3 - 16 Magma chamber of Unzen Volcano

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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.

Magma chamber inferred from seismic activities
The hypocenters of earthquakes went up repeatedly from the depth under Tachibana Bay to Mt. Fugen. Thereby it was considered that the main magma chamber was identified the hypocenter region deep beneath Tachibana Bay, and the second magma chamber the origin of subsidence of ground ( D in the lower figure) which has been confirmed during the eruption of this time. (SEVO, 1994)

Magma chamber inferred from the data of ground deformation
The origin of the ground subsidence after the eruption was measured to be 10km deep under the point D by a leveling, and 11.5km deep under the point G by a GPS surveying. (Kyoto University and Geodetic Survey Group, 1995)
(GPS = Global Positioning System)

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