桜島火山の火山性地震の性質について

Abstract

Observations of volcanic earthquakes with three components of seismographs and twosmall tripartite nets were carried out in August, 1962 and in July, 1963 at Sakurajima Volcano.Examining the wave-orbits and analyzing the apparent velocities and the directions ofapproach of waves, we obtained the results as follows: (1) The nature of origin of the observedearthquake is an explosive one, since the clear S-phase, which is to be found in tectonic seismic-waves, can not be observed. (2) The origins of the earthquakes are located beneath the summitcrater (Minami-dake). (3) The seismic waves are divided in two groups, the first group isP-waves having the apparent velocity ca. 2km/sec. And the second group is surface waveshaving their phase velocity ca.1 km/sec.. (4) The approaching directions of P-waves disagreewith those of surface waves. It seems that it is not only due to the crustal structure butrelated with the depth of earthquake origin. (5) Neverthless the lack of SH-motion, the surfacewaves of Love-type are found similarly to the case of the underground nuclear explosion.