No abstract

The northern Fossa Magna (NFM) is a back-arc rift basin filled 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 five sets of refraction/wideangle reflection 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 reflection 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.

No abstract

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.