Waveform Analysis of Volcanic Tremors Associated with the July 1989 Volcanic Activity off the East Coast of Izu Peninsula.

Abstract

Volcanic tremors have occurred since July 11, 1989, off the east coast of Izu Peninsula, and submarine eruptions were visually observed on July 13, 1989. Characteristic tremor (isolated events) occurred after very large amplitude tremors (main phase), which were associated with the submarine eruptions. The isolated events are analyzed instead of the main phase, since they have similar waveforms and spectral shapes (broad peaks at 0.4-0.8Hz). The main phase tremors consist of a succession of events similar to the isolated events. It was found that the radial and vertical components of the main phase and isolated events are those of Rayleigh-type surface waves and the transverse components are Love-type waves. Group velocity analysis reveals that a very thin and low-velocity layer (VS=0.6km/s, thickness=0.1km) exists on the top of the underground structure between the source and the station. Inversion is made for revealing the source mechanism of the isolated events. The result suggests that the source mechanism is an anisotropic explosive type and the source time function is the delta type with the duration time of 2s.