IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS007 Oral Presentation 1961
Imaging of Electrical and Thermal Structure of a Shallow Magmatic
Intrusion Associated with the 2000 Eruption of Usu Volcano
Dr. Takeshi Hashimoto
Institute of Seismology and Volcanology Faculty of Science, Hokkaido University IAGA
Yasuo Ogawa, Shinichi Takakura, Hiroyuki Inoue, Yusuke Yamaya, Mitsuru Utsugi,
Tetsuji Koike, Hiroshi Hasegawa, Hiroshi Ichihara, Hideyuki Satoh, Toru Mogi
1. Introduction * Usu Volcano, southwestern Hokkaido in northern , experienced the 4th (and the final)
eruption in the 20th century in March, 2000. The eruption resulted in the ground upheaval of about 80
m due to a shallow intrusion of magma beneath the western part of Nishiyama, the NW piedmont of the
volcano. Many geophysical investigations have been implemented in this area during and after the
eruption. None of them, however, have yet achieved a clear imaging of the intruded magma. As for the
electrical structure, Akita and Shibata (2003) conducted the shallow resistivity survey by audio
frequency magnetotellurics. They reported that the shallow part of this area is very conductive, and
therefore, it seemed that the investigation of the deeper structure down to the intrusion depth requires
lower frequency band. We thus planned a magnetotelluric survey up to 0.001 Hz in 2006. Six wide-band
MT sites aligned in the NE-SW direction (orthogonal to the regional strike), crossing the upheaval
center, were interfilled with several audio-frequency sites to achieve a high horizontal resolution as well
as the vertical reach. * 2. Electrical resistivity structure * Acquired MT responses were inverted to a 2D
resistivity structure by using the inversion process developed by Ogawa and Uchida (1996). As seen in
the previous survey, the general resistivity of this area is quite low (0.1 to 10 Ohm-m). This feature well
explains the poor variation of surface self-potential reported by Hase et al. (2007). Surface resistivity
shows about 10 Ohm-m, corresponding to the volcanic deposits. A very low resistivity (VLR: 0.1 to 1
Ohm-m) underlies with a thickness of some hundred meters. This layer is imaged at 200 to 400 m deep
beneath the upheaval center, while it does from 500 to 1000 m deep in the northern and southern side
of the cross-section, just like an umbrella-shaped structure. It is probable that a highly conductive clay
mineral (montmorillonite) immersed in high salinity fluid is responsible for the VLR. The bottom of this
VLR then possibly corresponds to the thermal transition (about 200 C) from montmorillonite to other
minerals such as illite of higher resistivity. The bump of the VLR below the upheaval center may be
related to the isotherm due to a magmatic intrusion. The intruded magma should be situated below this
VLR, though it is not imaged as an isolated body in the resistivity cross-section. * 3. Implication from
geomagnetic changes * Some of the authors (Hokkaido University ) have started magnetic repeat
measurements over the upheaval area since 2003, about three years after the surface manifestation
ended, anticipating the subsequent geomagnetic changes related to subsurface thermal activity.
Surprisingly, it has been revealed that markedly rapid magnetic changes up to 50 nT/yr was still going
on. The change looks quite linear with respect to the time, showing no plateauing even seven years
after the eruption calmed down. The magnetic total field has increased to the south, while it has
decreased to the north of the upheaval center, suggesting the increasing magnetization at 400 m deep.
Such change is normally interpreted as cooling at the source region; namely, the rock is getting more
magnetized as the magnetic minerals in the rock freeze themselves to the present geomagnetic field.
This mechanism requires that the source region must have been heated in advance to a high
temperature enough to produce the effective, enormous, and persisting thermal magnetization in the
subsequent cooling phase. Such an extensive pre-heating at 400 m deep is unlikely, taking account of
the discussion in the resistivity section. * One of the alternative mechanisms is the themo-viscous
magnetization (TVM) in a lower temperature range. Magnetic minerals exposed to an external field may