Earth Planets Space, Vol. 56 (No. 12), pp. 1293-1299, 2004
Tetsuya Takeda1, Hiroshi Sato1, Takaya Iwasaki1, Nobuhisa Matsuta1, Shin'ichi Sakai1, Takashi Iidaka1, and Aitaro Kato2
1Earthquake Research Institute, the University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
2The Institute for Frontier Research on Earth Evolution, Japan Marine Science and Technology Center, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan
(Received June 3, 2004; Revised December 30, 2004; Accepted December 30, 2004)
The northern Fossa Magna (NFM) is a back-arc rift basin °lled with thick Tertiary sediments, which show strong NW-SE shortening deformation. In the NFM, there exist two major active fault systems, the Itoigawa- Shizuoka Tectonic Line active fault system (ISTL) and the Western Nagano Basin active fault system (WNB), both of which have great potentials to cause destructive earthquakes. By reanalyzing °ve sets of refraction/wideangle re°ection data, we successfully obtained detailed and consistent models of the crustal structure in the NFM region. It was a very effective modeling procedure to incorporate vicinal seismic re°ection data and geologic information. The geometries of the active faults in the NFM region were revealed. The ISTL is east dipping, and the WNB is northwest dipping. The Tertiary sedimentary layer (<4.0 km/sec) west and adjacent to the ISTL extends to a depth of 4-5 km. The basement rocks below the Central Uplift Belt (CUB) form a wedge structure, which suggests the westward movement of the CUB basement rocks.
Key words: Crustal structure, northern Fossa Magna, refraction/wide-angle re°ection data, Itoigawa-Shizuoka Tectonic Line, wedge structure.