[S-SS11] Strong Ground Motion and Earthquake Disaster

[SSS11-04] Estimation of site amplification and S-wave velocity structure in Kumamoto Prefecture based on spectral inversion analysis of strong motion records

*Tomomi IIDA¹, Hiroaki Yamanaka¹, Kosuke Chimoto¹, Daiki Shinoyama², Seiji Tsuno³, Masahiro Korenaga³, Nobuyuki Yamada⁴, Hiroe Miyake⁵, Tatsuo Kanno⁶, Michiko Shigefuji⁶, Masayuki Yoshimi⁷, Shinichi Matsushima⁸, Hiroshi Kawase⁸, Kimiyuki Asano⁸, Tomotaka Iwata⁸ (1.Tokyo institute of Technology, 2.Central Research Institute of Electric Power Industry, 3.Railway Technical Research Institute, 4.Kochi University, 5.The University of Tokyo, 6. Kyushu University, 7.National Institute of Advanced Industrial Science and Technology, 8. Kyoto University)

Keywords:Spectral inversion, The 2016 Kumamoto Earthquake, Site amplification, S-wave velocity

In this study, we estimated site amplifications and S-wave velocity structures for shallow and deep soils using ground motion records in Kumamoto, Japan. We first apply a spectral inversion technique to the earthquake records to estimate the effects of source, path, and site amplification. And then these site amplifications are inverted to subsurface S-wave velocity. We used the data of 256 earthquakes obtained at strong motion stations of K-NET, KiK-net, seismic intensity-meters stations by local governments, and aftershock observation stations by Yamanaka et al. (2016). The estimated Q value of the propagation path is modeled as 72f^0.83 and it is similar to that from previous studies (e.g. Somei et al. 2019). The source characteristics can be approximated with the omega-square model. The site effects in the plain area show large values in a frequency range of 0.5-2.0Hz. At a high frequency above 5Hz, the site amplifications in the mountain are larger than those in the plain area. The amplifications of 0.5-1Hz are larger in the central area of the Kumamoto plain than in the other area because of the large sediments. The amplifications in Mashiki show different characteristic features around 2Hz due to the various thickness of the shallow soils. The Q value of deep soils in the plain area tends to be larger than in the mountainous area.



Acknowledgements

We used the data from the aftershock observation conducted as a part of the Comprehensive Study on the Futagawa-Hinagu fault zone after the 2016 Kumamoto earthquake, MEXT.