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| | Abstract | | Precise gravity measurements were conducted at 2767 stations with spacing of about 300m to investigate 3D bedrock topography of Kurashiki city, southwest Okayama Prefecture, west Japan. The structural model of basement obtained shows complicated configuration of granitic bedrock, which is considered to affect earthquake ground motion characteristics significantly. The effect of 3D seismic ray focusing was examined for plane S-waves, impinging on the bedrock from the supposed source region of the Nankai earthquake, to produce a map showing the degree of concentration of seismic rays arriving at the ground surface. The resultant map was compared to the seismic hazard map recently published by the local government of Kurashiki city, and then it was suggested that, not only the shallow soft soil but also the basement structure should be taken into account for seismic hazard assessment of the study area. |
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