日本海に発生する浅発小地震の発震機構

Abstract

Seismogenic stress field off the western coast of the Tohoku District, north-east Japan, is clarified by means of focal mechanism of small-to-moderate size earthquakes using polarity data of P-wave first motion. Foreshocks and aftershocks of some moderate size events show irregular patterns of direction of the maximum compressive stresses and of faulting style. Exclusive of such foreshocks and aftershocks, the directions of the maximum compressive stresses are concordant with those derived from two major events in the area, the 1964 Niigata earthquake and 1983 Akita-oki (Central Japan Sea) earthquake. East-west direction of the maximum compressive stress is dominant off the western coast of the Tohoku district. The direction gradually changes to northwest-southeast toward the northern edge of the Itoigawa-Shizuoka Tectonic Line.The isolated event of M 5.1 of May 16, 1975, which occurred in the southern edge of the middle part of the Japan Sea Basin appears to be peculiar in points that oean bottom topography in the source region shows no traces of seismotectonic deformation and that the maximum compressive stress of the event points to NNW-SSE, which is different from those of the seismogenic stress field off the western coast of the Tohoku district.