著者

Ymashina Ken'ichiro Miura Reiko

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.55, no.3, pp.621-633, 1981-03-07

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A sequence of earthquake activity occurred during May 26-29, 1978, in eastern Shimane, western part of the Honshu Island, Japan. The sequence was divided into four stages in activity; (1) foreshocks, (2) the M=3.9 main shock and its aftermaths, (3) precursory activity before the M=3.7 earthquake, and (4) the M=3.7 largest aftershock and its aftermaths. The aftershock activity of the M=3.7 event was much more active than that of the M= 3.9 event, both in number and in spatial distribution. The activity preceding the M=3.7 event was characterized by (1) a small b value, 0.4, (2) an increase in number of minor earthquakes, (3) successive increase in magnitude, and (4) the southeastward spreading of the active area, leaving a small region with no seismicity. If these had been recognized before the M= 3.7 event, we might have been able to predict the possibility of its occurrence. Although they were only two, the foreshocks of the M=3.9 event might also have suggested a subsequent major event because of (1) their occurrence in a previously inactive area, and (2) the successive increase in magnitude with a small difference (M=1.0 and 1.3). We may also admit a possible correlation between the present M=3.9 and 3.7 sequence and the M=6.1 earthquake which occurred nearby about one week later. In the hypocentral region of the M=6.1 event, the differential strain with respect to the P and T axes (namely twice as much as the shear strain on the fault plane) would have been enhanced by about 2×1O-9 by the preceding M=3.9 and 3.7 events.