Estimation of site effects in the Kumamoto area, Japan, using aftershock acceleration records of the 2016 Kumamoto Mj 7.3 earthquake

Assessing the influence of local site conditions on seismic ground motion is crucial for seismic hazard analysis and earthquake engineering research and applications. This study analyzes site effects in the Kumamoto area, Japan, using 985 high-quality horizontal strong-motion records from 45 aftershocks (Mj = 2.7-4.9) recorded within 24 hours following the 2016 Kumamoto M_j 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 this limitation, we apply the “transfer-station generalized inversion method (TSGI),” a modified GIT, to obtain the site responses for all stations and the average S-wave quality factor (Q_S) in the study area. It is found that Q_S is proportional to frequency in the 0.4-3 Hz range, while at frequencies above approximately 3 Hz, the dependence of Q_S on frequency becomes weak and Q_S can be regarded as constant. However, the results of GIT and TSGI are relative to the reference station, which may itself exhibit site effects. Therefore, we additionally apply a reference-independent technique, i.e., genetic algorithm (GA), to obtain the absolute site amplifications. Our result shows that at frequencies greater than about 1 Hz, the site response of the reference station is substantially 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. The GA method yields accurate absolute site amplification factors for the stations investigated this study, demonstrating the effectiveness of GA in site effect analysis. In addition, we analyze the characteristics of S-wave high-frequency attenuation parameter (κ) in the Kumamoto area, and establish κ models for different site conditions and an empirical κ₀-V_S30 relationship.