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

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.