Earth Planets Space, Vol. 56 (No. 12), pp. 1253-1259, 2004
Hiroshi Sato1, Takaya Iwasaki1, Yasutaka Ikeda2, Tetsuya Takeda1, Nobuhisa Matsuta1, Tomoko Imai1, Eiji Kurashimo1, Naoshi Hirata1, Sin'ichi Sakai1, Driss Elouai3, Taku Kawanaka4, Shinji Kawasaki4, Susumu Abe4, Takeshi Kozawa4, Takeshi Ikawa4, Yoshimasa Arai5, and Naoko Kato1
1Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Tokyo 113-0032, Japan
2 Department of Earth and Planetary Science, 7-3-1 Hongo, University of Tokyo, Tokyo 113-0033, Japan
3Department of Earth and Planetary Systems Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan
4JGI Inc., 1-5-21, Otsuka, Tokyo 112-0012, Japan
5Tokyo Electric Power Company, 212-1 Katsuyama, Ueno, Gunma 370-1613, Japan
(Received May 31, 2004; Revised December 27, 2004; Accepted December 27, 2004)
The northern Fossa Magna (NMF) is a Miocene rift basin formed in the final stages of the opening of the Sea of Japan. The northern part of Itoigawa-Shizuoka Tectonic Line (ISTL) bounds the western part of the NMF and forms an active fault system that displays one of the largest slip rates in the Japanese islands. Reflection and refraction/wide-angle reflection profiling and earthquake observations by a dense array were undertaken across the northern part of ISTL in order to delineate structures in the crust, and deep geometry of the active fault systems. The ISTL active fault system at depth (ca. 2 km) shows east-dipping low-angle in Omachi and Matsumoto and is extended beneath the Central Uplift Zone and Komoro basin keeping the same dip-angle down to ca. 15 km. The upper part of the crust beneath the Central Uplift Zone is marked by the high Vp and high resistivity zone. Beneath the folded zone of the NMF, the middle to lower crust shows low Vp, low resistivity and more reflective features. The balanced geologic cross-section based on the reflection profiles suggests that the shortening deformation since the late Neogene was produced by the basin inversion of the Miocene low-angle normal fault.
Key words: Crustal structure, geologic structure, active fault, Itoigawa-Shizuoka tectonic line, northern Fossa Magna, seismic reflection profile, seismic refraction profile, central Japan.