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Transient process of channel adjustment to tectonics: Field evidence from Iwaki, Fukushima, Japan
  • Naoya Takahashi
Naoya Takahashi
Tohoku University

Corresponding Author:naoya.takahashi.t1@dc.tohoku.ac.jp

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Local base-level change drives the transition of river morphology from one steady state to the new one. Although extracting tectonic information from river steepness assumes steady state, presence of knickpoints often complicates the stream profile analysis. This study aims to investigate the transient process of channel adjustment to tectonic perturbation using channel width, width to depth ratio, and normalized steepness index (θref=0.45). I focus on rivers along a normal fault in Iwaki, Fukushima, the eastern part of Japan. This fault ruptured on 11 April 2011, and the maximum vertical displacement was ~0.8 m. Paleoseismic trenches indicated that three surface-rupturing observed three types of rivers: one which is away from the fault and at old steady state, one which flows across the fault and at transient state characterized by a slope-break knickpoint, and one which flows across the fault and does not include any prominent knickpoints thus at new steady state. For a river at old steady state, channel width (W) increases with drainage area (A) (W~3.51A0.44), and width to depth ratio and steepness index are almost constant over the reach. For a river at transient state, width and width to depth ratio decrease at downstream of a knickpoint, and steepness index increases by a factor of two to four. For a river at new steady state, width, width to depth ratio, and steepness index show no discernible changes. Preliminary interpretation is that channel narrowing occurs after a channel starts steepening, and that channel width to depth ratio drops when a river is subjected to active deformation and becomes constant over an entire reach. Combining with ongoing field measurements, I will discuss the transient response of a river to tectonics.