Some Natures of Volcanic Earthquake at Sakura-jima Volcano （Part 1）

Abstract

The observation of volcanic earthquakes with three components of seismographs and a small tripartite net was carried out in August, 1962 at Sakura-jima Volcano. The volcanic earthquakes observed were closely related to the activity of the volcano. Examining the wave-orbits and analyzing the apparent velocities and the directions of approach of waves, we obtained the results as follows: (1) The nature of origin of the observed earthquake is an explosive one, since the clear S-phase which is to be found in tectonic seismic-waves could not be observed. (2) The origins of the earthquakes are located beneath the summit crater (Minami-dake). (3) The approaching directions of P-waves disagree with those of surface waves. It is due to the inclination of the tripartite-net plane to the horizontal and it is possible to estimate the depth of origin from the difference between the both directions under the knowledge of the crustal structure of Sakura-jima Volcano. (4) Neverthless the lack of SH-motion, the surface waves of Love-type are found similarly to the case of the underground nuclear explosion. (5) The effect of the feature of ground near the observing site on surface wave-orbit is clearly seen i.e. the polarized plane of Love-type wave trends along the ground surface. (6) The predominant period of seismic waves at Sakura-jima Volcano is in 0.3〜0.4sec. range. The apparent velocity of P-wave is ca. 1.9km/sec. and 0.8km/sec. for the phase velocity of surface wave (Rayleigh-type).