Abe et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-019-0318-6(2020) 7:7

RESEARCH ARTICLE

Progress in Earth andPlanetary Science

Open Access

Spatial distribution and sources of tsunami
deposits in a narrow valley setting - insightfrom 2011 Tohoku-oki tsunami deposits innortheastern Japan
Tomoya Abe1* , Kazuhisa Goto2,3 and Daisuke Sugawara2,4

Abstract
Sedimentary processes and spatial distributions of tsunami deposits in valleys have poorly been understood despite
many paleo-tsunami deposits have recently been discovered from sedimentary sequences in valleys. We conducted
an exhaustive investigation of 2011 Tohoku-oki tsunami deposits in a narrow valley at the south end of Sendai
Plain, northeastern Japan, to collect sedimentological data of the tsunami deposits in the valley and to correlatethem with tsunami inundation and associated sedimentary processes. The tsunami deposits were investigated at
174 sites to cover the entire inundation area of the narrow valley. We analyzed thickness, sedimentary structures,and grain size of the tsunami deposits to correlate their spatial variation with tsunami flow hydrodynamics andtopographic features. The tsunami deposits, composed of sand and mud layers, were found to be 0–40 cm thick.
Sand thickness generally decreased inland with local fluctuation. The sand layer was mostly composed of single
unit on the upper main valley and sub-valleys, although it was mainly composed of 2–6 sub-units on the lower
main valley and in the pond. Mud thickness is strongly controlled by local sources, namely, the rice paddy and thepond. The depositional area of the mud layer coincides with the distribution of the rice paddies and the pond
location. The mud layer also considerably thicker in and around the pond. These results indicate that sediment
thickness and sedimentary structures are highly varied on a local scale, but they are generally controlled by local
topography. Sediment budget in the valley was estimated, and it may provide implications to general sedimentary
process. Total depositional volume of the tsunami sand deposited on land is half of the volume of sediments
disappeared from the sandy beach and sand dune, suggesting the rest must have been discharged into the sea by
the backwash. Spatial distribution of sediment thickness and sedimentary structures as well as sedimentary
processes inferred from 2011 Tohoku-oki tsunami deposits will benefit searching and identification of paleo-tsunami
deposits in valleys of other coastal environments.
Keywords: 2011 Tohoku-oki tsunami, Narrow valley setting, Tsunami deposit, Spatial distribution, Sedimentary
processes, Sediment budget* Correspondence: tomoya-abe@aist.go.jp
1
Geological Survey of Japan, National Institute of Advanced Industrial
Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567,
Japan
Full list of author information is available at the end of the article© The Author(s). 2020 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

Abe et al. Progress in Earth and Planetary Science(2020) 7:7

Introduction
Tsunami deposits are useful for estimating recurrence
intervals and magnitudes of past tsunami waves (Goffet al. 2012). Ancient tsunami events have been elucidated since the 1980s using paleo-tsunami deposits(Atwater 1987; Dawson et al. 1988; Minoura and Nakaya1991). Nevertheless, identification criteria and sedimentological characteristics of tsunami deposits were notwell understood (Shanmugam 2012). Case studies ofrecent tsunami effects have proven to be extremely useful for understanding the sedimentary processes that
occur during tsunami flow and for refining the criteria
used to identify paleo-tsunami deposits in the geologicalrecord (Weiss and Bourgeois 2012). Therefore, many
field observations were conducted soon after recent tsunamis, especially after the 1990s (e.g., Shi et al. 1995;
Gelfenbaum and Jaffe 2003).
Many studies of recent tsunami events have clarified
sedimentological characteristics of tsunami deposits. For
instance, tsunami deposits generally become thinner andfiner inland (Peters and Jaffe 2010). However, the thickness, grain size, and sedimentary structures of tsunami
deposits fluctuate locally under the influence of hydraulic characteristics, topography, and sediment sources(e.g., Hori et al. 2007; Paris et al. 2007; Moore et al.
2011). Most studies of recent tsunami deposits, especially of the 2011 Tohoku-oki tsunami, were conducted
at low-lying coastal plains, where tsunami inundation
might have been generally unrestricted by topography(e.g., Goto et al. 2011; Abe et al. 2012; Richmond et al.
2012; Szczuciński et al. 2012; Takashimizu et al. 2012).
The tsunami deposits left by recent tsunamis in lowlying coastal plains generally exhibit landward thinningand fining trends (e.g., Gelfenbaum and Jaffe 2003; Horiet al. 2007; Fujino et al. 2010; Szczuciński et al. 2012). In
contrast, studies of recent tsunami deposits in valleys,
where tsunami inundation can be restricted by topography, are rare (Apotsos et al. 2011; Naruse et al. 2012;
Putra et al. 2013; Yamada et al. 2014; Goto et al. 2017).
These works have reported some characteristics of tsunami deposit in valleys as follows. Apotsos et al. (2011)
inferred that onshore transport and deposition of tsunami deposits were limited by the availability of sediment supply and a steep slope. Naruse et al. (2012)
evaluated onshore tsunami deposits as sourced not only
from eroded beach sands but also from the seafloor
judged from on rough estimation of erosional and depositional volumes by the tsunami and occurrence of ostracod assemblages in the tsunami deposit. Yamada et al.
(2014) demonstrated that sudden deceleration of thetsunami flow velocity, constrained by the local topography, caused rapid deposition of tsunami boulders andof layered sandy to gravelly tsunami deposits. Putra et al.
(2013) argued that tsunami flow forms onshore deposits

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that become finer and thinner landward and showed thatthey were laterally variable, being thinner in the topographic lows than in the surrounding higher areas. Goto
et al. (2017) described transportation of eroded clasts
back toward the valley bottom as associated with the return flow along a channel. These observations imply thatsource and transport processes in valleys possibly differ
from those found on low-lying coastal plains. From theabove, the distribution pattern and sedimentary process
in a valley are surely more complicated than those on a
low-lying coastal plain. It is possible that these sedimentological characteristics greatly differ among landform
types. Recent geological studies of tsunami deposits havespecifically emphasized low-lying coastal plains rather
than valleys. Some reasons might explain that dominant
preference: (1) Emergency surveys after tsunami events
have a time constraint related to post-disaster recovery
processes. (2) Tsunami deposits in low-lying coastal
plain present advantages for comparison with numericalsimulations and flume experiments. (3) Sedimentary
processes on low-lying coastal plains are easier to understand than those taking place in valleys. In fact, on lowlying coastal plains, observations along shore-normaltransects are an effective strategy to investigate tsunami
inundation and associated sedimentary processes. Because surveys of recent tsunami events in valleys havebeen few for the reasons described above, sedimentary
processes and spatial characteristics of tsunami depositin valleys still have remained poorly understood.
Earlier studies of tsunami deposits have typically been
conducted along shore-normal transects (e.g., Gelfenbaum and Jaffe 2003; Hori et al. 2007; Moore et al. 2011;
Takashimizu et al. 2012). Actually, spatial surveys arerare (Paris et al. 2007; Choowong et al. 2008; Naruse
et al. 2012) because distribution patterns of tsunami deposits are presumed to have a dominant shore-normal
trend. This assumption is probably valid for low-lying
coastal plains in cases where the tsunami inundation occurs in a direction perpendicular to the shoreline. By
contrast, tsunami flow and sedimentary processes in valleys can be complex because of topographic constraints.
Field observations along shore-normal transects might
be insufficient to characterize tsunami inundation andsedimentation occurring in valleys.
Before the 2011 tsunami, paleo-tsunami deposits on
low-lying coastal plains had been well studied (e.g., Atwater 1987; Minoura and Nakaya 1991). To enhancecapabilities of finding suitable places for paleo-tsunami
research, valleys should be regarded as target areas. In
fact, after the 2011 tsunami, reports of paleo-tsunami
deposits in valleys have also increased (e.g., Goto et al.
2015, 2019; Ishimura and Miyauchi 2015; Inoue et al.
2017; Fujino et al. 2018; Kusumoto et al. 2018; Shimada
et al. 2019), although recent examples of tsunami

Abe et al. Progress in Earth and Planetary Science(2020) 7:7

deposits in valleys, which can be a reference of interpreting paleo-tsunami deposits, still have been very few.
Regarding this point, improved understanding of sedimentary processes and spatial distribution of recent
tsunami deposit in valleys is crucially important for
paleo-tsunami deposit research.
The main objective of this study is to examine spatial
distributions and sedimentary processes of 2011
Tohoku-oki tsunami deposits in a valley setting using
exhaustive field observation and analyses of grain size
distributions and diatom assemblages. Furthermore, weestimated the sediment budget between depositional anderosional volumes based on a spatial thickness distribution and comparison of pre-tsunami and post-tsunami
aerial photographs and high-resolution digital elevation
models (DEM), to illustrate general sedimentary process
occurred in a valley.

Study area
Deposits of the 2011 Tohoku tsunami were surveyed ina narrow valley of Yamamoto Town, Miyagi Prefecture,northeastern Japan. The area is situated at the southernend of Sendai Plain (Fig. 1a). The valley is surroundedby hills of greater than 15 m height, comprising Pliocene
sandstone (Fig. 1b, Additional file 2: Fig. S1). The 2.3 km
long main valley stretches roughly east–west, with subvalleys developed in a nearly orthogonal direction to themain valley with lengths ranging 10–100 m. The land
use before the 2011 tsunami is characterized by (1) a 0–
80 m wide sandy beach (Fig. 2a), (2) 30–100 m wide
coastal forests (black pine trees) on 2–3 m high sanddunes, and (3) a 2 km wide wetland for rice paddies. The
6.2 m high seawalls were constructed at the boundary
between back beach and sand dune before the 2011 tsunami. These seawalls were mostly broken by the tsunami. A small coastal pond, named Suijin-numa (90 mlong, 200 m width, less than 3 m depth) (Fig. 2b), isplaced in lowland at 0.7–0.9 km from the shoreline.
Paleo-tsunami deposits such as those formed by the 869
Jogan and the 1611 Keicho tsunami were found earlier
from the pond sediment (Sawai et al. 2008). Besides, Shinozaki et al. (2015) reported that considerable bottom
erosion (approx. 1.4 m thick) took place in this lake by
the 2011 tsunami.
Matsumoto (1981) revealed that the post-glacial sea
level rose from 8 to 5 ka and that it reached close to thepresent sea level at around 5 ka; the alluvial lowland migrated seaward during the last 5 ka with stabilization of
the relative sea level in Sendai Plain. Processes of sediment transport and deposition can vary between the alluvial lowland and other portions of the valley, due tothe difference in the sloping. Based on analysis of aerial
photographs and DEM, topographic boundary between
flatter alluvial lowlands and steeper hill slopes in the

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valley was interpreted as around 3 m above Tokyo Peil
(T.P.) (Fig. 1c).
The landform types of the study area were divided into
the lower main valley (< T.P. 3 m), upper main valley (>T.P. 3 m), north sub-valley (> T.P. 3 m), south sub-valley(> T.P. 3 m), pond, sand dune, and sandy beach (Fig. 1c).
Furthermore, we subdivided sub-valley areas into N1–
N4 and S1–S3 (Fig. 1c).

Methods/experimental
Field survey

Field surveys were mostly conducted in July, October,and November in 2012, with additional works conductedin April 2013 and December 2014 (Table 1). We investigated 2011 tsunami deposits at 174 survey sites (land,
162 sites; pond, 12 sites) at 10–100 m spacing in the valley (Fig. 1b) to elucidate spatial variation, sedimentary
processes, and sediment budget of the tsunami deposit.
We conducted pit surveys with less than 1 m depth onland areas and coring with 1 m depth using a RussianPeat Borer on shipboard for pond areas. We observed
thickness, grain size, and sedimentary structures (normal
grading, inverse grading, lamination, mud (rip-up) clasts,
mud drape, and the number of sand sub-units) of thetsunami deposit at each site to clarify their spatial characteristics. We conducted additional survey for taking
beach sand sample (sampling site is shown in Fig. 1c) for
grain size analysis in December 2014.
We also observed soil erosion at the base of the tsunami deposit to estimate the erosion volume. According
to interviews with local farmers, the pre-tsunami thickness of rice paddy soil was approximately 0.2 m in thisarea. We assumed that this is the typical thickness of
paddy soil. We observed the thickness of the remaining
soil beneath the tsunami deposit. Besides, we identified
the numbers of sub-units in the sand layer judged fromexistence of the mud drape or an internal erosionalboundary within sand layers and abrupt change in grain
size and mineral composition within the sand layers.
The tsunami flow height at the shrine, called MizuShrine, placed at the center of the valley (Fig. 1b), wasmeasured by using Differential Global Positioning System (DGPS) survey equipment and hand-held laser rangefinders (Impulse 200; Laser Technology Inc.) in July,
2012. The run-up height at the inundation limit in thevalley head (Fig. 2c) was estimated using DEM with gridsize 2 m. The flow heights and run-up heights measured
by the 2011 Tohoku Earthquake Tsunami Joint Survey
Group (TETJSG; Mori et al. 2012) were used as supplemental data (Fig. 1b). The pond depth was measured
using a portable sounder (Hondex PS-7; Honda Electronics Co. Ltd.). Based on visual observations of thetsunami deposits, the deposits in the study area wereclassified into three types: a sand layer covered by a mudAbe et al. Progress in Earth and Planetary Science(2020) 7:7

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Fig. 1 Maps showing a the location of study area; b survey site, run-up height, flow height, inundation limit, and location of photograph sites inFig. 2; and c topographic type and sampling site for analysis. a The inundation area of the 2011 tsunami is based on data provided by the
Geospatial Information Authority of Japan (GSI). b, c The aerial photographs taken on the day after the tsunami (March 12, 2011) and theshoreline location were provided by GSI. Inundation limit, run-up height, and flow height were measured for this study and by the 2011 Tohoku
Earthquake Tsunami Joint Survey Group (TETJSG). c The contour line is based on the 2 m grid digital elevation model (DEM) data after thetsunami provided by GSI. No erosion/deposition means that no erosion of pre-existing sediments or no deposition caused by the 2011 tsunami

Abe et al. Progress in Earth and Planetary Science(2020) 7:7

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Fig. 2 Photographs taken in the study area. a Beach eroded and seawall broken by the 2011 tsunami (July 19, 2012). b Pond (Suijin-numa)(November 21, 2012). c Inundation limit at valley head in the upper main valley (July 18, 2012). d Tsunami sand layer covered with tsunami mud
layer in the pond (April 11, 2013). Photograph sites are portrayed in Fig. 1b. TS, tsunami sand layer; TM, tsunami mud layer; PS, pond sediment. e,f Tsunami sand layer covered with tsunami mud layer (April 11, 2013). g, h Tsunami sand layer with four sub-units (April 11, 2013). e, g Raw
photos. f, h Photos with interpretation of the structure of the tsunami deposit. Photograph sites are portrayed in Fig. 1b. TS, tsunami sand layer;
TM, tsunami mud layer; MC, mud clasts

Abe et al. Progress in Earth and Planetary Science(2020) 7:7

Table 1 List of survey dates and number of survey sites during
each period
Survey month

Number of survey sites

July 201237

October 201252

November 201274

April 201311

December 20141layer (Fig. 2d–f), a sand layer without an overlying mud
layer (Fig. 2g, h), and a mud layer without an underlying
sand layer.
Grain size distribution

Sandy deposit samples were collected at 48 sites in thevalley (beach sand, 1 site; sand dune, 3 sites; tsunami deposit, 43 sites; Fig. 1c). Respective units and sub-units ofthe sand layers were sampled separately for grain size
analyses to identify the sand source and to reveal landward change of grain size distribution. Before analysis,organic matter was removed using hydrogen peroxide.
Sieving was performed to separate sediments finer than
63 μm (4 phi) and coarser than 2 mm (− 1 phi). We used
a settling tube for analysis of grain size distribution of
sandy sediments ranging from 4 to − 1 phi. We calculated the average of median grain size and grain size
sorting at each sampling site. The average calculation isweighted by using a weighting coefficient of the thickness. Grain size data were compared with existing dataof the grain size distribution of the seafloor sands sampled from the offshore sea bottom (12–23 m depth) off
the coast of the valley (Ohshima et al. 1986).
Diatom assemblages

Diatom assemblages were analyzed to estimate the mud
source (Additional file 2: Fig. S2). We sampled the tsunami mud layers at 5 sites (GPS21, 0–6 cm depth;
GPS42, 4.5–5.0 cm depth; GPS172, 0–3 cm depth;
GPS182, 0–2.5 cm depth; GPS185, 0–3 cm and 3–9 cm
depth; Fig. 1c), rice paddy soil at 1 site (GPS12, 6.5–7.5
cm depth; Fig. 1c), and bottom sediment of the pond at
1 site (GPS 7, 29–30 cm depth; Fig. 1c). Diatom identification and nomenclature were based on methods described by Kosugi (1988) and Ando (1990).
Estimating erosional and depositional volume
attributable to the tsunami

Using densely collected sedimentary data, the onshore
deposit volumes of sand and mud based on the deposition area and interpolated thickness distribution were
estimated by the use of Geographic Information System(GIS). The natural neighbor interpolation method (Sibson 1981) was employed to estimate the onshore deposit

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volumes. In addition, we estimated the erosional volume
attributable to the tsunami using DEMs, aerial photographs, satellite images, topographic maps, and fielddata. Pre-tsunami (February 2006) and post-tsunami(March and October 2011) DEMs with grid size 2 m andaerial photographs provided by the Geospatial Information Authority of Japan (GSI) were used for comparing
between the deposit distribution and the topographic
features and for estimating the erosional volume. Satellite images, aerial photographs, and topographic maps
taken before and after the tsunami were analyzed to estimate the erosional features and erosion areas. Considering co-seismic subsidence by the 2011 earthquake,elevation of the post-tsunami was shifted by 0.26 mbased on geodetic observation data (GSI Maps provided
by GSI, https://maps.gsi.go.jp/).

Results
Hydrodynamic behavior of tsunami waves

Analysis of aerial photographs, taken on March 12,
2011, indicated that the 2011 Tohoku-oki tsunami fully
inundated the main valley and sub-valleys (Fig. 1b). The
maximum inundation distance was 2.2 km from theshoreline. The maximum run-up height is T.P. 14.6–
16.4 m at the hill along the shoreline (Fig. 1b). It reached
T.P. 12.8 m at the valley head of main valley (Fig. 2c).
The maximum flow height along the shoreline is T.P.
8.9–11.4 m.
Eyewitness accounts of local people described wavehydrodynamic behavior of the 2011 tsunami in this area.
(1) Before the arrival of the first incoming wave, the sea
level receded rapidly. The shallow seafloor was exposeda few hundred meters offshore from the coast. (2) About
1 h after the main shock, the tsunami inundated the valley and reached the top of Isozakiyama Park (Fig. 1b,
T.P. 17–18 m) (Kahoku Online News, published on May
14, 2011, https://www.kahoku.co.jp/special/spe1168/201
80226_01.html). (3) At least three incoming waves overtopped the seawall and sand dune. (4) Backwash discharged rubbles into the sea.
Erosion by the tsunami

Comparison of aerial photographs taken on March
12, 2011, and in October 2006 indicated extensive
erosional features along the coast (Fig. 1b). The tsunami severely eroded the sandy beach and sand dune
(Fig. 2a, Additional file 2: Fig. S1). In particular, thetsunami flow caused scouring of the ground and sanddune behind the seawall. Most of the pine trees onsand dune were bent landward. According to field observations, the soil erosion area in the rice paddy was
more than approx. 90% in the lower main valley. In
contrast, it was less than about 10% in the upper
main valley and sub-valleys.

Abe et al. Progress in Earth and Planetary Science(2020) 7:7

Deposition by the tsunami
Tsunami deposit thickness

Among 174 survey sites, traces of tsunami inundation anddeposition were well preserved at 166 sites in the valley.
The tsunami traces at 8 other sites were disturbed bypost-tsunami human activities. Although the tsunami deposits were identified at 155 sites, we observed no erosionof pre-existing sediments or no deposition caused by the2011 tsunami at 9 sites in the main valley and sub-valleys
except for 2 sites in sand dune (Fig. 1b). Tsunami deposits
generally comprise a combination of sand and mud layers.
Their total thickness (sum of thicknesses of sand and mud
layers) was 0–40 cm in the lower main valley and the pond
(Fig. 3a, b). Up to 0.1 km inland from the shoreline, either
erosion of the surficial soil or lack of deposition of tsunami sand/mud was evident on the sandy beach and thesand dune. Between 0.1 and 2.2 km inland, the tsunami
deposits were mostly distributed everywhere. The total
thickness of the deposits generally decreased landward,
but it fluctuated locally (Fig. 3a, b).
Sand layers were identified at 150 sites. The sandlayers reached near the inundation limit in the main valley (Fig. 3a). They were composed mainly of fine-grained
to medium-grained sand, showing either sharp or unclear erosional contact with underlying sediments. The
sand mostly comprised light minerals such as quartz andfeldspar grains, but it also contained heavy minerals suchas mica, pyroxene, and magnetite grains. Thin laminae(less than 1–2 mm thick), which are characterized bymagnetite concentration, were observed inside the sandlayer at some sites. Shell fragments were rarely observed
inside the sand layers. The sandy tsunami deposit wascomposed of either single or multiple sub-unit(s) (Fig.
2f, h). Sand layers with single unit were mostly observedon the upper main valley and sub-valleys. Sand layerswith multiple sub-units were observed on the lower
main valley and in the pond. Figure 3a shows the totalsand thickness, which is defined by the sum of the thickness of sub-units of the sand layer. The total sand thickness generally decreased landward (Figs. 3a and 4a).
Mud layers were observed at 102 sites. Distribution of
the mud layers was mostly limited to the rice paddy andthe pond (Fig. 3b). The mud layer was composed of clay
to silt, but it included floating debris and vegetation
fragments, with precipitation of salt on the surface. The
mud layers formed either a surface mud cap or a mud
drape between sub-units of sand layers. In Fig. 3b, thetotal mud thickness is defined by the sum of thicknessesof the surface mud cap plus the mud drape at each site.
The total mud thickness was 0–40 cm at each area (Figs.
3b and 5a). Up to 0.4 km inland from the shoreline,
which includes the sandy beach, sand dune, and residential areas, no mud layer was observed. Between 0.4 and0.7 km inland, the mud cap thickness was less than a few

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centimeters. The total mud thickness increased to 40 cm
in and around the pond. From 0.9 to 2.2 km inland, thetotal mud layer thickness was a few centimeters, withlocal fluctuation (Figs. 3b and 5a).
We compared the deposit thickness (sand, mud) andthe elevation of the bottom of the tsunami deposit using
the post-tsunami DEM with grid size 2 m (Figs. 4b and5b). The total sand thickness was well correlated to theelevation of the bottom in the upper main valley, and in
the south and north sub-valleys (Fig. 4b). Relations between mud thickness and the elevation of the bottom
are unclear in all topographic types (Fig. 5b).
Sedimentary structures

Normal grading, inverse grading, parallel lamination,
mud clasts, and mud drape were observed inside the tsunami deposit. Each sedimentary structure was observed
only inside the sand layer, although the mud layer was
massive, with no structures.
Sand layers with normal grading were observed at 42
sites. The layers were distributed between 0.27 and 1.72
km inland from the shoreline (Fig. 6a) and between − 2.0
and 8.4 m in elevation. Sand layers with normal gradingwere rarely observed at the places with a steep slope inthe main valley and sub-valleys (Fig. 6a).
Sand layers with inverse grading were observed at 4
sites. The layers were distributed between 0.6 and 1.2 km
inland from the shoreline (Fig. 6a), and between 0.7 and1.9 m in elevation. Distribution of the layers was concentrated at the center of the lower main valley (Fig. 6a).
Sand layers with parallel lamination were observed at
37 sites. The parallel lamination was observed as concentrations of coarse-grained particles or heavy minerals,with thicknesses of 1 mm to a few centimeters (Fig. 2e).
The laminated layer distributions extended between 0.2
and 1.2 km inland from the shoreline (Fig. 6b), and from
0.5 to 4.8 m in elevation. The layers were only observedat the lower main valley and the north sub-valley.
Sand layers with mud clasts (a few centimeters inlength) (Fig. 2f) were observed at 34 sites and were
mostly distributed on the rice paddies or in the pond between 0.5 and 1.8 km inland (Fig. 6b), and between − 2.0
and 5.8 m in elevation. The layers were rarely observedat the seaward side of the lower main valley.
Sand layers bounded by mud drapes (less than several
millimeters in thickness) were observed at 10 sites. Muddrapes were distributed at distances between 0.6 and 1.6
km from the shoreline (Fig. 6b). The layers were mostly
observed at the lower main valley and the pond.
Sand layers composed of multiple sub-units were observed at 51 sites. The sand layers with multiple subunits were generally thicker than the sand layers with asingle unit (Figs. 3a and 6c). The number of sub-units
decreased landward with increasing elevation (Fig. 6c).

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Fig. 3 Spatial thickness distribution of a tsunami sand and b tsunami mud. Elevation map and contour line are based on the 2 m grid DEM data
provided by GSI. N, north sub-valley; S, south sub-valley

Sand layers with multiple sub-units were characteristically observed in the lower main valley and the pond.
However, sand layers with a single unit were mostly observed in the upper main valley and sub-valleys.

Grain size distribution

In the lower main valley, median grain size and grain size
sorting respectively ranged 1.4–2.4 phi and 0.4–0.7, and in
the upper main valley and sub-valleys, respectively ranged

Abe et al. Progress in Earth and Planetary Science(2020) 7:7

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Fig. 4 Thicknesses of tsunami sand shown by each topographic type against the inland distance from the shoreline (a) and the base elevation(b). Control point of measurement of the distance from the shoreline is shown in Fig. 1b. Base elevation means the elevation (above T.P.) of thebottom of tsunami deposit. TS, tsunami sand layer; LM, lower main valley; UM, upper main valley; NV, north sub-valley; SV, south sub-valley;P, pond

1.7–2.1 phi and 0.4–0.5 (Fig. 7a–c and Additional file 1:
Table S1). Fluctuation of the median grain size with increasing shore-normal distance in the lower main valley was
found to be greater than that in the upper main valley andsub-valleys (Fig. 7b). In addition, the sand layer generally became finer landward in the upper main valley (Fig. 7b).
Grain size characteristics were similar among the tsunami

sand layer, beach sand, and dune sand (Fig. 7a). The seafloor
sands were clearly separated from these sands in terms ofthe median grain size and grain size sorting (Fig. 7a).
Diatom assemblages

Regarding the diatom analysis for the mud component,
species were generally similar among all sites (Fig. 8,

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Fig. 5 Thicknesses of tsunami mud shown by each topographic type against the inland distance from the shoreline (a) and the base elevation(b). Base elevation means the elevation (above T.P.) of the bottom of tsunami deposit. TM, tsunami mud layer; LM, lower main valley; UM, upper
main valley; NV, north sub-valley; SV, south sub-valley; P, pond

Table 2, and Additional file 1: Table S2). Freshwater species (e.g., Aulacoseira ambigua, Aulacoseira distans,
Aulacoesira italica, Aulacoseira spp., Fragilaria brevistriata, Fragilaria construens, Fragilaria pinnata, Fragilaria spp., Pinnularia spp.) dominated (88–93%) the
assemblages in all samples. Samples include a few marine diatom species (2–7%), and brackish species (1–5%)

were found in the pond sediment (14%) and tsunami
mud (9–19%), but not rice paddy soil (0%).
Estimating the sediment budget

The area and volume of the terrestrial erosion of sand,distributed on sandy beach and sand dune, were estimated respectively as 7.85 × 104 m2 and 1.34 × 105 m3

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Fig. 6 Spatial distribution of sedimentary structures: a normal grading and inverse grading; b parallel lamination, mud clasts, and mud drape; andc number of sand sub-units. Elevation map and contour line are based on the 2 m grid DEM data provided by GSIAbe et al. Progress in Earth and Planetary Science(2020) 7:7

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Fig. 7 Median grain size and grain size sorting of tsunami sand,dune sand, beach sand, and shallow marine sand. a Median grain
size of sand shown by topographic type against grain size sorting.
Results of shallow marine sand sampled off the coast of the valley
were from Ohshima et al. (1986). b Median grain size shown by
each topographic type against distance from the shoreline. c Grainsize sorting shown by each topographic type against distance fromthe shoreline. Locations of the sampling site are shown in Fig. 1c. TS,tsunami sand layer; LM, lower main valley; UM, upper main valley;NV, north sub-valley; SV, south sub-valley; P, pond

based on a comparison of pre-tsunami and post-tsunami
aerial photographs and DEMs (Fig. 9a and Table 3).
Average erosional depth in mud erosional areas (paddy
fields) was approximately estimated as 0.1 m based onfield observations and interviews to local farmers. The
area and volume of the terrestrial erosion of the mudwere estimated respectively as 2.55 × 105 m2 and 2.55 ×
104 m3 based on field observations, aerial photographanalysis, and interviews as described above (Fig. 9a andTable 3).
Sand and mud depositional areas were estimated respectively as 6.98 × 105 m2 and 6.94 × 105 m2 (Table 3)
by a field observation and spatial analysis using GIS. We
employed natural neighbor interpolation method to estimate spatial variations in thickness and onshore depositional volume (Fig. 9b, c). Consequently, depositional
volumes of sand and mud were roughly estimated respectively as 7.65 × 104 m3 and 1.34 × 104 m3. According to comparison between depositional and erosional
volumes, the estimated sand and mud depositional volumes respectively accounted for 57% and 52% of erosional volumes (Table 3).

Discussion
Sediment sources and budget
Sediment sources

Judging from a comparison of aerial photographs andDEMs obtained before and after the 2011 tsunami, tsunami flow eroded the sandy beach and sand dune (Figs.
1b and 9a). Grain size distributions of beach and dunesands and the sandy portion of tsunami deposits weresimilar (Fig. 7a, median grain size 1.5–2.5 phi). Grainsize distributions of the seafloor sands before the 2011
tsunami, which were sampled from offshore sea bottom(12–23 m depth) off the coast of the valley, were muchcoarser (Fig. 7a, median grain size − 0.1–0.7 phi) thanthe onshore deposits. Shell fragments were rarely observed in the tsunami deposits. These lines of evidence
suggest that sediment supply from the seafloor might
not have been dominant.
This interpretation was supported by earlier reported
data of mineral composition and microfossil assemblagesof 2011 tsunami deposits in coastal area along Sendai
Bay. Jagodziński et al. (2012) reported that heavy mineralAbe et al. Progress in Earth and Planetary Science(2020) 7:7

Page 13 of 21

Fig. 8 Diatom assemblages of tsunami mud, pond sediment, and rice paddy soil. The circular chart shows percentages of marine, marine orbrackish, brackish, and freshwater species. Base map is an aerial photograph taken on the day after the tsunami which was provided by GSI.
Diatom identification and habitat were based on methods described by Kosugi (1988) and Ando (1990)

concentrations and assemblages were similar between
the tsunami deposit and the sandy beach on Sendai
Plain. Both Szczuciński et al. (2012) and Takashimizu
et al. (2012) reported few brackish-marine diatoms of
the sandy tsunami deposit along a shore-normal transect
on Sendai Plain. Putra et al. (2013) described minor foraminifera populations in the sandy tsunami deposits andsimilar foraminifera densities between the beach sand,dune sand, and sandy tsunami deposits, which were
sampled along a transect on the coastal lowland 20 km
south of our study area. These earlier works indicate themain source of the sandy tsunami deposit not as offshore sediments but as onshore sediments including
beach and dune sands.
Onshore sediment sources are supported not only by
microfossil assemblages, but also by numerical modeling.

Table 2 Diatom assemblage analysis results
Sample nameSediment type

Depth (cm)Marine (%)

Brackish (%)

Marine-brackish (%)

Freshwater (%)

Unknown (%)

GPS7

Pond sediment

29–30

2.8

3.3

0.9

93.0

0.0

GPS12

Rice paddy soil

6.5–7.5

1.0

1.0

1.0

96.0

1.0

GPS21

Tsunami mud layer

0–6

2.9

6.8

2.0

87.8

0.5

GPS172

Tsunami mud layer

0–3

5.6

1.4

2.3

88.3

2.3

GPS182

Tsunami mud layer

0–2.5

1.9

1.4

0.5

94.7

1.4

GPS185-1

Tsunami mud layer

0–3

1.5

0.5

2.0

87.2

8.9

GPS185-2

Tsunami mud layer

3–9

5.3

2.4

1.4

90.3

0.5

GPS42

Tsunami mud drape

4–5

2.7

1.3

0.4

93.8

1.8

Sampling site locations were shown in Figs. 1c and 8. Major species of each sample were shown in Additional file 1: Table S2. Diatom identification and habitat
were based on methods described by Kosugi (1988) and Ando (1990)

Abe et al. Progress in Earth and Planetary Science(2020) 7:7

Page 14 of 21

Fig. 9 Estimation of erosion and deposition by the tsunami in study area: a a sand erosion depth in sandy beach and sand dune, and a soil
erosion area in a rice paddy; b spatial distribution of interpolated sand thickness; and c spatial distribution of interpolated mud thickness. Base
map is an aerial photograph taken on the day after the tsunami which was provided by GSIAbe et al. Progress in Earth and Planetary Science(2020) 7:7

Table 3 Estimation of erosion and deposition by the tsunami instudy area
ErosionArea (m2)4Depth (m)Volume (m3)Sand7.85 × 10

0–6.1

1.34 × 105

Mud2.55 × 105

0.1 (Ave.)

2.55 × 104

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the rice paddies and the pond location. It is unlikely thatthe mud was transported from offshore because theshallow seafloor sediment was mostly composed of sand(Ohshima et al. 1986). Therefore, the main source areaof the mud layer is identifiable as the rice paddies andthe pond (Fig. 9a).
Sediment budget

Deposition
Area (m2)Thickness (m)Volume (m3)Sand

6.98 × 105

0–0.4

7.65 × 104

Mud

5

0–0.4

1.34 × 104

6.94 × 10

Depositional volume / Erosional volume
Sand

57%

Mud

52%

Tsunami sediment transport modeling demonstrated
that the steep nearshore bathymetry in the central Sendai Bay promoted decreasing shear velocity at theseafloor in the offshore and preventing sediment resuspension from the seafloor around the coastal area (Sugawara et al. 2014). Yoshikawa et al. (2018) also conducted
a numerical simulation of tsunami sediment transport
near the area examined for the present study. Although
the local bathymetry is slightly shoaling comparing tothat of central Sendai Bay, they concluded that sedimentresuspension from the seafloor is minor even during
maximum inflow. Consequently, it is reasonable to inferthat the sand layer is predominantly sourced from the
sandy beach and sand dune, but not from the seafloor.
Locally, it seemed that the source area in sandy beachand sand dune was influenced by arrangement of artificial structures such as coastal dikes and developed land.
In fact, erosion depth reached to 6 m at landward side of
the coastal dike in studied area (Fig. 9a). Numericalsimulation demonstrated that sediment erosion was intense behind the coastal dikes due to the increased flow
speeds in Sendai Plain (Sugawara et al. 2014). Therefore,
it is required for application of paleo-tsunami to carefully consider effects of artificial structures on sedimenttransport by tsunami.
Diatom assemblages of muddy tsunami deposit mostly
comprise freshwater species and are similar with thoseof rice paddy soil and pond sediment (Table 2, Additional file 1: Table S2). Hence, diatom assemblages ofthe mud layer demonstrated that the mud was mostly
sourced from the rice paddies and the pond. Field observation and interviews of local farmers also revealed thatrice paddy soil, which might have typical thickness of
0.2 m, was approx. 0.1 m thick on average, eroded by thetsunami in paddy fields. In addition, the depositional
area of the mud layer coincides with the distribution of

The depositional volume of the sand layer (7.65 × 104
m3, Table 3) and mud layer (1.34 × 104 m3, Table 3) canbe explained well by the erosional volume of the beachand dune sands (1.34 × 105 m3, Table 3) and the mudfrom the paddies and the pond (2.55 × 104 m3, Table 3)
because the erosional volume is far larger than the depositional volume. As shown in Table 3, this finding inturn suggests that 57% and 52% of the erosional volumeof the sand and the mud were discharged into the sea by
the backwash. Udo et al. (2013) investigated beachmorphology changes along the 3 km long coast including
our study area and estimated that 75% of the total
amount of eroded coastal sand was transported seaward
by backwash. Differences in estimation between thisstudy and Udo et al. (2013) probably derived from theestimated area. Udo et al. (2013) estimated the total balance along 3 km long coast, but our estimation is limited
along the 1 km long coast. Based on high-resolution seismic surveys and coring, Yoshikawa et al. (2015) confirmed that considerable amounts of sediments havebeen transported by the backwash and they were deposited on the shoreface.
Control factors of thickness distribution

Generally speaking, tsunami deposits thinned inland in
low-lying coastal plains (e.g., Shi et al. 1995; Gelfenbaumand Jaffe 2003; Hori et al. 2007; Fujino et al. 2010; Richmond et al. 2012). In our studied valley, the sand thickness also generally decreased landward (Figs. 3a and 4a)and showed negative correlation with the elevation (Fig.
4b). This decreasing trend is probably related to the reduction of sediment supply and sediment transport capacity. The sediment supply possibly decreased during
tsunami inundation: the flow traveled longer distanceand reached a higher elevation from the sedimentsource. That decreased sediment supply probably engenders formation of the thinner deposit. Sediment transport capacity would have decreased concomitantly withdecreasing flow velocity and depth.
Besides, the sand thickness did not decrease linearly
landward (Fig. 4a). It spatially fluctuated because of
micro-landforms of the valley (Figs. 3a and 9). For example, the sand thickness decreased across stepwise
topographic features in north and south sub-valleys(N1–N4 and S1 in Fig. 3a). The sand thickness is widely
known to have fluctuated with topographic irregularities

Abe et al. Progress in Earth and Planetary Science(2020) 7:7

and obstacles, which locally caused changes in tsunami
behavior (Nishimura and Miyaji 1995; Hori et al. 2007;
Nakamura et al. 2012; Matsumoto et al. 2016). The sandthickness distribution pattern varied depending on the
topography in the valley. The sand layer generally
thinned landward with local fluctuation in the lower
main valley (Figs. 3a, 4a, and 9b). Flow velocity anddepth of the incoming wave might have been less affected by the flat topography of the lower main valley(Fig. 1b). Consequently, the sediment transport capacity
is apparently nearly constant in the lower main valley;decreased sand supply from the sand dune and sandy
beach might be key factors of inland thinning. By contrast, the sand layer thinned landward in the upper mainvalley and sub-valleys (Figs. 3a and 4a). The sand thickness is also related to the base elevation in these areas(Fig. 4b). Bedload transport is likely restricted by microlandforms such as high mounds and stepwise features;
most of the sand layer found from the higher places ispossibly deposited from suspension. Therefore, landward
and upward thinning trends in these areas might be affected by decreasing flow depth, velocity, and capacity ofthe suspended load (Hiscott 1994).
No clear landward trend of the mud layer thickness isapparent: it increased rapidly in and around the pond
(Figs. 3b and 9c). This increase occurs because the mudlayer was sourced not only from the paddy soil but also
from the bottom sediment of the pond. Therefore, local
sources played an important role in affecting the mudlayer thickness.
Factors affecting sedimentary structures

Sedimentary structures of sandy tsunami deposits provided useful information for estimating the sedimentary
processes and flow condition (Fujiwara and Tanigawa
2014). Distribution patterns of the respective sedimentary structures were controlled by the inland distanceand micro-landforms in our study area. Here, we describe major sedimentary structures in relation to the
topography.
Normal grading

Normal grading was common among tsunami deposits(Peters and Jaffe 2010). Grain sizes of the sand layersthat fine upward are usually explained as the product ofsuspended sediment settling from a decelerating flow
(Jaffe and Gelfenbaum 2007; Naruse et al. 2010; Moore
et al. 2011). From field observation and grain size analysis of the 2011 tsunami deposit, Szczuciński et al.
(2012) inferred forming process of upward fining inside
sandy tsunami deposit as follows. (1) Since sandy tsunami deposits with upward finning are at the same timelaminated, they were likely still in traction transport after
deposition from suspension. (2) The sand layer might be

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settled from flowing water—deceleration of flow reduces
the transport capacity of larger grains first, so they aredeposited first. In our study area, sand layers with normal grading were observed in the main valley (0.2–1.8
km inland from the shoreline), where the slope of topography was gentle (Fig. 6a, Additional file 2: Fig. S3). This
gradual slope in turn suggested that decelerating flow
dominantly occurred in this area: inundated water
tended to stagnate in the lower elevation area (Udo et al.
2013).
Inverse grading

Overall inverse grading of tsunami deposits is rare (Morton et al. 2008), but it was sometimes recognized in the2004 Indian Ocean tsunami and the 2011 tsunami sands(e.g., Naruse et al. 2010; Naruse et al. 2012; Szczuciński
et al. 2012; Koiwa et al. 2014). Naruse et al. (2010) inferred that the inverse graded layers are a consequenceof the waxing stage of the oscillatory flows caused by thetsunami waves. In our study area, a sand layer with inverse grading was found at the central part of the lower
main valley from 0.6 to 1.2 km inland from the shoreline(Fig. 6a, Additional file 2: Fig. S3). Sustained accelerationof the flow can be expected to lead to a sustained increase in transport rate of coarser grains. The local topography in the study area is characterized by a low-lying
depression near the pond. This topographic setting
might lead to acceleration of the flow toward the pondarea, particularly the inflow from the shoreline to thepond area and the return flow from the upper main valley and sub-valleys to the pond area, which induced formative condition of the inverse grading of the tsunami
deposits (Fig. 6a).
Parallel lamination

Parallel lamination is frequently identified in sandy tsunami deposits (Naruse et al. 2010; Peters and Jaffe 2010;
Nakamura et al. 2012). Parallel lamination is formed byplane beds, implying bedload transport of sediments(Bridge and Best 1988). Sand layers with parallel lamination were distributed between 0.2 and 1.2 km inland in
our study area (Fig. 6b). However, although parallel
lamination might be useful to estimate the minimum
velocity necessary to form it (Southward and Boguchwal
1990; Ohata et al. 2017), it is difficult to estimate themaximum velocity using laminated layers. Parallel
lamination in the valley was characterized by concentrations of coarser grains or heavy minerals. Landward
limits of the parallel lamination suggest inland decreasing in these grains as an indicator of the minimum flow
velocity. This interpretation is supported by grain size
sorting and median grain size of the sand (Fig. 7b, c).
Nakamura et al. (2012) and Jagodziński et al. (2012)
showed that a heavy mineral fraction of the 2011

Abe et al. Progress in Earth and Planetary Science(2020) 7:7

tsunami sand decreased landward, and that landwardchange in the ratio between light minerals and heavy
minerals has potential for use as an indicator of the sediment transportation modes (suspension/traction carpet)
and transport distance from the source of the sand suchas sandy beaches and sand dunes.
Mud clasts

Mud clasts, also known as rip-up clasts, were formed
from erosion of muddy bottom sediments (Li et al.
2017) and were related to increasing critical shear
stresses of the bottom (Talling et al. 2002). Mud clasts
were often observed in sandy tsunami deposits (e.g.,
Richmond et al. 2012; Putra et al. 2013; Ishizawa et al.
2018). In our study area, mud clasts (a few centimetersin length) were distributed from 0.5 to 1.8 km inland(Fig. 6b). It is highly likely that mud clasts were originated from the rice paddy soil based on the distribution(Fig. 9a) and components of the mud clasts. The marked
erosion of the paddy soil below the tsunami deposit also
supports this interpretation. Mud clasts in the sand layer
suggest that large traction by the tsunami inundation
flow was maintained at least up to 1.8 km inland fromthe shoreline.
Mud drape

Multiple sub-units of tsunami sand were occasionallyseparated by mud drapes up to several millimeters thick(e.g., Fujiwara and Kamataki 2007). Mud drapes wereformed in the process of suspension of muddy sediments
during the standing wave between the numbers of the
waves (Fujiwara and Kamataki 2007; Naruse et al. 2010).
In our study area, mud drapes in the sand layers existed
up to 1.8 km inland, where the topography is characterized by gentle slopes (Fig. 6b). Consequently, results suggest that multiple waves with long periods inundated thearea and that the water stagnated because of the gentle
slope up to 1.8 km inland. Lack of mud drapes in theupper main valley and sub-valleys suggests that thesehigher places were inundated by the main wave.
Multiple sand units

Multiple sand units were presumably formed by independent waves (Nanayama and Shigeno 2006). In ourstudy area, the number of sand sub-units decreased inland (Fig. 6c). The maximum inland distances of 1–6
sub-units were 0.6–1.9 km. In particular, the sand layer,
which was composed of 3–6 sub-units, was mostly distributed up to 0.6 km inland from the shoreline. They
also diminished with increasing elevation (Fig. 6c). The
maximum height of 1–6 sub-units was T.P. 1.5–8.9 m.
The sand layer with 3–6 sub-units concentrated at theplace of less than T.P. 3 m. The maximum number of
sub-units is a double of number of incoming waves

Page 17 of 21

based on eyewitness accounts. Although each sand subunit was likely to have been formed by independent
waves, reworking of earlier formed tsunami deposit(s)must be considered in any attempt to correlate the numbers of sub-units and waves. Therefore, the number ofthe sand sub-units should be regarded as the minimum
number of waves that formed the sand layers. Multiple
sand sub-units were mostly observed in the lower mainvalley, whereas single sand unit was observed in theupper main valley and sub-valley. Our findings of the effects of maximum waves, according to data observed
using GPS tide gauges (Kawai et al. 2013), suggest thatthe first wave was more than 3 m high. However, thedirection of run-up flow is known to be almost perpendicular to the shoreline, whereas backwash flow directions were likely affected strongly by local topography,
as reported by Umitsu et al. (2007).
Consequently, spatial fluctuations of the sand units inthe lower main valley of our study site might be attributable to differences in the influences on tsunami sedimentation process associated with the arrangement inspace of micro-landforms such as small mounds, steps,
old ponds, and old channels.
Relations among topography, tsunami behavior, andsedimentary process

Sedimentary processes of recent tsunami events were estimated based on tsunami deposit characteristics(Nanayama and Shingeno 2006; Paris et al. 2007; Choowong et al. 2008; Szczuciński et al. 2012; Takashimizu
et al. 2012; Putra et al. 2013). In the case of the 2011
tsunami, the flow height of first wave is greater than thatof later waves along Sendai Bay (Kawai et al. 2013). Consequently, sedimentation and erosion by the inflow andoutflow of the first wave are certainly greater than thoseby the following waves. In the valley, the sandy tsunami
deposits consisted of 1–6 sub-units; meanwhile, the sandlayer in a low-lying section in Sendai Plain mostly comprised single unit (Szczuciński et al. 2012; Takashimizu
et al. 2012). These differences suggest that steep slope
and narrow depositional area of valleys contribute to theformation of multiple sand sub-units.
In the valley, sediment supply from onshore sources
including sandy beach, sand dune, paddy field, and pond
is more dominant than that from offshore. This appears
to be similar to Sendai Plain (Szczuciński et al. 2012;
Takashimizu et al. 2012). However, several earlier reports of studies examining recent tsunami events havedescribed that onshore tsunami deposits include offshore
sediment (e.g., Gelfenbaum and Jaffe 2003; Nanayama
and Shingeno 2006; Naruse et al. 2012). Characteristics
of incoming tsunami waves, such as amplitude andperiod, and local bathymetry largely have been considered to control incorporation of marine components inAbe et al. Progress in Earth and Planetary Science(2020) 7:7

tsunami deposits (Goto et al. 2014). Sugawara (2017) examined differences in sediment sources of onshore tsunami deposits regarding the bottom shear velocity of
inflow based on numerical simulation, and concluded
that local bathymetry plays an important role for nearshore hydrodynamic characteristics of tsunami-induced
flow, which finally determine whether or not the shear
velocity can be large enough to transport seafloor sediments inland. The local bathymetry of the study area ischaracterized by a gentler offshore sloping, comparing
with that of Sendai Plain (e.g., Goto et al. 2012); nevertheless, the gentler bathymetry did not cause differencein the transport of seafloor sediments.
We estimated that 57% and 52% of the erosional volume of the sand and the mud were discharged into thesea by the backwash. The onshore slope from the shoreline to inundation limits (T.P. 12.8 m) at the valley headin the upper main valley was 5.9‰ in the valley.
MacInnes et al. (2009) examined tsunami deposits on
high-relief coastal topography of the Kuril Island alongfour shore-normal transects with 0.12–0.43 km in lengthand estimated that 76–98% of the sediments in thesource region were discharged into the sea by outflow(MacInnes et al. 2009). In their study sites, the onshore
slope ranged 25.5–54.9‰ from the shoreline to the inundation limit (5.7–18.1 m above mean sea level) (MacInnes
et al. 2009). MacInnes et al. (2009) argued that the tsunami was dominantly erosive in the Kuril Islands because
the high-relief topography of the shoreline accelerated tsunami outflow. The balance between the volume of erosionand deposition is likely controlled by the run-up height related to onshore slope of the inundation area and behavior
and height of the wave on the coast.
The balance between erosion and deposition was examined also by flume experiments (e.g., Hasegawa et al.
2001; Yoshii et al. 2017, 2018). Hasegawa et al. (2001)
carried out the flume experiment with land slope of50‰ and found that approx. 60% of onshore depositional volume, which was transported by run-up flow,
was discharged into the sea by return flow. Yoshii et al.
(2017, 2018) performed the flume experiment with threecases of land slope (0‰, 10‰, 20‰) and showed thatland slope plays an important role in balance between
sediment supply resulting from inflow and sediment remigration attributed to outflow. The role of tsunami as ageomorphological agent, which is reflected to patterns oftsunami-induced erosion and deposition, can sometimes
be very different between valleys and coastal plains. Estimations of the erosional volume of recent tsunamis arerare in the literature (e.g., Paris et al. 2009; Udo et al.
2013; MacInnes et al. 2009). In the future, not only deposition amounts, but also amounts of erosion must
be examined immediately after the tsunami to elucidate factors controlling sediment budget, combined

Page 18 of 21

with further knowledge from flume experiments andnumerical simulations.

Conclusions
This study investigated the spatial distribution of thickness, sedimentary structures, and grain sizes of the 2011
Tohoku-oki tsunami deposits in a narrow valley setting.
In the valley, spatial variations of them were likely controlled by local topography and the arrangement inspace of micro-landforms. This finding in turn suggeststhat transect surveys in a valley setting might not bevalid to elucidate sedimentary processes comprehensively. At our site, differences of observations between
the transect survey and spatial survey were similar inareas with higher than 3 m in elevation, although theydiffered in areas with lower than 3 m in elevation because multiple waves and backwash led to fluctuation of
the deposit thickness at lower elevation areas, whereas
the influence of sediment transport by multiple waves
was minor for higher elevations. Therefore, spatial variations of the deposit distribution should be investigatedcarefully in low elevation areas, where the deposition
pattern was more sensitive to multiple waves than thehigh elevation areas. Although the influence of sedimenttransport by multiple waves is probably more complicated in a narrow valley with steep slope than low-lying
coastal plains in general, there were advantages and disadvantages for understating sedimentary processes andreconstructing hydraulic characteristics of paleo-tsunami
with each setting. While a large (~ 2 km) gap was often
known between the inundation limit and the maximum
landward extent of the recognizable sand layer in lowlying plains (e.g., Goto et al. 2011; Abe et al. 2012), bothwere almost the same in the valley, as well as previous
study of recent tsunami deposits in steep valley(MacInnes et al. 2009). These findings imply that the
maximum inland extent of the tsunami sand is possible
to be good proxy for estimating the inundation area of
paleo-tsunami in steep valley. The issue of potential linkage between the number of sub-units and wave behavior
could be subjected to deeper analysis in the future. Our
results are presented as a template for recent examples
for studying paleo-tsunami deposits in narrow valleys.
Paleo-tsunami research in narrow valleys can be expected to open new avenues for collecting useful geological data from locations that have not been heretofore
regarded as amenable to research in this field.
Supplementary information
Supplementary information accompanies this paper at https://doi.org/10.
1186/s40645-019-0318-6.
Additional file 1: Table S1. Results of grain-size analysis by settlingtube method. All grain-size values are shown phi scale. The sampling sites

(2020) 7:7

Page 19 of 21

are shown in Fig. 1c. LM: lower main valley, UM: upper main valley, NV:
north sub-valley, SV: south sub-valley, P: pond, B: beach, SD: sand dune.
Table S2. Results of analysis of diatom assemblage. Only the common
species are presented. The sampling sites are shown in Fig. 1c.

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Abe et al. Progress in Earth and Planetary Science

Additional file 2: Fig. S1. Oblique aerial photographs in our study area
taken by The Geospatial Information Authority of Japan on May 25, 2011.
(A) View from offshore. (B) View from valley head. Fig. S2. Examples ofdiatom species in the muddy deposits: 1. Fragilaria pinnata, 2. Navicula
salinarum, 3. Rhopalodia operculata, 4. Navicula cincta. 5. Cyclotellameneghiniana, 6. Hyalodiscus scoticus, 7. Aulacoseira distans, 8. Synedra
tabulata, 9. Nitzschia plana, 10. Neidium alpinum, 11. Pinnularia gibba, 12.
Nitzschia calida, 13. Navicula elegans, 14. Navicula radiosa, 15. Navicula
crucicula, 16. Navicula cryptocephala, 17. Cymbella gracilis, 18. Pinnularia
viridis, 19. Diploneis ellipica, 20, Mastogloia pumila, 21. Rhopalodia
gibberula, 22. Thalassiosira bramaputrae, 23. Melosira undulata, 24.
Mastogloia pumila, 25. Synedra nanana, 26. Pinnularia nodosa, 27.
Nitzschia acuminata, 28. Gomphonema acuminatum, 29. Hantzschia
amphioxys. Fig. S3. Vertical variation of the tsunami deposit at GPS 206.
The sand layer is characterized by inverse-graded and normal-graded
multiple units. The observation site is shown in Fig. 1c. TS: Tsunami sandlayer. TM: Tsunami mud layer. Soil: Pre-tsunami soil.
Abbreviations
DEM: Digital elevation model; DGPS: Differential Global Positioning System;
GIS: Geographic Information System; GSI: Geospatial Information Authority ofJapan; T.P.: Tokyo Peil; TETJSG: The 2011 Tohoku Earthquake Tsunami Joint
Survey Group
Acknowledgements
We appreciate the local people and Yamamoto Town for their support
during the field survey. We also thank Dr. Witold Szczuciński and an
anonymous reviewer for their valuable suggestions and comments.
Authors’ contributions
TA proposed the topic, and conceived and designed the study. TA, KG, andDS conducted the field survey. TA analyzed the data. KG and DS
collaborated with the corresponding author in the construction ofmanuscript. All authors read and approved the final manuscript.
Funding
This research was supported by a research grant from the International
Research Institute of Disaster Science, Tohoku University.
Availability of data and materials
Please contact author for data requests.
Competing interests
The authors declare that they have no competing interests.
Author details
1
Geological Survey of Japan, National Institute of Advanced Industrial
Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567,
Japan. 2International Research Institute of Disaster Science, Tohoku University,
468-1 Aramaki Aza-Aoba, Aoba-ku, Sendai, Miyagi 980-0845, Japan.
3
Department of Earth and Planetary Science, The University of Tokyo, 7-3-1
Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. 4Curatorial Division, Museum of
Natural and Environmental History, Shizuoka, 5762 Oya, Suruga-ku, Shizuoka
422-8017, Japan.
Received: 12 June 2019 Accepted: 24 December 2019

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