Ground Motion Characteristics in Anamizu Town Obtained from Earthquake and Microtremor Observations

During the 2007 Noto Hanto earthquake, a K-NET station ISK005, which is located in Anamizu town and approximately 19 km far from the epicenter, recorded the ground velocity larger than 100 cm/s. A set of observational study is carried out to investigate spatial variation of ground motion amplification characteristics in Anamizu town. Firstly, the spatial variation of the amplification was observed by aftershock observations along a temporary linear seismic array across Anamizu town. In the center of the town, the spectral amplification factor is 10 to 20 between 1 Hz and 2 Hz with respect to the rock site. Then, dense single-station microtremor observations were carried out at 147 sites with average spacing of 100 m in Anamizu town to see the spatial variation in thickness of low-velocity layers. The peak frequency of the microtremor H/V spectral ratio varies from 0.8 to 2.0 Hz in the town. The velocity structure model of shallow portion in Anamizu town is estimated from the mircrotremor H/V spectral ratios. The thickness of low-velocity layers (VS =70 to 100 m/s) changes along the Omata and the Manai rivers. Finally, a three-dimensional ground motion simulation is conducted using the obtained velocity structure model in order to see relationship between shallow sedimentary layers and ground motion amplification in Anamizu town. The peak velocity in the frequency range below 2.5 Hz is three or four times larger in the area around ISK005, where the thickness of low-velocity layers is approximately 10 to 25 m, than that in the rock side. It could be concluded that the ground motion amplification characteristics in the frequency range between 1 Hz and 2 Hz is mainly controlled by the existence of such low-velocity sedimentary layers.