HR: 0830h
AN: V51J-0411    [PDF]
TI: Geomagnetic Change Associated With the Magma Intrusion at the Initial Stage of the 2000 Activity of Miyakejima, Central Japan
AU: * Ueda, H
EM: ueda@bosai.go.jp
AF: NIED, Tennodai 3-1, Tsukuba, 305-0006 Japan
AU: Matsumoto, T
EM: mtakumi@mext.go.jp
AF: NIED, Tennodai 3-1, Tsukuba, 305-0006 Japan
AU: Fujita, E
EM: fujita@bosai.go.jp
AF: NIED, Tennodai 3-1, Tsukuba, 305-0006 Japan
AU: Ukawa, M
EM: ukawa@bosai.go.jp
AF: NIED, Tennodai 3-1, Tsukuba, 305-0006 Japan
AU: Yamamoto, E
EM: yamamoto@bosai.go.jp
AF: NIED, Tennodai 3-1, Tsukuba, 305-0006 Japan
AU: Sasai, Y
EM: yosasai@zag.att.ne.jp
AF: Tokyo Metropolitan Government, Nishishinjuku 2-8-1, Shinjuku-ku, Tokyo, 163-8001 Japan
AU: Irwan, M
EM: irwan@eps.nagoya-u.ac.jp
AF: Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602 Japan
AU: Kimata, F
EM: fumikimata@yahoo.co.jp
AF: Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602 Japan
AB: A marked magnetic field change exceeding 200 nT was observed at the initial stage of the 2000 eruptive activity of Miyakejima Island, which is located 170 km to the south of Tokyo, Japan. The station recorded the magnetic change is one of the three continuous observations with three-component flux-gate type magnetometer in Miyakejima. The change began at 2100 LT on June 26 about 3 hours after the beginning of seismic swarm and crustal deformation in Miyakejima. A magma migration model obtained by Ueda et al. (2003, IUGG2003, Vol. A, p.553) from the crustal deformation data shows that magma was intruding very near the station at the time when the magnetic change was recorded. The crustal deformation has a strong correlation with the magnetic change. We evaluated a magnetic change due to the piezomagnetic effect from our magma migration model. The analysis indicates that the major part of the magnetic change can be interpreted quantitatively by the piezomagnetic effect of the magma intrusion. Other potential factors including thermal demagnetization, electorokinetic effect and movement of the magnetometer are unlikely to be the principal cause of it. The residual part that is not explained by the magma intrusion model may originate from piezomagnetic effect due to small cracks near the ground surface. Since the upper depth of the intruded magma is very shallow ($<$1km depth), small tensile cracks should be induced by the large stress changes around the upper bound of the magma. Such shallow small clacks, cause magnetic change especially near their edges, whose contribution to elastic deformation is small.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 8400 VOLCANOLOGY
DE: 8434 Magma migration
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting