奥羽脊梁山脈に発生した1970年10月16日秋田県南東部地震災害について
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概要
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A fairly strong earthquake occurred at 2 : 26 P. M., on October 16th, 1970, especially in the northern Ou backbone ranges. The entire Tohoku region, Hokkaido, Kwanto and the Chubu regions were shaked by the earthquake (Fig. 1). The Research Institute of Earthquake, Tohoku University named this earthquake the October 16th Earthquake. The depth of the focus was estimated as about 20 km by the Sendai Meterological Observatory and to be in the southern part of Akita Prefecture at about 60 km south of Mt. Komagatake which suddenly abrupted associated with volcanic explosions and shocks on September 18th, 1970, just about one month prior to the present earthquake. The eruptions amount to about 500 times a day, and the inhabitants in the vicinity fear that some quiet mountain(s) of the Ou backbone ranges may become active. The reactivity of the major tectonic lines in the northern Ou backbone range is suggested by the concentration of the earthquake shock, the strongest (seismic intensity 5) recorded in Shizukuishi-Town and Yuda-Town both in Iwate Prefecture, two towns situated in the depressional area on the east side of the major fault zones of N-S trend and also by the volcanic activity of Mt. Komagatake. The phenomena and disasters of the October 16th Earthquake are considered to have, had intimate relation with the major geo-tectonic lines of the northern backbone ranges. The concentration areas of the earthquake damages were observed at Higashinaruse-Village, Akita Prefecture and Yuda-Town, Iwate Prefecture both aligned in N-S direction and situated along the major tectonic lines such as the Kawafune-Warikurayama fault zone. The major cracks in the pavements due to the earthquake are of N-S trend whereas the destroyed houses, land-slides and road side cracks in the Sakanazawa and Iwaigawa areas in Higashinaruse-Village show random directions where the damages were concentrated but it is expected that the actual direction for a distant was similar to the above and is thought to be the same in the neighbouring areas. From the direction of falling the gravestones, pedestal stones and characteristic cracks of N-S directions, the areas are thought to have been shaken by seismic movements of E-W direction. In otherwords, by the tension of E-W and compression of N-S trend. From the view point of the regional subsurface geological structure, Yuda-Town, one of the areas of the concentrated damages, the disasters were found situated above the zone of the altered Miocene Kawashiri Tuff Member of the Oishi Formation. The main damages in Higashi-naruse-Village are concentrated along the fault zone of N-S direction, corresponding to the southern extension of the Kawafune-Warikurayama fault zone. This extends along the eastern side of the high dip zone of the east wing of the Wagatake and Warikurayama Anticlines, and northwards extends in the direction of Mt. Komagatake, though the exact relationship between the present activities of the volcano and the October 16th Earthquake should be examined in further detail.
- 1971-03-31