Transient process of channel adjustment to tectonics: Field evidence
from Iwaki, Fukushima, Japan
Abstract
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.