Softening and Recovery of Near‐Surface Layers During the 2024 M7.6 Noto Earthquake

Abstract On 1 January 2024, a devastating M J 7.6 earthquake occurred on the Noto Peninsula in Japan. When such a strong earthquake occurs, affected near‐surface soil layers behave nonlinearly and may undergo some structural changes driven by Flow Liquefaction, Cyclic Mobility, or Slow Dynamics phenomena. The structural changes can be manifested by short‐lasting coseismic and long‐lasting postseismic site‐response changes that are related to variations in near‐surface shear‐wave velocity. To examine this behavior, we perform a systematic analysis of Horizontal‐to‐Vertical (H/V) spectral ratios from regional earthquake waveforms recorded at 160 sites in the years 1996–2024. We identify significant H/V peaks and their directionality in the frequency range of 0.1–25 Hz separately for periods before and after the M J 7.6 earthquake. This allows us to measure long‐lasting relative changes in predominant frequency caused by the strong shaking, with maximum drops of −21% and a dependence on experienced ground motion levels. Next, the short‐lasting changes during the M J 7.6 earthquake reveal strongly nonstationary behavior. The frequency of spectral peaks decreases simultaneously and omnidirectionally with the strong shaking and then logarithmically recovers. The observed extreme short‐lasting predominant frequency drops reach −93% relative to the initial value, and their occurrence time divides the nonstationary behavior into elastic softening and recovery phases. This behavior is physically related to temporal changes in near‐surface shear‐wave velocity as a consequence of changes in shear moduli. The introduced phenomenon of elastic softening and recovery may have a significant impact on a broad scale of geophysical research topics.