Content uploaded by Daisuke Ishimura
Author content
All content in this area was uploaded by Daisuke Ishimura on Jan 02, 2020
Content may be subject to copyright.
Available via license: CC BY 4.0
Content may be subject to copyright.
R E S E A R C H A R T I C L E Open Access
Historical and paleo-tsunami deposits
during the last 4000 years and their
correlations with historical tsunami events in
Koyadori on the Sanriku Coast, northeastern
Japan
Daisuke Ishimura
1*
and Takahiro Miyauchi
2
Abstract
Large tsunamis occurring throughout the past several hundred years along the Sanriku Coast on the Pacific coast of
northeastern Japan have been documented and observed. However, the risk of large tsunamis like the tsunami
generated by the 2011 off the Pacific coast of Tohoku earthquake could not be evaluated from previous studies,
because these studies lacked evidence of historical and paleo-tsunami deposits on the coastline. Thus, we first
identified event deposits, which are candidates for tsunami deposits, from excavating surveys conducted on the
coastal marsh in Koyadori on the Sanriku Coast, northeastern Japan. Second, we determined the physicochemical
sediment properties of the deposits (roundness of grains, color, wet and dry densities, and loss on ignition) and
established their geochronology by radiocarbon dating and tephra analysis. Third, we identified event deposits as
tsunami deposits, based on their sedimentary features and origin, sedimentary environment, paleo-shoreline, and
landowner interviews. In this study, we report 11 tsunami deposits (E1–E11) during the past 4000 years, of which
E1, E2, E3, and E4 were correlated with the 2011 Tohoku-oki tsunami, the 1896 Meiji Sanriku tsunami, the 1611 Keicho
Sanriku tsunami, and the 869 Jogan tsunami, respectively. From age data and the number of tsunami deposits in the
trench, we estimated that tsunamis larger than the 1896 Meiji Sanriku tsunami occur and hit the study area on average
every 290–390 years. However, historical tsunami correlations revealed variable tsunami occurrence, indicating diverse
tsunami generation and/or the combination of several types of large earthquakes from different sources around the
Japan Trench.
Keywords: Historical and paleo-tsunami deposits; Sanriku Coast; 2011 Tohoku-oki tsunami; Historical tsunami
correlation; Geochronology
Background
Historical and paleo-tsunami research and its application
to geophysical study
The 2011 off the Pacific coast of Tohoku earthquake
(2011 Tohoku-oki earthquake) (M
w
9.0) that occurred
on March 11, 2011, triggered a large tsunami (2011
Tohoku-oki tsunami) along the east coast of Japan, causing
severe damage and loss of life. The Pacific coastal region,
in particular, the Sanriku Coast (Fig. 1), ranks among the
highest tsunami risk areas in Japan, in terms of both sever-
ity and frequency (Watanabe 1998). However, we could
not assess the risk of large tsunamis like the 2011
Tohoku-oki tsunami because we lack sufficient knowledge
about large earthquakes and the tsunami history in the
Japan Trench. Even after the 2011 Tohoku-oki tsunami,
the long-term tsunami history of the Sanriku Coast
remains obscured by inadequate field data (Sugawara
et al. 2012). Moreover, the geological and geophysical
problems revealed by the 2011 Tohoku-oki earthquake,
* Correspondence: ishimura@irides.tohoku.ac.jp
1
Disaster Science Division, International Research Institute of Disaster Science,
Tohoku University, 468-1 Aza-aoba, Aramaki, Aoba-ku, Sendai, Miyagi
980-0845, Japan
Full list of author information is available at the end of the article
© 2015 Ishimura and Miyauchi. This is an Open Access article distributed under the terms of the Creative Commons Attribution
License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any
medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://
creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
Ishimura and Miyauchi Progress in Earth and Planetary Science (2015) 2:16
DOI 10.1186/s40645-015-0047-4
including mechanisms of tsunami generation (Kawamura
et al. 2012; Tappin et al. 2014) and large earthquakes (Ikeda
et al. 2012; Goldfinger et al. 2013; Rajendran 2013), remain
unsolved. Recent studies have suggested that submarine
mass failure (e.g., submarine landslide) may have contrib-
uted to large tsunamis along the Japan Trench (Kawamura
et al. 2012; 2014; Tappin et al. 2014). To resolve these prob-
lems, we require long-term solid evidence (e.g., historical
and paleo-tsunami deposits).
Thebestsourcesofpreciselong-termtsunamidataare
coastal lowlands, in particular, marshes (Minoura and
Nakaya 1991; Witter et al. 2003; Sawai et al. 2009; Shennan
et al. 2014). In some regions (Hokkaido of Japan, Alaska of
USA, and the North Island of New Zealand), coseismic and
post-seismic crustal movements are recorded in sediments
as lithological and biological changes (Witter et al. 2003;
Sawai and Nasu 2005; Hamilton and Shennan 2005;
Hayward et al. 2005). Thus, in this study, we excavated
trenches at a coastal marsh in Koyadori, in the middle part
of the Sanriku Coast (Fig. 1). The aim was to provide new
geological evidence of historical and paleo-tsunami deposits.
Study site
Koyadori is located in the central part of the Sanriku Coast,
the easternmost part of Honshu Island (Figs. 1 and 2). Ap-
proximately 200 km east of Koyadori, the Pacific plate sub-
ducts underneath the Eurasian Plate, where rupture areas
of historical and observed earthquakes have been identified
on the plate boundary (Earthquake Research Committee
Headquarters for Earthquake Research Promotion Prime
Minister’s Office, Government of Japan 1999). In
Koyadori, the mouth of the valley is closed by beach
ridges. Prior to the 2011 Tohoku-oki tsunami, which hit
the valley (Fig. 3), the study site was used as paddy fields.
The geology differs on both sides of the valley (Fig. 2;
Yoshida et al. 1984). The east side comprises Early Cret-
aceous hornblende-biotite granodiorite, granite, granite
porphyry, and tonalite. The west side comprises dacite
to rhyolite lava and pyroclastic rock deposited during the
Early Cretaceous. Ishimura et al. (2014) drilled cores at the
study site (Fig. 4), revealing visible tephra layers such as
the Towada–Chuseri tephra (To–Cu: 6 ka; Machida and
Arai 2003) and Oguni Pumice (7.3–7.4 ka; Ishimura et al.
2014) erupted from the Towada Volcano (Fig. 1).
Tsunami history and previous study of tsunami deposits
Large historical earthquakes and tsunamis have been in-
strumentally recorded during the last several decades and
have been recorded in historical documents and legends
during the last 1300 years (Utsu 2004). Many earthquake-
generated tsunamis have hit the areas around Koyadori
(Table 1), nine of which were large events with runup
heights exceeding a few meters (Table 2). The 2011
Tohoku-oki tsunami exhibits the largest runup height
measured between AD 2011 and 1896. In contrast, based
on the estimated runup heights, the largest tsunami oc-
curring between AD 1611 and 1856 was probably the
1611 Keicho Sanriku tsunami (Table 2). Candidates of
Fig. 1 Study site location. aPlate tectonic map in and around northeastern Japan. The star indicates the epicenter location of the 2011 Tohoku-oki
earthquake determined by the Japan Meteorological Agency. SP Sendai Plain, IP Ishinomaki Plain. bStudy area and geomorphology along the Sanriku Coast
Ishimura and Miyauchi Progress in Earth and Planetary Science (2015) 2:16 Page 2 of 18
historical tsunamis older than AD 1611 are the 869
Jogan tsunami and the 1454 Kyotoku tsunami. The 869
Jogan tsunami, identified in the Sendai and Ishinomaki
Plains (Fig. 1) (Minoura and Nakaya 1991; Minoura
et al. 2001; Sawai et al. 2007; Shishikura et al. 2007),
has been noted as the penultimate large tsunami event
around these plains (after the 2011 Tohoku-oki earth-
quake and tsunami), because its inundation in the
Sendai Plain was similar to that of the 2011 Tohoku-oki
tsunami (Sugawara et al. 2012; Namegaya and Satake
2014). However, the northern and southern distribution
limits of the 869 Jogan tsunami deposits have not yet been
determined (Sugawara et al. 2012). Thus, elucidating
whether the 869 Jogan tsunami reached the Sanriku Coast
is essential for tsunami risk assessment and seismological
study. Consequently, this information is urgently required
(Sugawara et al. 2012). If the 869 Jogan tsunami and 2011
Tohoku-oki tsunami were of similar severity, a large
tsunami is also likely to have struck the areas along the
Sanriku Coast in AD 869. In recent times, the 1454
Kyotoku tsunami has been identified from tsunami
deposits in the Sendai and Ishinomaki Plains (Sawai
et al. 2012) and historical documents (Namegaya and
Yata 2014). However, at our study site, information on
the Kyotoku tsunami deposits and the size of the tsunami
has not yet been obtained.
Historical and paleo-tsunami deposits along the
Sanriku Coast have been studied at several sites by
The Headquarters for Earthquake Research Promotion
(2006; 2007; 2008; 2009; and 2010), Haraguchi et al.
(2006a, b; 2007), Haraguchi and Goya (2007), Imaizumi
et al. (2007), Torii et al. (2007), and Haraguchi and Ishibe
(2009) before the 2011 event. However, historical tsunami
deposits were identified at only one of the onshore sites at
Rikuzentakata (Fig. 1). This site has revealed four histor-
ical tsunami deposits during the past 700 years, the latest
of which was correlated with the 1960 Chile tsunami (The
Headquarters for Earthquake Research Promotion 2007).
The other events are not well correlated with historical
tsunamis. Recent sediments, in particular, those deposited
during the past 1000 to 2000 years, are badly preserved at
other onshore sites (Torii et al. 2007; The Headquarters
for Earthquake Research Promotion 2010). Therefore,
historical tsunami deposits are poorly understood along
the Sanriku Coast.
Methods
Geomorphological classification
Initial mapping of geomorphic surfaces around Koyadori
was based on interpretation of 1:8000- and 1:10,000-scale
aerial photographs taken by the Geospatial Information
Authority of Japan before and after the 2011 Tohoku-oki
tsunami, and anaglyph images prepared from 1 m and
5 m mesh DEM (Digital Elevation Model) provided by the
Geospatial Information Authority of Japan and the Iwate
Prefecture.
Fig. 2 Topography and geology around the Funakoshi Peninsula. Base map is based on the 5 m mesh DEM supplied by the Geospatial Information
Authority of Japan. Geology is modified after Yoshida et al. (1984)
Ishimura and Miyauchi Progress in Earth and Planetary Science (2015) 2:16 Page 3 of 18
Trench survey
In December 2012, we excavated a 12 m long, 3 m wide,
and 2 m deep trench (KYD-trench) approximately 3 m
above sea level (a.s.l.) and 300 m distant from the shore-
line (Figs. 4 and 5a, b). We logged and took photographs
of each trench wall. Block samples (50 cm long, 10 cm
wide, and 5 cm deep) were taken from the west, east, and
south walls. The block samples of the west wall overlapped,
while those of the other walls were separate (Fig. 6). We
also sampled deposits of each event from the east wall for
particle roundness analysis (Fig. 6). In 2013, a construction
company excavated an additional canal (the canal-trench;
approximate length, width, and depth 300, 1, and 0.5 m,
respectively) as a tentative drainage for field restoration
(Fig. 4). To confirm the continuity of each event, we logged
a 150 m-long section of the canal-trench wall and sampled
sediments for particle roundness analysis.
Dry and wet density, color, and loss on ignition
measurement
We sampled the entire 7 cm
3
cube (each side = 2.2 cm)
from the block samples and measured their wet and dry
bulk densities. The color of wet sediments in cubic samples
is quantified by the L*, a*, and b* parameters measured by
the Soil Color Reader SPAD-503 instrument (Konica
Minolta Sensing, Inc.). The a* and b* parameters specify
the red (+) to green (−) and yellow (+) to blue (−)content,
respectively, while L* represents lightness (0 = black, 100 =
white). The loss on ignition (LOI) was conducted in
each block sample following Bos et al. (2012) at 3–6cm
Fig. 3 Aerial photographs around Koyadori before and after the 2011 Tohoku-oki earthquake. Aerial photos (a)–(d) were taken on September
1977, March 2011, June 2011, and October 2012, respectively, by the Geospatial Information Authority of Japan
Ishimura and Miyauchi Progress in Earth and Planetary Science (2015) 2:16 Page 4 of 18
intervals, although this sampling was restricted to peat
and peaty silt.
Particle roundness
Furthermore, to reveal the origins of the event deposits
and to confirm tsunami deposits, we sampled fluvial and
beach sediments (Locations 1–8) (Fig. 4) for particle round-
ness analysis in 2012 and 2013. Samples were washed
and dry-sieved through 2 mm mesh and the gravels
were divided into six roundness categories (very angu-
lar, angular, sub-angular, sub-rounded, rounded, and
well-rounded).
Radiocarbon dating
Radiocarbon dating (30 samples) was conducted by accel-
erator mass spectroscopy (AMS) at the Institute of Ac-
celerator Analysis Ltd. and Geo Science Laboratory. The
obtained age data were calibrated using the OxCal 4.2
program (Ramsey 2009) and the calibration curve IntCal13
(Reimer et al. 2013).
Tephra analysis
The Towada-a tephra (To-a) (AD915; Machida and Arai
2003) is a useful indicator of the 869 Jogan tsunami de-
posits in the Sendai and Ishinomaki Plains (Minoura and
Nakaya 1991; Sawai et al. 2007; Shishikura et al. 2007).
From the presence and distribution of To-a along the
southern Sanriku Coast, Ishimura et al. (2014) suggested
that To-a had also been deposited at the central Sanriku
Coast. Therefore, we conducted a cryptotephra analysis
to identify the invisible To-a horizon.
Each block sample was sampled at 3–6 cm intervals.
These samples were washed using 60 μm nylon mesh
and dry-sieved using 124 μm nylon mesh. Thin sections
made with the 60–124 μm fractions revealed volcanic
glass contents. The refractive index of volcanic glass shards,
which is useful for identifying widespread tephras in Japan
Fig. 4 Geomorphological classification around Koyadori. Contour maps are based on the 1 m mesh DEM supplied by the Iwate Prefecture. The
contour interval in (a) and (b) is 5 m and 1 m, respectively. cTopographic profile along A–A’line based on the 1 m mesh DEM
Ishimura and Miyauchi Progress in Earth and Planetary Science (2015) 2:16 Page 5 of 18
(Machida and Arai 2003), was measured with a refractive
index measuring system (RIMS 2000: Kyoto Fission Track
Co., Ltd.). The RIMS system measures volcanic glass shards
to an accuracy of ±0.0002 (Danhara et al. 1992). The major
element compositions were analyzed by energy-dispersive
spectrometry using an electron probe microanalysis
(EPMA) system (Horiba Emax Energy EX-250) at the
FURUSAWA Geological Survey. The major elements
were measured by scanning a 4 μm grid of the targeted
grain under a counting time of 150 s and accelerating
voltage of 15 kV. The beam current and diameter were
0.3 nA and 150 nm, respectively. The atomic number
effect was corrected by the ZAF procedure.
Results
2011 Tohoku-oki tsunami and its deposits
The inundation and runup heights of the 2011 Tohoku-oki
tsunami at Koyadori ranged from 13 to 18 m a.s.l. and
Table 2 Historical tsunamis’runup height around Koyadori
Tsunami event Age [AD] Koyadori [m] Oura [m] Funakoshi [m] Uranohama [m] Yamada [m] Osawa [m] References
Measured runup height
Tohoku-oki 2011 26.0–29.4 7.7–11.3 13.5–16.1 13.6–14.9 8.7–10.4 5.8–8.9 Haraguchi and Iwamatsu
(2011)
Tokachi-oki 1968 3.3 1.1 0.7 Watanabe (1998)
Chile 1960 No inundation
a
3.5–4.4 3.5 4.0 Iwate Prefecture (1969)
Showa Sanriku 1933 6.6 1.2–4.2 3.7–5.0 3.0 3.0–4.0 1.1–2.5 Earthquake Research Institute,
the University of Tokyo (1934)
Meiji Sanriku 1896 15.0 6.0 10.0–12.0 6.0 5.5 Unohana and Ota (1988)
Estimated runup height based on historical documents
Ansei 1856 3 2 4–6 2 3 Tsuji and Ueda (1995),
Hatori (2000)
Kansei 1793 2 3–42–3 2 Hatori (2009)
Empo 1677 2–4 Hatori (1975), Tsuji and
Ueda (1995),
Keicho 1611 20–25 10–12 5–8 Hatori (2009)
We selected earthquakes generating tsunamis (runup height is larger than a few meters around Koyadori). Blank space means no data
a
Based on interviews of landowners
Table 1 Historical tsunamis along the Sanriku Coast during AD 1611–2011
Date Name Latitude [°N] Longitude [°E] Magnitude
Earthquakes in observed records (after 1896)
11 March 2011 Tohoku-oki 38.1 142.86 9.0
26 September 2003 Tokachi-oki 41.78 144.08 8.0
16 May 1968 Tokachi-oki 40.73 143.58 7.9
23 May 1960 Chile −38.17 −72.57 9.5
4 March 1952 Tokachi-oki 41.8 144.13 8.2
3 November 1936 Miyagiken-oki 38.26 142.07 7.5
3 March 1933 Showa Sanriku 39.23 144.52 8.1
1 November 1915 Miyagiken-oki 38.3 142.9 7.5
5 August 1897 Miyagkeni-oki 38.3 143.3 7.7
15 June 1896 Meiji Sanriku 39.5 144 8.2
Earthquakes in documented records (before 1896)
23 August 1856 Ansei 41 142.3 7.7
17 February 1793 Kansei 38.5 144.5 8.2
29 January 1763 Horeki 41 142.5 7.7
13 April 1677 Empo 41 143 7.9
2 December 1611 Keicho 39 144 8.1
Based on Utsu (2004). We selected major earthquakes generating tsunamis hitting the Sanriku Coast
Ishimura and Miyauchi Progress in Earth and Planetary Science (2015) 2:16 Page 6 of 18
from 26 to 29 m a.s.l., respectively (Table 2; Haraguchi and
Iwamatsu 2011). Figure 3 shows the landform changes be-
fore and after the 2011 event. Immediately following the
2011 Tohoku-oki tsunami (April 2011), the beach was not
yet re-established and the beach ridges may have been
shortened by the tsunami backwash (Fig. 3a, b). After the
beach was restored in June 2011, the shortcut channel was
filled with beach deposits (Fig. 3c). The poor drainage area
remained until October 2012 (Fig. 3d). The Tohoku-oki
tsunami hit the coastal levee originating from beach ridges,
destroying it and the trees on it, and eroding it to a depth
of 1–1.5 m (Fig. 5d). The eroded materials were trans-
ported landward and deposited as tsunami deposits. Ap-
proximately 9 m a.s.l. and 600 m landward, a boulder was
recognizable as a tsunami deposit because of the attached
oyster shells (Fig. 5e). Tsunami deposits composed of sand
and gravel sourced from the beach and beach ridges were
found up to 600 m landward in December 2012 (Fig. 5f ).
Description of the KYD-trench
Deposits in the trench wall were divided into five facies
(event deposits, marsh deposits, channel fill deposits, arti-
ficial fill deposits, and cultivation soil), based on their
sediment structure, continuity, and composition (Fig. 6).
All the walls contained marsh deposits and interbedded
event deposits.
The event deposits are composed of coarse sand and
granule, and are traceable in the trench (Figs. 6 and 7).
Deposits showing good continuity, horizontal sedimenta-
tion, erosional features, and loading structure at the
base were considered as potential candidates for tsunami
deposits, and were labeled E1 (youngest) to E11 (oldest).
The characteristics of each event deposit are presented in
Table 3. The E1 deposits (the 2011 Tohoku-oki tsunami
deposits) are divisible into two units. The lower unit is
composed of coarse sediments (pebble to coarse sand)
with normal grading. The upper unit comprises finer
sediments (granule to medium sand) and is partially lami-
nated. The E2 deposits are thin and composed of granule
to coarse sand. The E2 layer is interbedded with cultiva-
tion soil, indicating partial disturbance by cultivation. The
E3 deposits are well traceable and characterized by a
bluish color (Fig. 6). As these deposits thicken from grids
E–3toS–3, they also become coarser (cobble to pebble).
However, while the basal contact is very sharp, the upper
contact is partially disturbed by cultivation. The E4 de-
posits are partially disturbed and eroded by channel de-
posits. In general, their compositions are fine (granule
to fine sand), although some parts contain pebble to cob-
ble gravels. The E5 deposits are interbedded with low-LOI
organic sediments (Fig. 7). Their basal contact is sharp,
but their upper contact is disturbed. They are intermit-
tently distributed because of plant bioturbation. The E6
deposits are well traceable in the KYD-trench (Fig. 6), with
very sharp basal contacts and a loading structure at the
bottom of the layer. The E7 deposits are also well trace-
able and some of them have eroded the E8 deposits
(Fig. 6). The E7 layer also shows a loading structure at the
bottom. The E8 deposits are intermittently distributed be-
cause of erosion by the E7 deposits. Below the E8 deposits,
Fig. 5 Photographs around the trench sites. aThe KYD- and canal-trench sites, bthe KYD-trench, cthe canal-trench, derosion of beach ridges, e
tsunami boulder, and f2011 tsunami deposits
Ishimura and Miyauchi Progress in Earth and Planetary Science (2015) 2:16 Page 7 of 18
the upper and basal contacts of the event deposits have
been disturbed by plant bioturbation. The E9 and E10
deposits are very thin and similarly disturbed by plant
bioturbation, but are nonetheless traceable in the KYD-
trench (Fig. 6). The E11 deposits are traceable in the
southern half of the KYD-trench and distributed under
Fig. 6 Picture and sketch of the KYD-trench walls. aPhoto mosaics of the KYD-trench wall. bSketch of the KYD-trench wall
Ishimura and Miyauchi Progress in Earth and Planetary Science (2015) 2:16 Page 8 of 18
the trench bottom in the northern half (Fig. 6). The
E11 deposits are of medium thickness and contain fine
grains (coarse to medium sand).
The marsh deposits are composed of plant remains and
organic sediments. Their densities are inversely correlated
with their LOIs and indirectly indicate their organic
carbon content and degree of decomposition (Fig. 7).
Color, in particular, the L* and b* parameters, is correlated
with density, whereas the LOI fluctuates between event
deposits. Macroscopically, the LOI decreases from the
Fig. 7 Tephra analysis, color, dry/wet bulk density, and loss on ignition of the KYD-trench wall samples
Table 3 Characteristics of event deposits in the KYD-trench
Event
deposits
General
thickness
Comparison of
grain size among
event deposits
General grain size Measured
thickness
[cm]
Upper
contact
Basal contact Gravel
content
[wt%]
Roundness
(well-rounded +
rounded) [%]
E1 Medium Coarse Upper: granule to
medium sand, Lower:
pebble to coarse sand
3–20 –Sharp 10–28 45–66
E2 Thin Medium Granule to coarse sand 1–8 Disturbed Sharp 8–19 13–53
E3 Thick Coarse Granule to coarse sand 2–38 Sharp Sharp 11–55 48–86
E4 Medium Fine Granule to fine sand 3–16 Disturbed Sharp 2–18 32–41
E5 Medium Medium Granule to coarse sand 2–13 Disturbed Sharp 7–24 30–56
E6 Thick Medium Granule to coarse sand 1–24 Sharp Sharp 18–35 16–62
E7 Thick Medium Granule to coarse sand 2–28 Disturbed/sharp Sharp 2–31 26–67
E8 Thin Fine Coarse to medium sand 1–7 Disturbed Disturbed 8–10 16–45
E9 Thin Fine Granule to medium sand 2–18 Disturbed Disturbed/sharp 8–17 12–42
E10 Thin Fine Coarse to medium sand 1–9 Disturbed Disturbed 3–11 14–43
E11 Medium Fine Coarse to medium sand 2–13 Disturbed Disturbed 3–450–61
Ishimura and Miyauchi Progress in Earth and Planetary Science (2015) 2:16 Page 9 of 18
trench bottom to the E4 deposits and increases from the
E4 deposits to the E3 deposits.
The channel fill deposits exhibit two cross-sectional
geometries and compositions, categorized as Channel 1
and Channel 2 (Fig. 6). Channel 1 is distributed from
grid N–5toN–10 and from grid W–5.5 to W–7 (Fig. 6).
From the altitude of the channel bottom in both walls,
the flow direction of Channel 1 was determined as east to
west. Sediments are finer in Channel 1 than in Channel 2,
comprising coarse sand to fine pebbles, and interbedded
with peaty silt. Channel 2 is distributed from grid N–1to
W–10 and from grid E–5.5 to E–11.5 and flows from
northwest to southeast (Fig. 6). The composition is poorly
sorted pebble to cobble.
The artificial fill deposits with a buried PVC pipe, dis-
tributed from grid W–7toW–11 and from grid N–1.5
to N–2.5 (Fig. 6), were identified from interviews with
landowners as underdrains constructed 40–50 years ago.
The cultivation soil is distinguished from marsh deposits
by its different particle composition, color, and texture. This
soil type is interbedded between the E3 and E1 deposits
(Fig. 6). Event deposits, marsh deposits, and cultivation
soil are also easily distinguishable by their density and
color (Fig. 7). The dry bulk density of cultivation soil is
intermediate between low-density marsh deposits and
high-density event deposits.
According to radiocarbon dating of these marsh and
channel deposits (Figs. 6, 7, and 8; Table 4), the sediments
in the KYD-trench wall provide a continuous record since
approximately 4000 cal. BP.
Description of the canal-trench
Event deposits (the E1, E2, and E3 deposits), other event
deposits, cultivation soil/peat, and debris flow deposits
were identified in the canal-trench (Figs. 4 and 9). Debris
flow deposits are distinguished by poorly sorted gravel
beds interfingered with cultivation soil/peat in the geomet-
ric cross-section (Fig. 5c). Although the E1 deposits are
traceable, the original thickness of the E1 layer has been
obscured by artificial modification following the 2011
Fig. 8 Event age diagram in the KYD-trench
Ishimura and Miyauchi Progress in Earth and Planetary Science (2015) 2:16 Page 10 of 18
Table 4 Radiocarbon ages and calibrated ages
Sample no. Sample
name
Labo no. Sample
position
Grid no. Depth [m] Material δ
13
C
[‰]
Conventional
14
C age [yrBP]
Calibrated age (2σ) [calBP]
1 KYD-TrS-1 IAAA-131322 Above E2 S-1.95 0.27 Plant −25.3 110 ± 20 270–210 (27.4 %), 150–20
(67.9 %)
2 KYD-TrS-2 IAAA-131323 Below E2 S-1.95 0.31–0.33 Plant −26.7 190 ± 20 290–260 (19.6 %), 220–140
(52.9 %), 20–(22.9 %)
3 KYD-TrS-3 Beta-357404 Above E3 S-1.95 0.43 Plant −28.8 340 ± 30 490–310 (95.4 %)
4 KYD-TrE-1 IAAA-131320 Above E4 E-11.75 0.60–0.62 Plant −28.0 790 ± 20 740–670 (95.4 %)
5 KYD-TrE-2 IAAA-131321 Below E4 E-11.75 0.74–0.76 Organic
sediment
−25.8 1420 ± 20 1350–1290 (95.4 %)
6 KYD-TrE-3 Beta-341796 Below E7 E-11.75 1.23–1.24 Plant −24.8 1570 ± 30 1540–1390 (95.4 %)
7 KYD-TrE-4 Beta-341572 Above E8 E-11.75 1.31–1.32 Plant −25.1 2320 ± 30 2380–2300 (90.1 %), 2240–2180
(5.3 %)
8 KYD-TrE-5 Beta-341797 Below E8 E-11.75 1.35–1.36 Plant −26.8 2360 ± 30 2490–2330 (95.4 %)
9 KYD-TrW-1 IAAA-131324 Above E3 W-2.75 0.27–0.29 Plant −25.5 150 ± 20 290–250 (15.7 %), 230–130
(48.1 %), 120–70 (13.1 %),
40–0 (18.5 %)
10 KYD-TrW-2 Beta-339851 Below E3 W-2.75 0.39–0.41 Plant −27.7 370 ± 30 510–420 (55.0 %), 400–310
(40.4 %)
11 KYD-TrW-3 Beta-339855 Above E4 W-2.75 0.84–0.86 Plant −29.1 1090 ± 30 1060–930 (95.4 %)
12 KYD-TrW-4 Beta-339856 Below E4 W-2.75 0.94–0.96 Plant −25.7 990 ± 30 970–890 (57.4 %), 880–790
(38.0 %)
13 KYD-TrW-5 Beta-339857 Above E5 W-2.75 1.10–1.12 Plant −26.8 1030 ± 30 1050–1030 (2.8 %), 990–900
(91.9 %), 850–830 (0.7 %)
14 KYD-TrW-6 Beta-341794 Below E5 W-2.75 1.19–1.21 Plant −26.3 700 ± 30 690–640 (77.5 %), 590–560
(17.9 %)
15 KYD-TrW-7 Beta-339858 Above E6 W-2.75 1.30–1.31 Plant −26.1 1100 ± 30 1070–930 (95.4 %)
16 KYD-TrW-8 Beta-341795 Below E6 W-2.75 1.39–1.40 Plant −26.4 1420 ± 30 1370–1280 (95.4 %)
17 KYD-TrW-9 Beta-339859 Above E7 W-2.75 1.45–1.46 Plant −27.6 1680 ± 30 1700–1650 (10.2 %), 1630–1520
(85.2 %)
18 KYD-TrW-10 Beta-339861 Above E9 W-2.75 1.66–1.67 Plant −26.9 2410 ± 30 2690–2630 (11.2 %), 2620–2590
(2.9 %), 2500–2340 (81.3 %)
19 KYD-TrW-11 Beta-341798 Below E9 W-2.75 1.73–1.74 Plant −24.5 2530 ± 30 2750–2680 (35.8 %), 2640–2490
(59.6 %)
20 KYD-TrW-12 Beta-339862 Above E10 W-2.75 1.87–1.88 Plant −27.2 2810 ± 30 3000–2840 (95.4 %)
21 KYD-TrW-13 Beta-340343 Above E10 W-2.75 1.87–1.88 Charcoal −25.4 3500 ± 30 3860–3690 (95.4 %)
22 KYD-TrW-14 Beta-341799 Below E10 W-2.75 1.92–1.93 Plant −28.0 2780 ± 30 2960–2790 (95.4 %)
23 KYD-TrW-15 Beta-339863 Above E11 W-2.75 2.16–2.17 Plant −26.1 2870 ± 30 3080–2880 (95.4 %)
24 KYD-TrW-16 IAAA-131669 Above E11 W-2.75 2.16–2.17 Organic
sediment
−23.5 3500 ± 30 3860–3690 (95.4 %)
25 KYD-TrW-17 Beta-341800 Below E11 W-2.75 2.25–2.26 Plant −25.8 3020 ± 30 3340–3140 (92.0 %), 3130–3110
(1.4 %), 3100–3080 (2.0 %)
26 KYD-Tr-a Beta-357405 Below E3 E-2.40-2.60 0.50–0.70 Plant −28.2 500 ± 30 620–610 (0.7 %), 560–500 (94.7 %)
27 KYD-Tr-b Beta-339852 Channel 1 E-6.00 0.68–0.70 Plant −27.0 1190 ± 30 1230–1210 (2.9 %), 1190–1050
(89.0 %), 1030–1000 (3.5 %)
28 KYD-Tr-c Beta-339853 Channel 1 E-7.30 0.95 Seed
(Juglans sp.)
−25.3 1240 ± 30 1270–1070 (95.4 %)
29 KYD-Tr-d Beta-357406 Channel 1 E-6.00 0.94–0.99 Plant −28.9 1090 ± 30 1060–930 (95.4 %)
30 KYD-Tr-e Beta-339854 Channel 2 E-8.45 1.30 Plant −25.1 1040 ± 30 1050–1020 (5.2 %), 1000–910
(90.2 %)
Ishimura and Miyauchi Progress in Earth and Planetary Science (2015) 2:16 Page 11 of 18
event. The E2 deposits are intermittently distributed and
some of them have been modified by cultivation. The
E3 deposits are traceable and partially disturbed. The E3
layer is less than 20 cm thick, decreasing in the landward
direction.
Particle roundness
Event deposits were sampled from the KYD- and canal-
trenches (Figs. 6 and 9). Modern beach and river deposits,
and debris flow deposits, were sampled from the canal-
trench and outcrops (Figs. 4 and 9). Next, the origins of
the event deposits were inferred from their roundness
measures. Modern river and debris flow deposits consist al-
most entirely of angular/very angular and sub-rounded/
sub-angular gravels, with no well-rounded/rounded gravels
(Fig. 10a–c). Conversely, modern beach deposits con-
tain well-rounded/rounded and sub-rounded/sub-angular
gravels; angular/very angular gravels are absent (Fig. 10d).
The E1 and E3 deposits (Fig. 10e, f) are similar to mod-
ern beach deposits, with high contents of well-rounded/
rounded and sub-rounded/sub-angular gravels. The clear
roundness differences between modern river and beach
deposits are shown in the triangular diagram of Fig. 10g.
The roundness composition of all event deposits in the
KYD-trench is shown in Fig. 10h. In all samples, the pro-
portion of well-rounded/rounded gravel contents exceeds
10 %, while the angular/very angular gravel content is
below 40 %. Unlike the terrestrial deposits, all event de-
posits contain beach gravels.
Tephra analysis
From the radiocarbon dating, we determined that the in-
visible To-a (AD915 (1035 cal. BP)) lies between the E3
and E5 deposits. The volcanic glass contents in each
trench wall sample increase after the E4 deposition (Fig. 7).
In particular, in the east and west walls, the volcanic glass
content suddenly increases and gradually decreases from
the lower to upper parts, indicating an invisible tephra
horizon. However, this trend is absent in the south wall,
probably because it has been eroded by the E3 deposits.
The origins of the volcanic glass shards were determined
from their refractive indices. Above the E4 deposits, the
region of highest volcanic glass content, the glass refract-
ive index ranges from 1.504 to 1.511 (mode: 1.507–1.508).
Below the E4 deposits, the refractive index ranges from
1.509 to 1.514 (mode: 1.512). Furthermore, we analyzed
the major element compositions of the volcanic glass
shards above the E4 deposits (Table 5).
Discussion
Identification of tsunami deposits
The roundness similarities between the event and beach
deposits (Fig. 10) indicate that event deposits were trans-
ported from beach and beach ridges to the inland trench
sites. Landward transport from the sea is expected in tsu-
nami and storm events. The general characteristics of
tsunami and storm deposits have been reported by
many researchers (Morton et al. 2007; Kortekaas and
Dawson 2007; Switzer and Jones 2008; Goff et al. 2012;
Phantuwongraj and Choowong 2012). On average, tsu-
nami deposits are generally thinner than storm deposits
(Morton et al. 2007; Phantuwongraj and Choowong 2012),
and sedimentary structure is less common in tsunami
deposits than in storm deposits (Morton et al. 2007;
Kortekaas and Dawson 2007; Switzer and Jones 2008; Goff
et al. 2012). The basal contact of both sediments is uncon-
formable or erosional (Morton et al. 2007; Kortekaas and
Dawson 2007; Switzer and Jones 2008; Goff et al. 2012;
Phantuwongraj and Choowong 2012), although tsunami
deposits sometimes show a loading structure (Goff et al.
Fig. 9 Sketch of the canal-trench
Ishimura and Miyauchi Progress in Earth and Planetary Science (2015) 2:16 Page 12 of 18
Fig. 10 Photographs and roundness of tsunami deposits, channel deposits, debris flow deposits, modern beach deposits, and modern river deposits. a
Modern river deposits (Loc. 1). bModern river deposits (Loc. 2). cDebris flow deposits in the canal-trench (Loc. 7). dModern beach deposits (Loc. 8). e
The E1 deposits in the south wall of the KYD-trench. fThe E3 deposits in the south wall of the KYD-trench. gComparison of the 2011 Tohoku-oki tsunami
deposits, beach deposits, modern river deposits, and debris flow deposits; htsunami deposits in the KYD-trench wall. Scale in the photographs is 1 cm. In
the triangular diagrams, x-axis indicates well-rounded/rounded, y-axis indicates sub-rounded/sub-angular, and z-axis indicates very angular/angular
Ishimura and Miyauchi Progress in Earth and Planetary Science (2015) 2:16 Page 13 of 18
2012). On transect scales (several hundred meters), the
cross-shore geometries of tsunami and storm deposits
are characterized by “broad thin drapes with tabular or
landward thinning”and “narrow thick deposits with
abrupt landward thinning,”respectively (Morton et al.
2007). These characteristics of tsunami deposits are recog-
nized in the event deposits in the trenches. In the KYD-
trench (length = 12 m), all event deposits are generally
thinner than 20 cm and appear as draped or eroded paleo-
surfaces. Some of them exhibit a loading structure. In the
canal-trench (length = 150 m), the E1 and E3 deposits ap-
pear as draped deposits, with landward thinning in the E3
deposits. Furthermore, the KYD-trench is located 300 m
inland from the beach, and landowners reported no storm
deposits in the trench sites during the past 40–50 years. In
contrast, the paleo-shoreline after the To–Cu deposition
(about 6 ka) is estimated to be at least on the seaside of
the KYD-Br1 site (Fig. 4). The elevations of primary
To–Cu tephra within the KYD-Br1 to KYD-Br3 cores
are −1.60, −1.82, and −3.08 m a.s.l., respectively, show-
ing landward deepening, and sediments deposited after
the To–Cu deposition are consistent with non-marine
environments such as marsh (Ishimura et al. 2014). From
these data, we consider that the present depositional
setting (beach ridge and behind marsh) around the
trench sites was already established by 6 ka. Therefore,
prior to the To–Cu fall, the paleo-surface topography
places the paleo-shoreline on the seaside of the KYD-
Br1 core site. The features of the event deposits and
the geomorphological settings from 6 ka to the present,
together with the responses of interviewed landowners
regarding recent events, indicate that all event deposits
in the KYD-trench are sourced from tsunamis rather
than storms.
Ages of tsunami deposits and correlation to historical
tsunami event
Radiocarbon dating (Fig. 8, Table 4) suggests that the de-
posits in the KYD-trench are relatively close in age with
no large age gap. Radiocarbon dating of event deposits is
performed on plant fragments (such as reeds), because
these constitute the youngest material in a sampled hori-
zon. Plant materials in the trench are likely to be fragments
of in situ plants, but downward invasion of roots and
underground stems should not be ruled out. Thus, the ages
of the plant material were assumed to represent the youn-
gest ages of the sampled horizons. In contrast, a charcoal
and a hard-shell plant seed (Juglans sp.)(SamplesNo.21
and 28; Table 4) are assumed to be transported materials,
whose ages mark the older age limit of the sampled hori-
zons. Organic sediments (Samples No. 5 and 24; Table 4)
are older than plant fragments, consistent with our radio-
carbon dating interpretations. The true age of the sediment
is expected to lie between the ages of the plant and other
materials. Ishimura et al. (2014) identified To–Cu (6 ka)
tephra and Oguni Pumice (7.3 to 7.4 ka) in the KYD-Br3
core drilled next to the KYD-trench at depths of 4.41–
5.98 m (total thickness of primary and secondary tephra)
and 8.55–8.60 m, respectively. The horizons and ages
of these sediments are consistent with the radiocarbon-
dated geochronology of the KYD-trench determined in
this study.
We estimated the event ages based on radiocarbon
dating and the above criteria (Fig. 8, Table 6). The tsunami
deposit events are labeled E1–E11 in order of increasing
age, considering the age constraint of the next older event.
Since the ages of the E5 and E6 deposits were not deter-
mined by radiocarbon dating, we interpolated their ages
using the radiocarbon ages of the E4 and E7 deposits.
Table 5 Major element compositions of volcanic glass shards
SiO
2
TiO
2
Al
2
O
3
FeO MnO MgO CaO Na
2
OK
2
OnTotal
a
ONM-GS1-0.42 77.0 0.4 12.7 1.8 0.1 0.4 2.1 4.1 1.4 15 97.5
Ishimura et al. (2014) 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.6
NTR-Br-34.93 77.0 0.4 12.8 1.8 0.1 0.4 2.1 4.1 1.4 15 96.1
Ishimura et al. (2014) 0.3 0.1 0.2 0.1 0.1 0.0 0.1 0.3 0.1 0.7
To-a (35) 77.50 0.36 12.73 1.62 0.09 0.38 1.81 3.90 1.37 19 98.41
Aoki and Machida (2006) 0.7 0.0 0.4 0.2 0.0 0.1 0.2 0.1 0.1 1.4
To-a (36) 77.69 0.36 12.74 1.66 0.09 0.35 1.80 3.99 1.31 8 98.53
Aoki and Machida (2006) 0.6 0.1 0.3 0.1 0.0 0.0 0.1 0.2 0.0 1.3
To-a (37) 76.17 0.42 13.41 1.89 0.09 0.38 1.99 4.08 1.56 18 92.89
Aoki and Machida (2006) 0.3 0.0 0.2 0.1 0.0 0.0 0.1 0.1 0.1 1.1
KYD-TrW sec. 2 20–25 cm 77.1 0.4 12.7 1.8 0.1 0.4 2.1 4.1 1.4 15 94.4
(This study) 0.3 0.1 0.2 0.2 0.1 0.0 0.0 0.1 0.1 1.3
Number on upper line shows a mean value and that on lower line shows a standard deviation. Measured values are recalculated to 100 % on a water-free basis.
Results of Aoki and Machida (2006) are shown for comparison and numbers in brackets show sample number in the reference
a
Raw data before recalculations
Ishimura and Miyauchi Progress in Earth and Planetary Science (2015) 2:16 Page 14 of 18
From these chronological estimations, we can correlate
the E2, E3, and E4 deposits to historical tsunamis because
these deposits are younger than AD 600.
The ages of the E2 deposits range from modern times
to 290 cal. BP (i.e., they are younger than AD 1660).
Certainly, the E2 deposits can be correlated to one event
among the 1611, 1677, 1793, 1856, 1896, and 1933 events
(Table 2). The runup heights (Table 2) and stratigraphic
position of the E2 deposits suggest a correlation with the
1933 Showa Sanriku tsunami and the 1896 Meiji Sanriku
tsunami, because the height of the beach ridge at Koyadori
was approximately 5 m a.s.l. in both events. Although
both tsunamis inundated up to the trench sites, only
single-event deposits were identified from AD 2011 to
1660. Tsunami deposits can be absent for several reasons
stated as follows: 1) disturbance and/or removal by culti-
vation, 2) erosion by succeeding tsunami events, 3) sedi-
ment availability, and 4) tsunami size. The first cause is
easily explained. If sediments were deposited by the 1896
Meiji Sanriku and the 1933 Showa Sanriku tsunamis, the
latter deposits would first be disturbed and removed by
cultivation processes. In this case, we would correlate the
preserved tsunami deposits to the 1896 Meiji Sanriku
tsunami. Regarding the second cause, the 1896 deposits
might have been eroded by the 1933 deposits. However,
the E2 deposits show no clear base erosion in either the
KYD- or canal-trenches, and no remnants of eroded
tsunami deposits are evident between the E2 and E3 de-
posits. Thus, the second cause is inconsistent with the
observations. Meanwhile, the third cause is inconsistent
with the study site setting. If a large tsunami, with height
exceeding that of the beach ridge, hits Koyadori, sedi-
ments of beach and beach ridge must be transported
landward because there is much sediment in the coast
and the beach was re-established a few months after the
2011 event. The forth cause, tsunami size, relates to the
transportation and preservation of tsunami deposits. The
inundation heights were larger in the 1896 event than in
the 1933 event (Table 2). Thus, we can easily expect that
the volume of the 1896 tsunami deposits exceeded that of
the 1933 deposits. This also suggests that the 1896
tsunami deposits were better preserved than the 1933
tsunami deposits. From these considerations, we inferred
that the E2 deposits are correlated to the 1896 Meiji
Sanriku tsunami.
Considering the above correlation of the E2 deposits,
the age of the E3 deposits was estimated as 54–620 cal.
BP (AD 1896–1330). Thus, the E3 deposits can be corre-
lated to one event among the 1454, 1611, 1677, 1793,
and 1856 events (Table 2). Based on the tsunami runup
height of these events (Table 2), the E3 deposits are most
probably associated with the 1611 Keicho Sanriku tsunami.
The E3 deposits are thick and composed of coarse ma-
terials (Table 3), and are traceable in the canal-trench
(Fig. 9). Assuming a similar depositional setting from
about 6 ka onward, we considered that the feature dif-
ferences (thickness and grain size) among event deposits
roughly indicate the tsunami size. The features of the E3
deposits (Table 3) suggest a large, very energetic tsunami.
According to a local legend (Imamura 1934), the 1611
Keicho Sanriku tsunami inundated and surged through
the Koyadori–Oura pass (Fig. 2). This indicates that the
1611 Keicho Sanriku tsunami was at least as high as the
2011 Tohoku-oki tsunami, since the latter failed to reach
the geomorphic pass.
According to the radiocarbon dates of the channel de-
posits, the E4 deposits are aged 1000–1350 cal. BP (AD
950–600), and possibly correlate with the 869 Jogan tsu-
nami. By targeting our tephra analysis at the To-a (AD
915) horizon, we determined an absolute timing for the
E4 deposits. The increased content of volcanic glass above
the E4 deposits (Fig. 7) suggests a tephra fall after the
E4 sedimentation. The refractive index of volcanic glass
shards above the E4 deposits ranged from 1.504 to 1.511
(mode: 1.507–1.508), which includes the To-a tephra range
(Machida and Arai 2003; Ishimura et al. 2014). Similarly,
the chemical compositions of volcanic glass shards were
consistent with previously reported To-a compositions
(Aoki and Machida 2006; Ishimura et al. 2014). From
these data, we inferred that the To-a tephra fell between
the E4 and E3 deposits, and we assigned the E4 deposits
to the 869 Jogan tsunami. This identification based on
radiocarbon dating and tephra provides significant infor-
mation on the size and source of the 869 Jogan tsunami
and earthquake, indicating that this tsunami reached the
middle part of the Sanriku Coast and its inundation area
was possibly as large as the 2011 event. Since the Jogan
tsunami is not reported in historical records around
Table 6 Estimated ages of tsunami deposits and their
correlation with historical tsunami events
Age [BP] Most probable historical event Tephra
E1 2011 Tohoku-oki Earthquake
Tsunami
E2 Modern–290 1896 Meiji Sanriku Tsunami
E3 0–620 1611 Keicho Sanriku Tsunami
E3/E4 To-a (AD 915)
E4 1000–1350 869 Jogan Tsunami?(no historical
document around Koyadori)
E5 1000–1700
E6 1000–1700
E7 1520–1700
E8 2180–2490
E9 2340–2750
E10 2840–3860
E11 2880-3860
Ishimura and Miyauchi Progress in Earth and Planetary Science (2015) 2:16 Page 15 of 18
Koyadori and insufficient information is available for re-
gionally and chronologically identifying the tsunami de-
posits along the Sanriku Coast, this finding requires
confirmation in paleographical and geological researches.
Tsunami ages and their intervals
Conclusive age estimates and correlations of historical
tsunami events are summarized in Table 6. Although some
ambiguity of the ages remains, we calculated the average
interval of tsunami occurrence as 290–390 years. Before
considering the approximate age intervals of tsunami
events, we need to discuss the preservation potential of
tsunami deposits at this site. Szczucinski (2012) and
Spiske et al. (2013) mentioned the preservation potential
of tsunami deposits in tropical and temperate climate re-
gions, respectively, and showed that the characteristics of
tsunami deposits (thickness and sedimentary structure)
degrade over time. Spiske et al. (2013) emphasized the
significance of the preservation potential in assessing
the intervals and frequencies of tsunamis, because tsu-
nami deposits are not necessarily preserved in whole in-
undated areas. They identified five determining factors of
preservation potential as follows: 1) composition and
genetic type of the tsunami deposits, 2) coastal topography
and depositional environment, 3) co- and post-seismic
uplift or subsidence, 4) climate, and 5) anthropogenic
modification. In Koyadori, tsunami deposits originated
from beach and beach ridge deposits and are coarser
than those reported in Szczucinski (2012) and Spiske et al.
(2013), indicating larger resistance to post-tsunami surface
processes. As mentioned above, the sedimentary environ-
ment has remained largely unchanged since 6 ka, and the
beach ridge and behind-marsh environment have main-
tained accommodation space for tsunami and marsh
deposits. The 2011 event was followed by co- and post-
seismic subsidence (Ozawa et al. 2011), enhancing the
preservation environment of tsunami deposits. The Sanriku
Coast has a temperate climate and experiences fewer and
weaker storms and high tide events (such as typhoons) than
the western part of Japan. According to interviews with
landowners, no storm deposits have settled in the trench
sites during the past 40–50 years. Artificial modification is
limited to deposits younger than E3 at this site. Moreover,
2011 tsunami deposits were found in pits and coring sur-
veys conducted around the KYD-trench in 2013 and 2014.
These deposits were clearly identifiable, despite being par-
tially bioturbated by grass and reed. Such vegetation cov-
ered the tsunami deposits, preventing erosion and removal
by post-tsunami surface processes. Even in the event of
dense bioturbation, tsunami deposits are easily identified by
their grain composition, size, and roundness, which widely
differ from those of background deposits (e.g., peat and
debris flow deposits). Therefore, we conclude that the
preservation potential of tsunami deposits is very high
in Koyadori. Consequently, the calculated average inter-
val probably truly reflects the interval and frequency of
large tsunamis.
The calculated average interval (290–390 years) is shorter
than that obtained for the Sendai and Ishinomaki Plains
(Sawai et al. 2007; 2012; Shishikura et al. 2007), reflecting
thehighfrequencyoflargetsunamiscausingdestructive
damage along the Sanriku Coast. However, if we have
correctly correlated the historical deposits to the histor-
ical tsunami events, we can state the age intervals from
the E1 to E4 deposits as 115, 285, and 742 years, respect-
ively. This variability probably indicates the diversity of
the tsunami generation mechanism (e.g., large earthquake,
tsunami earthquake, submarine mass failure, and tsunami
of distant origin) and/or the combination of several types
of large earthquakes from different sources around the
Japan Trench.
On the other hand, the size of historical and paleo-
tsunamis can be estimated from our results because the
1896 Meiji Sanriku tsunami inundated the KYD-trench
site and transported tsunami deposits there. In contrast,
neither the 1968 Tokachi-oki tsunami (nearby source,
runup height approximately 3 m around Koyadori; Table 2)
nor the 1960 Chile tsunami (distant source, runup height
approximately 4 m around Koyadori; Table 2) inun-
dated, perhaps because they were blocked by beach ridges
(height approximately 5 m a.s.l.). Furthermore, the envir-
onmental setting at the study site has been established
since approximately 6 ka. These observations preliminarily
suggest that tsunamis larger than the 1896 Meiji Sanriku
tsunami occur at the calculated average interval, providing
a first step for assessing the risk and size of tsunamis along
the Sanriku Coast. To understand the tsunami generation
mechanism and earthquakes along the Japan Trench, we
require detailed information of ages, intervals, and sizes of
historical and paleo-tsunamis at multiple sites.
Conclusions
We identified eleven tsunami deposits, including the
2011 tsunami deposits, based on sedimentary structure
and continuity in two trenches and comparisons of the
roundness of the gravel composing the event deposits.
Radiocarbon dating and tephra analysis allowed us to
establish the geochronology in the KYD-trench wall sedi-
ments and to correlate tsunami deposits with historical
tsunami events. The four younger tsunami deposits (the
E1–E4 deposits) are correlated with the 2011 Tohoku-oki
tsunami, the 1896 Meiji Sanriku tsunami, the 1611 Keicho
Sanriku tsunami, and the 869 Jogan tsunami events, re-
spectively. The average interval of tsunami occurrence at
Koyadori is estimated at 290–390 years based on continu-
ous records in the KYD-trench. However, the age intervals
between the E1 to E4 deposits are variable (E1/E2:
115 years, E2/E3: 285 years, E3/E4: 742 years), likely
Ishimura and Miyauchi Progress in Earth and Planetary Science (2015) 2:16 Page 16 of 18
reflecting the diversity of the tsunami generation mechan-
ism and/or different earthquake sources around the Japan
Trench. By correlating the historical tsunami runup height
data with extant tsunami deposits, we could preliminarily
estimate the sizes of paleo-tsunamis at the study site. In
the future study, we need to confirm our tsunami correla-
tions by correcting many geological data along the Sanriku
Coast. Ultimately, we aim to assess tsunami risk and
understand the earthquake phenomena around the Japan
Trench.
Competing interests
The authors declare that they have no competing interests.
Authors’contributions
DI conducted preliminary surveys, chose the study site, collected samples,
performed experiments, and wrote the paper. TM designed and directed the
project and gave helpful comments for the paper. Both authors read and
approved the final manuscript.
Acknowledgements
We are grateful to Kazuomi Hirakawa, Toshifumi Imaizumi, Shuji Yoshida,
Heitaro Kaneda, Tomoo Echigo, and Shinsuke Okada for their comments and
helps in the fieldwork. We thank Hiroyuki Tustsumi for permission of using a
soil color meter. The landowners of the KYD-trench site are also thanked for
allowing us to conduct our surveys on their properties. The editor Ken Ikehara
and two anonymous reviewers provided constructive comments that improved
the manuscript. This study was a part of “Geophysical and geological studies of
earthquakes and tsunamis for off-Tohoku district, Japan”and supported by the
Ministry of Education, Culture, Sports, Science, and Technology, Japan (MEXT).
This work was supported by Intramural Research Grant for Special Project
Researches from International Research Institute of Disaster Science, Tohoku
University.
Author details
1
Disaster Science Division, International Research Institute of Disaster Science,
Tohoku University, 468-1 Aza-aoba, Aramaki, Aoba-ku, Sendai, Miyagi
980-0845, Japan.
2
Department of Earth Sciences, Graduate School of Science,
Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba, Chiba 263-8522, Japan.
Received: 11 November 2014 Accepted: 21 May 2015
References
Aoki K, Machida H (2006) Major element composition of volcanic glass shards in
the late Quaternary widespread tephras in Japan–Distribution of tephras
using K2O-TiO2 diagrams. Bull Geol Surv Japan 57:239–258
Bos AJ, Busschers FS, Hoek WZ (2012) Organic-facies determination: a key for
understanding facies distribution in the basal peat layer of the Holocene
Rhine-Meuse delta, The Netherlands. Sedimentology 59:679–703
Danhara T, Yamashita T, Iwano H, Kasuya M (1992) An improved system for
measuring refractive index using the thermal immersion method. Quat Int
13(14):89–91
Earthquake Research Committee Headquarters for Earthquake Research Promotion
Prime Minis ter’s Office, Government of Japan (1999) Seismic Activity in
Japan -Regional perspectives on the characteristics of destructive
earthquakes-. Association for the Development of Earthquake. Prediction,
Tokyo
Earthquake Research Institute, the University of Tokyo (1934) Reports on the 3 March,
Showa 8 Sanriku tsunami. Bull Earthq Res Inst Tokyo Univ, Suppl 1:1–250
Goff J, Chagué-Goff C, Nichol S, Jaffe B, Dominey-Howes D (2012) Progress in
palaeotsunami research. Sedi Geol 243–244:70–88
Goldfinger C, Ikeda Y, Yeats RS, Ren J (2013) Superquakes and supercycles. Seis
Res Lett 84:1–9
Hamilton S, Shennan I (2005) Late Holocene relative sea-level changes and the
earthquake deformation cycle around upper Cook Inlet, Alaska. Quat Sci Rev
24:1479–1498
Haraguchi T, Goya K (2007) Tsunami deposits from Goishi-hama, of Ofunato City,
Iwate Prefecture. In: Proceedings of Rekishi-Zishin Meeting 23. Ofunato Plaza
Hotel, Ofunato, 15–17 September 2007
Haraguchi T, Ishibe T (2009) Large earthquake model in the central part of Off-Sanriku
estimated f rom tsunami deposits and uplifted event layers. Chikyu Monthly
31:223–230
Haraguchi T, Iwamatsu A (2011) Detailed maps of the impacts of the 2011 Japan
tsunami: Vol. 1: Aomori, Iwate and Miyagi prefectures. Kokon-Shoin Publishers
Ltd, Tokyo
Haraguchi T, Fujiwara O, Shimazaki K, Imaizumi T (2006a) Tsunami deposits from
Ohtsuchi Bay of Sanriku Coast, North Eastern Japan. In: Abstract of Japan
Geoscience Union Meeting 2006. Makuhari Messe, Chiba, 14–18 May 2006
Haraguchi T, Torii K, Fujiwara O, Shimazaki K, Imaizumi T (2006b) Subaqueous
tsunami deposits from Otsuchi Bay of Sanriku Coast, Northeastern Japan.
Chikyu Monthly 28:539–545
Haraguchi T, Torii T, Imaizumi T, Miyauchi T, Shimazaki K (2007) Tsunami deposits
from Sanriku coast NE Japan in late Holocene. In: Abstract of Japan
Geoscience Union Meeting 2007. Makuhari Messe, Chiba, 19–24 May 2007
Hatori T (1975) Tsunami magnitude and wave source regions of historical Sanriku
tsunamis in northeast Japan. Bull Earthq Res Inst 50:397–414
Hatori T (2000) Reexamination of magnitude of historical tsunamis off the
Sanriku. Tsunami Eng Tech Rep Tohoku Univ 17:38–48
Hatori T (2009) Regional deviation of inundation heights due to the large Sanriku
tsunamis. Histo Earthq 24:41–48
Hayward BW, Grenfell HG, Sabaa AT, Carter R, Cochran U, Lipps JH, Shane PR,
Morley MS (2005) Micropaleontological evidence of large earthquakes in the
past 7200 years in southern Hawke’s Bay, New Zealand. Quat Sci Rev
25:1186–1207
Ikeda Y, Okada S, Tajikara M (2012) Long-term strain buildup in the Northeast
Japan arc-trench system and its implications for gigantic strain-release
events. J Geol Soc Japan 118:294–312
Imaizumi T, Miyauchi T, Haraguchi T, Shimazaki K, Kagohara K, Sasaki A, Kamal
PABM, Goya K (2007) Tsunami deposits along the Sanriku coast and historical
earthquakes. In: Proceedings of the 2007 General Meeting of the Association
of Japanese Geographers. Toyo University, Tokyo, 20–22 March 2007
Imamura A (1934) Past tsunamis of the Sanriku District. Bull Earthq Res Inst Tokyo
Univ, Suppl 1:1–16
Ishimura D, Yamada K, Miyauchi T, Hayase R (2014) Characteristics of tephras
interbedded with the Holocene sediments in the Sanriku Coast, northeast
Japan. J Geogra 123:671–697
Kawamura K, Sasaki T, Kanamatsu T, Sakaguchi A (2012) Large submarine
landslides in the Japan Trench: a new scenario for additional tsunami
generation. Geophys Res Lett 39, L05308
Kawamura K, Laberg JS, Kanamatsu T (2014) Potential tsunamigenic submarine
landslides in active margins. Marine Geol 356:44–49
Kortekaas S, Dawson AG (2007) Distinguishing tsunami and storm deposits: an
example from Martinhal, SW Portugal. Sedi Geol 200:208–221
Machida H, Arai F (2013) Atlas of Tephra in and around Japan [revised edition].
University of Tokyo Press, Tokyo
Minoura K, Nakaya S (1991) Traces of tsunami preserved in inter-tidal lacustrine
and marsh deposits: some examples from northeast Japan. J Geol 99:265–287
Minoura K, Imamura F, Sugawara D, Kono Y, Iwashita T (2001) The 869 Jogan
tsunami deposit and recurrence interval of large-scale tsunami on the Pacific
coast of northeast Japan. J Nat Dis Sci 23:83–88
Morton RA, Gelfenbaum G, Jaffe BE (2007) Physical criteria for distinguishing sandy
tsunami and storm deposits using modern examples. Sed Geol 200:184–207
Namegaya Y, Satake K (2014) Reexamination of the A.D. 869 Jogan earthquake
size from tsunami deposit distribution, simulated flow depth, and velocity.
Geophys Res Lett 41:2297–2303
Namegaya Y, Yata T (2014) Tsunamis which affected the Pacific coast of eastern
Japan in medieval times inferred from historical documents. J Seis Soc Japan
2nd ser. 66:73–81
Ozawa S, Nishimura T, Suito H, Kobayashi T, Tobita M, Imakiire T (2011) Coseismic
and postseismic slip of the 2011 magnitude-9 Tohoku-Oki earthquake. Nature
475:373–376
Phantuwongraj S, Choowong M (2012) Tsunamis versus storm deposits from
Thailand. Nat Hazards 63:31–50
Prefecture I (1969) Record of recovery process from the 1960 Chile earthquake
tsunami. Iwate Prefecture, Morioka
Rajendran K (2013) On the recurrence of great subduction zone earthquakes.
Curr Sci 104:880–892
Ishimura and Miyauchi Progress in Earth and Planetary Science (2015) 2:16 Page 17 of 18
Ramsey BC (2009) Bayesian analysis of radiocarbon dates. Radiocarbon 51:337–360
Reimer PJ, Bard E, Bayliss A, Beck JW, Blackwell PG, Ramsey BC, Grootes PM,
Guilderson TP, Haflidason H, Hajdas I, Hatte C, Heaton TJ, Hoffmann DL,
Hogg AG, Hughen KA, Kaiser KF, Kromer B, Manning SW, Niu M, Reimer RW,
Richards DA, Scott EM, Southon JR, Staff RA, Turney CSM, van der Plicht J
(2013) IntCal13 and Marine13 radiocarbon Age calibration curves 0–50,000
years cal BP. Radiocarbon 55:1869–1887
Sawai Y, Nasu H (2005) A 4500-year record of emergence events at Onnetoh,
Hokkaido, northern Japan, reconstructed using plant macrofossils. Mar Geol
217:49–65
Sawai Y, Shishikura M, Okamura Y, Takada K, Matsuura T, Aung TT, Komatsubara J,
FujiiY,FujiwaraO,SatakeK,KamatakiT,FujinoS(2007)Astudyonpaleotsunami
using handy geoslicer in Sendai Plain (Sendai, Natori, Iwanuma, Watari, and
Yamamoto), Miyagi, Japan. Ann Rep Act Fault Paleoearthquake Res 7:47–80
Sawai Y, Kamataki T, Shishikura M, Nasu H, Okamura Y, Satake K, Thomson KH,
Matsumoto D, Fujii Y, Komatsubara J, Aung TT (2009) Aperiodic recurrence of
geologically recorded tsunamis during the past 5500 years in eastern Hokkaido,
Japan. J Geophys Res 114, B01319
Sawai Y, Namegaya Y, Okamura Y (2012) Challenges of anticipating the 2011
Tohoku earthquake and tsunami using coastal geology. Geo phys Res Lett
39, L21309
Shennan I, Bruhn R, Barlow N, Good K, Hocking E (2014) Late Holocene great
earthquakes in the eastern part of the Aleutian megathrust. Quat Sci Rev
84:86–97
Shishikura M, Sawai Y, Okamura Y, Komatsubara J, Aung TT, Ishiyama T, Fujiwara O,
Fujino S (2007) Age and distribution of tsunami deposit in the Ishinomaki Plain,
Northeastern Japan. Ann Rep Act Fault Paleoearthquake Res 7:31–46
Spiske M, Piepenbreier J, Benavente C, Bahlburg H (2013) Preservation potential
of tsunami deposits on arid siliciclastic coasts. Earth-Sci Rev 126:58–73
Sugawara D, Goto K, Imamura F, Matsumoto H, Minoura K (2012) Assessing
the magnitude of the 869 Jogan tsunami using sedimentary deposits:
Prediction and consequence of the 2011 Tohoku-oki tsunami. Sed Geol
282:14–26
Switzer AD, Jones BG (2008) Large-scale washover sedimentation in a freshwater
lagoon from the southeast Australian coast: sea-level change, tsunami or
exceptionally large storm? The Holocene 18:787–803
Szczucinski W (2012) The post-depositional changes of the onshore 2004
tsunami deposits on the Andaman Sea coast of Thailand. Nat Hazards
60:115–133
Tappin DR, Grilli ST, Harris JC, Geller RJ, Masterlark T, Kirby JT, Shi F, Ma G,
Thingbaijam KKS, Mai PM (2014) Did a submarine landslide contribute to the
2011 Tohoku tsunami? Marine Geol 357:344–361
The Headquarters for Earthquake Research Promotion (2006) Research report of
high-priority observation and survey on miyagi-oki earthquakes, 2005 fiscal
year. The Headquarters for Earthquake Research Promotion, Tokyo
The Headquarters for Earthquake Research Promotion (2007) Research report of
high-priority observation and survey on miyagi-oki earthquakes, 2006 fiscal
year. The Headquarters for Earthquake Research Promotion, Tokyo
The Headquarters for Earthquake Research Promotion (2008) Research report of
high-priority observation and survey on miyagi-oki earthquakes, 2007 fiscal
year. The Headquarters for Earthquake Research Promotion, Tokyo
The Headquarters for Earthquake Research Promotion (2009) Research report of
high-priority observation and survey on miyagi-oki earthquakes, 2008 fiscal
year. The Headquarters for Earthquake Research Promotion, Tokyo
The Headquarters for Earthquake Research Promotion (2010) Research report of
high-priority observation and survey on miyagi-oki earthquakes, 2009 fiscal
year. The Headquarters for Earthquake Research Promotion, Tokyo
Torii K, Haraguchi T, Imaizumi T, Miyauchi T, Shimazaki K (2007) Survey of
tsunami deposits at Sanriku coast, North East Japan. In: Proceedings of
Japan society of engineering geolo gy 2007. Osaka City University, Osaka,
11–12 October 2007
Tsuji Y, Ueda K (1995) Evaluation of A.D. 1611 Keicho, A.D. 1677 Enpou, A.D. 1763
Houreki, A.D. 1793 Kansei, and A.D. 1856 Ansei Sanriku tsunamis. Histo Earthq
11:75–106
Unohana M, Ota T (1988) Disaster records of Meiji Sanriku tsunami by Soshin
Yamana. Res Rep Tsunami Disaster Control Center Tohoku Univ 5:57–379
Utsu T (2004) Catalog of damaging earthquakes in the world (through 2002), Tokyo.
http://iisee.kenken.go.jp/utsu/index_eng.html. Accessed 24 December 2014.
Watanabe H (1998) Comprehensive list of tsunamis to hit the Japanese Islands
2nd edn. University of Tokyo Press, Tokyo
Witter RC, Kelsey HM, Hemphill-Haley E (2003) Great Cascadia earthquakes and
tsunamis of the past 6700 years, Coquille River estuary, southern coastal
Oregon. Geol Soc Am Bull 115:1289–1306
Yoshida T, Ohsawa A, Katada M (1984) Geological Map of Japan 1:200,000.
Geological Survey of Japan, Tsukuba, Morioka
Submit your manuscript to a
journal and benefi t from:
7 Convenient online submission
7 Rigorous peer review
7 Immediate publication on acceptance
7 Open access: articles freely available online
7 High visibility within the fi eld
7 Retaining the copyright to your article
Submit your next manuscript at 7 springeropen.com
Ishimura and Miyauchi Progress in Earth and Planetary Science (2015) 2:16 Page 18 of 18
