Temporal variations of the “in-situ” nonlinear behavior of shallow sediments during the 2016 Kumamoto Earthquake sequence

Summary Strong ground shaking has the potential to generate significant dynamic strains in shallow materials such as soils and sediments, thereby inducing nonlinear site response resulting in changes in near-surface materials. The nonlinear behavior of these materials can be characterized by an increase in wave attenuation and a decrease in the resonant frequency of the soil; these effects are attributed to increased material damping and decreased seismic wave propagation velocity, respectively. This study investigates the “in-situ” seismic velocity changes and the predominant ground motion frequency evolution during the 2016 Kumamoto earthquake sequence. This sequence includes two foreshocks (Mw6, Mw6.2) followed by a mainshock (Mw7.2) that occurred 24 h after the last foreshock. We present the results of the seismic velocity evolution during these earthquakes for seismological records collected by the KiK-net (32 stations) and K-NET (88 stations) networks between 2002 and 2020. We analyze the impulse response and autocorrelation functions to investigate the nonlinear response in near-surface materials. By comparing the results of the impulse response and autocorrelation functions, we observe that a nonlinear response occurs in near-surface materials. We then quantify the velocity reductions that occur before, during, and after the mainshock using both approaches. This allows us to estimate the “in situ” shear modulus reduction for different site classes based on VS30 values (VS30 < 360 m/s, 360 <VS30 < 760 m/s, VS30 > 760 m/s). We also establish the relationships between velocity changes, shear modulus reduction, variations in predominant ground motion frequencies, and site characteristics (VS30). The results of this analysis can be applied to site-specific ground motion modeling, site response analysis, and the incorporation of nonlinear site terms into ground motion models.