Volume 39 (1991) Issue 1 Pages 79-92
Some characteristics of the earthquake swarm and the volcanic tremors of eastern Izu Peninsula in 1989 are investigated by using the data obtained by the permanent seismological network of the Japan Meteorological Agency (JMA). During the earthquake swarm activity, shallow low-frequency earthquakes were observed from July 5 and were frequently observed from July 10. The epicenters of the low-frequency earthquakes from July 10 are located near the eruption site but the low-frequency earthquake of July 5 occurred to the south of it. Volcanic tremors occurred when the earthquake swarm activity was on the decline. The spectra of the tremors have two peaks, one at about 1Hz and another at a higher frequency. The spectra of the tremors on July 13 associated with submarine eruptions also have two peaks but the predominant frequency of the low-frequency waves is lower than those of the tremors from July 11 through July 12. Four isolated events which occurred from 19:03 through 19:05 on July 13 have similar spectra to those of the tremors associated with submarine eruptions. It is recognized that the continuous tremors associated with submarine eruptions are composed of a number of such events that are superposed on a background low-frequency tremor. From the observation at KMT station, three types of volcanic tremors are recognized during the present activity. The first is high-frequency tremors which seem to be P-waves associated with the submarine eruptions. The second is low-frequency surface waves associated with the explosions. The third is a background low-frequency tremor which seems to be of the same type as the small-amplitude tremors observed after the eruptions through July 21, the predominant frequency of which was 1.0Hz and it changed into 1.5Hz from July 17. The change might reflect a reduction of the source size.