著者

佃 為成 大木 靖衛

出版者

東京大学

雑誌

東京大學地震研究所彙報 (ISSN:00408972)

巻号頁・発行日

vol.69, no.1, pp.19-38, 1994

被引用文献数

4

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A small earthquake of M4.5 severely attacked Tsunan Town, Niigata Prefecture on December 27,1992. Unexpectedly destructive damage to school buildings and gymnasiums was mainly attributed to an extremely shallow focal depth of 2-3km within the Tertiary layer, which is as thick as 5-6km. The accuracy of the depth determination is estimated to be within 1km. A high Vp/Vs ratio of 2.0 measured locally also supports the shallow focal depths. We have no seismometric reports of a shallow earthquake with magnitude greater than 4 whose fractured region is entirely within the Tertiary layer other than this paper. The epicentral region is tectonically complicated, where the Shinano River Seismic Zone and the Naoetsu-Choshi Line deduced from Bouguer gravity anomalies cross each other. Probably due to this complexity, the earthquake generating stress of the mainshock indicates NNW-SSE compression, while the major aftershocks are generated by WNW-ESE compresional stresses which are in harmony with the surrounding stress field. This earthquake is followed by a comparatively small number of aftershocks, and preceded by microearthquake activity about 3 months before and during the 12 hours before the mainshock. The aftershock region is separated into three spots, one of which was the source region of the precursory activity 3 months before.新潟県南部の長野県境に近い中魚沼郡津南町にて,1992年12月27日11時17分,M4.5の地震が発生した.小さい地震の割に中学校や小学校の建物などに大きな被害を及ぼした.被害地域は逆巻地区を中心に1km2の範囲に限られている.強い震動の原因は,震源の深さが2~3kmと非常に浅い直下地震であったためである.地震発生滅は花崗岩層の上の第三紀層内である.精密な震源は新潟大学積雪災害研究センターによる余震の臨時観測と東大地震研究所のテレメータ観測網のデータを組み合わせ,媒質の不均質性を補正して得られた.