Since August 2000, we have recorded the total intensity of the geomagnetic field at the summit area of
Kuchi-erabu-jima volcano, where phreatic eruptions have repeatedly occurred. A time series analysis has
shown that the variations in the geomagnetic field since 2001 have a strong relationship to an increase in
volcanic activity. These variations indicate thermal demagnetization of the subsurface around the presently
active crater. The demagnetization source for the early variations, until summer 2002, was estimated at
about 200 m below sea level. For the variations since 2003, the source was modeled on the basis of the
expansion of a uniformly magnetized ellipsoid. The modeling result showed that the source is located at
300 m above sea level beneath the crater. We carried out an audio-frequency magnetotelluric survey with
the aim of obtaining a relation between the demagnetization source and the shallow structure of the volcano.
A two-dimensional inversion applied to the data detected two good conductors, a shallow thin one which is
restricted to a region around the summit area, while the other extends over the edifice at depths between
200 and 800 m. These conductors are regarded as clay-rich layers with low permeability, which were
assumed to be generated through hydrothermal alteration. The demagnetization source for the early
variations was possibly located at the lower part of the deep conductor and the source after 2003 lies
between the two conductors, where groundwater is considered to be abundant. Based on these results, as
well as on seismological, geodetic, and geochemical information, we propose a heating process of the Kuchierabu-
jima volcano. In the initial stage, high-temperature volcanic gases supplied from the deep-seated
magma remained temporarily at the level around the lower part of the less permeable deep conductor since
the ascent path had not yet been established. Then, when the pathway developed as a result of repeated
earthquakes, it became possible for a massive flux of volcanic gases to ascend through the conductor. The
high temperature gases reached the aquifer located above the conductor and the heat was efficiently
transported to the surrounding rocks through the groundwater. As a consequence, an abrupt increase of the
gas flux and diffusion of the heat through the aquifer occurred and the high-temperature zone expanded.
Since the high-temperature zone is located beneath another conductor, which acts as caprock, we assume
that the energy of the phreatic explosion is accumulated there.
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