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International Journal of Geophysics
Volume 2010, Article ID 738139, 7 pages
http://dx.doi.org/10.1155/2010/738139
Research Article

Electrical Resistivity Structure and Helium Isotopes around Naruko Volcano, Northeastern Japan and Its Implication for the Distribution of Crustal Magma

1Tono Geoscientific Research Unit, Geological Isolation Research and Development Directorate, Japan Atomic Energy Agency, 959-31, Jorinji, Izumi, Toki 509-5102, Japan
2Volcanic Fluid Research Center, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro, Tokyo 152-8551, Japan
3National Institute of Advanced Industrial Science and Technology, GSJ, AIST, Tsukuba Central 7, Tsukuba, Ibaraki 305-8567, Japan

Received 13 May 2010; Accepted 31 August 2010

Academic Editor: Gary Egbert

Copyright © 2010 Koichi Asamori et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The two-dimensional electrical resistivity structure beneath Naruko volcano was determined using magnetotelluric soundings. The resulting model shows that a prominent conductor exists through the middle crust to the uppermost mantle beneath the volcano. The location of the conductor agrees closely with a seismic low-velocity zone. Low-frequency microearthquakes occur near the conductor around the Moho depth. The cutoff depth of crustal earthquakes is coincident with the upper boundary of the conductor, implying that the conductor has a temperature appreciably higher than 4 0 0 C. Furthermore, new helium isotope data from hot springs around the volcano were obtained. The spatial distribution of the observed 3 H e / 4 H e ratios reveals the extent of mantle-derived materials beneath Naruko volcano. Consequently, it is apparent that the conductor determined beneath the volcano reflects the presence of high-temperature mantle-derived materials such as magmas and/or related fluids derived from active magmatism in the northeastern Japan subduction zone.