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.

著者

Ymashina Ken'ichiro Miura Reiko

出版者

東京大学地震研究所

雑誌

東京大学地震研究所彙報 (ISSN:00408972)

巻号頁・発行日

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

---

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.1978年5月26~29に,島根県東部でM=3.9の地震が発生した.活動は,(1)前震,(2)M=3.9とそれに伴う余震,(3)M=3.7に先だつ前兆的活動,(4)M=3.7とそれに伴う余震の4つの段階に区分できる.M=3.9と3.7の余震活動を比較すると,後者の方が数,余震域の拡がりともはるかに活発であった.M=3.9発生後の活動経過は,本震と同程度の地震が新たに発生するかもしれないことを示唆していたと考えられる.M=3.7発生以前に,そうしたデータを検討できれば,ある程度の予測が可能だったかもしれない.その根拠は,(1)b値が約0.4とかなり小さかったこと,(2)いったんおさまっていたM=3.9以後の余震活動がふたたび活発化していたこと,(3)同時に,しだいにマグニチュードのより大きな地震が起こるようになってきたこと,(4)活動域が南東方へ拡がり,本震の破壊域と同程度の末破壊領域が推定されたこと,である.M=3.9に先だつ前震は2つ観測されただけであるが,(1)それまで活動のなかった場所に起きたこと(2)後者の方がマグニチュードが大きく,またマグニチュードの差が小さいこと(M=1.0と1.3)は注目される.M=3.9,3.7の地震から約1週間後に,近くでM=6.1の地震が発生している.何らかの関係があったかもしれない.なお,M=3.9,3.7の発生により,M=6.1の震源付近では2×10-9程度の(起震応力を増大させる方向の)差歪みの増加が推定される.