Earth Planets Space, Vol. 56 (No. 12), pp. 1315-1321, 2004
Yasutaka Ikeda1, Takaya Iwasaki2, Hiroshi Sato2, Nobuhisa Matsuta2, and Takeshi Kozawa3
1Department of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
2Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
3JGI Inc., 1-5-21 Otsuka, Bunkyo-ku, Tokyo 112-0012, Japan
(Received April 27, 2004; Revised October 1, 2004; Accepted December 7, 2004)
The Itoigawa-Shizuoka Tectonic Line (ISTL) in Central Japan is a fault zone with a very high slip rate during Pliocene-Quaternary time. Our seismic reflection and gravity surveys across the northern segment of the ISTL at Matsumoto have revealed its geometry to a depth of ~ 5 km. The fault plane was found to be of fairly low angle, in spite of the surface geologic observations that late Quaternary movements on this fault zone are dominantly strike slip. Partitioning of slip is taking place between the East Boundary Fault (thrust) and the Gofukuji Fault (left-slip), which constitute the fault zone and are parallel to and a few km apart from each other. However, these two faults are found to merge down-dip at a depth as shallow as 1.5 km below the surface. The geometry of the ISTL is significantly discordant with the orientation of the maximum shear stress acting regionally on Central Japan, indicating that the fault plane is of very low strength.
Key words: Itoigawa-Shizuoka Tectonic Line, active fault, seismic reflection profiling, slip partitioning, oblique slip, tectonic inversion.