Earth Planets Space, Vol. 56 (No. 12), pp. 1285-1291, 2004
Yasuo Ogawa1 and Yoshimori Honkura2
1Volcanic Fluid Research Center, Tokyo Institute of Technology, Tokyo, Japan
2Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan
(Received June 2, 2004; Revised October 12, 2004; Accepted November 3, 2004)
An active fault segment at the northern Itoigawa-Shizuoka tectonic line (ISTL), Central Japan, which will potentially cause M8-class intraplate earthquake, was imaged by wide-band magnetotellurics. Three parallel pro- files across ISTL revealed along-strike variation of the resistivity structure. Three resistivity models commonly showed the thickening conductors in the upper crust to east of ISTL which imply the heavily folded Miocene sediments with maximum thickness of several kilometers. Thus the upper crustal structure seems two-dimensional throughout the segment. We found mid-crustal conductors, top of which correlate well with the cutoff depth of seismicity. The seismicity clusters mainly in the resistive crust that is underlain by the mid-crustal conductors. This implies the local distribution of fluids below the brittle-ductile boundary and suggests that the fluid migration into resistive zone is triggering earthquakes. However, the distribution of these mid-crustal conductors is not consistent with the strike of ISTL, but rather it is better correlated with the negative dilatation anomaly inferred from GPS. This suggests the weakening of the crust by the existence of fluids.
Key words: Itoigawa-Shizuoka Tectonic Line, magnetotellurics, deformation, fluids.