Site amplification factors densely sampled over Okayama Prefecture, Japan

We extract site amplification factors at 120 strong-motion stations in Okayama Prefecture, southwest Japan. The area of Okayama prefecture is about 100 km x 90 km, and the average interval of the stations is roughly 5 km. We aim to elucidate the spatial variation of the site amplification factor over Okayama Prefecture from such dense station records. The stations belong to the K-NET, KiK-net, the JMA Shindo-kei network, and the seismic intensity network of Okayama Prefecture, Japan. We exploit the records of eight crustal and intra-slab earthquakes including the 18-June-2018 Osaka earthquake (M_JMA 6.1). We apply the spectral inversion technique developed by Kawase and Matsuo (2004) to extract the site amplification factors from the strong-motion records.

In the inversion results, we obtained the site amplification factors for all the 120 sites in the frequency range of 0.3 to 10 Hz, and found some remarkable features as follows. The area showing amplification values larger than 10 at a predominant peak of about 1-2 Hz is localized near southern coast of Okayama. This area faces the Inland Sea, and is a drained land or coastal lowland. The largest amplification values of almost 10 at a predominant peak frequency around 2 Hz are found in flood plain area of southern part of Okayama. We also found that most of stations in central and northern part of Okayama have low levels of amplification over the frequency range we analyzed. This area is on a large stable landmass, called Kibi Plateau. However, the amplification values of about 20 at lower peak frequencies under 1 Hz are observed at stations located in a local valley plain within the Kibi Pleteau. Soft and thick diatomaceous sediments had deposited in this local area, which was a lake about 350,000 years ago.

Acknowledgements: We used the strong-motion records observed at stations of the K-NET and KiK-net of the NIED, the JMA Shindo-kei network, and the seismic intensity network of Okayama Prefecture.