Undersea Seismic Observation of 1989 Volcano-Seismic Activity, off Ito, Izu Peninsula, Central Japan

Abstract

Volcano-related seismicity associated with the July 13, 1989, submarine eruption off Ito, Izu Peninsula, central Japan, began with a swarm activity on June 30. Two ocean bottom seismometers (OBSs) deployed near the future eruptive vent observed several precursor explosive activities during the two days preceding the surface eruption. OBS records show precursor waveforms, called "spindle packets, " which resemble those during the surface eruption. After the eruption, a sonobuoy was repeatedly deployed above the vent and, finally a cable OBS was deployed 1km NW of the vent. Isolated volcanic explosions show high frequency sound arrivals and low frequency (1.2Hz) coda on the vertical seismometer; this coda, which has a group velocity of approximately 330m/s, is identified as a Rayleigh wave. The high frequency arrivals result from the propagation of sound waves through the water column. Hypocenters before the surface eruption trended WNW-ESE and had depths between the ocean bottom and 6km. After the eruption, earthquakes occurred around the vent with depths a few kilometers below the surface. Sonobuoy records show an X phase. Assuming the X phase is generated by the conversion of an S wave to a P wave at the magma surface, the depth to the top of the magma is estimated as 1km below the hypocenter. Quiet vent site hydrothermal activity was observed one and half months after the surface eruption. Although rising bubbles were seen, no earthquake was located beneath the vent site.