Progress in Earth andPlanetary Science

Ishimura et al.
Progress in Earth and Planetary Science(2022) 9:69
https://doi.org/10.1186/s40645-022-00529-9

Open Access

RESEARCH ARTICLE

Washover deposits related to tsunami
and storm surge along the north coastof the Shimokita Peninsula in northern Japan
Daisuke Ishimura1* , Takashi Ishizawa2, Masaki Yamada3, Kaori Aoki4 and Kai Sato5

Abstract
A decade after the 2011 Tohoku-oki earthquake (Mw 9.0), geological surveys were conducted at multiple sites along
the Pacific Coast of the tsunami-inundated Tohoku region in Japan, providing thousands of years of tsunami history.
However, the challenges of correlation between historical records and geological tsunami deposits and identifying
sources of historical and paleotsunamis have newly surfaced. Particularly the simultaneity and source of the 1611
Keicho tsunami in the Tohoku region and the seventeenth-century tsunami in the Hokkaido region are problematic.
To solve such major issues, we conducted a tsunami-deposit survey at Sekinehama on the north coast of Shimokita
Peninsula, near the junction of the Japan and Kuril trenches. We performed nondestructive analyses (X-ray computed
tomography and micro-X-ray-fluorescence core scanning), grain-size analysis, tephra analysis, and radiocarbon datingof sediments from two coastal outcrops and inland drill cores. We identified five tsunami deposits (TD1–TD5) during the last 6 kyr and correlated them at a 200–400 m distance from the coast. They also correlate with previously
identified tsunami deposits around the Shimokita Peninsula. From our study on tsunami deposits, we found otherwashover deposits in the coastal outcrops that are not represented in the inland cores. These indicate minor washover events related to small tsunamis and infrequent storm surges. The modeled age of the latest tsunami deposit
is 500–300 cal yr BP (1450–1650 cal CE). This either correlates with two known tsunamis (the 1611 Keicho tsunami
and another seventeenth-century tsunami) or is a previously unknown tsunami that occurred in the fifteenth–seventeenth centuries. If the latest tsunami deposit is to be accurately correlated with tsunami deposits previously
identified within a 50-km distance from the study site, we need to consider an unknown fifteenth-century tsunami.
Our investigation yields insights regarding the tsunami source in the vicinity of the junction of the Japan and Kuril
trenches.
Keywords: Washover deposits, Tsunami, Storm surge, Shimokita Peninsula, B-Tm, To-Cu, ITRAX
1 Introduction
In the 10 years since the 2011 Tohoku-oki earthquake(Mw 9.0), tsunami-deposit research has shown dramatic
progress not only in Japan but also across the world(Goto et al. 2014, 2021; Costa and Andrade 2020; Sawai
2020; Sugawara 2021). In addition to the discovery of*Correspondence: ishimura@tmu.ac.jp
1
Department of Geography, Tokyo Metropolitan University, 1‑1
Minami‑Osawa, Hachioji, Tokyo 192‑0397, Japan
Full list of author information is available at the end of the article

new tsunami deposits, research on numerical modelingof tsunami and sediment transport (Sugawara 2021)
and on tsunami size estimation (Namegaya and Satake
2014; Ishimura and Yamada 2019; Naruse and Abe 2017;
Mitra et al. 2020, 2021) has been conducted. In particular, geological surveys were performed at multiple sites
along the Pacific Coast of the Tohoku region where the2011 tsunami inundated provided thousands of yearsof tsunami history (e.g., Goto et al. 2015; Ishimura andMiyauchi 2015, 2017; Takada et al. 2016; Inoue et al.
2017). Although long-term tsunami-deposit history was© The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

Ishimura et al. Progress in Earth and Planetary Science(2022) 9:69

obtained at multiple sites, correlation between tsunami
deposits from site to site and between historical records
and geological surveys and estimation of the probablesources of historical and paleotsunamis have been challenging (Takashimizu 2014; Goto et al. 2021; Ishizawa
et al. 2022).
Particularly for the fifteenth–seventeenth-century
tsunami deposits whose ages cannot be sufficiently constrained by radiocarbon dating owing to wiggles in thecalibration curve, the identification of the 1611 Keicho(e.g., Ebina and Imai 2014) and the 1454 Kyotoku (Namegaya and Yata 2014) tsunami deposits and the simultaneity and source of the 1611 Keicho tsunami in the Tohoku
region and the seventeenth-century tsunami in the Hokkaido region remain major foci for investigation. Ishizawa
et al. (2022) tackled the long-standing challenge of distinguishing the 1611 Keicho and 1454 Kyotoku tsunami
deposits by applying high-resolution radiocarbon dating
and concluded that only the 1611 Keicho tsunami deposit
is distributed along the northern Sanriku Coast (Fig. 1a).
Takashimizu (2014) discussed the simultaneity and wave
source of the seventeenth-century tsunami in Hokkaido
and the 1611 Keicho tsunami in Tohoku and the distribution and age of tsunami deposits in western Hokkaido,
such as along the Hidaka Coast (Fig. 1a), to determine
how widely distributed the seventeenth-century tsunami
deposits are in Hokkaido. The northern part of the Pacific
Coast of the Tohoku region, such as the Shimokita Peninsula, has high coastal dunes that limit appropriate sites
for tsunami-deposit surveys in the north of Hachinohe(Fig. 1a, b) (Minoura et al. 2013; Tanigawa et al. 2014).
In our study, we conducted a tsunami-deposit survey at Sekinehama on the north coast of the Shimokita
Peninsula in the northernmost part of the Pacific Coastof the Tohoku region (Fig. 1) to provide continuous andlong-term tsunami history and fill in blank areas of tsunami-deposit information near the junction of the Japanand Kuril trenches. The multiple event deposits that we
identified in the Holocene sediments and dated as from
6 ka or later are considered to be transported by tsunamis and storm surges. This study is in coordination with
Sato et al. (in press in this special issue), who investigated
the wave source of the latest tsunami deposit discussed inour study.

2 Settings
The geologic units along the north coast of the Shimokita Peninsula (Fig. 1b, c) consist of the Pliocene Sunagomata Formation, the Pleistocene Tanabu Formation, andsuperposed marine-terrace deposits of multiple ages
within the Pleistocene (Tsushima and Takizawa 1977).
Marine Isotope Stage 5e marine terraces are distributed
across the study area (Fig. 1c) (Koike and Machida 2001),

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and Holocene sediments are distributed in the lowlands
and incised valleys. Tephra and drift pumice layers werealready identified in the Holocene sediments at two outcrops along the coast (SKN-E and SKN-W in Fig. 1c).
In SKN-W, Kobayashi (2019) found fallout ash of B-Tm
tephra (946 CE; Oppenheimer et al. 2017) from Changbaishan. In SKN-E, fallout ash of To-Cu tephra (5986–
5899 cal yr BP; Mclean et al. 2018) from Towada volcano
was identified by Ishimura and Hiramine (2020), anddrift pumice of To-Cu and U-2 tephras (5681–5619 cal yr
BP; Mclean et al., 2018) from Ulleung Island, Korea, wasidentified by Hiramine et al. (in press).
Historical and observational records of tsunamis hitting this region (Table 1) show that they are of two types:
seismic and volcanic tsunamis. Most of the seismic tsunamis in Hokkaido and Tohoku regions have their sourcesalong the Pacific Coast. Records show that past tsunamis
were < 3 m high and may not have inundated inland in
the study area because the topographic height behind thepresent-day beach is 4–5 m (Fig. 1c). The recorded tsunami height of the 2011 Tohoku-oki earthquake 200 m
west of the study site was 2.2–2.7 m (Mori et al. 2012),
suggesting the absence of a large inundation of inlandarea in Sekinehama. Volcanic tsunamis are thought
to be generated by sector collapses of volcanoes due tovolcanic activity at Komagatake and Oshima-Oshima
volcanoes in Hokkaido (Fig. 1a) (Nishimura and Miyaji
1998; Satake 2007). The tsunamis’ inundation heights
were documented and estimated near their source areas(Nishimura and Miyaji 1998; Satake 2007), but no suchinformation is of record for on the Shimokita Peninsula.
Based on prior research or observational records, thefollowing summarizes information known or proposed
about widely recognized tsunami deposits along the
Pacific Coast in the Hokkaido and Tohoku regions. The
1611 Keicho tsunami deposits have been identified fromthe Sendai Plain to the northern Sanriku Coast (e.g.,
Sawai et al. 2012; Ishimura and Miyauchi 2015; Takada
et al. 2016; Ishizawa et al. 2022). The highest tsunami
run-up height along the Sanriku Coast was estimated tobe more than 25 m (Ebina and Imai 2014; Ishimura andEbina 2021). Recently, using high-resolution radiocarbon
dating, Ishizawa et al. (2022) confirmed that 1611 Keichotsunami deposits are distributed along the northern Sanriku Coast (Fig. 1a). The seventeenth-century tsunami
deposits are widely recognized in eastern Hokkaido (e.g.,
Nanayama et al. 2003; Sawai et al. 2009; Ishizawa et al.
2017). Tsunami source models for the seventeenth-century tsunami were proposed by Nanayama et al. (2003),
Satake et al. (2008), and Ioki and Tanioka (2016). The latest model proposed by Ioki and Tanioka (2016) setting
the fault off eastern Hokkaido calculated the total seismic
moment magnitude as Mw 8.8. The simultaneity of thisIshimura et al. Progress in Earth and Planetary Science(2022) 9:69

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Fig. 1 Study site. a Location of the Shimokita Peninsula. Ha: Hachinohe. Hk: Hakodate. Ko: Komagatake volcano. Os: Oshima-Oshima volcano. To:
Towada volcano. SC: Sanriku Coast. HC: Hidaka Coast. Star denotes the epicenter of the 2011 Tohoku-oki earthquake. Rectangle indicates area
represented by (b). b Northern Shimokita Peninsula. Shaded relief map is based on a 10-m-mesh digital elevation model (DEM) from the Geospatial
Information Authority of Japan (GSI). c Map of the Sekinehama Coast and vicinity. Contours are based on the 5-m-mesh DEM from GSI. Intervals ofgray and black contours are 1 m and 5 m, respectively. Curving arrow denotes the channel and flow direction of the Bitsuke River

Ishimura et al. Progress in Earth and Planetary Science(2022) 9:69

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Table 1 Historical tsunamis along the Pacific Coast in Hokkaido and Tohoku regions during 1611–2003 CE
Date [CE]

Tsunami nameSourceMagnitude (Utsu 2004)

Tsunami height (m)Location of measured site

2003

Tokachi-oki

Off Tohoku

8.0

1.5–2.41

West and south of Shiriyazaki

1994

Shikotan-oki

Off Shikotan

8.2

< ­12

Northern part of the Shimokita peninsula

1968

Tokachi-oki

Off Tohoku

7.9

< ­22

East coast of the Shimokita peninsula

1960

Chile

Chile

9.5

12

East coast of the Shimokita peninsula

1952

Tokachi-oki

Off Tohoku

8.2

(2)2

Hachinohe

1933

Showa-Sanriku

Off Tohoku

8.1

12

Sekinehama

1896

Meiji-Sanriku

Off Tohoku

8.2

(3)2

Hachinohe

1856

Ansei

Off Tohoku

7.7

1.53

Between Ohmazaki and Sekinehama

1843

Tempo

Off Nemuro

8.0

(2)3

Hakodate

1763

Horeki

Off Tohoku

7.7

(1)3

Hachinohe

1677

Empo

Off Tohoku

7.9

1.7–2.93

Between Ohmazaki and Sekinehama

1611

Keicho

Off Tohoku

8.1

–

1741

Oshima-Oshima

Oshima-Oshima volcano

Related to volcanic event

–

1640

Komagatake

Komagatake volcano

Related to volcanic event–

1: Tanioka et al. (2004); 2: Watanabe (1998); and 3: Tsuji et al. (2014)
The numbers in parentheses in “Tsunami height” column indicate the values at sites away from Sekinehama

tsunami with the 1611 Keicho tsunami has been debated
but is not yet fully understood (e.g., Tetsuka et al. 2020).
From research in the vicinity of the study site, twostudies, Minoura et al. (2013) and Tanigawa et al. (2014),
identified tsunami deposits along the east coast of the
Shimokita Peninsula (Fig. 1b). Minoura et al. (2013) identified one tsunami deposit as having occurred between
the late thirteenth and fifteenth centuries. Tanigawa et al.
(2014) identified five sandy event deposits interbedded
within marsh deposits during the last 6000 years, threeof which were attributed to inundation from the coast on
the basis of diatom assemblages. The latest one is estimated to have occurred 470–180 cal yr BP. Along theSanriku Coast, many tsunami-deposit surveys were conducted after the 2011 event. They show the relatively highfrequency of paleotsunamis from the north to the central
parts of this coast (Fig. 1), where their average recurrence
interval is ~ 300–500 years (Ishimura 2017; Ishimura and
Yamada. 2021; Ishizawa et al. 2022; Velasco-Reyes et al.
2022). On the Hokkaido side, many drilling surveys werealso conducted. In eastern Hokkaido, a short average
recurrence interval (close to 400 years) was estimatedfrom long-term tsunami history based on the tsunami
deposits (e.g., Sawai et al. 2009), whereas prior tsunamideposit researchers found a longer average recurrence
interval to the west of the Hidaka Coast than eastern
Hokkaido (Takashimizu et al. 2007; Takashimizu 2014;
Nakanishi et al., 2020, 2022). To the west of the Hidaka
Coast (Fig. 1a), only one tsunami deposit was identified
since ca. 3 ka and was correlated with the seventeenthcentury tsunami (Takashimizu et al. 2007). Additionally, the lateral correlation and sources of the tsunami

deposits formed during the last 500 years in the westof the Hidaka Coast (Fig. 1a) are not well understood(Takashimizu 2014). Recently, Kase et al. (2022) reported
tsunami deposits around Hakodate on the opposite sideof the Tsugaru Strait from the study area (Fig. 1a). They
drilled at four sites and identified tsunami deposits
formed since ~ 3000 cal yr BP.

3 Methods
We conducted a field survey to log two outcrops (SKN-E
and SKN-W) along the Sekinehama Coast and cores
drilled along the incised valley, which is a subsidiary of
the Bitsuke River (Fig. 1c). An additional coring survey
was conducted in the south of the SKN-W outcrop to
observe the younger sediments. A handy-Geoslicer (GS)(Takada et al. 2002) was used for the coring survey, bywhich we obtained a total of 11 GS cores.
For nondestructive analyses, we extracted block samples from outcrops and GS cores with a 50-cm-long,
4-cm-wide, and 3-cm-thick case. These block samples
were used for X-ray computed tomography (XCT) andhigh-resolution micro-X-ray-fluorescence (µXRF) scanning at the Center for Advanced Marine Core Research,
Kochi University. The XCT images were taken by using
an Aquilion PRIME/Focus Edition scanner (Canon
Medical Systems Corporation). For this analysis, asample was imaged according the X-ray absorption sothat high-density materials could be identified by theirrelatively light color (Falvard and Paris 2017). A vertical slice of the block sample was taken every 0.5 mm,
and these vertical slices were then processed using
Horos software (a free and open-source code software

Ishimura et al. Progress in Earth and Planetary Science(2022) 9:69

under the GNU Lesser General Public license, Version
3 (LGPL-3.0)) to build a 3D image of the block sample.
Using the ITRAX µXRF core scanner (Croudace et al.
2006), the block sample was measured at 1-mm intervals. The scanner, which irradiated the block sample
surface by applying a molybdenum X-ray source, operated at 60 kV and 50 mA and allowed us to obtain dataon 30 elements. We used 12 element intensities (Si, S,
K, Ca, Ti, Mn, Fe, Ni, Rb, Sr, Y, and Zr) after considering previous studies (Chagué-Goff et al. 2016; MartinPuertas et al. 2017; Ishizawa et al. 2019). Because theelemental data were influenced by the sample’s condition (Löwemark et al., 2011; Chagué-Goff et al. 2016),
we normalized each elemental datum over counts per
second for comparison between cores (e.g., Davies et al.
2015; Chagué-Goff et al. 2016; Ishizawa et al. 2019).
We determined major element compositions of volcanic glass shards for correlating known tephra. We
sampled sediments from GS cores for tephra analysis,
while considering the already-identified B-Tm and To-Cu
tephras in the coastal outcrops. For the cores with novisible tephra, we conducted continuous sampling andcalculated the content of volcanic glass shards. First, weused an ultrasonic generator to rinse the samples in waterand then sieved them with water through a 62-μm nylon
mesh. Next, we dried the samples at 50 °C and sieved
the dried samples through a 120-μm nylon mesh. Then,
using a polarizing microscope, we counted volcanic glass
shards in the 62–120-μm fraction. Finally, we determined the major element composition of the volcanic
glass shards in that size fraction by energy-dispersive
X-ray spectrometry (EDAX Genesis APEX2) and scanning electron microscopy (JEOL JSM-6390) following themethod and analysis conditions described by Suzuki et al.
(2014). Volcanic glass shards of the AT tephra sampled
at Chigaki, Toyama Prefecture (Machida and Arai 2013),
were used as a working standard to check data reproducibility and instrument stability.
For grain-size analysis, we collected all event-deposit
samples from the outcrops and cores, and present-day
sediment samples surrounding the study site (Fig. 1c). As
for the present beach sediments, we confirmed that there
has been no artificial beach nourishment by the interview
to the city office. First, each sample was pretreated with
3% ­H2O2 to eliminate organic matter and then rinsed
with water using an ultrasonic generator. Then, we sieved
the samples with water through a 62-μm nylon mesh andthen used a 2-mm sieve. Finally, we measured the weightof sand and gravel fractions to calculate mud, sand, andgravel contents. Grain-size analysis of sand fractions was
performed using Morphologi G3 (Malvern Instruments
Ltd.) according to Ishizawa et al. (2019), but in our study,
we used the 5X magnification setting.

Page 5 of 27

Radiocarbon dating using an accelerator mass spectrometer was performed at the Institute of Accelerator
Analysis Ltd., Japan. We obtained 42 radiocarbon dates:
32 for samples from GS cores and 10 for samples fromoutcrops. Radiocarbon dates were calibrated based on
the IntCal20 calibration curve (Reimer et al. 2020) using
the OxCal 4.4 program (Bronk Ramsey 2009). We used
the sequence model (Bronk Ramsey 2008) for calculating
depositional ages of tsunami deposits (Additional file 1).

4 Results
4.1 Outcrops along the Sekinehama Coast
4.1.1 SKN‑E outcrop

The SKN-E outcrop (Fig. 2) is ~ 300 m east of the mouthof the Bitsuke River (Fig. 1c) and is on a sea cliff that is
2.5 m high and 10 m wide. The sediments exposed inthis outcrop apparently belong to two different sedimentary facies. The lower part consists of blue-gray silt
(possibly marine sediments), and the upper part includes
marsh deposits consisting of organic silt and peat. The
uppermost part is artificially modified sediments that
contain concrete blocks and fishnets. In this study, we
described and sampled sandy event deposits in the marsh
sediments.
We recognized 12 sandy event deposits in the SKN-E
marsh sediments (SKN-E-E1 to SKN-E-E12, from youngest to oldest) (Fig. 2; Table 2). Except for SKN-E-E7,
they show good horizontal continuity. Most consist ofwell-sorted fine to medium sand. SKN-E-E5, SKN-E-E6,
SKN-E-E8, and SKN-E-E9 contain coarser grains suchas granules. SKN-E-E5 and SKN-E-E6 have an erosive
contact at the base and contain rip-up clasts in the lower
part. SKN-E-E8 also contains rip-up clasts. Some of the
sandy event deposits show normal grading. The bottom
shape of SKN-E-E7 is convex downward, suggesting achannel, and the deposit has parallel laminations.
From the grain-size analysis (Fig. 3), SKN-E-E1 to
SKN-E-E4 show grain-size distributions concentrated
in the range 1–2 phi. Meanwhile, SKN-E-E5 to SKN-EE10 show modes of 1–2 phi, but exhibit a wider grainsize distribution. In the event deposits below SKN-E-E11,
the mode changes from 1–2 phi to 1.5–2.5 phi with finer
grains. This is apparent from the mud content (Fig. 3).
The gravel contents also support the above descriptions
indicating that SKN-E-E5, SKN-E-E6, and SKN-E-E8
include coarser grains.
Three tephra-related layers in the marsh sediments
were already identified (Fig. 2) and described in detailin Ishimura and Hiramine (2020) and Hiramine et al.
(in press). To-Cu (5986–5899 cal yr BP) tephra (finegrained volcanic ash) is interbedded between SKN-EE9 and SKN-E-E10 (Ishimura and Hiramine 2020), anddrift pumice clasts that originated from To-Cu tephraIshimura et al. Progress in Earth and Planetary Science(2022) 9:69

Page 6 of 27

Fig. 2 SKN-E outcrop. a Photograph of SKN-E outcrop. b Sketch of SKN-E outcrop. Grayscale columns on the sketch are XCT images. a, b are of thesame section. The volcanic ash of the To-Cu tephra is too thin to depict in (b)are concentrated above the ash layer (Hiramine et al. inpress). Other drift pumice clasts, concentrated between
SKN-E-E8 and SKN-E-E9, were correlated with the U-2
tephra (5681–5619 cal yr BP) (Hiramine et al. in press).
Furthermore, they suggested that the depositional timings of these pumice concentrated layers are shortly after
the eruptions, and thus, their tephra dates can be used asages of the horizons.
4.1.2 SKN‑W outcrop

The SKN-W outcrop (Fig. 4) is ~ 150 m east of the mouthof the Bitsuke River and is exposed over a 2 m height anda 1 m width. There are 17 sandy event deposits (SKNW-E1 to SKN-W-E17, from youngest to oldest) interbedded within organic silt and peat (Fig. 3; Table 2). For

observing the uppermost sandy event deposit, two GS
cores (SKN-W-GS1 and SKN-W-GS2) were obtained
behind the outcrop. Except for SKN-W-E1 and SKN-WE8, which are notably thick (> 15 cm), the sandy event
deposits are ~ 10 cm thick or less.
Most of the sandy event deposits consist of well-sorted
fine-to-medium sand. SKN-W-E1, SKN-W-E2, SKN-WE11, and SKN-W-E14 include coarse sand. Our detailed
observation revealed that the 1–2-cm-thick black layer
between SKN-W-E3 and SKN-W-E4 was a sand layer
with a little organic material rather than soil. Therefore,
SKN-W-E3 and SKN-W-E4 were determined to be a single sandy event deposit (hereinafter SKN-W-E3/E4). The
bases of SKN-W-E3/E4, SKN-W-E11, and SKN-W-E14
are erosive contacts, but the upper and lower boundaries

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Table 2 Description of sandy event deposits in samples SKN-W, SKN-E, and SKN-GS
Location Event no.

Grain size

Thickness (cm) Sedimentary structure

SKN-E

E1

fs, ms5

E2

fs, ms3

E3

fs, ms3

E4

fs, ms3

E5

ms-granule 10–15

Normal grading, rip-up clasts (< 1 cm), pumice clast(< 1 cm), gravel (2–3 cm) at lower part, erosive contact
at baseE6

ms-granule 3–15

Normal grading, rip-up clasts, gravel (3 cm), erosive
contact at baseNormal grading

E7

ms, cs25–30

Channel fill, erosion up to E10, parallel laminae

E8

ms-vcs

5–10

Patchy distribution, normal grading, rip-up clasts

U-2 (drift pumice)
E9

Interpretation

U-2 (5681–5619 cal yr BP)
ms-granule 3–5

Poor sortingms

3

Patchy distribution

E11

silt, ms-cs

10–20

E12

fs, silt

To-Cu (drift pumice)
To-Cu (fallout ash)
E10

SKN-W

SKN-GS

To-Cu (5986–5899 cal yr BP)
Matrix rich, a little grading, bioturbation
Matrix rich, patchy distribution

E1

ms, cs> 16

E2

ms, cs

4

E3

ms

6–9

E4

Ms

12

E5

ms

5–10

B-Tm

fs

2

E6

fs

5

E7

fs, ms

5–8

E8

fs

6–20

E9

fs, ms

2–4

Parallel laminae in the lower part

Erosive contact at baseNormal grading

Parallel laminae

E10

fs, ms

4–8Erosive contact at baseE11

ms, cs

6–7

Normal grading, erosive contact at base

E12

ms

1–5

E13

ms

3

E14

ms, cs

5–9

Normal grading, erosive contact at base

E15

fs

1–2

Not continuous
Normal grading

E16

ms

7–12

E17

ms

4–10

B-Tm (946 CE)

GS9-E1ms

10Volcanic ash

fs

3

Composed of 2–3 units, erosive contact at base

GS9-E2

fs, ms2–3Erosive contact at base

GS9-E3ms

6Erosive contact at base

B-Tm (946 CE)

GS9-E4ms

4Unclear

GS8-E1ms

13

Composed of 2–3 units, erosive contact at baseVolcanic ash

fs3

Volcanic ash

fs2

Laminae

GS8-E2ms

2Erosive contact at base

B-Tm (946 CE)

GS8-E3ms, cs5Laminae, erosive contact at base

GS8-E4silt-granule

36

Erosive contact at base

GS8-E5

fs, ms3

Unclear laminae, erosive contact at base

GS8-E6

fs, ms>3

Parallel laminae

GS1-E1

fs, ms15

Composed of 2–3 units, erosive contact at base

B-Tm (946 CE)

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Table 2 (continued)
Location Event no.

Grain size

Thickness (cm) Sedimentary structureInterpretation

Volcanic ash

vfs

4

B-Tm (946 CE)

GS1-E2ms

3Unclear laminae, erosive contact at base

GS1-E3ms

9

Cross laminae, laminae tilting to inland, erosive contact
at baseGS1-E4

ms-granule 52

Parallel laminae, cross laminae, laminae tilting toseaward, fining upward in the upper part, coarsening
upward in the lower part, erosive contact at base

GS1-E5

fs

3

GS1-E6fs

1

GS7-E1ms

13

Composed of 2–3 units, erosive contact at baseVolcanic ash

fs<1

Patchy distribution

GS7-E2

fs

1

Unclear, identified in XCT image

GS7-E3ms-granule 45

B-Tm (946 CE)

Parallel laminae, cross laminae, laminae tilting to seaward, fining upward in the upper part

GS7-E4ms, cs

GS6-E1ms>5
12Volcanic ash

fs

2–3

GS6-E2ms, cs1

Erosive contact at base

GS6-E3ms, cs8

Cross laminae, laminae tilting to inland, normal grading,erosive contact at baseComposed of 2–3 units, erosive contact at base
B-Tm (946 CE)

GS6-E4ms

1Erosive contact at base

GS6-E5vcs

12Erosive contact at base

GS6-E6ms, cs5

Cross laminae, laminae tilting to inland, erosive contact
at baseGS6-E7

vcs, granule 3

GS5-E1

ms, fsContain rounded gravel, erosive contact at base

1

GS5-E2

ms, fs7

Erosive contact at base

GS5-E3csms

15

Composed of 2–3 units

GS5-E4

ms-granule 5

Contain rounded gravel, normal grading, erosive contact
at baseGS5-E5

ms-vcs6

Inversegrading, unclear laminae, erosive contact at base

GS5-E6ms

4Erosive contact at base

GS4-E1

ms, fs1

Patchy distribution

GS4-E2

ms, fs3Composed of 2 units

GS4-E3ms, fs1

Patchy distribution

GS4-E4fs

5

Unclear, identified in XCT image

GS4-E5ms, cs18

Composed of 2–3units

GS4-E6

fs-granule

53

Composed of multiple units, fining upward in the upper
part, coarsening upward in the lower part, erosive
contact at base

GS3-E1

ms

6

Patchy distribution

Volcanic ash

fs

10

Unclear, identified in XCT image

GS3-E2fs

2

Unclear, identified in XCT image

GS3-E3ms

1

GS2-E1

fs, ms10

Volcanic ash

fs

5

GS2-E2

fs, ms1

B-Tm (946 CE)

Unclear, identified in XCT image
B-Tm (946 CE)
Erosive contact at base

GS2-E3

fs, ms10Erosive contact at base

GS2-E4

fs, ms10Erosive contact at base

GS2-E5

fs, ms3

Unclear parallel laminae, erosive contact at base

Tephra ages are based on Oppenheimer et al. (2017) and Mclean et al. (2018)vfs very fine sand, fs fine sand, ms medium sand, cs coarse sand, vcs very coarse sand

Ishimura et al. Progress in Earth and Planetary Science(2022) 9:69

Page 9 of 27

Fig. 3 Grain-size analysis of event deposits in SKN-E and SKN-W outcrops and GS-W-GS2 core. Histograms show phi-scale grain-size distributions.
Vertical axis indicates the probability of particles. Pie charts show gravel, sand, and mud contents. N means the number of particle samples
measured. Mean, median, mode, skewness, and kurtosis of each histogram are shown in Additional file 2: Table S1of other event deposits are unclear. We recognizednormal grading in SKN-W-E11, SKN-W-E14, and
SKN-W-E16, whereas SKN-W-E1 and SKN-W-E8 characteristically have multiple parallel laminae. The GS coresbehind the outcrop also show alternating layers of lightand heavy minerals in the lower part of SKN-W-E1, while
the upper part is massive owing to bioturbation (Fig. 4c).
In the grain-size analysis (Fig. 3), almost all event
deposits show a mode at 1–2.5 phi, but their distribution ranges are variable. Below SKN-W-E14, event
deposits contained more fine sand fractions comparedto those above it. The event deposits in SKN-W-GS2
show a very sharp mode from SKN-W-E1 to SKN-WE5. The upper part of SKN-W-E1 has more fine-grainedmaterial compared to its lower part, possibly influenced
by bioturbation and aeolian deposits.
We recognized the volcanic ash layer between SKNW-E5 and SKN-W-E6 that was already correlatedwith B-Tm tephra (946 CE; Oppenheimer et al., 2017)by Kobayashi (2019), who also detected volcanic glass
shards of To-a tephra (915 CE; Machida and Arai 2013)
from the lower part of B-Tm tephra, suggesting that theIshimura et al. Progress in Earth and Planetary Science(2022) 9:69

Page 10 of 27

Fig. 4 SKN-W outcrop. a Photograph of SKN-W outcrop. b Sketch of SKN-W outcrop. c Photographs and sketches of GS cores drilled behind the
SKN-W outcrop. Grayscale columns on the sketch are XCT images. a, b Show the same area

age of sediments above and below this tephra is post946 CE and pre-915 CE, respectively.
4.2 Cores drilled in the incised valley

Nine GS cores (SKN-GS1 to SKN-GS9) were obtained
200–400 m inland from the shore (Figs. 1c, 5). Coring
depths were 1.3–2.8 m. We cut 50-cm-length block samples from each core for nondestructive analysis. Except
for SKN-GS2, the sediments mainly consisted of organicrich sediments (marsh deposits) with interbedded sandy
event deposits. Only SKN-GS2 contains a massive grayish-white clay at deeper than ~ 2.6 m depth.
We recognized sandy event deposits consisting of asingle unit or multiple units. Many of the single-unit
deposits were well sorted but lacked any sedimentary
structures. SKN-GS1-E3, SKN-GS6-E3, and SKN-GS6E-6 characteristically have laminae that tilt inland. Weinterpreted that these laminae may be cross lamination by considering the relationship between the bedding plane and laminae and by taking into account thesome deformations (e.g., dragging) due to GS coring,

although the observation window of GS cores is narrow compared to outcrops. Therefore, they indicatethat the paleocurrent direction was from seaside toinland (Fig. 6). The multiunit deposits were further
divided into thin (< 10–20 cm) and thick (> 35 cm)
deposits. Among the multiunit event deposits, thin layers were indistinct to the naked eye but could be identified clearly on XCT images. The uppermost event
deposit (E1 in each core) consists of a lower visible
sand layer and one or two unclear sand layers that canbe recognized in XCT images taken at the seaward sites
but that show as a single sand layer at the inland sites(Figs. 5, 6). Four multiunit event deposits (SKN-GS1E4, SKN-GS4-E6, SKN-GS7-E3, and SKN-GS8-E4) withnotable thicknesses (> 35 cm) exhibit alternations ofcoarse-grained and fine-grained layers. The sedimentary structures of SKN-GS1-E4 and SKN-GS7-E3 show
clear cross laminations, indicating a seaward paleocurrent, while those of SKN-GS8-E4 and SKN-GS4-E6 are
indistinct, possibly owing to bioturbation.

Ishimura et al. Progress in Earth and Planetary Science(2022) 9:69

Page 11 of 27

Fig. 5 Photographs, sketches, and XCT images of SKN-GS cores

From the tephra analysis, we identified visible tephra
layers and volcanic glass concentrated horizons (Table 3).
Visible tephra layers were identified below E1 deposits in
all SKN-GS cores except for SKN-GS4 and SKN-GS5, forwhich continuous sampling revealed volcanic glass concentrated horizons (Fig. 5, Table 3). The volcanic glass
shards in the visible tephra layers and volcanic glass concentrated horizons below E1 deposits in all cores show
mainly platy-type morphology. However, the horizon at
170–172 cm depth in SKN-GS5 showed the highest volcanic glass content within 163–186 cm depth (Table 3).
The volcanic glass shards at that horizon have a pumiceous (highly vesiculated) morphology.
The results of the grain-size analyses of the SKN-GS
samples (Fig. 7) show more variety than those of thecoastal outcrops SKN-E and SKN-W. Three types wereobserved: sandy event deposits with a 1–2-phi mode anda narrow grain-size distribution, those with a 3–4-phi
mode and a wide grain-size distribution, and those witha bimodal mode (1–2 phi and 3–4 phi). Some SKN-GS
samples contained a large amount of mud, but this may
be owing to the bioturbation and indistinct sand layers.

The uppermost E1 deposit in all cores with the same
stratigraphic position (above the visible tephra or volcanic glass concentrated horizon) has the same grain-size
distributions.
According to the ITRAX analysis results (Fig. 8 andAdditional file 2: Fig. S1) for the 12 elements that weinvestigated, the values of Si, K, Ca, Ti, Mn, Fe, and Sr are
higher in the sand layers. In the visible tephra layers andvolcanic glass concentrated horizons below E1 deposits,
K and trace elements (Rb, Y, and Zr) characteristicallyincrease. Data for Ni are somewhat noisy but are macroscopically stable. S is slightly elevated in a few event
deposits (SKN-GS1-E3, SKN-GS1-E6, and SKN-GS6-E6
in Fig. 8).
4.3 Grain‑size analysis of present‑day sediments

We did grain-size analysis of present-day sediments(Figs. 1c, 9) surrounding the study site to compare with
event deposits in the outcrops and SKN-GS cores. We
included foreshore and backshore beach sediments,
aeolian sediments, Pleistocene marine-terrace deposits, and fluvial sediments in the incised valley where weIshimura et al. Progress in Earth and Planetary Science(2022) 9:69

Page 12 of 27

Fig. 6 Close-up images and description of event deposits in SKN-GS cores. White dashed line shows the extent of the XCT image

obtained SKN-GS cores. We first observed the graincompositions and particle shapes of present-day sediments, which were composed mainly of quartz, pyroxenes, and rounded rock fragments with weatheringrind. The volcanic rocks and tephras from the volcanoes
located at the west of the study site contain quartz andpyroxenes (Tsushima and Takizawa 1977); thus, sourcesof the sediments are primarily the ejecta of volcanoes.
Furthermore, our findings suggest no substantial differences in the type and shape of the particles that comprise any deposits; therefore, the grain-size distribution
is considered important for determining the source ofthe sediments.
The foreshore sediments show a mode in the range
1.5–2.5 phi and are characterized by a narrow grainsize distribution. The backshore sediments containfiner-grained particles (3–4 phi), which may reflect acontribution of aeolian sand. Aeolian sediments on the
Pleistocene marine terrace have multiple modes: 2-phi,
2.5-phi, and 3–4-phi particles. These finer-grained particles are thought to be the result of wind-driven transport.
The mode of the Pleistocene marine-terrace deposits issimilar to that of present-day beach sediments but has awider grain-size distribution and contains coarser particles. Fluvial sediments from the creek in the incisedvalley show a similar size distribution to that of the

Pleistocene marine-terrace deposits, which is consistent
with the creek’s having incised the terrace.

5 Discussion
5.1 Tephra correlation

Using the major element compositions of volcanic glass
shards (Table 4; Additional file 2: Fig. S2), the visible
tephra at the 88–89 cm depth in SKN-GS1 was correlatedwith B-Tm tephra in SKN-W, and the volcanic glass concentration horizon at the 170–172 cm depth in SKN-GS5
included volcanic glass shards of To-Cu tephra. The former result means that the visible tephra layers and volcanic glass concentrated horizons below E1 deposits in
all GS cores are B-Tm tephra based on the stratigraphic
position and similarity of the morphology of volcanic
glass shards. Additionally, Nara et al. (2021) reported
that the B-Tm tephra characteristically included ­K2O andtrace elements, which is consistent with our tephra correlations and ITRAX results (Fig. 8 and Additional file 2:
Fig. S1). The latter means that the horizon at the 170–172 cm depth in SKN-GS5 is interpreted to be deposited at least after the To-Cu tephra (5986–5899 cal yrBP) as there is a possibility of its secondary deposition.
Although two visibly separate volcanic ash layers wereidentified at the 108–111 cm and 115–117 cm depths
in SKN-GS8, their volcanic glass morphology and grain

Ishimura et al. Progress in Earth and Planetary Science(2022) 9:69

Table 3 Tephra-analysis samples
GS no.

Depth (cm)

Volcanic glasscontent (%)

Visible (V) or
invisible (I)Correlative
tephraGS9

86.5–89

86.0

V

B-Tm

GS8

108–111

88.8

V

B-Tm

GS8

115–117

90.0

V

B-Tm

GS1

87–89

90.0

V

B-Tm

88.2

V

B-Tm

GS7

67–68

GS7

70–72

85.0

I

B-Tm

GS6

83–85

76.4

V

B-Tm

GS5

8–10

4.4

–

GS5

4.9

–

GS5

18–19

10.5

–

GS5

36–37

41.9

I

GS5

163–165

17.1

–

GS5

165–167

24.5

–

GS5

167–170

22.1

–

GS5

170–172

27.3

I

GS5

172–174

23.1

–

GS5

174–177

13.3

–

GS5

177–180

11.1

–

GS5

184–186

7.7

–

GS4

40–43

54.5

I

GS4

50–53

28.2

–

GS4
GS3
GS2

13–15

58–61
72–73
68–71

B-Tm

To-Cu

B-Tm

16.5

–

82.4

V

B-Tm

90.0

V

B-Tm

compositions are similar, and therefore, we interpreted
that both are B-Tm tephra and the lower tephra might
have migrated downward by bioturbation.
5.2 Cross‑checking radiocarbon dates of organic bulk
sediments

Because appropriate measurable organic materials (e.g.,
seed and leaf ) were not available, we used organic bulk
sediments for radiocarbon dating (Table 5). When datingorganic bulk sediments, if the origin of the organic material is uncertain, it must be cross-checked with known
ages. Therefore, in this study, the organic bulk sediments
were dated in the To-Cu horizon of SKN-GS5 and horizons above and below the B-Tm tephra in SKN-GS2. As aresult, the dates of the upper and lower parts of the B-Tm
tephra in SKN-GS2 were 1003 ± 21 yr BP (960–800 cal yrBP) and 1239 ± 21 yr BP (1270–1070 cal yr BP), respectively, which are consistent with the tephra age. However,
the To-Cu volcanic glass concentrated horizon of SKNGS5 has an age of 6673 ± 29 yr BP (7610–7470 cal yr BP),which is older than the To-Cu tephra (5986–5899 cal yrBP). Because we expected that the horizon contained
secondary deposits of To-Cu tephra, we assumed that

Page 13 of 27

a younger age than the tephra would not be a problem,
but the obtained age is older than the tephra age. Therefore, we considered the possibility that old carbon was
included in the horizon. Although by visual inspection
we thought the sedimentary facies consisted of clay-richsediments (Fig. 6), the sedimentary structure from the
XCT indicated that it was a poorly sorted clay with sand,indicating that the sediments were transported by fluvial
processes and contained old carbon. The horizons aboveand below B-Tm tephra are organic-rich sediments containing few sand grains (Fig. 6). Because organic-rich sediments deposited in a stable environment provide a moreaccurate age, we chose the horizon with little old organiccarbon for radiocarbon dating of organic bulk sediments.
We determined that horizons with less flow-in by fluvial
processes (i.e., with fewer clastic particles) were thosewith less old organic carbon and used XCT images toselect such horizons. Therefore, some radiocarbon dates
were obtained at some distance from the event deposits(Fig. 5).
5.3 Paleogeography of Sekinehama

Before the identification of the tsunami deposits, we
looked at the topographic change around the study site
since ~ 6 ka. Using an aerial photograph taken in 1952
and a corresponding geomorphic classification (Fig. 10a,b), we reconstructed the paleogeography of Sekinehama. The 1952 aerial photograph shows that dunes
used to be distributed along the coast where currently
the front of the coastal outcrops are located, while theother aerial photographs show that the topography in2001 was almost the same as today. This suggests that theoutcrops that we studied appeared at the surface owing
to coastal erosion in the last 50 years. Thus, until morethan 50 years ago, these outcrops had been in a lowland
behind the coastal dune and unexposed.
Hiramine et al. (in press) found two layers composedof abundant drift pumice in SKN-E and concluded
that SKN-E was under a lagoon-like wetland environment connected to the sea via a small channel at ~ 6 ka
(Fig. 10c). The presence of drift pumice and underlying
marine sediments at SKN-E roughly indicates a paleosea level at 2 m above the present sea level (Hiramine
et al. in press). Okuno et al. (2014) showed that theoretical relative sea level is 0–2 m in 6 ka at Ominato (15 km
southwest from the study site), indicating that study cite
is stable or slightly uplifted during the last 6 ka. This tendency is consistent with the long-term uplift rate of 0.1–
0.2 mm/yr based on the MIS 5 marine-terrace surfaces(Koike and Machida 2001). Even if this uplift is taken into
account, the sediments in the GS cores are considered tobe higher in elevation than the sea level at that time.

Ishimura et al. Progress in Earth and Planetary Science(2022) 9:69

Page 14 of 27

Fig. 7 Grain-size analysis of SKN-GS samples. Histograms show phi-scale grain-size distributions. Vertical axis indicates the probability of particles.
Pie charts show gravel, sand, and mud contents. N means the number of particle samples measured. In the histogram, blue means that the size
distribution is different from that of present-day beach sediments (presented in Fig. 9); pink means that it is similar to that of present-day beachsediments; and orange means that it is somewhat similar. Mean, median, mode, skewness, and kurtosis of each histogram are shown in Additional
file 2: Table S1

From the above-estimated paleogeography, the coastline at ~ 6 ka was estimated to be more seaward than atpresent, indicating that the present coastline position is
the nearest position to the GS core sites during the last
6 kyrs. Additionally, the layers of the drift pumice indicate the paleoshoreline position of ~ 6 ka in the inland
lagoonal environment (SKN-E), meaning that there isa distance of at least 200 m from the paleoshoreline of
the lagoon behind the spit or dune (Figs. 10c, 11) to the
GS core site and that there is a greater distance from thecoastline facing the sea (Tsugaru Strait). Although we
could not collect additional data on the paleogeography
after 6 ka, we interpreted that the study area has tended
to be land from 6 ka; therefore, the marine influence has
decreased. Additionally, the estimated tidal inlet was limited, and the GS core site was far from it (Fig. 10c). Therefore, a storm is not likely to reach the GS core site after
6 ka.

5.4 Interpretation of event deposits in GS cores

We compared source sediments surrounding the studysite with event deposits in the GS cores. From thegrain-size analysis of source sediments, only the beachsediments show a mode of 1.5–2.5 phi with a narrowgrain-size distribution (Fig. 9). This indicates that the
sandy event deposits with a grain-size distribution similar
to that of present-day beach sediments might have beentransported from the beach (pink histograms in Fig. 7).
However, coarse-grained sandy event deposits (including -1 to 1 phi particles) with a wider grain-size distribution likely originated from the Pleistocene marine-terrace
deposits and are similar to creek sediments. Therefore,
they are estimated to be deposited by fluvial processes.
Conversely, fine-grained particles (3–4 phi) could bederived from fine sediments above and below event
deposits due to bioturbation. If they are mixed by bioturbation to event deposits, especially thin event deposits,

Ishimura et al. Progress in Earth and Planetary Science(2022) 9:69

Page 15 of 27

Fig. 8 Results of the ITRAX micro-X-ray-fluorescence core scanning of SKN-GS1 and SKN-GS6. Each elemental datum was normalized by counts per
second (cps)Ishimura et al. Progress in Earth and Planetary Science(2022) 9:69

Page 16 of 27

Fig. 9 Grain-size analysis of present-day sediments. Histograms show phi-scale grain-size distribution. Vertical axis indicates the probabilityof particles. N means the number of particle samples measured. Mean, median, mode, skewness, and kurtosis of each histogram are shown in
Additional file 2: Table S1

Ishimura et al. Progress in Earth and Planetary Science(2022) 9:69

Page 17 of 27

Table 4 Major element compositions of volcanic glass shard samples and the AT tephra working standard
MgO

CaO

K2O

Na2O

n

Total""

0.1~  ~ 01

0.3

4.3

5.1

8

96.5

0.1

0.0

0.1

0.2

0.1

4.5

0.2

0.2

1.1

5.1

5.6

12

97.5

0.5

0.2

0.1

0.1

0.3

0.2

0.2

11.0

4.0

0.1

0.1

0.3

4.5

5.0

6

96.0

0.3

0.1

0.1

0.0

0.2

0.3

0.2

15.0

4.6

0.2

0.2

1.1

5.1

5.6

14

97.4

0.5

0.2

0.1

0.1

0.2

0.2

0.1

13.6

2.5

0.2

0.7

2.6

1.2

4.0

19

96.6

0.3

0.3

0.1

0.1

0.1

0.1

0.2

12.5

1.4

0.1

0.3

1.7

2.5

3.4

11

97.4

0.4

0.3

0.1

0.0

0.3

0.4

0.1

12.5

1.3

0.1

0.2

1.1

3.2

3.5

24

93.9

0.1

0.1

0.1

0.0

0.1

0.1

0.1

13.5

2.5

0.2

0.7

2.7

1.2

4.0

16

98.7

0.1

0.1

0.1

0.1

0.1

0.1

Sample

SiO2

TiO2

Al2O3

FeO"

MnO

B-Tm ash in SKN-W (1)

75.0

0.2

10.9

4.0

0.6

0.1

0.3

0.1

B-Tm ash in SKN-W (2)

67.6

0.4

15.1

1.2

0.1

88–89 cm in depth of SKN-GS1 (1)

74.8

0.2

B-Tm

0.7

0.1

88–89 cm in depth of SKN-GS1 (2)

67.8

0.4

B-Tm

1.1

0.1

170–172 cm in depth of SKN-GS5

74.7

0.5

To-Cu

0.3

0.1

170–172 cm in depth of SKN-GS5

77.9

0.2

Other tephra

0.4

0.1

AT

78.0

0.1

Working standard

0.4

0.1

To-Cu ash in SKN-E

74.5

0.5

0.3

0.1

0.3

Remark

0.6
0.5
0.9
0.7
0.7
1.6
1.4
Ishimura andHiramine(2020)

0.6

The number on the upper line is the mean value and that on the lower line is the standard deviation. Measured values were recalculated to 100% on a water-freebasis. FeO*, total iron oxide as FeO. Total**, raw data before recalculation

grain-size distribution becomes a little wider than thepresent beach sediments, such as SKN-GS1-E6, or shows
bimodal distribution, such as SKN-GS8-E5. If the contribution of fine-grained particles is minor, this possibility should be considered in the determination of source
sediments. Therefore, for thin event deposits, it is possible that the fine-grained component, if present, originally had a grain-size distribution similar to that of thepresent beach sand (orange histograms in Fig. 7). These
two event-deposit types (pink and orange histograms in
Fig. 7) are candidates for tsunami deposits, as discussed
below.
Some event deposits whose grain-size distributions are
similar to present-day beach sediments show cross lamination tilting inland (SKN-GS1-E3, SKN-GS6-E3,
and SKN-GS6-E6), indicating a paleocurrent directed
inland. Furthermore, SKN-GS1-E3, SKN-GS6-E3, and
SKN-GS6-E6 show sharp basal contacts, indicating erosional surfaces. These features indicate that they were
deposited by the strong flow from the coast. Considering
the distance from the coast and estimated paleogeography since ~ 6 ka (Fig. 10c) as discussed in Sect. 5.3, they
were deposited by tsunamis rather than storm surges.
The E1 deposits above the B-Tm tephra in each corealso show almost the same grain-size distribution as thepresent-day beach sediments, thin inland, and have multiple sand layers. These characteristics also suggest thatthey are a tsunami deposit and were formed by multiple
waves. Because other event deposits do not show any

relevant features other than grain-size distributions, weinterpret them based on their distribution in the incisedvalley. We discuss their correlations between GS cores
using radiocarbon dating later in this section.
SKN-GS1-E4, SKN-GS4-E6, SKN-GS7-E3, and
SKN-GS8-E4 are characterized by cross laminations
showing a seaward paleocurrent direction and havethicknesses of > 35 cm. Almost of their grain-size distributions are similar to the present creek sediments(Fig. 9). From these characteristics of sediments and thepresent creek topography (~ 1 m wide and 30–50 cmdeep), they are considered to be buried channel fills.
However, considering the tsunami behavior, such channel-fill deposits could have been formed during a tsunami return-wave period and thus could be either fluvial
or tsunami deposits. The grain-size distributions of lower
part of SKN-GS4-E6 (Fig. 7) showed similar distributionof present-day beach sediments, indicating the possibilityof tsunami return-wave sediments. Other event deposits
with a wide grain-size distribution and large amounts ofparticles both coarser and finer grain sizes than 1–2 phi
are more likely to be fluvial instead of tsunami deposits.
Finally, we conducted radiocarbon dating to determine the correlation between tsunami deposits as
inferred above and candidates in the GS cores in theincised valley. As a result, we identified five tsunami
deposits (TD1–TD5). The modeled ages of each tsunami deposit obtained using OxCal are shown in Table 6
and Additional file 2: Fig. S3. TD1 (500–300 cal yrIshimura et al. Progress in Earth and Planetary Science(2022) 9:69

Page 18 of 27

Table 5 Results of radiocarbon dating of samples in SKN-E, SKN-W, and SKN-GS
Sample no Lab. no

Location or Sample
core name position

Material

δ13C [‰]

Conventional Calibrated age
14
C age [yr BP] (2σ) [cal BP]

1

IAAA-181081 SKN-W

E4/E5

Organic sediment− 25.82 ± 0.23 424 ± 22

520–460 (95.4%)

2

IAAA-181082 SKN-W

E5/B-Tm

Organic sediment− 30.24 ± 0.20 979 ± 22

930–890 (33.4%),
880–790 (62.1%)

3

IAAA-181805 SKN-W

B-Tm/E6

Organic sediment− 28.44 ± 0.20 1315 ± 22

1300–1240(50.5%), 1220–
1170 (44.9%)

4

IAAA-181083 SKN-W

E6/E7

Organic sediment− 27.41 ± 0.21 1357 ± 23

1310–1260(87.0%), 1210–
1170 (8.5%)

5

IAAA-210212 SKN-W

below E10

Plant fragment− 30.13 ± 0.19 1815 ± 24

1820–1810(0.9%), 1800–
1760 (4.9%),
1750–1690(63.7%), 1670–
1620 (26.0%)

6

IAAA-210213 SKN-W

below E11

Organic sediment− 29.97 ± 0.21 2724 ± 24

2870–2760(95.4%)

7

IAAA-200962 SKN-W

below E14

Organic sediment− 29.79 ± 0.26 3992 ± 26

4530–4410(95.4%)

8

IAAA-210214 SKN-E

below E5

Organic sediment− 28.61 ± 0.15 3970 ± 27

4530–4400(89.9%), 4370–
4350 (2.2%),
4330–4300(3.3%)

9

IAAA-210215 SKN-E

below E6

Organic sediment− 24.25 ± 0.26 4320 ± 27

4960–4830(95.4%)

10

IAAA-211205 SKN-E

E11/E12

Organic sediment− 26.07 ± 0.23 5418 ± 29

6290–6180(93.0%), 6140–
6120 (2.5%)

11

IAAA-210649 SKN-GS1

58–59 cm in
depth

Organic sediment− 25.92 ± 0.24 542 ± 22

630–590 (18.0%),
560–520 (77.4%)

12

IAAA-210367 SKN-GS1

118–119 cm in
depth

Organic sediment− 25.83 ± 0.19 3931 ± 25

4510–4490(2.2%), 4440–
4250 (93.2%)

13

IAAA-210368 SKN-GS1

141–142 cm in
depth

Organic sediment− 25.39 ± 0.18 4209 ± 25

4850–4790(33.3%), 4760–
4690 (48.2%),
4680–4640(14.0%)

14

IAAA-210645 SKN-GS1

238–239 cm in
depth

Organic sediment− 26.77 ± 0.24 4783 ± 27

5590–5470(95.4%)

15

IAAA-210369 SKN-GS1

243.5–244.5 cm
in depth

Organic sediment− 26.72 ± 0.19 4902 ± 27

5720–5680(4.5%), 5670–
5580 (90.9%)

16

IAAA-202305 SKN-GS2

64–65 cm in
depth

Organic sediment− 27.5 ± 0.21

960–900 (78.3%),
870–820 (15.2%),
810–800 (1.9%)

17

IAAA-202306 SKN-GS2

70–71 cm in
depth

Organic sediment− 27.32 ± 0.19 1239 ± 21

1270–1200(40.0%), 1180–
1070 (55.5%)

18

IAAA-211211 SKN-GS2

91–92 cm in
depth

Organic sediment− 27.6 ± 0.21

2760–2690(72.7%), 2640–
2610 (7.7%),
2590–2510(15.0%)

1003 ± 212560 ± 24

RemarkIshimura et al. Progress in Earth and Planetary Science(2022) 9:69

Page 19 of 27

Table 5 (continued)
Sample no Lab. no

Location or Sample
core name position

Material

δ13C [‰]

Conventional Calibrated age
14
C age [yr BP] (2σ) [cal BP]

19

IAAA-210652 SKN-GS2

142–143 cm in
depth

Organic sediment− 29.82 ± 0.23 4410 ± 26

5260–5250(0.5%), 5240–
5190 (3.7%),
5060–4860(91.2%)

20

IAAA-200960 SKN-GS2

204–206 cm in
depth

Organic sediment− 27.37 ± 0.25 4979 ± 28

5860–5820(5.0%), 5750–
5600 (90.4%)

21

IAAA-211210 SKN-GS3

90–91 cm in
depth

Organic sediment− 26.76 ± 0.22 3268 ± 26

3570–3440(90.2%), 3430–
3400 (5.3%)

22

IAAA-211209 SKN-GS4

97–98 cm in
depth

Organic sediment− 27.1 ± 0.183405 ± 26

3820–3800(1.0%), 3720–
3570 (94.5%)

23

IAAA-210651 SKN-GS5

63–64 cm in
depth

Organic sediment− 27.1 ± 0.254005 ± 25

4530–4410(95.4%)

24

IAAA-202304 SKN-GS5

171–172 cm in
depth

Organic sediment− 22.62 ± 0.19 6673 ± 29

7610–7470(95.4%)

25

IAAA-210217 SKN-GS6

44–45 cm in
depth

Organic sediment− 28.52 ± 0.27 268 ± 21

430–370 (28.6%),
330–280 (62.3%),
170–150 (4.5%)

26

IAAA-210365 SKN-GS6

57–58 cm in
depth

Organic sediment− 28.87 ± 0.19 615 ± 21

650–550 (95.4%)

27

IAAA-211208 SKN-GS6

96–97 cm in
depth

Organic sediment− 25.6 ± 0.22

2782 ± 25

2960–2840(81.4%), 2830–
2780 (14.0%)

28

IAAA-210650 SKN-GS6

132–133 cm in
depth

Organic sediment− 27.9 ± 0.22

4160 ± 25

4830–4580(95.4%)

29

IAAA-210366 SKN-GS6

134–135 cm in
depth

Organic sediment− 25.77 ± 0.18 4125 ± 25

4820–4750(28.1%), 4730–
4520 (67.3%)

30

IAAA-210646 SKN-GS6

151–152 cm in
depth

Organic sediment− 27.2 ± 0.23

5290–5160(47.3%), 5140–
4960 (48.1%)

31

IAAA-210218 SKN-GS6

165–166 cm in
depth

Organic sediment− 30.01 ± 0.20 4643 ± 28

5470–5310(95.4%)32

IAAA-210219 SKN-GS6

173–174 cm in
depth

Organic sediment− 27.84 ± 0.19 5102 ± 29

5920–5840(36.2%), 5830–
5740 (59.2%)

33

IAAA-210220 SKN-GS6

195–196 cm in
depth

Organic sediment− 28.69 ± 0.18 5099 ± 29

5920–5840(35.7%), 5830–
5740 (59.8%)

34

IAAA-210221 SKN-GS6

228.5–229.5 cm
in depth

Organic sediment− 29.06 ± 0.18 5211 ± 29

6110–6080(5.5%), 6010–
5900 (89.9%)35

IAAA-200961 SKN-GS6

262–264 cm in
depth

Organic sediment− 26.88 ± 0.25 5524 ± 28

6400–6280(95.4%)36

IAAA-210643 SKN-GS7

40–41 cm in
depth

Organic sediment− 25.69 ± 0.21 555 ± 21

630–590 (36.7%),
560–520 (58.8%)

37

IAAA-210644 SKN-GS8

74–75 cm in
depth

Organic sediment− 26.9 ± 0.21

503 ± 21550–500 (95.4%)

38

IAAA-210216 SKN-GS9

62–63 cm in
depth

Organic sediment− 27.8 ± 0.21412 ± 22

520–450 (90.5%),
360–330 (4.9%)

39

IAAA-211206 SKN-GS9

118–119 cm in
depth

Organic sediment− 25.2 ± 0.22

3264 ± 25

3570–3440(87.6%), 3430–
3400 (7.8%)4457 ± 26

RemarkIshimura et al. Progress in Earth and Planetary Science(2022) 9:69

Page 20 of 27

Table 5 (continued)
Sample no Lab. no

Location or Sample
core name position

Material

δ13C [‰]

Conventional Calibrated age
14
C age [yr BP] (2σ) [cal BP]

Remark40

IAAA-211207 SKN-GS9

165–166 cm in
depth

Organic sediment− 25.2 ± 0.21

3940 ± 26

4520–4480(8.8%), 4450–
4290 (85.3%),
4270–4250(1.4%)

41

IAAA-210647 SKN-GS9

175–176 cm in
depth

Organic sediment− 27.9 ± 0.244017 ± 26

4570–4550(1.2%), 4530–
4410 (94.2%)

42

IAAA-210648 SKN-GS9

201–202 cm in
depth

Organic sediment− 29.5 ± 0.244350 ± 27

5030–5010(1.3%), 4980–
4850 (94.1%)

43

IAAA-181806 SKN-E

below U-2 drift
pumiceOrganic sediment− 22.9 ± 0.214710 ± 30

5580–5500(17.2%), 5490–
5430 (23.9%),
5420–5320(54.4%)

Hiramine et al. (inpress)

44

IAAA-181607 SKN-E

below To-Cu ash Organic sediment− 24.6 ± 0.395180 ± 30

6000–5900(95.4%)

Ishimura andHiramine (2020)

Fig. 10 Geomorphic change in Sekinehama. a Aerial photograph in 1952, obtained from GSI. It was geo-referenced in ArcGIS. b Geomorphic
classification in 1952. c Paleogeography in ~ 6 ka, based on the work of Hiramine et al. (in press)

BP (1450–1650 cal CE)) is the latest tsunami deposit(E1 deposit in all cores) in this study and is distributed
above B-Tm tephra. TD1 in many of the cores shows agrain-size distribution similar to that of the beach sediments, with multiple units and thinning inland. TD2(4490–4300 cal yr BP; SKN-GS1-E2, SKN-GS8-E2, and
SKN-GS9-E3), found up to SKN-GS1 and not farther
inland, shows a grain-size distribution similar to that of
the beach sediments, with an erosive contact at the basein SKN-GS1. TD3 (4790–4580 cal yr BP; SKN-GS1-E3,
SKN-GS6-E2, SKN-GS8-E3, and SKN-GS9-E4) shows agrain-size distribution similar to that of the beach sediments, with an erosive contact at the base in SKN-GS1.
The sedimentary structure of SKN-GS1-E3 shows a paleocurrent direction inland. SKN-GS7-E3 was a channel-filldeposit that may have been formed by the return wave
during the TD3 deposition. TD4 (5420–5030 cal yr BP;
SKN-GS2-E2 and SKN-GS6-E3) has a slightly finergrain-size distribution in the beach sediments. In
SKN-GS6-E3, the paleocurrent direction inland was estimated from the sedimentary structure. SKN-GS1-E4 and
SKN-GS8-E4 were channel-fill deposits and may havebeen formed by the return wave during TD4 deposition.
TD5 (6030–5790 cal yr BP; SKN-GS1-E5, SKN-GS2-E5,
SKN-GS6-E6, and SKN-GS8-E5) shows a grain-size distribution similar to that of the beach sediments, with anerosive contact at the base. The sedimentary structureof SKN-GS6-E6 shows a paleocurrent direction inland.
SKN-GS4-E6 was channel-fill deposit and may have been
formed by the return wave during TD5 deposition. In theIshimura et al. Progress in Earth and Planetary Science(2022) 9:69

Page 21 of 27

Fig. 11 Lateral correlation of tsunami deposits (TD1–TD5) between coastal outcrops and SKN-GS cores. A–A′ and B–B′ profile lines are shown in
Fig. 1c. Vertical exaggeration is 25X. Sketches of SKN-E, SKN-W, and SKN-GS are from Figs. 2, 4, and 5, respectively. Sandy event deposits in orange in
SKN-E and SKN-W indicate candidate tsunami deposits, showing relatively strong wave currents among the sandy event deposits in the outcrops

Table 6 Modeled ages of TD1–TD5
Tsunami deposits

Modeled age (cal yr BP)

TD1

500–300 (1450–1650 cal CE)

TD2

4490–4300

TD34790–4580

TD45420–5030

TD5

6030–5790

ITRAX results (Fig. 8), some sand layers (e.g., SKN-GS1E3 and SKN-GS6-E6) show an increase in S, which is anindicator of marine intrusion (e.g., Chagué-Goff et al.
2016), supporting the above identifications of tsunami
deposits.
In addition, SKN-GS1-E6, SKN-GS6-E7, and
SKN-GS8-E6 may be correlated. SKN-GS6-E7 showsa finer-grain-size distribution than that of the beachsediments, while SKN-GS1-E6 and SKN-GS8-E6 show
grain-size distributions similar to those of the beach sediments. The ITRAX results show a slight increase in S at
SKN-GS1-E6 (Fig. 8). However, because they are near thebase of the cores, and the underlying sediments were notfully observable, they are not identified here as tsunami
deposits.
5.5 Interpretation of event deposits in SKN‑E and SKN‑W
outcrops

Candidates for tsunami deposits in SKN-W are SKNW-E3/E4, SKN-W-E10, SKN-W-E11, and SKN-W-E14

because all of them have erosional contacts at the baseand are distinct sand layers compared to the other event
deposits (Table 2). In contrast, SKN-W-E1 and SKNW-E8 characteristically exhibit thicker layers and parallel laminae. These features are commonly observed inwashover deposits caused by storm surges behind thecoast (e.g., Morton et al. 2007). Additionally, SKN-W-E1
is distributed up to the ground surface (Fig. 4). Considering the most recent topographic changes (Fig. 10a, b),
it could be a washover deposit formed by a storm surgethat eroded the coastal sand dune in the past 50 years.
In SKN-E, SKN-E-E5, SKN-E-E6, and SKN-E-E8 are
candidates for tsunami deposits because all of them
have erosive contact at the base with rip-up clasts. The
rip-up clast is often used as one of the indicators for
identifying tsunami deposits (e.g., Goff et al. 2012) andfor distinguishing tsunami and storm-surge deposits(e.g., Kortekaas and Dawson 2007). SKN-E-E5 in particular includes a few centimeters of gravels and pumice clasts, suggesting a stronger flow compared to otherevent deposits in the outcrop. SKN-E-E7 is a channel-filldeposit with parallel laminae. Taking into account the
paleogeography (Fig. 10c), it could be fluvial sedimentsthat buried the creek flowing through the wetland or
tidal-channel sediments that filled the channel connecting to the sea.
To correlate the sediments between the two outcrops,
we used radiocarbon dating on deposits below the tsunami-deposit candidates mentioned above, which are
expected to be widely distributed in the study area. Dates

Ishimura et al. Progress in Earth and Planetary Science(2022) 9:69

for samples collected below SKN-E-E5 and SKN-W-E14
are 3970 ± 27 yr BP and 3992 ± 26 yr BP, respectively.
This coincidence means that they are the sandy deposits related to the same event. These dates added to thetephra ages and other radiocarbon dates confirmed having continuous sediments from the present to 6300 cal yr
BP exposed in the outcrops.
Grain-size analyses of the outcrop samples showthat almost all the event deposits have a mode at 1–2phi, similar to the present-day beach (Figs. 3, 9). In the
SKN-E outcrop, SKN-E-E1 to SKN-E-E4 (younger
than ~ 4500 cal yr BP) show a narrow grain-size distribution with a 1–2-phi mode. In contrast, SKN-E-E5
to SKN-E-E10 (4500–6000 cal yr BP) exhibit 1–2-phi
modes but wider grain-size distributions. Below SKNE-E11 (older than ~ 6000 cal yr BP), the mode shifts toa somewhat-smaller size (2 phi), and the proportionof fine-grained particles increases. A similar trend ofgrain-size variation was observed in the SKN-W outcrop. From SKN-W-E1 to SKN-W-E13 (present to4500 cal yr BP), they show narrow grain-size distributions with 1–2-phi modes. SKN-W-E14 to SKN-W-E16(older than 4500 cal yr BP) show 1–2-phi modes butwith wider grain-size distributions. SKN-W-E17 has asmaller mode (2 phi) and contains a higher proportionof fine-grained particles compared to the event deposits
above it. This similarity in the grain-size distribution of
the event deposits at the two sites according to age suggests the existence of common event deposits at the twosites. However, the numbers of event deposits between
the two sites do not necessarily match, and correlations
with radiocarbon dates are difficult because of the close
distributions of event deposits. In Sect. 5.6, we correlate
the tsunami deposits (TD1–TD5) identified in the SKNGS cores to the event deposits in these outcrops.
5.6 Tsunami deposits distribution in Sekinehama

From the dating results, we correlated tsunami-deposit
candidates among the coastal outcrops to TD1–TD5 in
the GS cores (Figs. 2, 4, 11). TD1 correlates with SKN-WE3/E4, which has an erosional base, and was traced in the
SKN-W-GS cores drilled behind the outcrop. In outcrop,
they present as two separate units, indicating the multiple waves. This feature is consistent with the sedimentary
facies of E1 deposits of the SKN-GS cores. Accordingly,
TD1 was deposited by a tsunami with multiple waves
during 500–300 cal yr BP (1450–1650 cal CE).
TD2, TD3, and TD4 were correlated with SKN-E-E5,
SKN-E-E6, and SKN-E-E8, respectively. Because thesethree layers have coarse grains, an erosional base, andrip-up clasts, we consider them to have been depositedby strong wave currents. Considering the radiocarbon
dates, TD5 may correlate with SKN-E-E9 or SKN-E-E10.

Page 22 of 27

In SKN-W, we correlated SKN-W-E14 to TD2. SKN-WE16 may correspond to TD3, but the age could not bemeasured owing to the low organic carbon content. Considering the age of the sediments, it is unlikely that TD4
and TD5 are represented in SKN-W.
These results allowed us to correlate tsunami deposits
from the coastal outcrops to deposits 200–400 m inland(Fig. 11). These distributions also indicate that TD1–TD5
were deposited by large tsunamis compared to historical tsunamis (Table 1). TD1, TD4, and TD5 are distributed the farthest inland among the five tsunami deposits.
However, the fact that the sediments in the coastal outcrops were deposited in lowlands behind dunes and thatthe other event deposits not correlated with TD1–TD5
show similar grain-size distributions with those of thebeach sediments suggests that they may have been deposited by smaller tsunamis or by low-frequency stormsurge events. This is consistent with our interpretationthat the thick event deposits with multiple laminations
and limited distributions (SKN-W-E1 and SKN-W-E8)
are washover deposits associated with storm surges.
Between TD2 and B-Tm tephra, multiple event deposits are commonly recognized in the coastal outcrops,
whereas no distinct event deposits are recognized
between them in the GS cores. The GS cores yield datesof ~ 3000 cal yr BP ~ 50 cm below the B-Tm tephra. The
sedimentary facies show no unconformity below B-Tm,
but this indicates that the sedimentation rate was low or
it represents a hiatus. Given the absence of event deposits between TD2 and B-Tm tephra in the GS core, apparently event deposits either are distributed only near thecoast or are not well preserved in the GS cores.
5.7 Tsunami deposits correlation in Tohoku and Hokkaido
regions

Considering the correlation to more-distant tsunami
deposits, TD1 (500–300 cal yr BP (1450–1650 cal CE))can be correlated with the 1611 Keicho tsunami deposits
on the Sanriku Coast, the seventeenth-century tsunami
deposits in western Hokkaido, and the 1640 Komagataketsunami deposits. The 1454 Kyotoku tsunami (Namegaya
and Yata 2014) also appeared to be a possible correlative
event linked to TD1, but Ishizawa et al. (2022) showedthat there is no deposit from the 1454 Kyotoku tsunamion the northern Sanriku Coast (Fig. 1a). As for the 1640
Komagatake tsunami, numerical simulations conductedby Nakanishi and Okamura (2019) to explain the tsunami-deposit distributions show that with distance fromthe source the wave heights decreased. As Sekinehamais ~ 100 km from the Komagatake, the calculated tsunami
height was < 5 m, which is below the present sea-cliff
height of ~ 5 m (Fig. 11), meaning that the 1640 Komagatake tsunami likely did not inundate the locations of theIshimura et al. Progress in Earth and Planetary Science(2022) 9:69

studied GS cores. Also, Sato et al. (in press in this special
issue) confirmed that the simulated 1640 Komagataketsunami would not have inundated the GS core sites inour study.
To explain the tsunami deposits near the study site
(Minoura et al. 2013; Tanigawa et al. 2014; Kase et al.
2022; Fig. 1), it was necessary to assume the occurrenceof an unknown tsunami in the fifteenth century, considering that only one layer of tsunami deposits at each site
has been identified during the last 1500 years (Fig. 12),the topographical settings (distance from the coastline
and elevation) of each site, and the uncertainties of ages
due to differences in dating materials. We determined thedepositional ages of the latest tsunami deposits within
a 50-km distance from Sekinehama (Figs. 1, 12) to be as

Page 23 of 27

follows: Ta1 deposit (589–516 cal yr BP (1361–1434 calCE)) at the Takadai-cho site in Hokkaido (Elevation is
6.4 m, and distance from the coast is 200 m; Kase et al.
2022), TD1 (500–300 cal yr BP (1450–1650 cal CE)) inthis study (Elevation is ~ 5–7 m, and distance from thecoast is ~ 200–400 m), a tsunami deposit (681–490 cal yr
BP (1269–1460 cal CE)) on the east coast of Shimokita
Peninsula (Elevation is ~ 21–22 m, and distance from thecoast is ~ 1350 m; Minoura et al. 2013), and S1 deposit(470–180 cal yr BP (1480–1770 cal CE)) on the east
coast of Shimokita Peninsula (Elevation is ~ 3–5.5 m,
and distance from the coast is ~ 700–1050 m; Tanigawa
et al. 2014.). Although not completely overlapping, they
are very close to the mid-fifteenth century (Fig. 12). If
they were all deposited by the same tsunami event, weFig. 12 Depositional ages of tsunami deposits along the Pacific Coast in Hokkaido and Tohoku regions. Ages of Us-b and Ta-b tephras are from
Machida and Arai (2013). For study-site locations, see Fig. 1

Ishimura et al. Progress in Earth and Planetary Science(2022) 9:69

assume they were deposited in the mid-fifteenth century.
Accordingly, and given the seventeenth-century tsunamiin Hokkaido and the 1611 Keicho tsunami on the Sanriku
Coast, the latest tsunami deposits at Sekinehama could
have been deposited by an indeterminate fifteenth-century tsunami that has yet to be recognized in the historical record.
We correlated TD2–TD5 to tsunami deposits identified near the Sekinehama Coast. On the other side of the
Tsugaru Strait, in addition to Ta1 mentioned above, Kase
et al. (2022) identified two tsunami deposits (Ta2 andTa3) at the Takadai-cho site (Fig. 1a), which is the closest
to our study site among their study sites. The ages of Ta2
and Ta3 are 1745–1639 cal yr BP and 2401–2265 cal yr
BP, respectively (Fig. 12). Although they do not correlatewith TD2–TD5, radiocarbon dates indicate that they may
correlate with SKN-W-E7 to SKN-W-E10. However, Kase
et al. (2022) stated that the areal distributions of Ta1–Ta3
were not confirmed and were limited to a narrow area of
10 m or less. Therefore, more excavation and drilling surveys similar to our current study are needed to confirm
the inland distribution and correlate laterally.
In addition to the already-described S1, Tanigawa et al.
(2014) identified four sandy event deposits (S2–S5) thatformed during the last 6000 years and concluded that S1,
S3, and S4 were deposited by seawater inundation on thebasis of diatom analysis. The ages of the four deposits are
as follows (Fig. 12): S2 (4800–4500 cal yr BP), S3 (5300–
4870 cal yr BP), S4 (5540–5320 cal yr BP), and S5 (5890–
5490 cal yr BP). We correlated TD4 (5420–5030 cal yrBP) to S3 or S4. If S2 and S5 are also tsunami deposits,
we would correlate TD3 (4790–4580 cal yr BP) to S2 andTD5 (6030–5790 cal yr BP) to S5. Tanigawa et al. (2014)
also found no seawater inundation over a long period
between S1 and S2 or S3. Thus, the depositional ages andintervals that they determined for the tsunami deposits
are similar to those in our study. In addition, S1 and S3
are distributed farthest inland in the drilling area, whichis also consistent with the distribution of TD1 and TD4
in our study (Fig. 11).
We also investigated correlation to deposits inmore-distant areas. West of the Hidaka Coast in Hokkaido, tsunami deposit during 4000–6000 cal yr BP is
unknown (Takashimizu, 2014). In the southern Hidaka
Coast area, dates for tsunami deposits that formed during 5.5–2.2 ka were obtained by Nakanishi et al. (2020):
U1: 2360–2060 cal yr BP, U2: 2745–2445 cal yr BP, U3:
2856–2745 cal yr BP, U4: 3553–3233 cal yr BP, U5:
3905–3705 cal yr BP, U6: 4407–4009 cal yr BP, U7: 5014–
4830 cal yr BP, and U8: 5399–5054 cal yr BP (Fig. 12).
Although tsunami deposits that correlate with our results
are distributed, Nakanishi et al. (2021) considered that
their tsunami sources were likely to be along the Kuril

Page 24 of 27

Trench (Fig. 1a). Therefore, to confirm correlation, tsunami sources need to be addressed. Similarly, although
TD2–TD5 could be correlated with tsunami deposits identified along the Sanriku Coast (Fig. 12; Ishimura
2017; Ishimura and Yamada 2021; Ishizawa et al. 2022;
Velasco-Reyes et al., 2022), we need to assess whether
tsunamis off the Sanriku Coast could generate tsunamis
inundating Sekinehama. Thus, although there is a possibility that the tsunami deposits in Sekinehama correlatewith those in the Hokkaido and the Sanriku Coast, further investigation involving tsunami sources is needed tocorrelate the Sekinehama deposits to more-distant tsunami deposits.

6 Conclusions
In this study, we performed nondestructive analyses(XCT and ITRAX µXRF core scanning), grain-size analysis, tephra analysis, and radiocarbon dating of sediments
exposed in the two coastal outcrops and GS cores at
Sekinehama on the north coast of the Shimokita Peninsula, which is near the junction of the Japan and Kuril
trenches. By comparing grain-size distribution between
sandy event deposits and present-day source sediments
and by investigating detailed sedimentary structures
observed in XCT images, we found that some event
deposits show transportation inward from the coast.
Finally, we identified five tsunami deposits (TD1–TD5)
that formed since ~ 6 ka and correlated them from coastal
outcrops to the incised valley based on radiocarbon dating and tephrochronology. We also found that otherwashover deposits were among the coastal outcrops but
not in the GS cores. This limited distribution indicatesminor washover events related to small tsunamis andlow-frequency storm surges.
The modeled age of the latest tsunami deposit is500–300 cal yr BP (1450–1650 cal CE). This may becorrelative with two known tsunamis (seventeenthcentury tsunami in Hokkaido and 1611 Keicho tsunami
in Tohoku) or may be an unknown tsunami that possibly occurred during the fifteenth–seventeenth centuries. If the latest tsunami deposit is correlated witheither or both of the seventeenth-century and 1611 Keicho tsunamis, the tsunami inundation area would be
expanded, resulting in the need for a complex tsunami
source model. However, if the latest tsunami deposit iscorrelated with tsunami deposits previously identified
within a 50-km distance from the study site, we needto consider an unknown fifteenth-century tsunami
and use a different source model set off the Shimokita
Peninsula. Nonetheless, our determination of the latest tsunami deposit is one constraint with high reliability for constraining the tsunami source models in the
future even if not correlated with any specific tsunamis.

Ishimura et al. Progress in Earth and Planetary Science(2022) 9:69

To tackle possible simultaneity and the as-yet-indeterminate source of the seventeenth-century tsunami
in the Hokkaido region and the 1611 Keicho tsunami
in the Tohoku region, we need to conduct tsunamideposit surveys in other areas along the Pacific Coastnear the junction of the Japan and Kuril trenches. The
tsunami deposits TD2–TD5 could be correlated withthose on the east coast of the Shimokita Peninsula bytheir depositional ages. To correlate with more-distanttsunami deposits in the Hokkaido and Tohoku regions,tsunami sources need to be assessed because there aremany correlative tsunami deposits in those regions.
Abbreviations
cps: Counts per second; DEM: Digital elevation model; GSI: Geospatial Information Authority of Japan; TD: Tsunami deposits; XCT: X-ray computed tomography; µXRF: Micro-X-ray-fluorescence.

Supplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​022-​00529-9.
Additional file 1. OxCal code.
Additional file 2. Supplementary figures and table. Fig. S1a. Resultsof the ITRAX micro-X-ray fluorescence core scanning of SKNGS9. Eachelemental datum was normalized by counts per second (cps). Fig. S1b.
Results of the ITRAX micro-Xray fluorescence core scanning of SKN-GS5.
Each elemental datum was normalized by counts per second (cps). Fig.
S1c. Results of the ITRAX micro-X-ray fluorescence core scanning of SKNGS4. Each elemental datum was normalized by counts per second (cps).
Fig. S1d. Results of the ITRAX micro-X-ray fluorescence core scanning of
SKN-GS2. Each elemental datum was normalized by counts per second(cps). Fig. S2. Major element compositions of volcanic glass shards. Fig.
S3. OxCal output of TD1 to TD5. We performed OxCal modeling using theSequence model (Bronk Ramsey, 2008). Table S1. Results of grain analysis.
Acknowledgements
We would like to thank Reona Hiramine for help in the preliminary field survey.
This study was partially performed under the cooperative research program of
the Center for Advanced Marine Core Research, Kochi University (Accept No.
20B052, 21B010). The thoughtful comments from the two anonymous reviewers improved the article.
Author contributions
DI designed this research, conducted tephra and grain analyses, and drafted
the manuscript. All authors conducted the GS drilling survey. MY and TI
discussed the sedimentary features of event deposits. TI checked the age
modeling with OxCal. KA conducted tephra analysis in SKN-W and discussed
the tephra correlation. KS compiled information on the tsunami-deposit
research and tsunami source modeling. All authors read and approved thefinal manuscript.
Funding
This research was supported by the Sasakawa Scientific Research Grant fromthe Japan Science Society and JSPS KAKENHI Grant Number JP21H00631.
Availability of data and materials
The datasets supporting the conclusions of this article are included within thearticle and its additional files.

Page 25 of 27

Declarations
Competing interests
The authors declare that they have no competing interests.
Author details
1
Department of Geography, Tokyo Metropolitan University, 1‑1
Minami‑Osawa, Hachioji, Tokyo 192‑0397, Japan. 2 International ResearchInstitute of Disaster Science, Tohoku University, 468‑1 Aoba, Aramaki, Aoba‑ku,
Sendai, Miyagi 980‑8572, Japan. 3 Department of Geology, Faculty of Science, Shinshu University, 3‑1‑1 Asahi, Matsumoto, Nagano 390‑8621, Japan.
4
Research Center for Volcanic Hazards and Their Mitigation, Tokyo Metropolitan University, 1‑1 Minami‑Osawa, Hachioji, Tokyo 192‑0397, Japan. 5 Department of Earth and Planetary Science, The University of Tokyo, 7‑3‑1 Hongo,
Bunkyo‑ku, Tokyo 113‑0033, Japan.
Received: 29 April 2022 Accepted: 1 December 2022

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