4 - 6 Tilt measurement

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Tilt measurement is to measure changes in the tilt of the Earth' surface, which is one of methods to know the ground deformation.
We are now able to detect very slight changes in the ground tilt of the volcano.
Our highly sensitive bubble-tube tiltmeter is so small in size that it is very suitable for the filed observations.
We put tiltmeters around the Fugendake crater and succeeded in obtaining signs indicating repetitive rise of magma mass through the vent to the crater (see Fig. 1).
We also observed how the pressure changed inside the volcano during explosions (see Fig. 2).
Furthermore, based on tiltmeter data, we succeeded in continuous estimation of the magma volume supplied to the crater. This enabled us to get immediate grasp of activity levels of the volcano.

Change in the ground tilt before and after the growth of a lava dome

The tiltmeter at 700 meters to the west of the crater recorded a lifting of the summit of the Fugendake cone about a week before the appearance of a lava dome. One micro-radian change of the tiltmeter corresponds to 1 millimeter uplifting or subsidence of the ground one kilometer ahead.

The magma seems to ascend through a dyke at depths of few hundreds meters beneath the crater.

Tilt variation associated with an explosive eruption, and the suggested movement of magma mass beneath the volcano

Associated with an explosive eruption on June 8, 1991, the quick subsidence of the summit and the subsequent gentle recovery in several hours were both indicated by the tiltmeter.

In association with explosive eruptions, a total of about 40,000 m3 of magma including compressed volcanic gas was discharged from a chamber at a depth of 600-800 meters. This chamber was, however, recharged in following several hours with magma of about two thirds of this volume from the depth.

Top A bubble-tube tiltmeter
The cylindrical cover: 10 cm in diameter.

The principle of a bubble-tube tiltmeter is simple. The bubble-tube tiltmeter consists of electrolysis fluid and an air bubble holded in a glassware. Three electrodes A, B and C are set in the glassware as shown in Fig. 3. When the ground is inclined as shown in Fig. 3, the bubble in the glassware moves to the right so that the electrode A becomes more covered by the electrolysis fluid, and the electrode B becomes more covered by the bubble. This results in the reduction and the increase of the electric conductivity between the electrodes B and C and between the electrodes A and C, respectively. We can detect the change in the ground tilt in this way.

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