著者

松田 時彦 恒石 幸正

出版者

東京大学地震研究所

雑誌

東京大學地震研究所彙報 = Bulletin of the Earthquake Research Institute, University of Tokyo (ISSN:00408972)

巻号頁・発行日

vol.48, no.6, pp.1267-1279, 1971-02-27

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A locally destructive earthquake with magnitude 6.6 occurred in Okumino area, Gifu Prefecture, Central Japan on September 9, 1969 (Fig. 1). The damage area is underlain mainly by Paleozoic formations and Cretaceous rhyolitic pyroclastic deposits, which are partly covered with dissected Quaternary volcanoes (Figs. 2 and 3). A number of Quaternary faults which are dominantly of strike-slip nature, occur in Central Japan which includes the epicentral area (Fig. 6). The epicentral area is located about 10km west of a main fault of the Atera fault system, which is an active left-lateral strikeslip fault of Central Japan. Although a few active faults are in the meisoseismal area, no definite evidence of surface faulting was found along any active fault, except some echelon cracks suggesting a left-slip on a fault line at one locality. The push pull distribution of P waves (Fig. 1), the distribution of aftershocks and the shape of the meisoseismal area (Fig. 5) suggest the concealed seismic fault to be a northwest-trending left-lateral strike-slip fault, which strikes parallel to the Atera fault and has the same sense of displacement. The direction of maximum pressure of the present earthquake corresponds well with the Quaternary stress direction, which has been inferred from the study of active fault systems in Central Japan.