抄録
Recently an outcrop of welded tuff in Omine group (Pliocene) were found in the River Takase-gawa, Tokiwa area, Shinano-Omachi, Nagano Prefecture. Since this welded tuff plays an important role on clarifying the subsurface structure of Northern Fossa Magna, we conducted a seismic reflection survey in this area. As a result, the following is found. There are some strata in the sections, which show aparent dips of 20-30° to the east. Judging from their dips and velocities, these belong to the Omine group. A fault is found about 200m west of River Takase-gawa by the analyses of reflection and refraction methods. It may be related to the Itoigawa-Shizuoka Tectonic Line. The welded tuff in the River Takase-gawa has a welded structure of westward dip, contrary to the dip of surveyed strata. Therefore to avoid contradiction, we have to assume the presence of an anticline or two other faults beneath the River Takase-gawa.
