** Progress in Earth and Planetary Science is the official journal of the Japan Geoscience Union, published in collaboration with its society members.
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Research
Solid earth sciences
202407202407
Assessing the relative activity of faulting along both flanks of the Ou Backbone Range, Tohoku Region, Japan, from fluvial geomorphic analyses
J. Bruce H. Shyu, Jhih-Hao Liao, Chia-Yu Chen, Hiroyuki Tsutsumi, Yasufumi IryuJ. Bruce H. Shyu, Jhih-Hao Liao, Chia-Yu Chen, Hiroyuki Tsutsumi, Yasufumi Iryu
Active faults, Normalized river steepness index, Ou Backbone Range, Tohoku Region, Japan
Basin-wide mean local steepness (MLS) of the 31 basins analyzed in this study. The highest values of MLS are generally found in river basins along the northwestern Ou Backbone Range (OBR), whereas river basins along the southwestern and northeastern OBR generally have lower MLS values. The characteristics of knickpoints identified in our analysis are also shown. YD: Yakeishi Dake
The Ou Backbone Range in the Tohoku Region of Japan is bounded on its both sides by two major active fault systems: the Western Fault Zone of Kitakami Lowland in the east and the Eastern Fault Zone of Yokote Basin in the west. Although these two systems are primary active fault systems in the region, information on their long-term activity characteristics is still quite limited. Therefore, we analyzed the normalized channel steepness indexes of river valleys trending perpendicular to the range front along both flanks of the Ou Backbone Range. Our results show that the eastern flank has gentler river valleys, whereas rivers along the northwestern flank are steeper. Our field investigation shows that knickpoints in this area are mostly related to local lithologic boundaries or are check dams along the valleys, thus the river systems are likely under steady-state conditions. Hence, the steeper river valleys in northwestern Ou Backbone Range indicate a higher uplift rate of the area. Because both fault systems are primarily dip-slip reverse faults and do not have significant variations in their subsurface geometry, the faster uplift suggests that the northern segment of the Eastern Fault Zone of Yokote Basin has a higher slip rate. This is consistent with results of previous studies, and the fact that the rupture of the 1896 Rikuu earthquake, the only historical surface-rupturing event in this region, was only limited along the northern segment of this fault system.
