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.

The Northern Fossa Magna is a Miocene failed rift and due to subsequent shortening, its basin-fill forms a fold-belt associated with active faults. Seismic reflection data across the middle part of the northern Fossa Magna acquired in late 1990s were reprocessed to reveal the deep geometry of active faults. The reprocessed seismic sections portray the folded and faulted structure of the Neogene basin-fill. The deeper extension of the Western Nagano Basin active Fault (WNBF), which has been revealed for the first time, can be traced down to 4 km, as a reverse fault dipping 40• westward. In the western part, the Itoigawa-Shizuoka Tectonic Line (ISTL) active fault is presented as an emergent thrust dipping 30-35• eastward. Based on the seismic profiles, surface geology and well data, the balanced geologic cross section was constructed. Using simple-shear model of the basin formation, the total amount of Miocene extension is calculated to be ca. 27 km and the total amount of late Neogene to Quaternary shortening is ca. 11 km. The basin formation and shortening deformation are well explained by the tectonic inversion model and fault reactivation.

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