Shimada et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-019-0279-9(2019) 6:33

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open AccessGeological record of prehistoric tsunamis
in Mugi town, facing the Nankai Trough,western Japan
Yumi Shimada1,3* , Shigehiro Fujino2, Yuki Sawai3, Koichiro Tanigawa3, Dan Matsumoto3, Arata Momohara4,
Megumi Saito-Kato5, Masaki Yamada6, Eri Hirayama1, Takahiro Suzuki1 and Catherine Chagué7

Abstract
Stratigraphic and paleontological investigations in Mugi Town, on the Pacific coast of Shikoku Island, revealed evidenceof as many as five tsunami inundations from events along the Nankai Trough between 5581 and 3640 cal yr BP. Nine
event deposits (E1–E9) were identified in cores ranging in length from 2 to 6 m, consisting of sandy and gravelly layersinterbedded with organic-rich mud. Sedimentary structures in the event deposits observed by computed tomography
included normal grading and sharp lower stratigraphic contacts. Event deposits E3, E6, E7, and E8 contained mainly
brackish-marine diatom species, suggesting that they had been deposited during inundation by seawater. In addition,
fossil diatom assemblages were markedly different above and below event deposits E3, E4, E6, and E8. For example,
assemblages below event deposit E6 were dominated by a freshwater species (Ulnaria acus), whereas assemblages above
it were predominantly brackish-marine (Diploneis smithii, Fallacia forcipata, and Fallacia tenera). We attributed thesechanges to the increase of marine influence due to coastal subsidence associated with subduction-zone earthquakes, as
documented in the 1946 Showa-Nankai earthquakes. We conclude that event deposits E3, E6, and E8 and perhaps E4 andE7 were deposited by tsunamis generated by subduction zone earthquakes along the Nankai Trough. The ages of these
event deposits, as constrained by ten radiocarbon ages, suggest that some of the tsunamis that impacted Mugi Town
were correlated with those reported elsewhere along the Nankai Trough, thereby complementing the existing but still
incomplete geological record for these events.
Keywords: Tsunami deposits, Subduction zone earthquake, Environmental change, Coastal deformation, Diatom, Nankai
Trough, Tokushima Prefecture

Introduction
The Pacific coast of Japan has been repeatedly affected by
subduction zone earthquakes and tsunamis. Long-term
forecasts of subduction zone earthquakes are calculatedby the Japanese government on the basis of earthquake
histories (The Headquarters for Earthquake Research Promotion 2013). The long-term forecasts for the Nankai
Trough region in western Japan relied mainly on historical
documents from the last 1300 years (The Headquarters
for Earthquake Research Promotion 2001), although there* Correspondence: yumi-shimada@geol.tsukuba.ac.jp
1
Graduate School of Life and Environmental Sciences, University of Tsukuba,
1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan
3
Geological Survey of Japan, National Institute of Advanced Industrial
Science and Technology (AIST), 1-1-1-C7 Higashi, Tsukuba, Ibaraki 305-8567,
Japan
Full list of author information is available at the end of the article

were a few reports on geological records of prehistoric
tsunamis (Okamura et al. 2000). After 2001, geologicalstudies discovered evidence of unusually large prehistoric
tsunamis in other subduction zones, including Thailand(Jankaew et al. 2008), Sumatra (Monecke et al. 2008), andHokkaido (Nanayama et al. 2003; Sawai et al. 2009a).
Along the Japan Trench, a tsunami deposit associatedwith the 869 CE Jogan earthquake was identified (e.g., Abeet al. 1990; Minoura and Nakaya 1991; Minoura et al.
2001) that later was recognized as a predecessor of the2011 Tohoku earthquake and tsunami (Sawai et al. 2012).
The most recent evaluation for the Nankai Trough region
sought to incorporate geological records of prehistoric
earthquakes (The Headquarters for Earthquake Research
Promotion 2013). However, while more than 70 recent
publications have presented paleoseismological evidence© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

(2019) 6:33

Page 2 of 15(Garrett et al. 2016), only a few have provided thewell-dated evidence needed to understand past fault behavior through regional correlations (Fujino et al. 2018;
Tanigawa et al. 2018).
This study was carried out in a coastal lowland in
Mugi Town, adjacent to the Nankai Trough, with theaim of identifying prehistoric tsunami deposits and estimating event ages. Our findings provide new information on the history of subduction zone earthquakes
along the Nankai Trough, complementing the meager
existing record.

Committee of the History of Mugi Town 1976). The
post-disaster survey report of the Showa-Nankai earthquake (Central Meteorological Observatory 1947) indicated that the tsunami inundation area included thewhole study site. The Showa-Nankai earthquake also
caused coastal subsidence of at least 0.3 m or more in thisarea, such that groins built to stand above spring hightides were submerged to about 0.3 m depth at spring tides
after the earthquake (Editing Committee of the History ofTokushima Prefecture 1963). Other detailed information
on damages and coastal deformation caused by these four
earthquakes is summarized in Table 1.

Shimada et al. Progress in Earth and Planetary Science

Study area
Site selection

The study was carried out in a small and isolated freshwater wetland (about 250 × 150 m) in Mugi Town insoutheastern Shikoku Island (Fig. 1). The wetland is separated from the sea by a sand barrier through which anarrow drainage channel runs (Fig. 1d). The barrier
shields the wetland from waves and tidal flows that
could lead to deposition of coarse marine sediments. No
rivers flow into the wetland, and therefore there is no input of alluvial sediment. These topographical conditions
favor the preservation and identification of tsunami deposits because only extreme high-energy events arelikely to deliver sand or gravel into the wetland. Sandy
and gravelly event deposits interbedded within muddy
sediments are left undisturbed, making their identification easier. The wetland is surrounded by coastal hills
mainly dominated by shale and sandstone (Kumon 1981;
Onishi and Kimura 1995; Yamaguchi et al. 2009) of late
Cretaceous to Eocene age (Suyari and Yamasaki 1987;
Shibata et al. 2008). The elevation of core locations(Fig. 1d) ranges from 2.37 m at MG12 to 3.10 m at
MG11 (all elevations in this paper are reported withrespect to sea level at the Tokyo Peil). The present elevation of the sand barrier is approximately 4.0 m; however,the natural elevation of the sand barrier is not known
because an embankment has been constructed upon it.
The site was formerly a rice paddy field, but it was abandoned about 10–15 years before the time of this study.
According to the town’s official history (Mugi Choshi),the wetland has been isolated from recent flooding andstorm surges, but the barrier is low enough to have been
overtopped by the Showa-Nankai tsunami in 1946 (waveheight of 4.5 m in Mugi Town; Editing Committee ofthe History of Mugi Town 1976, Central MeteorologicalObservatory 1947).
Historical tsunamis in Mugi town

The last four great earthquakes along the Nankai
Trough (the 1605 CE Keicho, 1707 CE Hoei, 1854 CE
Ansei-Nankai, and 1946 CE Showa-Nankai earthquakes)
generated tsunamis that hit Mugi Town (Editing

Methods/Experimental
Sediment samples were collected at 13 locations(MG01–MG13) along 2 transects, 1 perpendicular andthe other parallel to the shoreline (Fig. 1d). Samples at
all sites except MG05 were obtained with a gouge auger.
At MG05, sediment was sampled successively with a
Handy Geoslicer (Nakata and Shimazaki 1997) from 0 to
120 cm, a thin-wall sampler from 120 to 400 cm, and a
Russian sampler from 400 to 450 cm depth below theground surface. Surface elevations of each location wereobtained with a GNSS survey instrument, Leica Viva
GS08 Plus GNSS receiver (Leica Geosystems).
Sedimentary structures in the MG05 samples were observed using X-ray computed tomography (LightSpeed
Ultra 16, GE Healthcare Japan) at the Center for
Advanced Marine Core Research, Kochi University.
Cores were then split and peels were prepared using ahydrophilic glue (SAC-100) for further observation of
sedimentary structures.
Subsamples for paleontological analysis were taken
every centimeter from cores MG05 and MG10. Subsamples were also taken from the other cores to provide
plant macrofossils for radiocarbon dating. Diatom analysis was carried out at 1–5 cm intervals in core MG05.
Samples were prepared with the bleaching method proposed by Nagumo and Kobayasi (1990) and Nagumo(1995) and placed on slides for examination under an
optical microscope (× 600 magnification) and a scanningelectron microscope (JEOL JSM-6510). More than 200
valves were identified and counted on each slide. Diatoms
were identified and paleoenvironments were assigned on
the basis of standard sources (e.g., Patrick and Reimer
1966, 1975; Krammer and Lange-Bertalot 1986; Witkowski
et al. 2000; Kobayasi et al. 2006; Levkov 2009).
Plant macrofossils (e.g., fruits, seeds, needles, andleaves) from core MG10 were picked out under a binocular microscope for interpretation of paleoenvironments and radiocarbon dating (Table 2). All samples
were measured by accelerator mass spectrometry (AMS)
at Beta Analytic. Radiocarbon ages were calibrated byusing OxCal 4.3.2 software (Bronk Ramsey 2017) with

Shimada et al. Progress in Earth and Planetary ScienceA(2019) 6:33

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B

C

D

Fig. 1 a Index map of Japan, showing the location and tectonic setting of the study area. b Map of Shikoku Island, showing localities where coastal
deformation was recognized from the 1946 CE Showa-Nankai or 1854 CE Ansei-Nankai earthquakes. c Map of Mugi Town, showing the geography,
landmarks mentioned in the text, and the area inundated by the 1946 CE Showa-Nankai tsunami (stippling). d Aerial photograph of the study site,
showing the geography and sediment sampling points and transects (photograph CSI20121-C10-13 by the Geospatial Authority of Japan, taken 21
February 2013)

the IntCal13 radiocarbon calibration dataset (Reimer et
al. 2013). The limiting ages were then used to constrain
the depositional ages of the events based on Bayesianstatistics (Table 3). This calculation was performed in
OxCal with the procedure proposed by Lienkaemper andBronk Ramsey (2009).

isolated beds of sand and gravel (Figs. 2, 3, and 4). The
sand and gravel layers were interpreted as event deposits(E1 to E9, from top to bottom), indicating rapid deposition of allochthonous sediment into quiescent wetland
environments, and are described below. Radiocarbon
ages from the sedimentary succession ranged from 6750
to 6931 to 1421–1690 cal yr BP (Fig. 2 and Table 2).

Results
Stratigraphy and radiocarbon ages

Diatom assemblages and plant macrofossils

Beneath a top layer of paddy soil ranging in thickness
from 38 to 98 cm, the sedimentary sequences were composed of organic silt (decomposed peat) containing

The diatom assemblages from core MG05 differed aboveand below ~ 0.6 m elevation (Fig. 5). In the lower part ofthe sediments, which included event deposits E2 to E9,

Shimada et al. Progress in Earth and Planetary Science(2019) 6:33

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Table 1 Information of the last four historical earthquakes and tsunamis
Name ofearthquake

Date (CE) a

MomentRupture zonea
magnitude (Mw)

Coastalb, d deformation Number of tsunami Other descriptions inmonuments in
Mugi Town
Mugi Town

Keicho3 February1605

No data

Variousinterpretations
suggested

No data

No data

Mantokuji temple was washedaway.f (The temple sat at
3.5 m T.P.g)

Hoei

28 October1707

8.6b

Z+A+B+C+ D (+E?)

No data

No data

Tsunami waves ran up threerivers.f More than 700 houses
were washed away and morethan 100 people died.f
Mantokuji temple was washedaway.f The tsunami ran up to
about 1.5 km on the Uchizuma
River. f,hAnsei-Nankai

24 December1854 8.4b

Z+A+B

Uplift in Cape Muroto(1.2 m)Subsidence inKochi City (1.1 m)

2 Playground andTeba Islande

640 houses were carried awayand 39 people died.f The
tsunami ran up to the stepsof the Mantokuji temple along
the Mugi River.f

A+B

Uplift in Cape
Shionomisaki(0.6 m) Cape Muroto(0.5–0.9 m) Subsidence
in Mugi Town (0.3 m
or more)Shishikui (0.6 m)

3 Two are in aplayground, andthe other is in anelementary school.

The whole study site wasinundated.d 151 houses were
washed away and 52 people
died.f The western part of Mugi
Bay was more strongly affected
than the eastern part because
the tsunami came from theeast.fShowa-Nankai 21 December1946 8.0b–8.3caFrom Sangawa (2007) and Ishibashi (2014)
From Usami et al. (2013)c
From Tanioka and Satake (2001)
d
From Central Meteorological Observatory (1947)eTeba Island is located about 4 km south of Mugi Bay
f
From Editing Committee of the History of Mugi Town (1976)g
From Murakami et al. (1996)hUchizuma River is located about 1 km west of the study siteb

the diatom assemblages were dominated by brackish andmarine species. Brackish planktonic species included
Cyclotella atomus var. gracilis and Thalassiosira lacustris.
Among the benthic diatoms, brackish-marine species suchas Amphora coffeaeformis, Amphora pseudoholsatica,
Diploneis smithii, Diploneis suborbicularis, Fallacia forcipata, and F. tenera were most abundant. Below event deposit E6 (from − 1.25 m elevation to the bottom of thecore), freshwater-brackish species such as Ctenophorapulchella and Cocconeis placentula were abundant. Freshwater species such as Eunotia spp. and Gomphonema spp.
were found around event deposit E4. Plant macrofossils
used for radiocarbon dating from core MG05 included submerged and floating-leaved plants, such as Ruppia maritima, Zannichellia palustris, Najas spp., and Potamogeton
spp. (Table 2).
In the upper part of the sediments (above ~ 0.6 m),
diatom assemblages were characterized by an upward
decrease in brackish planktonic species and a drastic increase in the proportion of freshwater species such as
Cymbella proxima, Cymbella tumida, Pinnularia spp.,
Stauroneis spp., and Surirella spp. In addition, the concentration of diatom fossils in the upper part, particularly at 0.8–1.1 m elevation (Fig. 5), was lower than thatin the lower part. Samples above 0.8 m were characterized by mostly broken valves.
Given the sparseness of diatoms near the top of thecores, plant macrofossils were quantitatively analyzed
below and above event deposit E1 at site MG10 (Fig. 6).
Below E1, plant macrofossils were dominated by Cladium chinense, a brackish emergent plant. Above E1,they included abundant emergent and wetland plants
typical of a shallow freshwater environment, such as
Persicaria pubescens, Eriocaulon spp., and Monochoria
plantaginea.
Event deposits, their ages, and diatom assemblages

Event deposits (E1 to E9) and sedimentary structures
were identified in tomographic images of core MG05
(Fig. 4), with the sedimentary characteristics of each
event deposit summarized in Table 4. The event deposits
were correlated across the cores based on their elevations, sedimentary features, and radiocarbon ages fromplant macrofossils below and above the event deposits(Table 2). Ages from below event deposits were interpreted as limiting maximum ages, and ages from above
them were interpreted as limiting minimum ages. For
example, a radiocarbon age from unidentified leaves

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Table 2 Radiocarbon ages from Mugi Town
No. Site

Elevation (m,T.P.)

Conventional age(14C yr BP)

Calibrated age2σ range(cal yr BP)

Material

Lab no.
(Beta-)1

MG01 0.24 to 0.21

1940 ± 30 BP

1970–1820

Needles4653452

MG03 0.26 to 0.23

3460 ± 30 BP

3831–3642

Leaf

465341

3

MG03 − 0.03 to −0.06

3760 ± 30 BP

4237–3994

Needles, Leaves, and Najas spp. & unidentified taxa

483201

4MG03 − 2.63 to −2.66

6000 ± 30 BP

6931–6750

Ruppia maritima

465342

5MG05 1.65 to 1.64

1660 ± 30 BP

1690–1421

Cladium chinense

473445

6MG05 1.24 to 1.21

2340 ± 30 BP

2457–2316

Seeds

5035007MG05 1.15 to 1.14

2740 ± 30 BP

2920–2765

Cladium chinense

4734468MG05 0.64 to 0.63

3340 ± 30 BP

3678–3480

Cladium chinense

4748189MG05 − 0.15 to −0.163680 ± 30 BP

4139–3914

Leaves47436910MG05 − 0.68 to −0.69

4030 ± 30 BP

4572–4422

Needles

473447

11

MG05 − 0.72 to −0.73

4380 ± 30 BP

5040–4863

Najas spp.

474796

12

MG05 − 0.86 to −0.87

4170 ± 30 BP

4831–4585

Leaves47344913

MG05 − 0.92 to −0.934260 ± 30 BP

4870–4711

Leaf

47437014

MG05 − 1.28 to −1.29

4750 ± 30 BP

5586–5331

Zannichellia palustris47345015

MG05 − 1.79 to −1.80

4780 ± 30 BP

5590–5468

Ruppia maritima and Zannichellia palustris473451

16

MG06 0.99 to 0.97

2890 ± 30 BP

3145–2928

Cladium chinense

48164317

MG06 − 0.83 to −0.89

4140 ± 30 BP

4824–4571

Moss stems, Najas spp., Zannichellia palustris and un
identified taxa

48342518

MG06 − 1.76 to −1.79

4470 ± 30 BP

5289–4976

Zannichellia palustris48282819

MG06 − 2.88 to −2.91

5450 ± 30 BP

6301–6205

Najas spp., Zannichellia palustris and unidentified taxa

48320220

MG08 1.06 to 1.03

2610 ± 30 BP

2776–2721

Cladium chinense

46534321

MG08 − 0.64 to −0.67

4020 ± 50 BP

4570–4421

Leaf

46534422

MG08 − 2.00 to −2.03

4740 ± 50 BP

5589–5326

Najas spp., Ruppia maritima,Zannichellia palustris andunidentified taxa

483,427

23

MG10 1.48 to 1.462280 ± 30 BP

2353–2161

Needles503498

24

MG10 1.29 to 1.27

2870 ± 30 BP

3077–2881

Cladium chinense

50349925

MG10 − 0.39 to −0.42

3570 ± 30 BP

3972–3731

Cladium chinense

483203

26

MG10 − 0.75 to −0.78

5010 ± 30 BP

5892–5656

Leaves, Najas spp. and unidentified taxa

483426

27

MG10 − 0.66 to −0.69

5780 ± 30 BP

6657–6500

Zannichellia palustrisRuppia maritima,and unidentified
taxa

48320428

MG13 0.99 to 0.97

2720 ± 30 BP

2868–2761

Cladium chinense

48164429

MG13 − 2.68 to −2.71

4890 ± 30 BP

5662–5587

Najas spp., Zannichellia palustris and unidentified taxa

482824

Ages were calibrated with OxCal version 4.3 (Bronk Ramsey 2017) and ranges at two standard deviations were computed with the calibration data of IntCal13(Reimer et al. 2013)

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Table 3 Depositional ages of event deposits at MG05
Core location Name of event Age (cal yr BP, 1σ range, 68.2%) Age (cal yr BP, 2σ range, 95.4%) Maximum conventional Minimum conventional
age nos. in Table 2
age nos. in Table 2
MG05

E1

2775–2451

2826–2350

7, 8

5, 6

MG05

E2

3813–3577

3954–3514

9, 10

7, 8

MG05

E3

4030–3789

4073–3640

9, 10

7, 8

MG05

E4

4656–4503

4730–4452

12

9, 10

MG05

E5

4841–4738

4853–4657

13

12

MG05

E6

5327–4878

5366–4838

14

13

MG05

E7

5494–5349

5541–5338

15

14

MG05

E8

5542–5430

5581–5388

15

14

MG10

E1

2867–2346

3009–2263

24

23

Radiocarbon ages in Table 1 were constrained by OxCal version 4.3 (Bronk Ramsey 2017) with the calibration data of IntCal13 (Reimer et al. 2013). The age
estimation method is based on Lienkaemper and Bronk Ramsey (2009)

obtained below event deposit E4 represents a limiting
maximum age of 4831–4585 cal yr BP (No. 12 inTable 2), whereas two ages from unidentified leaves andneedles above E4 correspond to limiting minimum agesof 4139–3914 and 4572–4422 cal yr BP (Nos. 9 and 10
in Table 2).
The following subsections describe sedimentary features and diatom assemblages of the nine named
deposits as well as the ages of the events responsible forthe deposits as constrained by radiocarbon dating. Constrained ages are reported with their 2σ range as given
in Table 3.
E1 (event deposit 1)

Event deposit E1 consisted of medium to very coarse
sand (Fig. 4a) and was found at nearly all locations(MG03 to MG13, Figs. 2 and 3). Diatom assemblages

were very sparse and mainly dominated by freshwater
diatom species such as Cymbella proxima, C. tumida,
Pinnularia spp., Stauroneis spp., and Surirella spp. Its
depositional age was constrained at two locations byfour and two radiocarbon ages, respectively (Nos. 5–8 at
MG05 and Nos. 23 and 24 at MG10 in Table 2). The
depositional age was 2826–2350 cal yr BP at MG05 and
3009–2263 cal yr BP at MG10 (Table 3).
E2 (event deposit 2)

Event deposit E2 consisted of poorly sorted medium orvery coarse sand (Fig. 4b) and was found at sites MG03 to
MG06 (Fig. 2). It contained some subangular gravel at
MG05 (Fig. 4b). Benthic diatom assemblages consisted of
freshwater species (e.g., Eunotia spp. and Gomphonema
spp.) and brackish species (e.g., Navicula peregrinopsis),while planktonic diatoms included brackish species such

Fig. 2 Lithostratigraphy of the study site along transect A–A′ in Fig. 1d. Radiocarbon ages are detailed in Table 2. The correlation of event deposits
between core sites is shown by solid lines, with tentative correlations shown by dotted lines

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correlated with E4 at MG03 and MG13. Therefore, thecorrelation of E4 at MG05 with the sand layers at
MG03, MG04, MG12, and MG13 is tentative. A sandcouplet consisting of very fine to medium sand was observed at MG07 and MG08. At MG05, this sand layer had
two subunits, an upper layer of sand 5 cm thick and alower layer of sand 8 cm thick, each with normal gradingfrom very coarse to fine sand (Fig. 4d). Two pieces ofwood were observed just above it. The sand particles were
the same size in the two subunits. Diatom assemblages
had a mixed composition of freshwater (Pinnularia spp.),brackish (Navicula peregrinopsis), and brackish-marine(Diploneis smithii and D. suborbicularis) species. The
depositional age of E4 was constrained by three radiocarbon ages (Nos. 9, 10, and 12 in Table 2) as 4730–4452 cal yr
BP (Table 3).
E5 (event deposit 5)

Fig. 3 Lithostratigraphy of the study site along transect B–B′ in Fig. 1d.
Radiocarbon ages are detailed in Table 2. The correlation of event
deposits between core sites is shown by solid lines, with tentative
correlations shown by dotted lines

as Thalassiosira lacustris. Its depositional age was constrained by four radiocarbon ages (Nos. 7–10 in Table 2)
as 3954–3514 cal yr BP (Table 3).

Event deposit E5 was found at MG05 and MG06 and possibly at seven other locations (MG03, MG04, MG07,
MG08, MG09, MG12, and MG13; Figs. 2 and 3). As with
E4, radiocarbon ages did not constrain the event ages wellenough to correlate with the event age of E5 at MG05,
and multiple event deposits could be correlated with E5 at
MG03, MG09, and MG13. Therefore, the appearance of
E5 at MG03, MG04, MG07, MG08, MG09, MG12, andMG13 is tentative. At MG05, where this unit consisted ofpatchy sand, it did not yield enough material for diatom
analysis. Its depositional age was constrained by two
radiocarbon ages (Nos. 12 and 13 in Table 2) as 4853–
4657 cal yr BP (Table 3).

E3 (event deposit 3)

Event deposit E3 consisted of poorly sorted angulargravel (Fig. 4c) and was found at MG02 and MG04
through MG13 (Figs. 2 and 3). The gravel consisted of
granules and pebbles less than 3 cm in diameter, and thematrix was gray sandy mud. Mud clasts were observedin MG05 (Fig. 4c). The thickness of E3 varied from 4 cm
(MG13) to 74 cm (MG08). It contained more specimensof brackish-marine species (e.g., Thalassiosira lacustrisand Diploneis smithii) than freshwater species. Its depositional age was constrained by four radiocarbon ages
(Nos. 7–10 in Table 2) as 4073–3640 cal yr. BP (Table 3).
E4 (event deposit 4)

Event deposit E4 was a single layer of well-sorted sandat six locations (MG05, MG06, and possibly MG03,
MG04, MG12, and MG13; Figs. 2 and 3). The grain size
ranged from very fine to very coarse sand. At MG03,
MG04, MG12, and MG13, a sand layer was observed
below E3 at the same elevation as E4 at MG05, but
radiocarbon ages did not constrain the event ages wellenough to correlate them with the event age of E4 at
MG05. In addition, multiple event deposits could be

E6 (event deposit 6)

Event deposit E6, composed of poorly sorted sand with
granules, was found at MG05 and possibly at MG06,
MG08, and MG13 (Figs. 2 and 3). At MG06, the sandlayer with a limiting maximum age of 5289–4976 cal yr
BP (No. 18 in Table 2) could be correlated with E6, E7, or
E8. At MG08 and MG13, multiple event deposits could becorrelated with E6. Therefore, the appearance of E6 at
MG06, MG08, and MG13 is tentative. Tomographic images showed that its lower stratigraphic contact was deformed due to bioturbation or the presence of a load cast(Fig. 4e). Diatom assemblages included brackish planktonic species (e.g., Cyclotella atomus var. gracilis andThalassiosira lacustris) and brackish-marine benthic species (e.g., Amphora coffeaeformis and A. pseudoholsatica).
Its depositional age was constrained by two radiocarbon
ages (Nos. 13 and 14 in Table 2) as 5366–4838 cal yr BP(Table 3).
E7 (event deposit 7)

Event deposit E7 was found at MG05 and possibly at
MG06, MG08, and MG13 (Figs. 2 and 3). It had threeShimada et al. Progress in Earth and Planetary Science(2019) 6:33

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D

B

E
C

F

G

Fig. 4 Tomographic images (left), photographs (center), and interpretive sketches (right) of core MG05 showing sedimentary features of event
deposits a E1, b E2, c E3, d E4 and E5, e E6, f E7, and g E8

subunits at MG05, an upper subunit of fine sand 0.6 cm
thick, a middle subunit of sandy organic silt 2.0 cm
thick, and a lower subunit of fine sand with parallel laminae 2.8 cm thick (Fig. 4f ). Diatom assemblages were
dominated by brackish-marine species Fallacia forcipata
and F. tenera. Its depositional age was constrained by
two radiocarbon ages (Nos. 14 and 15 in Table 2) as
5540–5338 cal yr BP (Table 3).

fine to medium sand at MG05 and medium to coarse sandat MG08. It displayed parallel laminae and a sharp lower
stratigraphic contact at MG05 (Fig. 4g). Diatom assemblages were characterized mainly by brackish-marine
species Fallacia forcipata and F. tenera and a brackish
species Navicula peregrinopsis. Its depositional age wasconstrained by two radiocarbon ages (Nos. 14 and 15 inTable 2) as 5581–5388 cal yr BP (Table 3).

E8 (event deposit 8)

E9 (event deposit 9)

Event deposit E8 was found at MG05 and possibly at
MG06, MG08, and MG13 (Figs. 2 and 3). It consisted of

Event deposit E9, a layer of poorly sorted angular pebbles, was found at MG03, MG04, MG05, MG09, MG10,

Shimada et al. Progress in Earth and Planetary Science(2019) 6:33

Page 9 of 15

Fig. 5 Fossil diatom assemblages between − 2.02 and 1.28 m elevation (between 120 and 450 cm depth) in core MG05. F-B means freshwater-brackish

and MG11, but we did not find any correlated gravel
layers at MG06, MG07, or MG08 (Fig. 2). No diatom
analysis was possible in this material. Based on a radiocarbon age above this deposit (No. 15 in Table 2), thisevent is older than 5590–5468 cal yr BP.

Discussion
Paleoecology

Changes in the diatom assemblages below and above theevent deposits, as well as plant macrofossils within theorganic silt, suggest that the depositional environment at

Fig. 6 Plant macrofossil assemblages between 1.18 and 1.58 m elevation in core MG10. Numbers of specimens exceeding 10 appear above the bars

Shimada et al. Progress in Earth and Planetary Science(2019) 6:33

Page 10 of 15

Table 4 Characteristics of event deposits at Mugi Town
Eventcharacteristics

E1

E2

E3

E4

E5

E6

E7

E8

E9

Number oflocations observed

114

11 or 12

4–8

2–9

1–4

1–4

1–4

3–6

Type of sediment

Sand

Sand

Granulepebble

Sand

Sand

Sand

Sand

Sand

Granulepebble

a

No

No

No

Multiplenormalgrading

No

No

No

No

No

Gradinga

Mud clast

No

No

Yes

No

No

No

No

No

No

a

Laminae

No

No

No

No

No

No

Parallel

ParallelNoaLower contact

UnclearUnclearUnclearSlightly
sharp

UnclearSharp (deformed Unclear
by bioturbation
or load cast)

Sharp

Not foundaSubunits

No

No

No

b

No

No

b

Yes (3)

No

No

a

No

Poorly sorted

Angular gravels, Sorted other Patchy
Poorly sorted
than MG05 sandPoorly sorted

No

No

Angular gravels,
poorly sorted

Other
sedimentologicalcharacteristics

Yes (2)aDiatoms
Freshwater Freshwater
Brackish-marine Mixture
Not
Brackish-marine
assemblages within benthic
benthic, brackish
composition analyzed
event deposits
plankton

Brackish- Brackish- Not analyzed
marine marineaThese items are applied to MG05 core
Numbers of subunits are indicated in bracketsbthe study site changed from brackish to freshwater
marsh during the last ~ 5500 years (Fig. 5 and Table 2).
The samples from below ~ 0.6 m elevation containedbrackish planktonic diatoms that are common in brackish water bodies in Japan (Fig. 5), and plant macrofossils
included hydrophytic taxa, suggesting standing water at
the study site. In samples from above ~ 0.6 m, brackish
and marine diatom species were scarce and freshwater
species were dominant, indicating that the environment
had changed to a freshwater marsh. Radiocarbon ages
show that this change occurred just before 3480–
3678 cal yr BP (Fig. 2 and Table 2).
Based on the change in plant macrofossils, it is likelythat after about 3000 cal yr BP, the study site was used
for agriculture. Before the deposition of event deposit
E1, plant macrofossils were dominated by Cladiumchinense, a common species in salt marshes (Matsushita
et al. 2004; Tanabe et al. 2016). However, above E1, it
was replaced by Monochoria plantaginea, a well-known
weed in paddy fields (Takeuchi et al. 1995) (Fig. 6). The
emergence of M. plantaginea means that the study sitechanged to a terrestrial cultivated area after E1 was deposited, although it is impossible to ascertain whether
the deposition of E1 was related to agricultural activity
or merely coincidental with it.
Origin of event deposits

Tsunamis, flash floods, and storm surges were considered as possible causes of the event deposits based on
their fossil assemblages, sedimentary features, andpaleoenvironmental settings. Event deposits E5 and E9lacked sediment for diatom analyses; therefore, their origin was not considered further in this study. However,they were most probably deposited by one of these three
types of events.
Diatom assemblages provide important information toestimate the origin of event deposits (Hemphill-Haley
1996; Sawai et al. 2009b; Szczuciński et al. 2012).
High-energy events such as tsunamis, flash floods, andstorm surges erode and transport sediment from thesurrounding area, producing sediment with a mixture of
allochthonous organisms, then redeposit it into
low-energy environments such as wetlands, lakes, andponds (Sawai et al. 2008). Diatoms in these deposits areuseful organisms for analyses because they have distinct
assemblages that are adapted to freshwater, brackish,and marine environments.
The diatom assemblages within event deposits E3,
E6, E7, and E8 had large proportions of brackish andmarine species (Fig. 5). These included Fallacia forcipata and F. tenera, common in sandy beaches (Sabbe
et al. 1999; Sawai et al. 2009b; Joh 2013), and Diploneis
smithii, common in marine and tidal flats with highsalinity (Hirose et al. 2004; Chiba and Sawai 2014).
Therefore, these four deposits were not emplaced by
sedimentary processes that supply terrestrial material,
such as flash floods and slope collapses. Event deposit
E4 contained a mixture of freshwater, brackish, andmarine species, and deposits E1 and E2 contained
mainly freshwater species.

(2019) 6:33

Page 11 of 15

Sedimentary features in some of the event deposits
were consistent with those in modern tsunami deposits.
For example, the normal grading seen within event deposit E4 is also common in deposits from the 2011
Tohoku tsunami (e.g., Yamada and Fujino 2013; Matsumoto et al. 2016), the 2004 Sumatra tsunami (Choowong
et al. 2008a, 2008b; Naruse et al. 2010), and the 1993
southwest Hokkaido tsunami (Nanayama and Shigeno2006). Parallel laminae like those in deposits E7 and E8
are also documented in the 2011 Tohoku tsunami
deposit (e.g., Abe et al. 2012; Nakamura et al. 2012; Matsumoto et al. 2016). However, some criteria for distinguishing tsunami deposits from deposits produced byother extreme waves are difficult to apply at the study
site. For instance, tsunami deposits tend to extend overwide regions and can be found more than a kilometer
inland, much farther than storm deposits (Goff et al.
2004; Morton et al. 2007). However, the core locationsof the study site were all within a few tens of meters of
the shoreline.
Changes in fossil diatom assemblages between samples
from below and above event deposits have been used as
possible indicators of synchronous seismic coastal
deformation along subduction zones in Washington andOregon (Atwater and Hemphill-Haley 1997; Atwater et
al. 2005; Nelson et al. 2008), Alaska (Shennan and
Hamilton 2006; Hamilton and Shennan 2005) and Hokkaido (Sawai et al. 2004). Coastal deformation suggeststhat the source of the event deposit is not an unusual
storm surge or a distantly generated tsunami, as neither
is associated with deformation. In this study, for example, the relative abundance of the brackish-marine
diatom species Fallacia forcipata and F. tenera increased
above event deposits E4, E6, and E8, and Diploneis
smithii and D. suborbicularis, typically reported as marine diatoms in Japan (Kosugi 1988; Chiba and Sawai2014), increased above deposits E3 and E6. Thesechanges indicate an increased marine influence after
these four events (E3, E4, E6, and E8). Interpreting themas the result of coastal subsidence is consistent with thedeformation associated with historical subduction-zone
earthquakes on the Nankai Trough, like the 1946 CE
Showa-Nankai earthquake (Editing Committee of theHistory of Tokushima Prefecture 1963).
Diatom assemblages may change for reasons otherthan subsidence. For example, breaching and closing
barriers can cause changes in salinity that may affect thecomposition of diatom assemblages. However, the stable
dominance of planktonic diatoms (Thalassiosira lacustris and Cyclotella atomus var. gracilis) (Fig. 5) and submerged plants Najas spp. (Table 2) throughout the coresuggests that an enclosed brackish water environment
persisted between ~ 5500 and ~ 3600 cal yr BP without
interruptions due to breaching and closing of coastal

barriers. As another example, meandering channels
could cause changes in diatom assemblages, but the
sedimentary sequences did not exhibit any cross-stratification or reactivated surfaces of sandy sediment typicallyrecognized in tidal inlets and deltas.
We consider event deposits E3, E6, and E8 most likely
to be tsunami deposits associated with earthquakes along
the Nankai Trough. These three deposits met the criteriaof sedimentary characteristics, a large proportion ofbrackish-marine species within the event deposit, and environmental change after the event. Together with radiocarbon ages from sediments above and below these
deposits, the evidence suggests that earthquakes accompanied by tsunamis occurred at least three times between
5581 and 3640 cal yr BP (Fig. 5 and Tables 2 and 3).
Evidence from deposits E4 and E7 did not strongly
qualify them as tsunami deposits, but did not rule outthat origin. Within E4, freshwater species were moreabundant than brackish-marine species, but some modern
tsunami deposits also contain mainly freshwater species(Tuttle et al. 2004; Szczuciński et al. 2012; Takashimizu etal. 2012). Deposit E7 contained abundant brackish-marine
species, but there was no evidence of environmental
changes after the event. Although E7 may have been the
result of a storm surge or a remote tsunami, tsunamis
caused by events with no coastal deformation cannot be
ruled out (e.g., submarine slides and tsunami earthquakes), nor can events causing coastal deformation too
subtle to lead to a change in diatom assemblages. If E4and E7 are accepted as tsunami deposits, then at least fivetsunamis may have left extensive sand sheets within ourstudy site between 5581 and 3640 cal yr BP.
Based on the archives of local history from Anan and
Hiwasa, storm inundation is unlikely to have resulted in
the event deposits recorded in this study. In the southeastern part of Tokushima prefecture, at least 62 storms(described as “strong wind and rain” or “wind and rain”)
have affected the coastal area between 1626 and 1866
CE (Editing Committee of the History of Anan City
1995). In addition, recent records show that over 100
typhoons have struck the coast of Hiwasa between 1872
and 1955 CE (Editing Committee of the History of
Hiwasa Town 1984). If we assume that such storms havecontinuously occurred over hundreds of years, and that
if they represented the possible origin of the event deposits in our study site, then more event deposits should
exist interbedded with mud layers. We thus conclude
that the five event deposits (E3, E4, E6, E7, E8) are notassociated with century-scale storms.
Event deposit E2 may have been generated by anevent, such as flooding, which supplied mainly terrestrial
material. Consistent with this interpretation are thestrong predominance of freshwater diatoms and theabsence of change in diatom assemblages after the event.

Shimada et al. Progress in Earth and Planetary Science(2019) 6:33

Page 12 of 15

The evidence related to event deposit E1 is inconclusive. Fossil diatom assemblages below, within, and above
E1 consist mainly of fragments of the freshwater genus
Pinnularia (Fig. 5). However, the composition of plant
macrofossils shows that the environment changed from asalt marsh to a freshwater submerged environment (Fig. 6).
The appearance of the paddy field weed Monochoria
plantaginea suggests that the environmental change wasdue to human cultivation. Another possibility is the
non-tectonic regional sea-level change known as the Yayoi
Regression, which affected the Japanese coast between
2000 and 3000 yr BP (e.g., Ariake Bay Research Group
1965; Sakaguchi et al. 1985; Umitsu 1991; Tanabe et al.
2016). This regression may have led to coastal emergenceand desalination at the study site. Although coastal emergence due to coseismic uplift from a subduction-zone
earthquake is conceivable, an uplift would be inconsistent
with the subsidence documented for other prehistoric
events in this study and for the 1946 CE Showa earthquake (Editing Committee of the History of Tokushima
Prefecture 1963). Further study is needed to assess
whether such an anomalous seismic event could haveoccurred in this region.

event in Nankoku City, although either correlation must
be considered tentative given the large errors of theradiocarbon ages. However, if either event in Mugi Town
is correlated with a tsunami event in Nankoku City, itsuggests that a prehistoric tsunami affected a wide area
along the eastern coast of Shikoku Island, rather than amore local tsunami generated by submarine mass movement (Baba et al. 2017). The oldest tsunami deposit inNankoku City was constrained to two possible ages,
5510–4440 and 5980–4870 cal yr BP (Tanigawa et al.
2018), and thus could be correlated with any one of four
event deposits in Mugi Town (E4, E6, E7, and E8).
Fujino et al. (2018) described tsunami deposits on the
Shima Peninsula, to the east of Mugi Town in Mie Prefecture (Fig. 1), that may be correlated with our event
deposit E3, although they presented no environmental
analysis of fossil species. Depositional ages of their sandH were 4040–3730 and 4368–3921 cal yr BP at two different locations, both of which overlap with that of our
E3. In addition to these examples, Okamura and Matsuoka (2012) described sandy event deposits interbedded
within lacustrine sediments in coastal areas elsewhere in
Kochi, Tokushima, and Mie Prefectures. However, theirdepositional ages were not based on published radiocarbon ages, which prevent a detailed correlation of these
event deposits (Garrett et al. 2016).
Paleoseismological evidence other than tsunami deposits has also been reported by previous studies, alongwith radiocarbon dating of marine carbonates. Shishikura et al. (2008) recorded two episodes of coastal uplift in the southern part of the Kii Peninsula (Fig. 1)
around 4500 and 5200 cal yr BP, based on fossil recordsof the sessile polychaete Pomatoleios kraussii. These episodes could be correlated with the events that led tothe deposition of E4 and E6, respectively. Ikehara(1999) reported turbidites and Sakaguchi et al. (2011)
reported mud-breccia units, attributable to seismic
shaking, from the seafloor east of the Kii Peninsula anddated them with radiocarbon ages from foraminifers. A
turbidite (turbidite 10) was bracketed by ages of 4270 ±50 and 4050 ± 50 yr BP (Ikehara 1999). However, ourrecalculation of these ages using OxCal 4.3.2 (Bronk
Ramsey 2017) with Marine13 (Reimer et al. 2013) andthe local marine reservoir value ΔR = − 7 ± 0 (Yoneda etal. 2000) yielded a range of 3909–4513 cal yr BP, which
does not match the ages of tsunami deposits in Mugi
Town. A date from above the mud-breccia units provided a limiting minimum age of 3512 ± 34 cal yr BP(Sakaguchi et al. 2011). However, these correlations are
weak given the indefiniteness of the local marine reservoir effect. Local marine reservoir values are known tovary among sites and materials (Yoneda et al. 2000,
2007; Shishikura et al. 2007; Hirabayashi et al. 2017)
but there is only one published estimate of the local

Shimada et al. Progress in Earth and Planetary Science

Correlation of tsunami deposits with those at other sites

The five tsunami deposits identified in Mugi Town (E3,
E4, E6, E7, and E8) have the potential to be correlatedwith tsunami deposits and other paleoseismologicalevidence recognized in other regions. However, interregional correlations are hampered because of the uncertainty in radiocarbon dating, which usually has an error
greater than 100 years. Tephra layers have sometimes
been used as the basis for regional correlation (e.g.,
Nanayama et al. 2003; Sawai et al. 2012), but there are
no reports of tsunami deposits associated with tephra
layers in this region. Therefore, interregional correlationof tsunami events currently depends only on radiocarbon dating. Given these circumstances, this study allows
possible correlations between our results and the resultsof previous studies that may shed light on the simultaneity and rupture zones of subduction zone earthquakes.
Tsunami deposits contemporary with those found in
this study have been reported in two regions. A coastal
lowland in Nankoku City, about 70 km west of Mugi
Town in Kochi Prefecture (Fig. 1), was inundated by atsunami between 4560 and 3730 cal yr BP (Tanigawa et
al. 2018). This age was constrained by six radiocarbon
dates from plant macrofossils below and above the event
deposit in two core locations and overlaps with the ageof our event deposit E3 (4073–3640 cal yr BP). It also
could possibly be correlated with the age of our event
deposit E4 (4730–4452 cal yr BP), but the probability
distributions of these two ages overlap only slightly.
Thus, E3 or possibly E4 is correlated with the tsunami

Shimada et al. Progress in Earth and Planetary Science(2019) 6:33

reservoir effect (ΔR = − 7 ± 0) along the Nankai Trough(Yoneda et al. 2000).

Conclusions
This study investigated the geological record of prehistoric
tsunamis from Holocene coastal sediments in Mugi
Town. Tsunami deposits associated with earthquakes
along the Nankai Trough were identified from stratigraphic and microfossil evidence, and their depositional
ages were estimated using radiocarbon ages from plant
macrofossils based on Bayesian statistics. We inferred thefollowing findings.
(1) As many as three or possibly five tsunami inundations
between 5581 and 3640 cal yr BP are recorded in thiscoastal lowland in event deposits E3, E4, E6, E7, and E8.
Sedimentary features recognized in all five event deposits
are consistent with modern tsunami deposits. Events E3,
E6, and E8 featured both a predominance of brackish-marine diatom species within the event deposit and evidence ofcontemporaneous environmental changes probably associated with coastal subsidence. Event E4 featured environmental changes like those associated with events E3, E6,
and E8, but had mixed freshwater, brackish, andbrackish-marine diatom species. Event E7 featured abundant brackish-marine diatom species, but no evidence ofenvironmental changes coincident with the event.
(2) Tsunami deposits E3 (4073–3640 cal yr BP), E4(4841–4486 cal yr BP), and E6 (5364–4854 cal yr BP) can
be correlated with tsunamis reported in other regions
along the Nankai Trough. Further geological studies areneeded in other regions to map the distributions of prehistoric tsunami deposits and coastal deformation.
(3) This site lacks evidence of tsunami inundation after
event deposit E1, probably because agricultural activities
continuously disturbed the surface sediments beginningabout 3000 years ago. A full history of tsunami inundations at this locality will require further research inneighboring areas.
Abbreviations
cal yr BP: Calibrated radiocarbon years before the present; CE: Common era;
Mw: Moment magnitude
Acknowledgements
We appreciate the landowners who gave us permission to access their properties
and collect samples. We also thank Yuichi Namegaya for providing information
on historical documents of earthquakes and tsunamis along the Nankai Trough.
The tomographic image analysis was performed under the cooperative research
program of the Center for Advanced Marine Core Research, Kochi University(Accept Nos. 17A008, 17B008). We acknowledge two anonymous reviewers fortheir constructive comments on an earlier version of this manuscript.
Funding
This work was financially supported by the Ministry of Education, Culture,
Sports and Technology of Japan as part of the Research Project for Compound
Disaster Mitigation on the Great Earthquakes and Tsunamis around the Nankai
Trough Region and by Japan Society for the Promotion of Science KAKENHI
grant 16 K17818.

Page 13 of 15

Availability of data and materials
Data sharing is not applicable to this article. Please contact the authors fordata requests.
Authors’ contributions
YSh, SF, YSa, KT, DM, MY, EH, TK, and CC participated in the fieldwork. YSh
took core logs at 12 locations and SF took core logs at one location. YSh
and MSK performed SEM imaging of diatoms. YSa and YSh identified
diatoms and interpreted paleoenvironments. YSh picked out plant
macrofossils for radiocarbon dating, and YSa identified the materials. AM
identified and interpreted plant macrofossils from MG10.KT taught YSh theprocedure to constrain event ages by OxCal and YSh constrained them. YSh
led the writing of the text and preparation of the figures. All authors readand approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.
Author details
1
Graduate School of Life and Environmental Sciences, University of Tsukuba,
1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan. 2Faculty of Life andEnvironmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba,
Ibaraki 305-8572, Japan. 3Geological Survey of Japan, National Institute ofAdvanced Industrial Science and Technology (AIST), 1-1-1-C7 Higashi,
Tsukuba, Ibaraki 305-8567, Japan. 4Graduate School of Horticulture, Chiba
University, 648 Matsudo, Matsudo, Chiba 271-8510, Japan. 5Department of
Geology and Paleontology, National Museum of Nature and Science, 4-1-1
Amakubo, Tsukuba, Ibaraki 305-0005, Japan. 6Earthquake Research Institute,
The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan.
7
School of Biological, Earth and Environmental Sciences, UNSW Sydney,
Sydney, NSW 2052, Australia.
Received: 22 November 2018 Accepted: 13 March 2019

References
Abe F, Sugeno Y, Chigama A (1990) Estimation of the height of the Sanriku
Jogan 11 earthquake-tsunami (A.D. 869) in the Sendai Plain. J Seismol Soc
Japan 43:513–525 https://doi.org/10.4294/zisin1948.43.4_513 (in Japanesewith English abstract). 2nd ser
Abe T, Goto K, Sugawara D (2012) Relationship between the maximum extent oftsunami sand and the inundation limit of the 2011 Tohoku-oki tsunami on
the Sendai plain, Japan. Sediment Geol 282:142–150 https://doi.org/10.1016/j.
sedgeo.2012.05.004
Ariake Bay Research Group (1965) Quaternary system of the Ariake and the
Shiranui Bay areas, with special reference to the Ariake soft clay. Earth Sci 11:
1–86 (in Japanese)
Atwater BF, Hemphill-Haley E (1997) Recurrence intervals for great earthquakes ofthe past 3,500 years at northeastern Willapa Bay, Washington. United States
Government Publishing Office, Washington D.C
Atwater BF, Musumi-Rokkaku S, Satake K, Tsuji Y, Ueda K, Yamaguchi DK (2005)
The orphan tsunami of 1700—Japanese clues to a parent earthquake inNorth America. University of Washington Press, Seattle
Baba T, Okada T, Ashi J, Kanamatsu T (2017) A possible source model of the 1512
Eisho tsunami described in an ancient document, Abstract HDS12–10
presented at the JpGU-AGU Joint Meeting 2017, Makuhari, Japan, pp 20–25
May 2017. https://confit.atlas.jp/guide/event/jpguagu2017/subject/HDS12-10/
detail. Accessed 10 Sept 2018
Bronk Ramsey C (2017) Methods for summarizing radiocarbon datasets.
Radiocarbon 59:1809–1833. https://org/https://doi.org/10.1017/RDC.2017.108
Central Meteorological Observatory (1947) Showa 21 nen 12 gatsu 21 nichi
Nankaido dai jishin chosa gaiho (survey overview of the December 21,
Showa 21 Nankaido earthquake). Central Meteorological Observatory, Tokyo(in Japanese)
Chiba T, Sawai Y (2014) Reexamination and updating of diatom species forpaleoenvironmental reconstructions. Diatom 30:17–30 https://doi.org/10.
11464/diatom.30.17 (in Japanese with English abstract)(2019) 6:33

Page 14 of 15

Choowong M, Murakoshi N, Hisada K, Charoentitirat T, Charusiri P, Phantuwongraj S,
Wongkok P, Choowong A, Subsayjun R, Chutakositkanon V, Jankaew K,
Kanjanapayont P (2008a) Flow conditions of the 2004 Indian Ocean tsunami in
Thailand, inferred from capping bedforms and sedimentary structures. Terra
Nova 20:141–149 https://doi.org/10.1111/j.1365-3121.2008.00799.x
Choowong M, Murakoshi N, Hisada K, Charusiri P, Charoentitirat T, Chutakositkanon V,
Jankaew K, Kanjanapayont P, Phantuwongraj S (2008b) 2004 Indian Ocean
tsunami inflow and outflow at Phuket, Thailand. Mar Geol 248:179–192 https://doi.
org/10.1016/j.margeo.2007.10.011
Editing Committee of the History of Anan City (1995) Anan shishi. Anan City,
Anan City (in Japanese)
Editing Committee of the History of Hiwasa Town (1984) Hiwasa choshi. Hiwasa
Town, Hiwasa Town (in Japanese)
Editing Committee of the History of Mugi Town (1976) Mugi choshi. Mugi Town,
Mugi Town (in Japanese)
Editing Committee of the History of Tokushima Prefecture (1963) Tokushima
Kenshi. Tokushima Prefecture, Tokushima City (in Japanese)
Fujino S, Kimura H, Komatsubara J, Matsumoto D, Namegaya Y, Sawai Y, Shishikura M(2018) Stratigraphic evidence of historical and prehistoric tsunamis on the Pacificcoast of Central Japan: implications for the variable recurrence of tsunamis in the
Nankai trough. Quat Sci Rev 201:147–161 https://doi.org/10.1016/j.quascirev.2018.
09.026
Garrett E, Fujiwara O, Garrett P, Heyvaert VMA, Shihikura M, Yokoyama Y, HubertFerrari A, Brückner H, Nakamura A, De Batist M, the QuakeRecNankai team(2016) A systematic review of geological evidence for Holocene earthquakes
and tsunamis along the Nankai-Suruga trough, Japan. Earth-Sci Rev 159:337–
357 https://doi.org/10.1016/j.earscirev.2016.06.011
Goff J, McFadgen BG, Chagué-Goff C (2004) Sedimentary differences between the2002 Easter storm and the 15th-century Okoropunga tsunami, southeastern
North Island, New Zealand. Mar Geol 204:235–250 https://doi.org/10.1016/S00253227(03)00352-9
Hamilton S, Shennan I (2005) Late Holocene relative sea-level changes and theearthquake deformation cycle around upper cook inlet, Alaska. Quat Sci Rev
24:1479–1498 https://doi.org/10.1016/j.quascirev.2004.11.003
Hemphill-Haley E (1996) Diatoms as an aid in identifying late-Holocene tsunami
deposits. The Holocene 6:439–448 https://doi.org/10.1177/
095968369600600406
Hirabayashi S, Yokoyama Y, Suzuki A, Miyairi Y (2017) Takahiro Aze short-term
fluctuations in regional radiocarbon reservoir age recorded in coral skeletonsfrom the Ryukyu Islands in the North-Western Pacific. J Quatern Res 32:1–6
https://doi.org/10.1002/jqs.2923
Hirose K, Gotoh T, Sato H, Yoshikawa S (2004) Diatoms in surface sediments from
northeastern part of Osaka Bay, southwestern Japan. Diatom 20:229–240 https://doi.org/10.11464/diatom1985.20.0_229
Ikehara K (1999) Recurrence interval of deep-sea turbidites and its importance forpaleoseismicity analysis. J Sediment Soc Japan 49:13–21 https://doi.org/10.
4096/jssj1995.49.13 (in Japanese with English abstract)
Ishibashi K (2014) Nankai Trough kyodai jishin: Rekishi, Kagaku, Syakai (Nankai
Trough great earthquake history, science and society). Iwanami Shoten,
Tokyo (in Japanese)
Jankaew K, Atwater BF, Sawai Y, Choowong M, Charoentitirat T, Martin ME,
Prendergast A (2008) Medieval forewarning of the 2004 Indian Ocean tsunami
in Thailand. Nature 455:1228–1231
Joh G (2013) Species diversity of the old genus Navicula Bory (Bacillariophyta) on
intertidal sand-flats in the Nakdong River estuary, Korea. J Ecol and Environ
36:371–390 https://doi.org/10.5141/ecoenv.2013.371
Kobayasi H, Idei M, Mayama S, Nagumo T, Osada K (2006) H. Kobayasi’s atlas ofJapanese diatoms based on electron microscopy 1. Uchida Rokakuho, Tokyo(in Japanese)
Kosugi M (1988) Classification of living diatom assemblages as the indicator ofenvironments, and its application to reconstruction of paleoenvironments.
The Quaternary Research (Daiyonki-Kenkyu) 27:1–20 https://doi.org/10.4116/
jaqua.27.1 (in Japanese with English abstract)
Krammer K, Lange-Bertalot H (1986) Süßwasserflora von Mitteleuropa.
Bacillariophyceae 1. Teil: Naviculaceae. Gustav Fischer Verlag, Stuttgart
Kumon F (1981) Shimanto Supergroup in the southern part of Tokushima
Prefecture, Southwest Japan. J Geol Soc Japan 87:277–295 https://doi.org/10.
5575/geosoc.87.277 (in Japanese with English abstract). pl 1.
Levkov Z (2009) Amphora sensu lato. In: Lange-Bertalot H (ed) Diatoms ofEurope: diatoms of the European inland waters and comparable habitats.
Volume, vol 5. A. R. G. Gantner Verlag K. G, Ruggell

Lienkaemper JJ, Bronk Ramsey C (2009) OxCal: versatile tool for developing
paleoearthquake chronologies—a primer. Seismol Res Lett 89(3):431–434
https://doi.org/10.1785/gssrl.80.3.431
Matsumoto D, Sawai Y, Tanigawa K, Fujiwara O, Namegaya Y, Shishikura M,
Kagohara K, Kimura H (2016) Tsunami deposit associated with the 2011
Tohoku-oki tsunami in the Hasunuma site of the Kujukuri coastal plain,
Japan. Island Arc 25:369–385 https://doi.org/10.1111/iar.12161
Matsushita M, Sato H, Suzuki S, Yukumoto K, Momohara A, Ueda Y, Katoh S,
Maeda Y (2004) Paleoenvironmental analyses of the buried peat deposit
during the mid-Holocene at the Desaki coast in Tamano City, Okayama
prefecture, Western Japan. Okayama University Earth Science Reports 11:
39–47 (in Japanese with English abstract)
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 Pacificcoast of Northeast Japan. J Nat Disaster Sci 23:83–88
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 https://doi.org/10.1086/629488
Monecke K, Finger W, Klarer D, Kongko W, McAdoo BG, Moore AL, Sudrajat SU(2008) A 1,000-year sediment record of tsunami recurrence in northern
Sumatra. Nature 455:1232–1234
Morton RA, Gelfenbaum G, Jaffe BE (2007) Physical criteria for distinguishingsandy tsunami and storm deposits using modern examples. Sediment Geol
200:184–207 https://doi.org/10.1016/j.sedgeo.2007.01.003
Murakami H, Shimada T, Itoh S, Yamamoto N, Ishizuka J (1996)
Reexamination of the heights of the 1605, 1707 and 1854 Nankai
tsunamis along the coast of Shikoku Island. J Nat Disaster Sci 15:39–52(in Japanese with English abstract)
Nagumo T (1995) Simple and safe cleaning methods for diatom samples. Diatom
10:88 https://doi.org/10.11464/diatom1985.10.0_88 (in Japanese)
Nagumo T, Kobayasi H (1990) The bleaching method for gently loosening
and cleaning a single diatom frustule. Diatom 5:45–50 https://doi.org/10.
11464/diatom1985.5.0_45
Nakamura Y, Nishimura Y, Putra PS (2012) Local variation of inundation,
sedimentary characteristics, and mineral assemblages of the 2011
Tohoku-oki tsunami on the Misawa coast, Aomori, Japan. Sediment Geol
282:216–227 https://doi.org/10.1016/j.sedgeo.2012.06.003
Nakata T, Shimazaki K (1997) Geo-slicer, a newly invented soil sampler, for highresolution active fault studies. J Geogr 106:59–69 https://doi.org/10.5026/
jgeography.106.59 (in Japanese with English abstract)
Nanayama F, Satake K, Furukawa R, Shimokawa K, Atwater BF, Shigeno K,
Yamaki S (2003) Unusually large earthquakes inferred from tsunami
deposits along the Kuril trench. Nature 424:660–663
Nanayama F, Shigeno K (2006) Inflow and outflow facies from the 1993 tsunami
in Southwest Hokkaido. Sediment Geol 187:139–158 https://doi.org/10.1016/j.
sedgeo.2005.12.024
Naruse H, Fujino S, Suphawajruksakul A, Jarupongsakul T (2010) Features andformation processes of multiple deposition layers from the 2004 Indian Ocean
tsunami at ban Nam Kem, southern Thailand. Island Arc 19:399–411 https://doi.
org/10.1111/j.1440-1738.2010.00732.x
Nelson AR, Sawai Y, Jennings AE, Bradley A, Gerson L, Sherrod BL, Sabean J,
Horton BP (2008) Great-earthquake paleogeodesy and tsunamis of the past
2000 years at Alsea Bay, Central Oregon coast, USA. Quat Sci Rev 27:747–768
https://doi.org/10.1016/j.quascirev.2008.01.001
Okamura M, Matsuoka H (2012) Tsunami taisekibutsu kara wakaru Nankai jishin
no kurikaeshi (Nankai earthquake recurrences from tsunami sediment).
Kagaku 82:182–191 (in Japanese)
Okamura M, Matsuoka H, Tsukuda E, Tsuji Y (2000) Kogan kosho taisekibutsu ni
yoru kako ichiman nen no chikaku hendou to rekishi tsunami monitoring(tectonic movements of recent 10000 years and observations of historicaltsunamis based on coastal lake deposits). Chikyu Mon 28:162–168 (inJapanese)
Onishi CT, Kimura G (1995) Change in fabric of melange in the Shimanto Belt,
Japan: change in relative convergence? Tectonics 14:1273–1289 https://doi.
org/10.1029/95TC01929
Patrick RM, Reimer CW (1966) The diatoms of the United States exclusive of
Alaska and Hawaii, vol 1. Academy of Natural Sciences of Philadelphia,
Philadelphia
Patrick RM, Reimer CW (1975) The diatoms of the United States exclusive of
Alaska and Hawaii, vol 2. Academy of Natural Sciences of Philadelphia,
Philadelphia

Shimada et al. Progress in Earth and Planetary Science(2019) 6:33

Page 15 of 15

Reimer PJ, Bard E, Bayliss A, Beck JW, Blackwell PG, Ramsey CB, Buck CE, Cheng H,
Edwards RL, Friedrich M, Grootes PM, Guilderson TP, Haflidason H, Hajdas I,
Hatté 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,
CSM T, JVD PI (2013) Cal13 and Marine13 radiocarbon age calibration curves
0–50,000 years cal BP. Radiocarbon 55:1869–1887 https://doi.org/10.2458/
azu_js_rc.55.16947
Sabbe K, Vyverman W, Muylaert K (1999) New and little-known Fallacia species(Bacillariophyta) from brackish and marine intertidal sandy sediments inNorthwest Europe and North America. Phycologia 38:8–22 https://doi.org/10.
2216/i0031-8884-38-1-8.1
Sakaguchi A, Kimura G, Strasser M, Screaton EJ, Curewitz D, Murayama M (2011)
Episodic seafloor mud brecciation due to great subduction zone
earthquakes. Geology 39:919–922 https://doi.org/10.1130/G32043.1
Sakaguchi Y, Kashima K, Matsubara A (1985) Holocene marine deposits in
Hokkaido and their sedimentary environments. Bull Dept Geogr Univ Tokyo
17:1–17
Sangawa A (2007) Jishin no nihonshi (earthquakes in Japanese history).
Chuokoron-shinsha, Tokyo (in Japanese)
Sawai Y, Fujii Y, Fujiwara O, Kamataki T, Komatsubara J, Okamura Y, Satake K,
Shishikura M (2008) Marine incursions of the past 1500 years and evidence oftsunamis at Suijin-numa, a coastal lake facing the Japan trench. The
Holocene 18:517–528 https://doi.org/10.1177/0959683608089206
Sawai Y, Jankaew K, Martin ME, Prendergast A, Choowong M, Charoentitirat T(2009b) Diatom assemblages in tsunami deposits associated with the 2004
Indian Ocean tsunami at Phra Thong Island, Thailand. Mar Micropaleontol 77:
70–79 https://doi.org/10.1016/j.marmicro.2009.07.003
Sawai Y, Kamataki T, Shishikura M, Nasu H, Okamura Y, Satake K, Thomson KH,
Matsumoto D, Fujii Y, Komatsubara J, Aung TT (2009a) Aperiodic recurrenceof geologically recorded tsunamis during the past 5500 years in eastern
Hokkaido, Japan. J Geophys Res 114:B01319 https://doi.org/10.1029/
2007JB005503
Sawai Y, Namegaya Y, Okamura Y, Satake K, Shishikura M (2012) Challenges ofanticipating the 2011 Tohoku earthquake and tsunami using coastal geology.
Geophys Res Lett 39:L21309 https://doi.org/10.1029/2012GL053692
Sawai Y, Satake K, Kamataki T, Nasu H, Shishikura M, Atwater BF, Horton BP,
Kelsey HM, Nagumo T, Yamaguchi M (2004) Transient uplift after a 17thcentury earthquake along the Kuril subduction zone. Science 306:1918–1920
https://doi.org/10.1126/science.1104895
Shennan I, Hamilton S (2006) Coseismic and pre-seismic subsidence associatedwith great earthquakes in Alaska. Quat Sci Rev 25:1–8 https://doi.org/10.
1016/j.quascirev.2005.09.002
Shibata T, Orihashi Y, Kimura G, Hashimoto Y (2008) Underplating of mélange
evidenced by the depositional ages: U–Pb dating of zircons from the
Shimanto accretionary complex, Southwest Japan. Island Arc 17:376–393
https://doi.org/10.1111/j.1440-1738.2008.00626.x
Shishikura M, Echigo T, Heitaro K (2007) Marine reservoir correction for the Pacificcoast of Central Japan using 14C ages of marine mollusks uplifted duringhistorical earthquakes. Quat Res 67:286–291 https://doi.org/10.1016/j.yqres.
2006.09.003
Shishikura M, Echigo T, Maemoku H, Ishiyama T (2008) Height and ages of
uplifted sessile assemblage distributed along the southern coast of the Kii
peninsula, south-central Japan: reconstruction of multi-segment earthquake
history along the Nankai Trough. Ann Rep Active Fault and Paleoearthquake
Res 8:267–280 (in Japanese with English abstract)
Suyari K, Yamasaki T (1987) Boundary between the north and South Shimanto
subbelts in Tokushima Prefecture. J Sci Coll Gen Educ Univ Tokushima 20:37–
46 Pls 1–9 (in Japanese)
Szczuciński W, Kokociński M, Rzeszewski M, Chagué-Goff C, Cachão M, Goto K,
Sugawara D (2012) Sediment sources and sedimentation processes of 2011
Tohoku-oki tsunami deposits on the Sendai plain, Japan: insights fromdiatoms, nannoliths and grain size distribution. Sediment Geol 282:40–56
https://doi.org/10.1016/j.sedgeo.2012.07.019
Takashimizu Y, Urabe A, Suzuki K, Sato Y (2012) Deposition by the 2011
Tohoku-oki tsunami on coastal lowland controlled by beach ridges near
Sendai, Japan. Sediment Geol 282:124–141 https://doi.org/10.1016/j.
sedgeo.2012.07.004
Takeuchi Y, Sassa T, Kawaguchi S, Ogasawara M, Yoneyama K, Konnai M (1995)
Stimulation of germination of Monochoria vaginalis seeds by seed coatpuncture and cotylenins. J Weed Sci and Technol 40:221–224 https://doi.org/
10.3719/weed.40.221

Tanabe S, Hori K, Momohara A, Nakashima R (2016) Verification of the “Yayoi
regression” in the Tonegawa Lowland, central Japan. J Geol Soc Japan 122:
135–153 (in Japanese with English abstract)
Tanigawa K, Shishikura M, Fujiwara O, Namegaya Y, Matsumoto D (2018) Mid-to
late-Holocene marine inundations inferred from coastal deposits facing the
Nankai Trough in Nankoku, Kochi Prefecture, southern Japan. The Holocene
28:867–878 https://doi.org/10.1177/0959683617752837
Tanioka Y, Satake K (2001) Coseismic slip distribution of the 1946 Nankai
earthquake and aseismic slips caused by the earthquake. Earth Planets andSpace 53:235–241 https://doi.org/10.1186/BF03352380
The Headquarters for Earthquake Research Promotion (2001) Evaluations ofoccurrence potentials or subduction-zone earthquakes. https://www.jishin.go.
jp/main/chousa/kaikou_pdf/nankai.pdf Accessed 1 Feb 2019 (in Japanese)
The Headquarters for Earthquake Research Promotion (2013) Evaluations ofoccurrence potentials or subduction-zone earthquakes. https://www.jishin.go.
jp/main/chousa/kaikou_pdf/nankai_2.pdf Accessed 14 Sept 2018 (inJapanese)
Tuttle M, Ruffman A, Anderson T, Jeter H (2004) Distinguishing tsunami fromstorm deposits in eastern North America: the 1929 grand banks tsunami
versus the 1991 Halloween storm. Seismol Res Lett 75:117–131 https://doi.
org/10.1785/gssrl.75.1.117
Umitsu M (1991) Holocene sea-level changes and coastal evolution in Japan.
Holocene sea-level changes and coastal evolution in Japan, vol 30, pp 187–
196 https://doi.org/10.4116/jaqua.30.187
Usami T, Ishii H, Imamura T, Takemura M, Matsumura R (2013) Materials forcomprehensive list of destructive earthquakes in Japan, 599–2012. Universityof Tokyo Press, Tokyo (in Japanese)
Witkowski A, Lange-Bertalot H, Metzeltin D (2000) Diatom flora of marine coasts I,
Iconographia Diatomologica 7. A.R.G. Gantner Verlag K. G, Ruggell
Yamada M, Fujino S (2013) Sedimentary characteristics of the onshore tsunami
deposits formed by the 2011Tohoku-oki tsunami in coastal lowlands, Ibaraki
and Chiba prefectures. J Sediment Soc Japan 72:13–25 https://doi.org/10.
4096/jssj.72.13 (in Japanese with English abstract)
Yamaguchi A, Shibata T, Ujiie K, Kimura G (2009) Deformation and fluid flow inseismogenic subduction zone: the Mugi Mélange in the Shimanto Belt. J
Geol Soc Japan 115(Supplement):21–36 https://doi.org/10.5575/geosoc.115.
S21 (in Japanese)
Yoneda M, Kitagawa H, Plicht JVD, Uchida M, Tanaka A, Uehiro T, Shibata Y,
Morita M, Ohno T (2000) Pre-bomb marine reservoir ages in the western
North Pacific: preliminary result on Kyoto University collection. Nucl Instr
Meth Phys Res B 172:377–381 https://doi.org/10.1016/S0168-583X(00)00361-X
Yoneda M, Uno H, Shibata Y, Suzuki R, Kumamoto Y, Yoshida K, Sasaki T, Suzuki
A, Kawahata H (2007) Radiocarbon marine reservoir ages in the western
Pacific estimated by pre-bomb molluscan shells. Nucl Instr Meth Phys Res B
259:432–437 https://doi.org/10.1016/j.nimb.2007.01.184

Shimada et al. Progress in Earth and Planetary Science