Estimation of site effects in the Kumamoto area, Japan, using aftershock acceleration records of the 2016 Kumamoto <i>M</i> j 7.3 earthquake

Summary Site amplification in the Kumamoto area, Japan, is analyzed using 985 high-quality horizontal strong-motion records from 45 aftershocks (Mj = 2.7–4.9) recorded within 24 hours following the 2016 Kumamoto Mj 7.3 earthquake, as observed by 51 K-NET and KiK-net stations. For the generalized inversion technique (GIT), a reference station is required as a standard. In the GIT process, the number of events available for analysis is limited to those recorded by the reference station, and the stations whose site effects can be estimated are restricted to those that record common events with the reference station. To overcome the limitation of the GIT, the ‘transfer-station generalized inversion method (TSGI),’ a modified GIT, is introduced to increase the number of analyzed events and stations. The site responses obtained from GIT and TSGI for the same stations exhibit a high degree of consistency, thereby demonstrating the effectiveness of the TSGI. The discrepancies between the ${{Q}_S}$ estimates of GIT and TSGI can be attributed to the gradual expansion of the region represented by ${{Q}_S}$ as more events and stations are included in the inversion. However, the results of GIT and TSGI are relative to the reference station that may itself exhibit site effects. Thus, a reference-independent technique, i.e. genetic algorithm (GA), is also introduced to obtain the absolute site amplifications. The results show that at frequencies greater than about 1 Hz, the site response of the reference station is significantly lower than the theoretical amplification factor of 2, resulting in an overestimation of the site responses at other stations. When the results of GIT are corrected with the site response of the reference station obtained from GA, these two results agree very well for most of the stations. This indicates that the results of GIT are reliable if the reference station is an ideal surface rock station, and that the GA produces accurate absolute site amplification factors for the stations investigated in this study. In addition, we analyze the high-frequency attenuation characteristics of S-waves in the Kumamoto area, and establish $\kappa $ models for different site conditions and an empirical ${{\kappa }_0}$-${{V}_{S30}}$ relationship.