Temporal Variations of Shallow Material Properties During the Kumamoto Earthquake Sequence

 Strong ground motion can generate a large dynamic strain in shallow materials, lead to a nonlinear response, and cause permanent damage in near-surface materials. The nonlinear behavior of soils subjected to strong vibrations leads to an increase in wave attenuation and a decrease in shear modulus. These effects lead to a decrease in the resonance frequency of the soil and a decrease in the propagation speed of S-waves. This work investigates, using deconvolution and autocorrelation methods, “in situ” seismic velocity changes and predominant ground-motion frequency evolution during the 2016 Kumamoto earthquake sequence. The Kumamoto sequence contains two major foreshocks (Mw6, Mw6.2) and a mainshock (Mw7.2) that occurred 24 hours after the last foreshock. We present results of the seismic velocity evolution during this sequence for seismological records collected by Kik-Net and K-Net stations between 2002 to 2020. The results indicate that nonlinearity response is profoundly occurring in the damaged material close to the surface. We quantify these velocity reductions occurring during the mainshock and show that the healing process lasted about three months after the mainshock. We finally quantify the relationships between velocity changes, ground-motion predominant frequency variations, and site condition characteristics (Vs30).