resonance frequency at several instrumented sites.
Using data from 160 surface stations throughout the Noto Peninsula and surrounding areas, we compute Horizontal-to-Vertical Spectral Ratios (HVSR) from three-component strong-motion recordings, which were obtained from seismic networks operated by local governments (Ishikawa-ken and Toyama-ken Shindo-kei), the National Research Institute for Earth Science and Disaster Resilience (NIED K-NET and KiK-net), the Japan Meteorological Agency (JMA Shindo-kei), the Port and Airport Research Institute (PARI), and temporary stations around the Morimoto-Togashi Fault Zone operated by the Disaster Prevention Research Institute (DPRI) of Kyoto University. We processed waveform data for the MJ7.6 event as well as moderate earthquakes recorded within the period from 1996 to the end of January 2024. By comparing HVSR before, during, and after the MJ7.6 earthquake, we track site-specific resonance frequency shifts that provide insights into the evolving mechanical properties of near-surface layers. This was done separately for transversal, radial, E-W, N-S, SE-NW, and NE-SW horizontal components of ground motion.
Results indicate a pronounced coseismic reduction in predominant resonance frequency, occurring synchronously with the strong ground shaking at all horizontal components (i.e., omnidirectional effect), which is recovering logarithmically over the long term. Some locations near the source fault plane exhibit extreme resonance frequency drops exceeding a relative difference of -70% (i.e., change to 30% of the initial frequency value). The magnitude of these shifts correlates with recorded peak ground velocity (PGV) and peak ground acceleration (PGA); however, the relationship is complex due to the simultaneous influence of PGV, PGA, strong-shaking duration, and site conditions. The observed frequency variations reflect transient reductions in shear-wave velocity and shear modulus, emphasizing the nonstationary and nonlinear response of near-surface layers under high-intensity shaking. These findings contribute to a deeper understanding of soil and rock behavior under extreme seismic loading and have implications for earthquake-resistant design. Recognizing the time-dependent nature of strong-motion site response is crucial to improving infrastructure resilience in seismically active regions.
Acknowledgements: This research is supported by the Japan Society for the Promotion of Science (JSPS) through Fellowship number P23070 and Grant-in-Aid for JSPS Fellows number 23KF0149. We use earthquake waveforms recorded by seismic stations operated by the NIED, JMA, PARI, DPRI, and local governments.
Using data from 160 surface stations throughout the Noto Peninsula and surrounding areas, we compute Horizontal-to-Vertical Spectral Ratios (HVSR) from three-component strong-motion recordings, which were obtained from seismic networks operated by local governments (Ishikawa-ken and Toyama-ken Shindo-kei), the National Research Institute for Earth Science and Disaster Resilience (NIED K-NET and KiK-net), the Japan Meteorological Agency (JMA Shindo-kei), the Port and Airport Research Institute (PARI), and temporary stations around the Morimoto-Togashi Fault Zone operated by the Disaster Prevention Research Institute (DPRI) of Kyoto University. We processed waveform data for the MJ7.6 event as well as moderate earthquakes recorded within the period from 1996 to the end of January 2024. By comparing HVSR before, during, and after the MJ7.6 earthquake, we track site-specific resonance frequency shifts that provide insights into the evolving mechanical properties of near-surface layers. This was done separately for transversal, radial, E-W, N-S, SE-NW, and NE-SW horizontal components of ground motion.
Results indicate a pronounced coseismic reduction in predominant resonance frequency, occurring synchronously with the strong ground shaking at all horizontal components (i.e., omnidirectional effect), which is recovering logarithmically over the long term. Some locations near the source fault plane exhibit extreme resonance frequency drops exceeding a relative difference of -70% (i.e., change to 30% of the initial frequency value). The magnitude of these shifts correlates with recorded peak ground velocity (PGV) and peak ground acceleration (PGA); however, the relationship is complex due to the simultaneous influence of PGV, PGA, strong-shaking duration, and site conditions. The observed frequency variations reflect transient reductions in shear-wave velocity and shear modulus, emphasizing the nonstationary and nonlinear response of near-surface layers under high-intensity shaking. These findings contribute to a deeper understanding of soil and rock behavior under extreme seismic loading and have implications for earthquake-resistant design. Recognizing the time-dependent nature of strong-motion site response is crucial to improving infrastructure resilience in seismically active regions.
Acknowledgements: This research is supported by the Japan Society for the Promotion of Science (JSPS) through Fellowship number P23070 and Grant-in-Aid for JSPS Fellows number 23KF0149. We use earthquake waveforms recorded by seismic stations operated by the NIED, JMA, PARI, DPRI, and local governments.