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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
be gradually magnetized with time along the present geomagnetic field. Heating of a rock body, even in
a range far below the Curie temperature, may cause dramatic decrease of the relaxation time, resulting
in considerable amount of TVM acquisition (e.g. Dunlop, 1983). The TVM therefore seems more
consistent with the temperature information deduced from the magnetotellurics than simple cooling
magnetization process. * It is worth considering such TVM effect on a rock with initial remanent
magnetization of opposite sense to the present geomagnetic field (i.e. reversely magnetized body). In
such a situation, magnetic field change due to TVM will be similar to the one due to thermal
demagnetization (TD). TVM effect can, however, be even more efficient, especially in a low temperature
range, since it rotates the reversed component in place of simple erasing as in the case of TD. It is still
an open question whether the magnetization of the upheaved mound of Usu is normal or reversed.
However, the latter is probable since the relevant area is situated at the geological boundary between
the reversed Tertiary volcanic basement and recent Usu somma basalt. Aeromagnetic survey of this
area (Okuma et al., 2002) and a marked geomagnetic change in the initial stage of the 2000 eruption
(Satoh et al., 2002; Hashimoto et al., 2007) also support the reversal. The ongoing rapid and persistent
changes in the total field may be accounted for such effective TVM at low temperature. * 4. Summary
and conclusions * We conducted MT resistivity survey over the magmatic intrusion of the 2000 eruption
of Usu Volcano. An umbrella-shaped very low resistivity (VLR: 0.1 to 1 Ohm-m) layer was found
beneath the upheaval center at some hundred meters deep. This umbrella structure is probably related
to the intrusion. Montmorillonite is a plausible candidate for the VLR, while the bottom of the VLR may
correspond to the isotherm (c.a. 200 C) of transition to more resistive clay mineral. Meanwhile, results
from the geomagnetic monitoring in the post-eruption stage suggest that a rock body at about 400 m
deep is acquiring the magnetization to the present geomagnetic field. It is more consistent with the
resistivity results to interpret this ongoing change is due to the TVM at a low temperature range rather
than considering the simple thermal demagnetization.
Keywords: usu volcano, magnetotellurics, geomagnetic field