T Waves Associated with Submarine Volcanic Eruptions in the Marianas Observed by Ocean Bottom Seismographs.

Abstract

In September and December of 1989, swarms of T waves were observed by ocean bottom seismographs (BSOBS) off the Boso Peninsula of Japan, by a SOFAR hydrophone array near Wake Island, and by a station at Rangiroa. Using arrival times observed by these networks, the source location of the T waves was determined to be in the northern Marianas, where submarine eruptions had been observed frequently in recent years.The T waves were not accompanied by corresponding body waves. The T waves had shorter duration (from a few to ten seconds) and higher prominent frequency than those of tectonic earthquakes which also occurred in the northern Marianas. The amplitude variation of the T waves observed by BSOBS showed a different pattern from those from tectonic earthquakes. This is a manifestation of a difference in the spectral contents between them. The spectra of these T waves showed conspicuous harmonic peaks. These characteristics are similar to those of volcanic T waves as previously reported. Therefore we concluded that they are excited by submarine volcanic eruptions.From the detailed analysis of the T wave events, this volcanic activity is considered to proceed through three stages: the first which was not so explosive, the second which was composed of intermittent explosive eruptions, and the third which was small and similar to the first. The observed fundamental frequencies of the T waves varied with time. This could be caused by changes in the eruption site or in the size of the area where reverberations occurred.