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 1973
A shallow resistivity structure of Kuju Volcano, Central Kyushu, Japan
Dr. Wataru Kanda
Sakurajima Volcano Research Center DPRI, Kyoto University IAGA
Yoshikazu Tanaka, Mitsuru Utsugi, Yasuaki Okada, Hiroyuki Inoue
Kuju volcano is a composite volcano, consisting of more than 20 lava domes or cones, located on
central Kyushu, Japan. In Oct. 1995, a phreatic explosion occurred after a few hundred years of
dormancy with opening of several new vents at the eastern flank of Mt. Hossho, one of the domes of
the Kuju complex. The new vents were located about 300m south of a pre-existing fumarolic field called
Iwo-yama. The activity continued to middle of 1996 with occasional ash ejections. Although seismicity
around Mt. Hossho showed a decreasing tendency after 1995 to 1996 eruptive activities, fumarolic
activity around Iwo-yama has been still in high level. Heat discharge rate has been estimated as 500-
1000 MW that is several times higher than that before eruption (~100MW). Proton-precession
magnetometers were installed shortly after the 1995 eruption to obtain the continuous records of the
total intensity (Tanaka et al., 1996). Total intensities showed rapid increase at sites located south of the
pre-existing fumarolic area, while it decreased at a site located north, indicating that magnetization of
the volcanic rocks occurred beneath around Iwo-yama area by cooling. We conducted Audio-frequency
MagnetoTelluric (AMT) surveys in Aug. 2005 and 2006 around Iwo-yama area The data were
successfully collected at 22 locations and interpreted by 2-D inversion (Ogawa and Uchida, 1996).
Although the data quality was not good at the marginal sites of the section, following features of
resistivity structure were obtained. (1) High resistivity of several hundreds to a thousand ohm*m is
found near the surfaces of the profile, which correspond to the andesitic lavas of Mt. Hossho and Nakadake,
This surface layer is thicker at the northern flank of Mt. Hossho where two lava-flow units are
seen. (2) Conductive layer of less than 3 ohm*m is seen at the depths between 200m and 600m
beneath the northern flank of Mt. Hossho and beneath Naka-dake. This conductive layer is mainly
composed of hydrothermally altered rocks and also contains ground water. It appears to be shallower
near the fumarolic area, indicating that high temperature zone is inflated due to higher heat flux related
to intense fumarolic activity of Iwo-yama.(3) Relatively resistive zone of 10 - 30 ohm*m lies beneath
Iwo-yama area. This less conductive body seems to decouple the conductive layer. Magnetization
source is located at this part. Since the highest fumarolic temperature still exceeds 200 degrees, this
less conductive body may be vapor rich zone. Gravity data suggested that a great amount of water has
flowed into the shallow part beneath the pre-existing fumarolic area. The estimated resistivity section is
consistent with this idea. Since water contained in the conductive layer has flowed into and cooled the
less conductive zone, geomagnetic variations have shown a cooling process. However, high temperature
magmatic gases have been still supplied from the deep thermal source to Iwo-yama area, so that the
central part of the less conductive zone is still in high temperature state.
Keywords: resistivity, fumarolic activity, phreatic eruption