The Tsugaru Mountains are bounded by active faults and flexures along both western and eastern margins. The western marginal fault zone are reexamined by large-scale airphotograph interpretation, leveling and seismic refractive survey. This fault zone are characterized by west-dipping flexure with east-dipping fault (back thrust) in the southern part, and is not recognized in the northern and central part. Thus, the parameters of this fault zone was revised to be ca.15km long and in the vertical slip rate of ca.0.2m/ka in the late Quaternary time. The Namioka flexure near the fault zone has relatively high vertical slip rate of 0.7m/ka. These active tectonic structures are possibly sesimogenic, but their last events still unclear with implication of the historical 1766 earthquake (M7_1/4).