Volcanic Tremor Associated with the 1989 Submarine Eruption of Ito, Japan.

Abstract

Volcanic tremor accompanied by sound was recorded on July 11 and 13, 1989, by two strong-motion accelerographs located at the east coast of Ito City in the Izu Peninsula, Japan. The tremor on July 13 was resulted from successive explosion earthquakes during a submarine volcanic eruption off Ito. Through a comparison of the tremor episodes on July 11 and 13, we found the same types of sources. Therefore, it is reasonable to postulate that volcanic explosions, small-scaled compared with those on July 13, occurred during the late evening of July 11, though no surface evidence was reported. The tremor accompanied by sound on July 11 and 13 was characterized by both high-frequency (3-30Hz) and low-frequency (0.5-1Hz) motion. The high-frequency motion corresponded to water waves and low-frequency one to both Rayleigh and Love waves. Several types of volcanic earthquakes were observed by temporary measurements during the late evening of July 11 and the early morning of July 12. These observations suggest that volcanic activity became intensified during this period and was strongly related to the creation of the Teisi Knoll.