| 講演要旨(和文) | | 岡山県倉敷市の地震防災対策に資することを目的として，倉敷平野と水島工業地帯を含む高梁川下流域両岸の南北18km，東西13kmの地域において，約300m間隔，2767地点で精密重力測定を実施し，3次元の花崗岩質基盤構造を推定した．その結果，(1)倉敷平野北部に東北東-西南西方向の深度100m超の谷が存在し，高梁川西岸の玉島地区北部に続いていること，(2)水島地区東部にも南北方向の谷が存在することなどが明らかになった．次に，基盤構造が地震動特性に及ぼす影響をみるために，想定南海地震震源域から基盤に入射する平面S波の3次元的な焦点効果を考え，地表における波線集中度を見積もった．その結果を「倉敷市地震ハザードマップ」(2008)と比較・検討し，「工学的基盤」以浅の地盤情報だけでなく花崗岩質基盤の影響をも考慮した，よりきめ細かい地震ハザードマップの作成が必要であることを指摘した． |
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| | 講演要旨(英文) | | 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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