大島における重力変化の研究(1)

抄録

A gravity survey in Ooshima Island was carried out by the Geographical Survey Insti tute in September 1967 for detecting the secular variation of gravity in connection with the volcanic activity. The remeasurements by means of the LaCoste and Romberg gravitimeter No. 29 have been made at about thirty stations, among which 4 stations were previously occupied two or three times from 1950 and other stations were first occupid in 1963. Assuming that the gravity value at the station of Ooshima Meteorological Observatory has been constant during the period from 1950 to 1967, the maximum gravity decrease of about 0.9 mgals has been found between this period at Gozinka tea house near the crater, and the distribution of gravity decrease seems to become smaller in proportion to the distance from the crater. This result might indicate the sinking of magmatic substance under the volcano in recent years. Considering the observed gravity variations and other results of geophysical observations, it may be concluded that after the great explosions of 19501951, volcanic activity at Mt. Miharayama, Ooshima Island, has been declining.