小地震に対する空白域の形成と発生率および<i>b</i>値の時間的変化

Abstract

Spatio-temporal seismic pattern for shallow and small events which occurred during 1974-1980 in the Shikoku District, western part of the Median Tectonic Line in Japan, is investigated to elucidate similarity of precursory phenomena in seismic pattern between relatively small earthquakes and greater events. The biggest event during 1974-1980 in the district is on Sep. 8, 1977 and its magnitude is 4.6; this was followed by some aftershocks but not preceded by forerhocks in a narrow sense. The main event was obviously associated with “seismicity gap” around the focal zone before 3 months of the event. Size of the gap is 30-40km in diameter. Before the seismic gap, there was “precursory swarm” not only in an adjacent area of the impending main event but also around the gap area. The precursory swarm continued approximately 2 years before the seismic gap. Spatial migration of activity in the swarm area is also seen. In another word, a sort of “seismic gap” before the precursory swarm could also exist. A linearment of seismic activity along a strike of composite focal mechanism solution is found in a period during the time of precursory swarm.Temporal variation of b-value of the seismic zone is also shown. b-value anomalously increased up to 1.8 during the first half of the precursory swarm period. The high b-value decreased in the last half down to 0.6, which followed the seismic gap that prepared the main event.