地盤変動連続観測で捕捉された山頂噴火の前駆現象

Abstract

An underground tunnel was constructed in the lava dome 2.8 kilometers northwest ofthe summit crater of Sakurajima Volcano in order to collect the tilt data and strain data ofhigh quality and to detect the ground deformation associated with the summit eruption. A2-component water tube tiltmeter and a 3-component extensometer were installed in August1985. The precursor of the summit eruption was observed by the tiltmeter and the extenso-meter for 125 explosions, which corresponds to about 60% of the events, including minor ex-plosions, during the period of the observation from August 1985 to January 1986. Upwardtilt in the radial direction to the summit crater ranged from 10 minutes to 7 hours in dura-tion and from 0.01 to 0.2 microradians in amount. The extension in the tangential directionand the extension or minor contraction in the radial direction ranged from 10 8 to 10 7 instrain. After the eruption, the downward tilt in the radial direction and the contraction inevery direction were observed. The depth of the origin which generates the inflation of thesummit is estimated to be from 2 to 6 kilometers beneath the active crater by comparing theobserved ratio of the dilatation to the tilt (from 0 to 1.4) with the theoretical value calculat-ed according to Mogi's model.Ground tilt data in the radial component were continuously monitored and processed inreal time by a computer in order to judge the stage of the eruptive activity from the rate oftilt change and tilt duration and to forecast the explosive eruption automatically. The prac-tice of the preliminary program has been done for a month. The statistical results of the ex-periment indicate that the number of critical stage of activity judged from tilt data is 27 outof 39 explosions, 4 out of 5 short eruptions, and 7 out of 22 continuous eruptions. Therewere 21 critical stages reported when no significant event occurred. Most of these excep-tions were caused by temporary relaxation in tilt due to minor eruptions, for example, the gasemission.