Seismic Swarm Activity in and around the Izu Peninsula Preceding the Volcanic Eruption of July 13, 1989

We have investigated features of the seismics 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. Earthquakes associated with these two groups are related respectively to possible magma intrusions and to the largest earthquake occurring on Jult 9 and its aftershock sequence.