Shyu et al.
Progress in Earth and Planetary Science(2024) 11:41
https://doi.org/10.1186/s40645-024-00644-9

Progress in Earth andPlanetary Science

Open Access

RESEARCH ARTICLE

Assessing the relative activity of faulting
along both flanks of the Ou Backbone Range,
Tohoku Region, Japan, from fluvial geomorphic
analysesJ. Bruce H. Shyu1* , Jhih‑Hao Liao1, Chia‑Yu Chen1,2, Hiroyuki Tsutsumi3 and Yasufumi Iryu4,5

Abstract
The Ou Backbone Range in the Tohoku Region of Japan is bounded on its both sides by two major active fault sys‑
tems: 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 activ‑
ity characteristics is still quite limited. Therefore, we analyzed the normalized channel steepness indexes of rivervalleys trending perpendicular to the range front along both flanks of the Ou Backbone Range. Our results showthat 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 primar‑
ily dip-slip reverse faults and do not have significant variations in their subsurface geometry, the faster uplift sug‑gests 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 surfacerupturing event in this region, was only limited along the northern segment of this fault system.
Keywords Active faults, Normalized river steepness index, Ou Backbone Range, Tohoku Region, Japan*Correspondence:
J. Bruce H. Shyu
jbhs@ntu.edu.tw
1
Department of Geosciences, National Taiwan University, Taipei, Taiwan
2
Department of Earth and Environmental Sciences, National Chung
Cheng University, Chiayi, Taiwan
3
Department of Environmental Systems Science, Faculty of Scienceand Engineering, Doshisha University, Kyotanabe, Japan
4
Institute of Geology and Paleontology, Graduate School of Science,
Tohoku University, Sendai, Japan
5
Advanced Institute for Marine Ecosystem Change (WPI‑AIMEC), Tohoku
University, Sendai, Japan

1 Introduction
The Ou Backbone Range (OBR) is one of the mostprominent geomorphic features in the Tohoku (northeastern) Region of Japan, which is located within one of
the most seismically active plate boundary belts in theworld (Headquarters for Earthquake Research Promotion
2009; Fig. 1). Bordering the OBR on both sides are the
Yokote Basin and the Kitakami Lowland, both of whichhave a long history of agriculture development and settlement establishment. The mountain fronts between therange and the basin/lowland on its both sides are characterized by two active fault systems, the Eastern Fault
Zone of Yokote Basin (EFZYB) along the western frontof the range and the Western Fault Zone of Kitakami
Lowland (WFZKL) along the eastern front of the range© The Author(s) 2024. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://creativecommons.org/licenses/by/4.0/.

Shyu et al. Progress in Earth and Planetary Science(2024) 11:41

Page 2 of 15

Fig. 1 Map showing the study area. The Ou Backbone Range (OBR) is one of the most prominent geomorphic features in the Tohoku Regionof Japan. The mountain range is bounded on its both sides by the Eastern Fault Zone of Yokote Basin (EFZYB) along the western front of the range
and the Western Fault Zone of Kitakami Lowland (WFZKL) along the eastern front of the range. In the central part of the northern OBR, the Eastern
Fault Zone of Mahiru Mountains (EFZMM) on the western side of the intermontane Yuda Basin is generally considered as a backthrust that branches
out from the EFZYB (e.g., Kagohara et al. 2009; Arora et al. 2021). The 1896 M7.2 Rikuu earthquake was produced by ruptures of the northern
segment of the EFZYB, with some surface ruptures observed along a segment of the EFZMM. Fault traces are from Research Group for Active Faultsof Japan (1991), Nakata and Imaizumi (2002), and Headquarters for Earthquake Research Promotion (2001, 2005a, b). Location of the 1896 epicenterand the surface ruptures during the earthquake (shown in blue) are from Matsuda et al. (1980). Location of the 2008 epicenter is from Matsu’ura
and Kase (2010), and the 2008 surface rupture locations (shown as blue squares) are from Toda et al. (2010). Orange triangles indicate two active
volcanoes in this region (AK: Akita Komagatake; KY: Kurikoma Yama), and the yellow triangle indicates the Yakeishi Dake (YD), a prominent
Quaternary volcano. Volcano data are from Nakano et al. (2013) and Geological Survey of Japan, AIST (2020)(e.g., Research Group for Active Faults of Japan 1991;
Nakata and Imaizumi 2002; Headquarters for Earthquake Research Promotion 2001, 2005a). The EFZYB is
an east-dipping reverse fault, and the WFZKL is a reverse
fault that dips to the west (Headquarters for Earthquake
Research Promotion 2001, 2005a; Sato et al. 2002). These
two fault systems likely converge at a depth of ~ 13 km,
making a pop-up structure that forms the OBR (e.g., Satoet al. 2002).

Both of these two fault zones are considered and evaluated among the 115 major active faults by the Headquarters for Earthquake Research Promotion (HERP) of Japan(https://​w ww.​jishin.​go.​jp/​e valu​ation/​long_​term_​e valu​
ation/​major_​active_​fault/). These faults pose significant
seismic hazard for the settlements of the basin/lowland,as demonstrated by the 1896 M7.2 Rikuu earthquake,
which was produced by ruptures of the northern segmentof the EFZYB and was one of the largest on-land reverseShyu et al. Progress in Earth and Planetary Science(2024) 11:41

faulting earthquakes in the history of Japan (e.g., Matsuda et al. 1980; Figs. 1 and 2).
There have been many previous studies to understandthe long-term ­(103 to ­105 years) activity of the two major
fault systems, including paleoseismological studies on
various sites along the faults and geomorphic analysesof fluvial terraces along rivers flowing out of the range
to quantify the incision/uplift rate of the range (e.g.,

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Matsuda et al. 1980; Research Group for the Senya Fault
1986; Imaizumi et al. 1997, 1989a, b; Tajikara and Ikeda
2005; Kagohara et al. 2009; AIST 2010). However, being
the seismogenic structure of the 1896 Rikuu earthquake,
most of the efforts have been focused on the northern
segment of the EFZYB. Information on the long-term
characteristics of other segments of these two fault systems is still quite limited. Furthermore, many of theFig. 2 Detailed fault segments of the EFZYB system and the WFZKL system. Fault names are adapted from Research Group for Active Faultsof Japan (1991), Nakata and Imaizumi (2002), AIST (2010), and Sawa et al. (2013). Surface ruptures during the 1896 Rikuu earthquake (shown in blue)
are from Matsuda et al. (1980). YD: Yakeishi Dake

Shyu et al. Progress in Earth and Planetary Science(2024) 11:41

data were obtained from isolated trenching sites along
the faults, or from one of the fault branches that may
not be representative for the entire fault system. As aresult, different ideas exist for the 1­ 03-to-105-year activity characteristics of these faults. For example, whereas
many previous studies suggest that the slip rates along
the southern segment of EFZYB is lower than that along
the northern segment (e.g., Headquarters for Earthquake Research Promotion 2005a), a recent study alonga branch fault of its southern segment indicate that theslip rates along the entire EFZYB may be similar (Aroraet al. 2021).
For the past two decades, fluvial geomorphic indiceshave been widely applied to study active deformations
in orogenic belts (e.g., Kirby and Whipple 2012). This isbased on the assumption that rivers, as the connection
between the uplifting mountain belts and the base level,
are the first-order control of the landscape relief and totransmit the tectonic forcing signals of the orogen. Oneof the most frequently used indices is the channel steepness index of river valleys that flow out of mountain belts,
which has been shown to provide information about thegeneral long-term uplift patterns of the mountain belt
(e.g., Kirby and Whipple 2012; Chen et al. 2015). Since
the channel steepness index generally reflects the conditions of the entire drainage basin, it would also provide information with a wider spatial extent than thatobtained near the fault trace. Patterns of fluvial channel
steepness indexes have been applied to understand characteristics of active faults (e.g., Kirby and Ouimet 2011;
Harvey et al. 2015), uplift patterns along mountain belts(e.g., Kirby and Whipple 2012; Miller et al. 2013; Chenet al. 2015), and the relationship between tectonic upliftand erosional processes across various mountain belts(e.g., Harkins et al. 2007; Cyr et al. 2010; DiBiase et al.
2010; Chen et al. 2021; DeLisle et al. 2022).
Despite the fact that this method has been applied tomany orogenic belts in the world, it has only been applied
scarcely in Japan (e.g., Regalla et al. 2013; Ogami 2015;
Takahashi et al. 2022, 2023). Therefore, this study aims
to understand the long-term activity characteristics ofboth the EFZYB and the WFZKL based on the analysis of channel steepness indexes of river valleys flowing
out of the OBR to delineate the variation of uplift ratesof the hanging-wall block along these fault systems. We
have also compared our results with data from previous
studies.
Our study provides one of the few results of geomorphic indexes applied to understanding the upliftof mountain ranges in Japan. Not only are these results
important in assessing future earthquake hazard in the
Tohoku Region of Japan, but they also demonstrate theapplicability of fluvial geomorphic analysis in providing

Page 4 of 15

additional constraints for understanding active faults inother parts of this important orogenic belt.

2 Geological background of the Ou Backbone
Range
The OBR is one of the most prominent topographic features in northern Japan, spanning ~ 100 km and forms
the boundary between the Iwate and Akita Prefectures(Fig. 1). On the western side of the range is the Yokote
Basin that separates the OBR and the Dewa Mountains
further to the west. On the eastern side of the range is
the Kitakami Lowland, which is also a young fluvial basinthat separates the OBR and the Kitakami Mountains in
the east.
Rocks within the OBR generally consist of Miocene
andesitic lava and pyroclastic rocks that are overlain by
Miocene to Pleistocene marine to terrestrial sedimentary rocks (e.g., Kagohara et al. 2009; Geological Surveyof Japan, AIST 2023; Fig. 3). Quaternary volcanoes arepresent mostly in the eastern part of the range, with theYakeishi Dake having the most prominent landform. The
most recent activity of this Pleistocene volcano is at ca.
0.2 Ma, and its surrounding area is composed of lava flow
and pyroclastic flow deposits from the volcano (Nakano
et al. 2013; Geological Survey of Japan, AIST 2020,2023). Two active volcanoes (Akita Komagatake, AK, andKurikoma Yama, KY) are present at the northern andsouthern end of the OBR (Nakano et al. 2013; Geological
Survey of Japan, AIST 2020; Fig. 1).
Along both sides of the OBR are two major active
fault systems identified and listed by the Headquarters
for Earthquake Research Promotion (HERP) of Japan.
Between the OBR and the Yokote Basin in the west, theEFZYB is composed of the Shiraiwa fault, the Ota fault,
the Senya fault, the Kanazawa fault, the Yokote fault,
and the Omoriyama fault from north to south (e.g.,
Research Group for Active Faults of Japan 1991; Nakata
and Imaizumi 2002; AIST 2010; Fig. 2). Some previous
studies have also mapped the Sugisawa fault to the eastof the Kanazawa fault north of Yokote (e.g., Sawa et al.
2013). West of the Yokote fault, the northern extension of the Omoriyama fault along the western edge ofa series of low-lying hills has also been named as the
Akasaka fault (e.g., Sawa et al. 2013). South of the Akasaka fault, the Higashi Chokai San fault along the western edge of the Yuzawa Hills is sometimes considered
as a frontal branch of the EFZYB system (e.g., ResearchGroup for Active Faults of Japan 1991; Sawa et al.
2013). In the east between the OBR and the Kitakami
Lowland, the WFZKL consists of the Nanshozan faultgroup, the Uwandaira fault group, the Yokomoriyama fault, the Houryouno-Urasawa fault, the Tengumori fault, and the Dedana fault from north to southShyu et al. Progress in Earth and Planetary Science(2024) 11:41

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Fig. 3 A Simplified geologic map of the OBR region. Data from https://​gbank.​gsj.​jp/​seaml​ess/​v2/​viewer/ (Geological Survey of Japan, AIST 2023).
YD: Yakeishi Dake. B Annual precipitation patterns of the study area. Data from https://​nlftp.​mlit.​go.​jp/​ksj/​gml/​datal​ist/​KsjTm​plt-​G02.​html(Research Group for Active Fault of Japan 1991; Nakata
and Imaizumi 2002; Fig. 2).
Both the EFZYB and the WFZKL are considered
as primarily dip-slip reverse fault, and no strike-slip
component has been identified or reported (Headquarters for Earthquake Research Promotion 2001, 2005a).
Based on seismic reflection survey data, the northern segment of the EFZYB dips eastward at 20°–35° at
depths shallower than ~ 1.5 km, and becomes ~ 45° at
deeper depths (Sato et al. 2006; Kagohara et al. 2009).
Data for the southern segments of the EFZYB is limited, but the fault appears to dip at about 45° to theeast (AIST 2010). However, Arora et al. (2021) foundthat the shallow part of the Omoriyama and Higashi
Chokai San faults may have similar gentle dip (20°–30°)

as the northern EFZYB. The WFZKL has a similar fault
geometry as the EFZYB. Near surface, the fault dips
westward at 20°–30°, and becomes 30°–40° to a depthof 7–8 km (Headquarters for Earthquake Research Promotion 2001; Kagohara et al. 2011).
The northern segment of the EFZYB, including the Shiraiwa fault, the Ota fault, and the Senya fault, ruptured in1896 in the M7.2 Rikuu earthquake (e.g., Matsuda et al.
1980; Research Group for the Senya Fault 1986; Fig. 2). In
contrast, the other segments of these two fault systems
do not have any historical surface-rupturing earthquake
record, but the M7.2 (Mw 6.9) Iwate-Miyagi Nairiku
earthquake in 2008 was accompanied by discontinuous
surface ruptures southwest of the Dedana fault, probably
along a previously unmapped fault in the hanging-wallShyu et al. Progress in Earth and Planetary Science(2024) 11:41

block of WFZKL’s southern extension (e.g., Matsu’ura
and Kase 2010; Toda et al. 2010; Fig. 1).
There are many previous studies on the Holocene
earthquake history and the slip rates of these two fault
systems. Along the northern EFZYB, for example, paleoseismological studies along the 1896 rupture revealed
that the penultimate event prior to 1896 occurred at ca.
3.5 ka, thus the earthquake recurrence interval for thenorthern EFZYB is proposed to be around 3400 years(Research Group for the Senya Fault 1986; Imaizumi et al.
1989a, 1989b; Headquarters for Earthquake Research
Promotion 2005a). Along the southern EFZYB, the mostrecent earthquake event was found to be after ca. 6–5 ka,
but the fault was not activated in 1896 (Akita Prefecture
1998a, b, 1999). Along the WFZKL, on the other hand,paleoseismological studies found that the most recent
earthquake event was ca. 4.5 ka, with a penultimate event
sometime between 30 and 7 ka (Watanabe et al. 1994;
Iwate Prefecture 1996).
Late Pleistocene or Holocene fault slip rates or vertical separation rates of the two fault systems have been
determined mainly using the deformation amount ofterraces across the faults and their ages. For example, anaverage late Holocene slip rate of the northern EFZYB
was estimated at about 0.8 mm/yr (Matsuda et al. 1980).
Late Pleistocene dip-slip rate of this segment has been
calculated at about 0.9–1.2 ± 0.3 mm/yr (Imaizumi et al.
1997; Kagohara et al. 2009; Matsu’ura and Sugaya 2017),
slightly higher than the late Holocene rate. Along thesouthern EFZYB, data on the slip rates are fewer, but it
is generally considered that the faults slip slower than
the northern segments, at about 0.2–0.3 mm/yr (Akita
Prefecture 1998a, b; AIST 2010). However, Arora et al.
(2021) recently reported a slip rate of 1.6–1.9 mm/yr overthe past 3.5 ka of the Higashi Chokai San fault, based on
borehole studies across the fault. This rate appears to be
similar to, or even greater than that along the northern
segment of the EFZYB.
Along the WFZKL, geomorphic analyses have determined that the Late Pleistocene to Holocene slip rate ofits northern segment is about 0.2–0.6 mm/yr (Nakata
1976; Iwate Prefecture 1998a, b). Based on the horizontal
shortening amount reconstructed from the faulted strata,
Kagohara et al. (2011) estimated a vertical separation rateof ~ 0.4 mm/yr since ca. 700 ka for the northern WFZKL,
which would correspond to a slip rate of ~ 0.6 mm/yr if
the fault dips at ~ 40°. For the southern segment of theWFZKL, Iwate Prefecture (1997) proposed a Late Pleistocene to Holocene slip rate of about 0.2 mm/yr. Further tothe south, in the areas of the Dedana fault (the southernmost segment of the WFZKL) and the surface ruptures ofthe 2008 Iwate-Miyagi Nairiku earthquake, vertical separation rates of ~ 0.1–0.5 mm/yr across the fault traces have

Page 6 of 15

been estimated from fluvial terrace studies (e.g., Watanabe
1989; Matsu’ura et al. 2008; Tajikara et al. 2009; Goto andSasaki 2019).
In the central part of the northern OBR, an intermontane
Yuda Basin is present (Fig. 1). Along the western edge ofthis basin, there is an Eastern Fault Zone of Mahiru Mountains (EFZMM; Headquarters for Earthquake Research
Promotion 2005b), which is generally considered as a
backthrust that branches out from the EFZYB at a depth
around 10 km (e.g., Kagohara et al. 2009; Arora et al. 2021).
In the middle segment of the EFZMM, some surface ruptures were also observed during the 1896 Rikuu earthquake(Matsuda et al. 1980).

3 Methods
Normalized channel steepness (ksn) has been shown tohave a positive relationship with erosion rate and hence canbe used to obtain a first-order view of the tectonic patternfrom the landscape (DiBiase et al. 2010; Kirby and Whipple2012). In this study, we used a simplified form of the stream
power model for erosion rate, which states that erosion rate
is a function of rock erodibility (K), upstream area (A), andchannel slope (S) (Howard and Kerby 1983; Whipple andTucker 1999), and considered mass balance that the changeof elevation (z) of a channel point through time (t) andspace (x) is determined by the differences between uplift
rate (U) and erosion rate (Eq. 1):∂z(t, x)
= U (t, x) − KAm∂t∂z(t,x)∂x(1)

where m and n are positive constants relating to themechanical process of erosion, hydraulic geometry, andbasin hydrology.
Instead of using the convention way to derive channel steepness with slope-area relation of channels (e.g.,
Kirby and Whipple 2012), we used the integral approach,χ analysis, to bedrock rivers proposed by Perron and
Royden (2012), which uses elevation data directly insteadof its derivatives. Assuming uplift rate and rock erodibility are spatially and temporally uniform, the steady-stateelevation of a channel point can be calculated from
 n1
 the
base level z(xb) as a function of channel steepness, U,
Kand χ:

z(x) = z(xb ) +



U
KAm
0

1

n

χ

(2)

where:

χ=

x 

xb

A0
A(x′ )

m
n

dx′

(3)

Shyu et al. Progress in Earth and Planetary Science(2024) 11:41

and A0 is an arbitrary scaling area to give χ dimensions oflength.
We used a 10-m resolution DEM of Japan obtained
from the Geospatial Information Authority of Japan.
To avoid the analyses of colluvial and alluvial channels
where the stream power model may not be valid, we set
a critical area of 1 k­ m2. This critical area is set according
to the change of relations between slope and area, whichsuggests that channels with drainage basin area smaller
than this are dominated by debris flows and colluvial processes. We also removed the alluvial area, including footwall regions of both the EFZYB and the WFZKL, and the
Yuda Basin in the middle of the OBR, so that the analysis
is focusing on the bedrock channels. We used TopoToolbox 2 that was developed by Schwanghart and Scherler(2014), to extract channels and calculate χ values with am/n value of 0.45 and an A0 value of 1 ­km2. The ksn values
were then calculated by dividing the difference in z by thedifference in χ for each channel reach with a moving window of 250 m.
We analyzed a total of 31 river basins along the twosides of the OBR (Fig. 4). We primarily chose the drainage
basins that flow out of the OBR across either the EFZYB
or the WFZKL to compare the river steepness patterns of
the hanging-wall blocks of these two fault systems. Sincerivers flowing into the Yuda Basin do not flow across thefaults, we excluded those river basins in our analysis. On
the other hand, the river systems north of our analyzed
basins have upstream tributaries flowing out of the Akita
Komagatake volcano (AK in Fig. 1), which is an active
volcano (Nakano et al. 2013; Geological Survey of Japan,
AIST 2020). Therefore, we also excluded those river systems from our analysis. Among the 31 basins, 12 arelocated along the western OBR and flow into the Yokotebasin, and 19 are located along the eastern OBR and flow
into the Kitakami Lowland.
In addition to deriving channel steepness for eachchannel reach, we also calculated basin-wide mean local
steepness (MLS) for the 31 basins in our study area inorder to characterize the spatial variance of uplift rate
along both flanks of the OBR. MLS is calculated following
the method proposed by Chen and Willett (2016). Each
local channel steepness is weighted by its local upstream
area, and the MLS is the mean of the local area-weighted
channel steepness of all channel reaches in a basin.

4 Results
4.1 River steepness patterns along both flanks of the Ou
Backbone Range

The ksn results of the 31 river basins we analyzed areshown in Fig. 4. Along both flanks of the OBR, the ksn
value distribution show a difference between the northern part and southern parts of the range flanks. Along

Page 7 of 15

the eastern flank of the OBR, the ksn values are generally
higher in basins in the south than those in the north. In
Fig. 4, this is illustrated by the generally warmer colorsof ksn values that appear along the river segments inthe drainage basins in the southern part of the eastern
OBR mountain front, south of the Waga River. In contrast, along the western flank of the OBR, the ksn values
in basins north of Yokote are generally higher, especiallythose flowing across the segment of the EFZYB that ruptured in the 1896 Rikuu earthquake (Fig. 4).
4.2 Knickpoints observations of the Ou Backbone Range
riversIn our analysis, we have identified many knickpoints
along the 31 river valleys that drain the two sides of the
OBR. Since the presence of certain types of knickpoints
may indicate that the river valleys are out of topographic
steady state (e.g., Kirby and Whipple 2012), we also conducted field investigations to understand the characteristics of the knickpoints.
Our investigation results indicate that almost all of the
knickpoints identified in the OBR rivers are related to
local lithologic boundaries or are check dams along thevalley (Fig. 5; Additional file 1: Fig. S1). We were unable
to access a few knickpoints, but most of those are locatedalong upstream segments of the rivers. Therefore, even
if those knickpoints that we did not investigate weretectonic knickpoints, they do not significantly affect thegeneral ksn patterns of the river basins. As a result, weconclude that the OBR river basins are generally under
topographic steady state.
4.3 Basin‑wide mean local steepness of the Ou Backbone
Range rivers

To better compare the ksn patterns of river basins along
the two sides of the OBR, we further calculated thebasin-wide mean local steepness (MLS) of the 31 basins.
The MLS values of the 12 river basins along the western
OBR front are between ~ 19 and ~ 70, and the MLS values of the 19 basins in the east are between ~ 16 and ~ 55
(Table 1). The MLS values have similar trends as the ksn
values, that is, along the western OBR front, the northernbasins north of Yokote have higher MLS values than thesouthern basins; along the eastern OBR front, the southern basins south of the Waga River have higher MLS values (Fig. 5). Among them, the northwestern OBR river
basins have generally the highest values of MLS.

5 Discussion
5.1 Mechanisms affecting steepness indexes of rivers

Nontectonic processes, especially climatic changes andlithological differences, may affect the ksn values of riversegments. However, based on the weather information

Shyu et al. Progress in Earth and Planetary Science(2024) 11:41

Page 8 of 15

Fig. 4 River steepness index (ksn) of the 31 analyzed river basins on both sides of the OBR. We excluded the rivers flowing into the Yuda Basinsince they do not flow across the fault systems. YD: Yakeishi Dake

available from the Japan Meteorological Agency, the river
basins we analyzed are all located at regions with similar
precipitation (Fig. 3B), and the pattern of precipitation is
generally reflecting the changes in elevation from lowland or basins to the drainage divides, without an obvious
latitudinal trend. Furthermore, we calculated the meanannual precipitation for each basin and compared it with
MLS (Additional file 2: Fig. S2). The result shows that
there seems to be a weak positive relationship between

these two values. However, based on the stream powermodel, higher precipitation will result in a higher rockerodibility (K) and lower MLS. This is opposite to theirpositive relationship, and suggests that the precipitation
is not the primary control of the MLS values. Therefore,
it is unlikely that the ksn differences among the OBR river
basins are primarily produced by differences in climate
conditions.

Shyu et al. Progress in Earth and Planetary Science(2024) 11:41

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Fig. 5 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 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

To further test if the ksn patterns are produced by differences of lithology, we analyzed ksn values of river
basins in two nearby regions west of the OBR, one to
the north and the other to the south of the Yokote Basin,
where the lithology is similar to the OBR but no active
faults were identified (Fig. 6). In these two areas, rivers flow through four major types of bedrocks that are

similar in age: sedimentary rocks with some tuff, volcanic
rocks, volcanic rocks with some sedimentary rocks, andtuff. These rock types are also the major rock types foundin the OBR (Figs. 3 and 6). In each of these two areas,
the average ksn values of river segments flowing throughthese different rock types vary, but overlap within uncertainties, showing no systematic variations caused by rockShyu et al. Progress in Earth and Planetary Science(2024) 11:41

Page 10 of 15

Table 1 Basin-wide mean local steepness (MLS) data of the 31 river basins analyzed in this study, together with previously publishedvertical separation rates of the two fault systems in the corresponding areas
No

Area

1

NW

2

NW

Basin area ­(km2)

Mean local steepness

Weighted average MLS

Published fault
vertical rates(mm/yr)9.65578

60.67

60.66

0.4–1.3

39.7108

70.05

34.41

0.1–0.4

34.40

0.2–0.6

48.31

0.4–0.5

3

NW

17.4968

70.26

4

NW

10.8431

58.64

5

NW

10.9370

23.89

6

NW

42.2993

57.89

7

SW

6.75494

36.37

8

SW

7.42205

29.99

9

SW

101.0420

31.67

10

SW

87.5279

28.02

11

SW

5.7846

19.04

12

SW

257.6540

38.08

13

NE

6.7463

41.49

14

NE

2.3849

36.34

15

NE

50.1790

27.29

16

NE

33.0572

36.40

17

NE

19.6266

26.79

18

NE

14.9837

27.32

19

NE

101.2140

37.02

20

NE

9.9853

34.97

21

NE

5.3751

25.54

022

NE

15.3485

35.60

23

NE

31.9513

37.70

24

NE

12.0159

42.73

25

NE

77.6148

35.01

26

SE

21.1544

55.58

27

SE

51.1843

51.87

28

SE

9.2057

16.59

29

SE

20.4161

36.22

30

SE

34.5379

41.52

31

SE

191.2260

50.60

type differences (Fig. 6). Therefore, we conclude that thedifferences in rock types do not produce significant ksn
differences in the OBR rivers.
We propose that the ksn patterns in OBR rivers are primarily produced by differences in the activities of theactive faults along the OBR mountain fronts. The generally higher MLS values for the river basins along thenorthwestern OBR front indicate that rivers there havegenerally steeper valleys, whereas the generally lower
MLS values for the southwestern OBR rivers indicate thatthey have gentler river valleys. Under similar climatic andlithological conditions, this would suggest that the northwestern OBR has higher uplift rates than the southwestern OBR.

The OBR is bounded on its two sides by the EFZYB and
the WFZKL, both of which are primarily dip-slip reverse
faults (Headquarters for Earthquake Research Promotion 2001, 2005a). Therefore, the uplift patterns of the
OBR flanks are likely reflecting the variations of verticalseparation rates of the two fault systems. The northern
EFZYB, for example, would have a higher vertical separation rate than the southern EFZYB to produce a higher
uplift rate for the northwestern OBR. The vertical separation rates of faults are related to their actual slip rates
and their dip angles. Although there are not many data
about the subsurface geometry of the two fault systems,
all available information indicates that their subsurface
geometries are similar. Both fault systems have gentle

Shyu et al. Progress in Earth and Planetary Science(2024) 11:41

Page 11 of 15

Fig. 6 Analysis of the relationship between lithology and channel steepness. We analyzed ksn values of river basins in two nearby regionsof the OBR, where the lithology is similar to the OBR but no active faults were identified, to test if the ksn patterns are produced by differencesof lithology. In each of these two areas, the average ksn values of river segments flowing through different rock types overlap within each other’s
uncertainties, showing no systematic variations caused by rock type differences

dips of ~ 20°–35° at shallow depths, and become steeper
at 30°–45° at deeper depths (Headquarters for Earthquake Research Promotion 2001; Sato et al. 2006; Kagohara et al. 2009, 2011; AIST 2010; Arora et al. 2021).
There is also no apparent difference in the fault geometry from north to south. As a result, we propose that the
MLS pattern illustrated in Fig. 5 is primarily produced byvarying slip rates along different segments of the EFZYB
and the WFZKL.

5.2 Structural activity patterns of the Ou Backbone Range

Based on the discussion above, we propose that the different MLS values shown in the river basins of OBR(Fig. 5) reflect different slip rates along the EFZYB and
the WFZKL. We therefore divided both OBR fronts into
northern and southern segments, and calculated theaverage MLS values (weighted with drainage basin areas)
for the four mountain front segments. The northwestern
OBR front has the highest average MLS value of 60.66,

Shyu et al. Progress in Earth and Planetary Science(2024) 11:41

albeit it also has a large scattering (Table 1). On the other
hand, the southwestern and northeastern OBR fronts
have significantly lower average MLS, and the average
MLS value for the southeastern OBR front is somewhat
in between.
Because the channel steepness is proportional to the
uplift rate (e.g., Kirby and Whipple 2012), the nearly
twice higher value of the average MLS along the northwestern OBR front than that along the southwestern
OBR front suggests that the northwestern OBR is uplifting much faster than the southwestern OBR. Along the
Shiraiwa fault and the Ota fault in the northern EFZYB,
Mizumoto (2006) proposed a Holocene vertical separation rate of ~ 0.4–1.1 mm/yr based on the ages and offset
amount of river terraces across the faults. Similarly, along
the Senya fault in the northern EFZYB, Holocene and late
Pleistocene vertical separation rates of ~ 0.9–1.3 mm/yr
and ~ 0.8–0.9 mm/yr, respectively, have been proposed(Nakata 1976; Matsuda et al. 1980; Hirano 1984; Mizumoto 2006). Data points along the southern EFZYB aremuch fewer, but Akita Prefecture (1998a, b, 1999) found amuch lower late Pleistocene to Holocene vertical separation rate across the fault of ~ 0.1–0.4 mm/yr. The patternsof these numbers are consistent with the variations of theaverage MLS values of the river basins. A similar uplift
pattern is also reported by Tajikara and Ikeda (2005)
using the height differences between fluvial terraces. As
discussed above, the subsurface geometry of the EFZYB
does not change significantly from north to south, andthe slower uplift along the southern EFZYB likely represents lower slip rates of the fault. Therefore, based on
our observations of river valley steepness, we suggest thatthe less constrained southern EFZYB does slip at a much
lower rate than the northern segment.
On the eastern side, the southern segment of the eastern OBR front has higher average MLS than the northern
segment. This pattern would suggest that the southeastern OBR is uplifting at a higher rate than the northeastern OBR, and the southern segment of the WFZKL
slips at a higher rate than the northern segment. However, existing data do not show significant difference for
uplift rates along the WFZKL (e.g., Tajikara and Ikeda
2005). For example, along the Nanshozan fault group,
the Uwandaira fault group, and the Yokomoriyama fault
in the northern WFZKL, Mizumoto (2006) calculated
a Holocene vertical separation rate of ~ 0.3–0.5 mm/yr,
and the late Pleistocene rate was suggested to be ~ 0.2–
0.6 mm/yr (Watanabe 1991; Iwate Prefecture 1996,
1998a, b; Mizumoto 2006; Kagohara et al. 2011; Kosaka
et al. 2013). These numbers are much lower than the rates
along the northern EFZYB, consistent with the MLS patterns. Along the southern WFZKL, data points are alsomuch fewer. Based on the ages of offset river terraces,

Page 12 of 15

Mizumoto (2006) calculated a Holocene vertical separation rate of ~ 0.5 mm/yr, and Iwate Prefecture (1998a, b)
proposed a late Pleistocene rate of ~ 0.4 mm/yr. These
numbers are all similar.
We suggest that the slightly higher average MLS values
in southeastern OBR may have been produced by young
pyroclastic materials in the vicinity of the Yakeishi Dake
(Fig. 3). In our analysis, we have avoided the areas with
Holocene active volcanoes to prevent the influences of
volcanism on the uplift and incision of rivers. However,
although the Yakeishi Dake is not active in the Holocene,
its last activity at ca. 0.2 Ma had produced a large amountof pyroclastic materials (Nakano et al. 2013; Geological Survey of Japan, AIST 2020, 2023). These pyroclastic deposits may have filled up previous topography
and rejuvenated river valleys in this area. Therefore, thepresence of young pyroclastic materials from the Yakeishi Dake may have contributed to the steepening of river
channels in the southeastern OBR.
5.3 Implications for future earthquake hazards

The OBR is one of the most prominent topographic features in the Tohoku Region of Japan. The two active fault
systems bounding the two sides of the OBR are bothmajor active fault systems in Japan, and pose importantearthquake hazard potentials for the region. The northern segment of the EFZYB, for example, is the seismogenic structure of the 1896 M7.2 Rikuu earthquake,
which had resulted in more than 200 casualties (Matsuda
et al. 1980). Although there have been many studies on
the activities of these two major active fault systems,
information on segments other than the northern EFZYB
is still limited. As a result, it is still unclear whether
the southern segment of the EFZYB is as active as thenorthern segment and poses similar earthquake hazard
potential.
Our geomorphic analysis results on river basins flowing out of both OBR fronts suggest that the northwestern OBR front has the steepest river valleys. This implies
that the northern segment of the EFZYB has the highest
slip rates among the different segments of the EFZYB
and the WFZKL. This is consistent with the available slip
rate data of the two fault systems, and the fact that thesurface rupture during the 1896 Rikuu earthquake waslimited in the northern segment of the EFZYB. If this iscorrect, the earthquake recurrence intervals for the othersegments of the two fault systems would be longer thanthe ~ 3500 years proposed for the northern EFZYB (e.g.,
Imaizumi et al. 1989a, 1989b). For example, if we assume
the slip rates of those segments are only about a half ofthe rate of northern EFZYB as implies by the MLS patterns, and the average slip per earthquake event is similar
for all segments, the earthquake recurrence intervals forShyu et al. Progress in Earth and Planetary Science(2024) 11:41

those segments would be around 7000 years, and longer
if the average slip per event is larger. This would be consistent with previous estimations (e.g., Headquarters for
Earthquake Research Promotion 2001, 2005a).
However, since there are not many constraints for thetiming of the last rupture event for those fault segments(e.g., Headquarters for Earthquake Research Promotion 2001, 2005a), a longer recurrence interval does notmean that the probability for an earthquake occurringin the near future is low. More detailed paleoseismological information on these two major fault systems is stillneeded for a better understanding of future earthquake
hazard potentials for this region.

6 Conclusions
In this study, we analyzed the steepness indexes of 31
river basins that flow out of the two flanks of the OBR.
Along the western OBR front, river basins in the north
have steeper valleys than those in the south, and river
basins along the northeastern OBR front have slightly
gentler valleys than those along the southeastern OBR
front. We have found knickpoints in these river basins,
but most of them are related to local lithologic boundaries or are check dams. Therefore, we suggest that theriver steepness patterns are reflecting variations in uplift
rates along the OBR flanks, which in turn reflecting thestructural activities of the EFZYB and the WFZKL, thetwo major active fault systems on both sides of the OBR.
Our results suggest that the northern segment of the
EFZYB has a higher slip rate than the southern segment. This is consistent with results of previous studies
and the fact that the 1896 M7.2 Rikuu earthquake only
ruptured the northern segment of the EFZYB. Therefore,
the earthquake recurrence intervals for the southern
segment of the EFZYB or the WFZKL would be longer
than the published ~ 3500 years for the northern EFZYB.
However, since there is no good constraint for the lastearthquake event along these segments, more paleoseismological studies on these two major fault systems is stillneeded.
Supplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​024-​00644-9.
Additional file 1
Additional file 2
Acknowledgements
We are grateful for the enlightening discussions with T. Imaizumi, T. Nakata,and Y. Ota. We would like to thank C.-H. Chen, S.-C. Liu, C.-H. Tsai and Y.-H. Yin
for the assistance in the field investigations, and H.L.B. Chou for preparing
the figures of this paper. We appreciate the comments and suggestions
from N. Toke and an anonymous reviewer, which significantly improved thisPage 13 of 15

manuscript. This study was partly supported by the World Premier Interna‑
tional Research Center Initiative (WPI), MEXT, Japan.
Author contributions
JBHS contributed to conceptualization, investigation, visualization, project
administration, funding acquisition, writing-original draft preparation, andwriting-review and editing. JHL contributed to formal analysis, investigation,
visualization, and writing—original draft preparation. CYC contributed tomethodology, software, investigation, and writing—review and editing. HT
contributed to data curation and writing—review and editing. YI contributed
to investigation and writing—review and editing. All authors have read andapproved the final version of the manuscript.
Funding
This research was supported by the National Science and Technology Council(NSTC) of Taiwan (108-2116-M-002-016-MY2, 110-2116-M-002-019, and 1112116-M-002-044 to JBHS).
Availability of data and materials
The 10-m resolution DEM of Japan analyzed in this study is available from theGeospatial Information Authority of Japan at https://​www.​gsi.​go.​jp/​top.​html.
The geologic map of the study area is available from the Geological Survey ofJapan at https://​gbank.​gsj.​jp/​seaml​ess/​v2/​viewer/. The annual precipitation
data are from https://​nlftp.​mlit.​go.​jp/​ksj/​gml/​datal​ist/​KsjTm​plt-​G02.​html. The
channel steepness index data generated in this study are included in Table 1.
Competing interests
The authors declare that they have no competing interests.
Received: 11 December 2023 Accepted: 28 June 2024

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