Excitation mechanism of large-amplitude volcanic tremor associated with the 1989 Ito-oki submarine eruption, central Japan

Abstract

Unusually large-amplitude volcanic tremor was observed in association with the 1989 Ito-oki submarine eruption off the east coast of Izu Peninsula, central Japan. The largest-amplitude tremor occurred on July 13, 1989, and was studied by using digital seismic data (National Research Institute for Earth Science and Disaster Prevention), in the distance range of 6–250 km. Spectral analysis shows that the seismograms have two kinds of predominant spectral peaks, low-frequency peaks around 1 Hz and middle-frequency peaks between 2 and 7 Hz. The relative amplitude ratios of these peaks show distinctive spatial variation depending primarily on the distances from the crater. Low-frequency waves are predominant at close stations (distances less than 50 km from the crater) similar to typical low-frequency volcanic tremor, while at more distant stations the middle-frequency waves are more prominent than low-frequency waves. Propagation velocity analysis for narrow-frequency bands on the basis of correlation of envelopes of seismograms reveals that the middlefrequency waves are compressional body waves and the excitation of shear wave was ineffective, suggesting an explosive or implosive source as the simplest source model. In contrast to the middle-frequency waves, our analysis showed no evidence for the low-frequency waves to have propagated with any body wave velocity. Consideration of the spatial pattern of both waves suggests that surface waves were trapped in sedimentary layers from the shallow source and that no lowfrequency oscillations dominated at the source.