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of 65 m high accompanying numerous faults. We made repeated measurements of ground temperature, Self-Potential
(SP) and electrical resistivity, in order to clarify the mechanism of development of the newly formed geothermal
field on the fault zone. Prior to the expansion of the geothermal field, we detected a resistive zone at
the center of the geothermal zone and it supposed to evidence that the zone involving dry steam phase had been
formed beneath the fault zone. A rapid expansion of the geothermal field followed along the fault zone away
from the craters. The place of maximum amplitude of the SP field also migrated following the expansion of the
high ground temperature zone. The high resistive part has shrunk as a consequence of the progress of condensation
to warm the surroundings. Based on the observations, we delineated the process of the hydrothermal circulation.
Considering the topographic effect of the SP field observed on the highly permeable zone in the Usu somma,
the potential flow along the slope of the soma was expected to play an important role to promote the rapid
expansion of the geothermal field and the migration of the most active part.
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