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Considering Tachibana Bay
on the western side of
Unzen Volcano as
"Chijiwa Caldera",
we may think that the magma chamber
hides itself about 10km
below the caldera.
This is "Tachibana-Bay Theory"
claimed in 1972
by Prof. K. Ohta,
the former director of SEVO.
Various data obtained
by observations
during the period
of the 1990-1995 eruption
accord well with this theory.
Professor Ohta proposed
the Tachibana-Bay theory
by looking about
the results of research
on the properties
of the hot springs
in Unzen Volcano and
by considering
the data of the earthquake swarm
occurred around 1970.
In the area of Unzen Volcano,
there gush up salt springs
with high temperature
on the west seashore,
sulphur springs
with medium temperature
in the central area,
and carbonic-acid springs
with low temperature
on the east seashore.
The property of
a hot spring depends
on the gas ingredients
coming from magma.
They are separately
dissolved in the groundwater
and change
their own properties
in the process of their movement.
Accordingly the properties
of the hot springs vary
depending on the distance
and/or relative height
from the location of the magma.
Assuming that the hot
springs in the area
of Unzen Volcano
are due to a single
magma chamber, looking about
the difference
in the properties
of these hot springs,
investigating the geological
structure of the Unzen area,
and taking into account
that the hypocenters of
the earthquake swarm
occurred around 1970 have
repeatedly moved
from the deep domain
beneath the western part
to the east side,
we can reach Prof. Ohta's
Tachibana-Bay theory;
namely, the location
of the magma chamber
of Unzen Volcano
should exist about 10km
deep beneath Tachibana Bay.
The data on the distribution
of the hypocenters of the seismic
activities observed
before and during the eruption
(Nov. 1980 - Dec. 1991)
and those on the
ground deformation
in the Unzen area
after the eruption are
shown below.
These data can be
thought to be in accord with
the Tachibana-Bay Theory.
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