Distribution of Active Faults and Fault Topography around the Southwestern Foot of the Chokai Volcano, Yamagata Prefecture, Northeast Japan

Abstract

The southwestern foot of the Chokai volcano is located in the northern part of the eastern boundary fault system of the Shonai plain, northeastern Japan. The author classified geomorphic surfaces and clarified the distribution of active faults and fault topography characteristics on the basis of aerial photo analysis and field surveys, to reveal active tectonics in the area. West-facing major fault scarps develop on the western flank of the Tohyama, Butai, and Tengumori hills distributed along the fringe of the volcanic fan, forming a clear topographic break. Back-tilted terraces or east-facing scarplets frequently develop on the eastern flanks of these hills. These findings suggest that the faults along the western margin of these hills are east-dipping, lowangle main reverse faults, and those along their eastern margin are west-dipping secondary back thrusts (conjugate faults). However, on the Marumori hills along the Sakata thrust belt, located in the southeastern part of the study area, range-facing major fault scarps are assumed to develop and delineate the eastern flank of the hills. The average vertical slip rate of the active faults, based on the displacement and age of the topographic surfaces, is estimated to be less than 0. 5 mm/year in the Tohyama hills, and more than 0.7–0. 8 mm/year in the Butai and Tengumori hills.