抄録
We have investigated features of the seismic swarm activity preceding the volcanic eruption of July 13, 1989, at the Teisi Knoll located off the east coast of the Izu Peninsula. The whole activity can be divided into two groups: the eastern group around off coast of Ito City and the western group near Ito City. The eastern activity consists of earthquakes primarily related to possible magma intrusions by taking several features into account: burst-like activities with quasi-periodic occurrences, rapid epicentral migrations with short steps, and the coincidence of linear epicentral trends with the tectonic compressional direction. On the other hand, the western activity may have been caused by the largest earthquake occurring on July 9, and its sequence of aftershocks. Seismic activity comprising both magma-originated earthquakes and tectonic earthquakes is recognized as one of several typical seismicity patterns observed around the Izu area.
