To quantitatively evaluate the nonlinear characteristics near the downtown of Kumamoto City, located in the plain where several building damages occurred during the (Asano, and Iwata, 2016) Kumamoto earthquake sequence, Japan, we investigated the variation of site amplification factors using the earthquake ground motion data of over 200 records and the nonlinear characterization of the shallow subsurface structure. At first, the earthquake ground motions at the seismic station near the downtown of Kumamoto City, which was selected as the test site for the blind predictions in ESG6 (Effects of Surface Geology on Seismic Motion), were categorized into weak ground motions with the observed PGAs (Peak ground accelerations) of smaller than 50 cm/s2 and strong ground motions with the observed PGAs of over 50 cm/s2, respectively, by site amplification factors and DNL (Degree of nonlinearity). Secondly, we optimized Q-factors by fitting to the site amplification factor for weak ground motions. Finally, we applied the equivalent linear method using the nonlinear properties derived from the laboratory test to the ground motions at the rock site observed during the foreshock and the mainshock. As a result, the predictions were in good agreement with the observations in the frequency range of 0.1 to 5 Hz, as the GOF (Goodness-of-fit) shows the reliable scores. In this study, we concluded that nonlinearity does not appear much in the superficial layers at the site, but significantly appears in the middle layer with sand (Vs 190 cm/s2) at a depth of about 10 to 20 m. The effective shear strain of about 1% was estimated at the site near the downtown of Kumamoto City, located in the plain during both the foreshock and the mainshock of the (Asano, and Iwata, 2016) Kumamoto earthquake.
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