Observations of the Crustal Deformations at Hakamagoshi, Sakurashima Volcano

Abstract

Sakurashima Volcano which erupted in October 1956 after a long quiescent period is still active accompanying more than one hundred explosions since then. From August 1956, we began observations of crustal deformations at Hakamagoshi, western part of the volcano, by means of a sillica tube extensometer. Results of the observation from August to October were compared with daily mean sea level, rainfall, air temperature etc. Although we have experienced several times of volcanic eruption during the period of our observation, it seems that there is no marked correlation between the deformation of the earth’s surface and the eruption. But there is a very interesting and important fact which seems to be relevant to the structure of the volcano. The fact is that the time lag between the oceanic tide and the crustal deformation is about 4 hours and a half, which is abnormally larger than those hitherto observed at several other places. Furthermore, when visited by three typhoons during the period the time of the maximum extension of the earth’s surface was from 10 to 20 hours after the time of the highest high water of the sea. Thus the observed time lag of the earth’s deformation takes place in depending on the period of external disturbances. We shall not enter in detail the mechanism of such observed phenomena but discuss in the forthcoming paper.