[S-SS10] Strong Ground Motion and Earthquake Disaster

[SSS10-P18] Site amplification factors at highly dense positions over Okayama Prefecture

Ryoji Kaneshima², *Tomotsugu Watanabe¹, Hiroshi Takenaka¹, Keiichi Nishimura³, Mitsutaka Oshima⁴, Hiroshi Kawase⁵ (1.Okayama University, 2.Okayama University (former), 3.Okayama University of Science (former), 4.Shimizu Corporation, 5.Kyoto University)

Keywords:site amplification factor, strong motion, spectral inversion, Okayama Prefecture

We updated site amplification factors at local strong motion stations densely distributed over Okayama Prefecture, southwest Japan. The strong motion stations belong to K-NET, KiK-net of NIED, JMA, Okayama University of Science, or the seismic intensity network of Okayama Prefecture. We employed a spectral inversion technique to extract the site effects at the stations from the observed seismic records. We used 1470 three-component acceleration records of 20 events with 4.0 ≦ M_JMA ≦ 6.3. The target frequencies range from 0.1 to 20 Hz. We selected a KiK-net station (OKYH05), located in the center part of Okayama Prefecture as a reference station of the inversion, whose site amplification was estimated by Nakano et al. (2014). The evaluated site effects show the amplification by the sedimentary layers above the seismic bed rocks. It was found that the site amplifications obtained here have characteristics as, follows. The site amplifications at many stations in the southern part have a peak between 1 and 3 Hz, and their shapes show gentle curves. Most stations located in the center or northern part of Okayama have the site effects representing a flat portion without peaks and a gradually increase over 1 Hz. At some stations around Hiruzen plateau or Kojima Bay, their amplifications have a peak over 10 times at lower than 1 Hz, which has a good agreement with other studies such as the geological surveys and gravity explorations.

Acknowledgements: We used the strong motion records observed at the seismic intensity stations of the JMA and the seismic intensity network of Okayama Prefecture and the strong-motion stations of the K-NET and KiK-net of the NIED and Okayama University of Science. This study was partially supported by the Core-to-Core Collaborative research program of the Earthquake Research Institute, The University of Tokyo and the Disaster Prevention Research Institute, Kyoto University.