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 Poster presentation 1968
Shallow Resistivity Structure of Asama Volcano and Its Implication for
Magma Ascent Process of 2004 Eruptions
Dr. Koki Aizawa
Tokyo Institute of Technology Tokyo Institute of Technology IAGA
Yasuo Ogawa, Takeshi Hashimoto, Takao Koyama, Wataru Kanda, Electromagnetic
Research Group For Asama Volcano
Asama volcano, which sits 150km northwest of Tokyo, is one of the most active volcanoes in Japan.
Historical records show many eruptions that are mainly characterized by Vulcanian eruptions. Its recent
eruptions took place in 2004 at the summit crater without remarkable precursors. Geodetic data and the
hypocenter distributions [e.g., Takeo et al., 2006] suggest that magma ascent route is not simple, and
the conduit does not directly link the summit crater to deep magma chamber. Because shallow structure
may give the constraints on volcanic activities [Tanaka et al., 2002] and can control type of eruptions
[Kagiyama et al., 1999], imaging the upper part of volcanoes is important. In this paper, we will argue
the resistivity structure shallower than 5km obtained by dense magnetotelluric (MT) survey. The MT
data were taken along the four survey lines across the volcano. The total number of measurement sites
was 74. The resistivity profiles obtained by two-dimensional inversions are mainly characterized by
resistive surface layer and underlying conductive layer. Important feature of the profiles is that in a
depth range of a few hundred meters to a few kilometers, three resistive bodies exist, being surrounded
by highly conductive regions. Two of three resistive bodies correspond to the old eruption centers; one
corresponds to the 24ka collapse caldera, and the other does to the 21ka lava dome. Taking into
account that the geothermal activities exist near the old eruption centers, the highly conductive zone is
considered to be hydrothermal system. In this study, we interpret the resistive bodies as old and
solidified intrusive magma that are still driving hydrothermal system in the surrounding area. The
solidified magma bodies are considered to be resistive due to its low porosity. Self-potential survey was
also conducted along the MT profiles. The obtained SP data show the distinct "W"-shaped SP profiles
[Ishido, 2004], and suggests the existence of hydrothermal and hydrogeological zone within the
volcano. The SP profiles seem to have correlation with the top surface of the highly conductive regions,
which is interpreted as hydrothermal system. The SP data may support our interpretation of the
resistivity structure. The resistive body beneath the 24ka collapse caldera is located above the swarm of
tectonic earthquakes at the depth of 3km. Geodetic data suggests that magma intruded beneath this
resistive body. We suggest that magma ascent was hampered by old and solidified remnant magma at
the depth of 3km beneath the collapse caldera, and that a part of the magma migrated horizontally to
the east and finally ascended to the summit, resulting in the 2004 eruptions. This study raises the
possibility that solidified remnant magma in volcanoes is controlling present magma movement.
Keywords: magnetotellurics, resistivity, self potential