Volcanic Tremor Accompanying the 1989 Submarine Eruption off the East Coast of Izu Peninsula, Japan.

Abstract

The volcanic tremor accompanying the submarine eruption on July 13, 1989, off the east coast of Izu Peninsula, Japan, was recorded by the temporary observation. The observed tremor contains around 0.8Hz wave and a pair of waves around 7Hz and around 20Hz. A pair of waves characterizes the volcanic tremor accompanying the eruption. It shows intermittent activity in the early period and isolative activity in the latter period. In the middle period, a pair of waves is not discerned. Hypocenters of them are near the erupted position and their depths are shallower than those of earthquakes occurring in this region. A pair of waves has the distinguishing features of the absence of S-phase and of the existence of dilatational and compressional first motions, although its mechanism is not yet fully understood. The waves with the frequency of about 0.8Hz, on the other hand, are observed in all observation periods, which are thought to be the same kind of tremors observed in the period of no eruption. The temporal change in amplitude of the tremor accompanying the eruption is caused by a pair of waves and the wave around 0.8Hz whose amplitude is increased by the occurrence of a pair of waves.