桜島火山における地震観測 : 高度差を考慮に入れた多点観測

抄録

An improved method is introduced to analyze travel time data from a seismic array composed of stations at various elevations, and is applied to the data of volcanic earthquakes or tremors occurred in Sakurajima Volcano. By this method, arrival azimuths and apparent velocities of seismic waves, and wave velocities in the surface layer are estimated with their errors. Several events are observed with anomalously low wave velocity in the surface layer. This is probably attributed to apparent late arrival times at the station which is highest in altitude and is nearest to the Minami-dake Crater of the Volcano. From the analysis of explosion earthquakes, the existence of such low wave velocity is denied and P wave velocity in the surface layer is estimated as 500m/sec or over. Predominant frequencies of waves are 4 to 6 Hz for the events of which relatively dominant initial phases are recognized. 0n the other hand, so-called “B-type” earthquakes or continuous tremors have the frequency around 2.5 to 4 Hz.