抄録
Fukuoka District Meteorological Observatory has continued seismic observations at Satsuma-Iojima volcano since September 1997. Of the daily number of seismic events at the volcano, which sometimes exceeded 100, several different types have been observed. Hypocenters of A-type earthquakes mostly existed in a shallow area within the volcanic island. Numerous peculiar earthquakes that have waveforms different from usual volcanic earthquakes were observed with the ascent of seismic activity. They have an emergent phase that precedes the appearance of the high-amplitude main phase by 5-8 seconds and consists of weak seismic waves increasing gradually in amplitude. This type of earthquakes is considered to be excited by a bubbling of gas and a succeeding shear fracture within the highly viscous rhyolitic magma.
