Notable Phenomena Registered by the Borehole Type Volume Strainmeter at Izu-Oshima before and after the 1983 Miyakejima Eruption

Abstract

The volume strainmeter, installed at the bottom of a drill hole at Izu-Oshima, an insular volcano, Izu Islands, Japan shows the characteristic phenomena such as pauses of its contraction tendency and of the intermittent occurrence of minor step-like recordings, which have been detected since the beginning of the observation in 1981, before the taking place of the 1983 October 3 Eruption of Miyakejima, about 80 km SSE of Izu-Oshima. Besides, the amplitude of earth tide recorded by the strainmeter has shown continuous increase almost without phase variations during the observation period. It is confirmed that there is no alteration of the characteristics of the instrumentation on sensitivity etc., through the analysis of the strainmeter’s response against variations of atmospheric pressure and calibrations of the amplification of the system. The phenomena are not seen at the neighbouring strainmeter stations, and there are no relationships between the notable phenomena and the variations of atmospheric pressure, ocean tide level and the temperature at the hole-bottom. It may be possible that the notable phenomena detected by the strainmeter at Izu-Oshima are generated by proceeding volcanic activity in a broad sense at Izu-Oshima or the alternative change of the nature of volcano edifice as the medium materials. However, it will be also possible that there are close relationships between the characteristic phenomena detected by the strainmeter and the active tectonic process in the region of Izu Islands, which shows frequent occurrences of seismic events and the outbreak of volcanic eruption since the end of 1982.