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Fig.1 Location map of Izu-Oshima Island (Okuma et al., 1994)
Contour interval is 100 m. The central square shows the study area. Knotted lines indicate flight paths of the aeromagnetic surveys in 1978 and 1986.
Izu-Oshima Volcano is situated
about 110 km SSW of Tokyo, Japan (Fig.1). It is an active insular
volcano mainly of basalt with two connected calderas, northeastern
and southwestern ones, and a post-caldera central cone, Mt. Mihara,
on the summit. Major eruptions have taken place periodically
with an interval of about 135 - 50 years during the last 1,500
years (Nakamura , 1964) and the latest eruption happened in 1986.
Nakamura (1987) indicated that the ambient stress field in this
region is of strike-slip type with the P axis in the NW - SE
direction, based chiefly on focal mechanism solutions of large-scale
earthquakes in and around Izu-Oshima Island and directions of
dyke intrusions in the island. This idea is supported by the
fact that the flank fissures of the 1986 eruption were aligned
in the NW - SE direction (SakaguchiI et al., 1988) and
epicenters of the 1986 earthquakes during and after the flank
fissure eruption were distributed linearly in the same direction
(Yamaoka et al., 1988).
Fig. 1 Lava flow (a) of 1986
This lava came from the vent A in the central cone, Mt. Mihara (behind). Orthopyroxene-augite basalt lava and associated andesite lava. This picture was taken from Gojinkajaya on the northwestern rim of the southwestern caldera in 1989.
Fig. 2 Inside the southwestern caldera of Izu-Oshima volcano
Mt. Kushigata is behind. See also Fig. 1.
Fig.3 IGRF residual aeromagnetic anomaly map of Izu-Oshima volcano and its surrounding area (Okuma et al., 1994)
The datum plane is 1,070 m (3,500 ft) above sea level. Contour interval is 100 nT.
Makino et al. (1988) and Nakatsuka et al. (1990) compiled a precise aeromagnetic anomaly map in and around Izu-Oshima Volcano by the data obtained before and soon after the eruption. Their analyses of magnetic anomalies of the area revealed that the magnetic anomalies were caused by the uniformly magnetized terrain, a dyke-like body trending NW- SE and many small prismatic bodies. They thought that this dyke-like body corresponds to a macroscopic representation of numbers of dyke intrusions in the NW - SE direction. This direction coincides with that of P axis of the regional stress field in this region. On the other hand, many small prismatic bodies apparently suggest that the magnetization intensity of the rocks varies laterally in some parts of the island. Therefore, we carried out a magnetization intensity mapping in and around Izu-Oshima Island to obtain more detailed information about the subsurface structure.
Fig.4 Apparent magnetization intensity map of Izu-Oshima volcano and its surrounding area (Okuma et al., 1994) .
Contour interval is 0.5 (A/m).
Our result showed a good correlation with the surface geology, generally. To give a few instances, magnetization highs coincide with the exposure of spatter ramparts at Yuba and volcanics of Fudeshima volcano, in addition to the distribution of the post-caldera lava flows. Magnetization lows predominate on the northeastern coast, which correspond to the distribution of the older edifice of the main stratovolcano at the pre-caldera stage and volcanics of Gyojanoiwaya volcano.However some areas of the island lack geologic information in spite of presence of characteristic distribution of magnetization intensity. For instance, a low intensity area of magnetization ranges from Motomachi to Nomashi on the western side of the island, where no relevant surface geologic features are known. This low intensity area of magnetization may relate to the hydrothermal alteration of volcanics by hot ground water or the demagnetization by the heat of the magma chamber or conduits, in addition to the inferred distribution of the older edifice of the main stratovolcano at the pre-caldera stage and a complex of altered volcanics.
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