Infrared absorption spectra were taken to determine concentration of water dissolved in rhyolitic glass of pumice, scoria and andesite fragments ejected by the Asama 2004 September eruptions. As the groundmass in the fragments includes a lot of microlites of plagioclase and pyroxene, thickness of the volcanic glass itself is difficult to measure directly. However, an absorption peak, assigned to volcanic glass, is observed at 1,850cm^-1, and its peak intensity obtained by a curve-fitting method gives the net thickness of the glass. This net thickness provides the intensities of water per glass thickness without microlite contamination. The relationship between water concentration and two absorption peak-intensity (3,570, 1,630cm^-1) is derived from infrared analyses of the aphyric Wada obsidian, whose water content was determined by Karl Fischer titrator. Following the above procedure, the water contents in the glass of the pumice of the September 1st eruption, and scoria and andesite fragments of the September 23rd eruption are estimated to be 0.66, 0.1, 0.1-0.2wt%, respectively. The magma represented by the quenched marginal crust of the pumice preserves considerable amount of water (0.66wt%) for its shallow location beneath the crater floor, prior to the September 1st eruption. The tensile strength of the cap rock presumably kept sufficient pressure in the top of the magma column to dissolve water into the magma, whereas the magma providing the scoria and andesite fragments of the September 23rd eruption lost most of water.

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A magnetic inversion algorism for constructing a 3D-magnetic structure of a volcanic edifice was developed and applied to the airborne magnetic anomalies of Miyakejima Volcano surveyed in 1987, 1999, and 2001. The method consists of two steps. First, a uniformly magnetized terrain model is assumed for calculating the mean magnetization intensity, and the residual field was calculated by subtracting the terrain effect. In the second step, the magnetization deviations (ΔJ) from the mean value are calculated for each prismatic block constituting the three dimensional volcanic edifice, by the refined Conjugate Gradient iteration method under the condition of 10A/m＜ΔJ＜10A/m. Characteristics of the derived 3D-magnetic structures are summarized as foliowing four features. (1) Miyakejima Volcano has rather uniform magnetization, whose non-uniform magnetization is less than 10～20% of total bulk-magnetization (total summation of the product of block volumes and magnetization intensities).(2) A nearly N-S trending magnetic basement is estimated at the deeper part of the volcano.(3) Relatively weak magnetization zones are estimated beneath the eastern coastal zone and its offshore area, and beneath the western flank of the edifice.(4) A relatively high magnetization, zone is estimated beneath the northern offshore area. Comparison of the 3D-magnetic structures derived from surveys in 1987 and 1999 made apparent that the zone of relatively weak magnetization intensity had spread wider from 1987 to 1999, with the most demagnetized zone at the base (0m～300m b.s.1.) in about 750m southeast from the center of the crater. The 3D-magnetic structure in 2001 indicates the demagnetized zone is considerably restricted in comparison with that in 1999. This feature may suggest that the thermal demagnetization has been depressed after the eruption in 2000. These results suggest that the detection of demagnetization process associated with volcanic activity might be promising by conducting repeated airborne magnetic surveys.

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A helicopter-borne, aeromagnetic survey was performed in October 2004 over the western Unzen region, Kyushu, Japan. The survey was flown at an altitude of about 1100m above sea level along north-south survey lines and east-west tie lines, spaced 500m and 1000m apart, respectively. The observed aeromagnetic anomalies were reduced onto a constant surface of 1500m above sea level. Then, a magnetization intensity distribution was estimated, assuming that it varies only laterally. The characteristics of the magnetization intensity distribution are summarized as follows;(1) The hydrothermally altered areas of Unzen hot spring, Ii-Dake, Shimo-Dake, and Kami-Dake show magnetization lows, which indicates that high temperature fluids associated with hydrothermal activities reduce the rock magnetization. These fluids are likely to migrate through E-W trending faults developed inside the Unzen graben.(2) Magnetization lows lie outside of the Unzen graben, corresponding to debris flow deposits and pyroclastic-flow deposits of the Older Unzen (500-150ka). In contrast, magnetization highs are distributed in the northern part region of Azuma-Dake and Hachimaki-Yama and around Takaiwa-Yama, where the lava of the Older Unzen (500-300ka) is exposed. (3) Magnetization lows, corresponding to fan deposits of the Older Unzen (500-300ka), are distributed along the Chijiwa fault.

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