九重火山で観測されたS波の先行相

抄録

Kuju Volcano is located in the center of Kyushu Island, Japan. This volcano is a complex volcano formed of andesitic lava domes. A small phreatic eruption occurred at the northeastern side of Mt. Hossho in the center region of the volcano on October 1995. Since then, a seismic network has been installed in this area. Clear precedent phases of S wave are found in many seismograms recorded by the network. Precedent phases show following features; 1) the arrival time difference between the precedent phase and S wave is independent of hypocenteral distance, 2) the phase is significant in a vertical component, 3) the amplitude of the phase is lager than that of P wave, 4) the phase appears for the case of which the seismic station is situated in the direction of relatively large amplitude of S wave from hypocenter. Because of these features, their precedent phases are concluded to be S-P converted phase which is excited in the boundary of S wave velocity. Conversion points distributed within the depth of 0.5 km to 1.5 km from the surface beneath the western region of Kuju Volcano, which are estimated by means of a travel time analysis. Conversion points correspond to a clear boundary of elastic velocity in well data which were obtained around this area.