| 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.
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Change in the ground tilt
before and after
the growth of a lava dome
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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.
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The magma seems
to ascend through
a dyke at depths of
few hundreds meters
beneath the crater.
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Tilt variation associated
with an explosive eruption,
and the suggested
movement of magma mass
beneath the volcano
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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.
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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.
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A bubble-tube tiltmeter |
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The cylindrical cover: 10 cm in diameter.
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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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