Effects of Surface Geology on Seismic Motion

Site amplification factors at period ranges of 0.1-0.2, 0.2-0.3, 0.3-0.5, 0.5-1, 1-2, and 2-10 s were determined for 27 stations of KiKnet and K-NET in and around the source area of the 2008 Iwate-Miyagi Nairiku earthquake. The ratio of the geometric mean values of the two maximum horizontal amplitudes of acceleration and velocity at the uphole to that at the downhole is considered as the amplification factor of KiK-net stations. The bedrock borehole ground motions of KiK-net were used as the reference motions to get the amplification factors for K-NET stations. Amplification factors of 3 to 33 at shortperiod ranges of 0.1 to 0.3 s are observed at many stations such as AKTH04, IWTH22, MYG004, and so on. The stations of MYG004, MYG005, and MYG006 have shown amplification factors of 3 to 8 at longperiod ranges of 1 to 10 s. On the other hand, AKT023, IWT010, and IWT011 stations show flat response spectra compared to hard rock sites. Larger acceleration amplification than velocity amplification is observed at stations located on deep soft soil structures such as MYG004, MYG006, and YMT006. The results are compared to other results by H/H and H/V spectral ratios of the mainshock and five aftershocks data.