Higaki et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-021-00415-w(2021) 8:28

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open AccessThree thousand year paleo-tsunami historyof the southern part of the Japan Trench
Hokuto Higaki1* , Kazuhisa Goto2,3, Hideaki Yanagisawa4, Daisuke Sugawara2,5 and Takashi Ishizawa2

Abstract
This study conducted a field survey and multiproxy analyses on sediment cores retrieved from the Kobatake-ike pond
in Choshi City, Chiba Prefecture, Japan. Kobatake-ike pond is located at a high elevation (i.e., 11 m above present-day
sea level) and faces the southern part of the Japan Trench. Three event sand layers were detected within thecontinuous mud and peat sequences of 3000 years. Based on the multi-proxy analyses, including mineralogicalcomposition, diatom assemblages, and geochemical markers, these sedimentological events were associated with pasttsunamis. The most recent event was a sandy layer and is attributed to the AD 1677 Enpo tsunami, which was reported
by an earlier study conducted in the pond. Our results demonstrated that two older sand layers are associated withlarge tsunamis that struck the Choshi area in AD 896–1445 and in BC 488–AD 215. In addition, the age ranges of these
events seem to overlap that of large earthquakes and tsunamis known from the central part of the Japan Trench. Thisimplies a possible spatiotemporal relation of earthquake generations between the central and southern parts of theJapan Trench. However, since the error ranges of the ages of tsunami deposits at the southern and central parts of theJapan Trench are still large, further investigation is required to clarify the relations of large earthquakes in both areas.
Keywords: Tsunami deposit, Radiocarbon dating, ITRAX, Southern Japan Trench

1 Introduction
The Tohoku-oki earthquake and tsunami on March 11,2011 caused severe damage to the Pacific coast from Tohoku to the Kanto region (Toda 2012). Seismologicalstudies by Simons et al. (2011) and Toda et al. (2011) reported that the strain balance has changed in the southern part of the Japan Trench due to the earthquake. The
studies also indicated the possibility of an increase in therisk of future occurrence of large earthquakes in this region. Therefore, historical records and tsunami deposits
need to be investigated to understand the recurrence
interval and magnitudes of large earthquakes in thisregion and risk assessment.
Tsunami deposits are useful for identifying tsunami
events with more extended time scales, i.e., more thanhundreds of years. Therefore, they are effective for detecting huge tsunami events with prolonged recurrence* Correspondence: hokuto.higaki.p2@dc.tohoku.ac.jp
1
Department of Earth Sciences, Graduate School of Science, Tohoku
University, Sendai 980-8578, Japan
Full list of author information is available at the end of the article

intervals (Goto et al. 2021; Sawai 2012, 2020; Sugawara
et al. 2012). Due to the Tohoku-oki earthquake and tsunami in 2011, considerable effort by scientific communities has been dedicated to elucidating paleo-tsunami
events using tsunami deposits. Moreover, the findings
from the analyses of the deposits have been critical forthe long-term risk assessment of huge tsunami events.
Since the occurrence of Tohoku-oki event in 2011, manystudies have been conducted along the coasts facing thecentral to northern parts of the Japan Trench to reevaluate paleo-tsunami histories (e.g., Goto et al. 2015, 2019;
Inoue et al. 2017; Ishimura and Miyauchi 2015; Ishizawa
et al. 2018, 2019; Kusumoto et al. 2018; Minoura et al.
2013; Sawai et al. 2012, 2015; Takada et al. 2016; Takeda
et al. 2018; Tanigawa et al. 2014a, 2014b; Watanabe
et al. 2014; see also Sawai 2017, 2020). For instance,
Tanigawa et al. (2014b) reported two event layers of thepast 6000 years in a meadow of Misawa City, Shimokita
Peninsula, Aomori Prefecture (Fig. 1). The recurrence
interval of the massive tsunamis in the Sanriku region
was estimated to be 290–390 years (Ishimura and© The Author(s). 2021 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
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Higaki et al. Progress in Earth and Planetary Science(2021) 8:28

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Fig. 1 a Map of Tohoku and Kanto regions showing the location of Choshi City and known paleo-tsunami deposits (red circles) identified byearlier studies. b Digital elevation model showing the topography around the Kobatake-ike pond (data from the Geospatial Information Authorityof Japan). c Detailed digital elevation model showing the topography from the Kimiga-hama beach to the Kobatake-ike pond (same data to (b)).
d Survey points (red circles) in the Kobatake-ike pond. KBT1 to KBT11 are core numbers in this study. A black cross mark shows the location of
the P-7 and P-14 cores sampled by Yanagisawa et al. (2016). Generated by ArcGIS (Environmental Systems Research Institute, Inc.)

Miyauchi 2015), 500–750 years (Takada et al. 2016), or
400–600 years (Inoue et al. 2017). Around the Sendai
and Ishinomaki Plains, near the central part of the Japan
Trench, the paleo-tsunami history for the past 3000
years was obtained, including historically well-known
tsunamis, i.e., the 869 Jogan, the 1611 Keicho, and possibly the 1454 Kyotoku tsunamis (Sawai et al. 2012).
Others (Matsumoto et al. 2013) have reported a prehistoric tsunami occurrence approximately 2000 years ago
in the Sendai and Ishinomaki Plains.
In contrast, known historical and prehistoric tsunami deposits in the coasts facing the southern partof the Japan Trench (i.e., Ibaraki to Chiba Prefectures) are limited because of the modern urbanizationin the coastal regions. Sawai et al. (2012) reported
three event layers from approximately 1000 years ago
at northern Ibaraki in Juo Town (Fig. 1a, Sawai et al.
2012). Yanagisawa et al. (2016) identified the deposits
formed by the 1677 Enpo Boso-oki earthquaketsunami (Mw 8.34–8.63) from historical and geological studies at the Kobatake-ike pond in Choshi
City (Fig. 1a). To date, there is no unified explanationof paleo-tsunami history in the southern part of theJapan Trench.

Higaki et al. Progress in Earth and Planetary Science(2021) 8:28

Historical records show that large tsunamigenic earthquakes occurred previously in the southern part of theJapan Trench. One such event is the Enpo Boso-oki
earthquake and tsunami in 1677, which affected an area
extending over 600 km in Choshi City, with a maximum
run-up height of approximately 13 m (Hatori 2003;
Takeuchi et al. 2007; Tsuji et al. 2012). Although no predecessor of this event is known, a local tradition at theTokai Shrine in the city (Fig. 1b) indicates that theshrine was affected by a tsunami occurring around AD
976, after which the shrine was relocated from Hiyoriyama in Tokawamachi (Fig. 1) to the Takagami area
(Choshi Geopark Promotion Council 2016). The
tsunami-related relocation time is based on a local traditional knowledge rather than written records; therefore,
its reliability is likely lower than other historical tsunamis (e.g., 1677 Enpo Boso-oki tsunami).
The Kobatake-ike pond is a good site for paleotsunami research in this region, considering the discovery of the deposits from the 1677 Enpo Boso-oki tsunami. Coastal lakes and marshes are regarded as suitablelocations for paleo-tsunami research because the sediments are not disturbed by human activities, even in urbanized areas (Sawai 2012). The Kobatake-ike pond is
one such coastal lake located at high elevation (Fig. 1b),thus offering prospects for estimating massive tsunamis’
histories and magnitudes. Furthermore, the local traditional knowledge regarding the possible AD 976 event
also exists. In this study, a geological survey and sedimentological and geochemical analyses of sediments retrieved from the Kobatake-ike pond were conducted toelucidate the paleo-tsunami history in the southern partof the Japan Trench.

2 Study area
Choshi Peninsula, which includes the Kobatake-ike pond,
is located at the Kanto Plain’s eastern edge (Fig. 1a). The
late Pleistocene marine terraces consisting of upper andlower Shimousa surfaces are distributed in the southernpart of the peninsula and surround Mt. Atago-yama,
where a Mesozoic formation is exposed (Kashima 1985).
The Kobatake-ike pond is located at the valley plain, called
the Takagami lowland, which developed along these terraces (Fig. 1b). The Takagami lowland is approximately
500 m wide and 3 km long, with a sharply bent shape(Kashima 1985). The eastern margin of the lowland isclosed by beach ridges. Ota et al. (1985) report that the
Takagami lowland had emerged above sea level by approximately 5000 yr BP and marine terraces were formed.
Subsequently, the Takagami lowland continued to rise to
its present elevation. The average uplift rate is estimatedto be approximately 0.4 m/kyr (Ota et al. 1985). Based on
the diatom analysis results, Kashima et al. (1990) showedthat the marine transgression started at 10,000 yr BP.

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They also reported that the sea level in the Takagami lowland likely reached almost present-day levels at about
7000 yr BP and temporary desalination may have occurred
at 7000–6000 yr BP. At approximately 3000 yr BP, the desalination across the Takagami lowland, was complete.
Since then, no evidence of transgression has been confirmed (Kashima 1985).
The Kobatake-ike pond in Choshi City is located 500
m inland from the shoreline at a high elevation (T.P. 11
m, Fig. 1b). Furthermore, 13–20 m high beach ridges
comprise the eastern side of the coast. No historical or
instrumental records indicate that the pond was inundated by storm surges and waves over the last 400 years,
except for the 1677 event. Hence, neither storm waves
nor small- and medium-scale tsunamis might have inundated the pond (Yanagisawa et al. 2016). Therefore, evidence of seawater invasion in the pond directly indicatesthe occurrence of a past large tsunami similar to the AD
1677 event.
The pond does not receive major river input and thepond water is supplied by a spring of Mt. Atago-yama, located southwest of the pond. This pond is a freshwater environment used as an agricultural reservoir with an area of
0.03 m2 and average water depth of 0.5 m. The present
pond’s outer rim is covered by reeds. There is no high cliffaround the Kobatake-ike pond that supplies sand into thepond (Fig. 1c). Kashima et al. (1990) reported that the
Takagami lowland is covered by 0 to 5m thick peat, which
has been accumulated since ca. 5000 yr BP.
Yanagisawa et al. (2016) identified two sand layers
from cores collected from the pond. The upper sandlayer was identified as the AD 1677 Enpo tsunami deposit; meanwhile the origin and age of the lower sandlayer have been uncertain. Based on tsunami numericalsimulations, they demonstrated that the magnitude of
the AD 1677 earthquake can be estimated at Mw 8.34–
8.63. Their simulation suggested that the rupture zoneof the 1677 earthquake is located near the trench axis,
which is similar to the AD 1896 Sanriku earthquake(Mw 8.3–8.6) that occurred along the northern Japan
Trench (Fig. 1a).

3 Samples and methods
3.1 Field survey

In August 2016, 11 sediment cores were obtained fromthe pond floor (Fig. 1d). To retrieve the cores, a 100 cm
long Russian peat sampler (Jowsey 1966) was used. Aftermeasuring the water depth, we obtained the first core at
a depth of 100 cm from the pond surface. Cores wereobtained side-by-side with at least 20 cm overlap in
depth to guarantee lithostratigraphic correlation. Then,
we obtained samples reaching up to 400 cm depth ateach point to produce a composite lithostratigraphic section. We used the core P-14, which was the longest core

Higaki et al. Progress in Earth and Planetary Science(2021) 8:28

among all cores obtained from the Kobatake-ike pond
and taken by Yanagisawa et al. (2016), for radiocarbon
dating, diatom analysis, and geochemical measurements.
3.2 Laboratory analyses

The upper and lower boundaries of the event deposits
are sometimes unclear to visual observations (e.g., Bondevik et al. 1997). To ascertain appropriate boundaries,
we adopted X-ray computed tomography (CT) scanning,
the brightness of which reflects the density or changes
in the chemical composition of the sample (Boespflug
et al. 1995). The application of CT scanning to tsunami
deposits helps to understand characteristics such as
grain size distribution and sedimentary structures in a
non-destructive way (e.g., Cuven et al. 2013; Falvard
et al. 2018; Falvard and Paris 2017). We used CT scanning for all core samples that we obtained and the P-14
core. Then, we defined the sand layer thickness. Analyses were performed using a CT scanner (LightSpeed
Ultra 16; GE Healthcare Japan Company, and Aquilion
NEW PRIME; Toshiba Medical Systems Corp.) at theCenter for Advanced Marine Core Research at Kochi
University.
To identify the sand source, we examined the mineralogical composition of the sands in the event deposits.
Samples from the event deposits in the Kobatake-ike
pond and the beach sand at Kimiga-hama (Fig. 1b) wereused for examination. Before this analysis, 10% hydrogen
peroxide was added to remove organic material, and the
mud component was removed using a 63 μm sieve.
Then, the samples were dried and observed using a stereo microscope.
Grain size analysis was performed to investigate thegrading trend of the representative event deposit. Sediments were sampled at 1 cm vertical intervals and cut
into halves to use analyses of grain size and sand content. Mud components were removed using 63 μm sieve.
The samples were dried and then 10% hydrogen peroxide was added to remove organic material. Subsequently,
they were immersed in a dispersion solution (0.01 mol/Laqueous solution of sodium diphosphate) and stirred todisperse mud particles. Grain size was then measured
using a laser granulometer (SALD-2300; Shimadzu
Corp.). The results are shown as the volume percentageof the grain size distribution.
Diatom assemblages were investigated to estimate thesource of the event deposits. The presumed 1677 Enpo
tsunami deposit had already been studied by Yanagisawa
et al. (2016), which reported the presence of marine diatoms. Therefore, we focused on the analysis of older two
event deposits. Samples were basically collected at equalintervals above and below the event deposits. In
addition, when the boundary of the event deposit was
unclear, an additional sample was retrieved. Then,

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diatom analysis was commissioned to Paleo Labo Co.,
Ltd., Japan. Each sample was observed under a microscope at a magnification of 600 to 1000. More than 200
diatom fossils were identified and counted. Diatom tests
were categorized into four taxa, i.e., marine, marinebrackish, brackish, and freshwater based on descriptions
reported by Kosugi (1988) and Ando (1990).
Among all cores obtained from the Kobatake-ike pond,
the P-14 core was the longest, and there were preliminary radiocarbon data (Yanagisawa et al. 2016). Therefore,
we used it for additional high-resolution radiocarbon(14C) dating to construct an age–depth model. Due to alack of organic matter, we retrieved seven samples ofseeds and plant fragments from the mud layer in the P14 core. Further, in the peat layer, bulk samples werecollected at 12 horizons of the P-14 core and 1 horizonof the KBT4 core based on the methods described by
Ishizawa et al. (2017). The samples from the KBT4 core
were used for correlation of the cores. Then, 14C dating
was conducted using accelerator mass spectrometry(AMS, Beta Analytic Inc. in Miami, USA) measurements.
The 14C ages were calibrated to the calendar ages using
the OxCal 4.3.2 program (Bronk Ramsey 2009a) and the
IntCal13 dataset (Reimer et al. 2013). For the P-14 core,
we constrained dating results based on stratigraphic
order using a sequence model (Bronk Ramsey 2008) anda general outlier model in the OxCal program (Bronk
Ramsey 2009b). Then, we constructed an age–depth
model. Furthermore, δ13C values of organic materials(Table 2) were measured by the elemental analyzer/isotope ratio mass spectrometer (EA/IRMS).
Geochemical signatures are useful as evidence of inland
seawater invasion for paleo-tsunami research (Chagué-Goff
et al. 2017). High-resolution (1 mm interval) non-destructive
X-ray fluorescence (XRF) analysis was conducted using an
ITRAX core scanner (e.g., Croudace et al. 2006) (ITRAX;
COX Analytical Systems) at the Center for Advanced Marine
Core Research at Kochi University to examine the chemical
characteristics of event deposits. The Mo tube was used tomeasure the scattering ratio (Mo inc/Coh), i.e., a proxy fororganic matter (Guyard et al. 2007). The voltage and current
were set to 30 kV and 55 mA, respectively, and the exposure
time was 10 s. The ITRAX elemental data are semiquantitative and affected by core properties such as minerals,
grain size, and moisture (Croudace et al. 2006). Therefore,
we normalized the measurement values by over total counts(kcps) following the procedures described by Bouchard et al.
(2011) and Judd et al. (2017).

4 Results
4.1 Sedimentary facies and stratigraphy

The lower half of the core was predominantly composedof black peat, which contained several plant fragments(Fig. 2a). Then, a 100–200 cm thick black mud layer

Higaki et al. Progress in Earth and Planetary Science(2021) 8:28

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Fig. 2 a Stratigraphic correlation of the geological column of the cores in a sea-land direction. Satellite photographs show the thickness distributionof the b upper sand layer (event 1) and c lower sand layer (event 3) in this study

covered the peat layer up to the pond floor. Hereafter,
we call the boundary at the mud and peat layers the“mud–peat boundary” because it is expected to indicate
significant environmental change at the pond. At theother coring sites, one to three anomalous sand layers
consisting of fine to medium sand and locally showing
normal grading and inverse grading were interbedded
within the peat and mud layers (Table 1). Contacts between the sand and the mud or peat were visually unclear. The CT images showed either clear or blurboundary (Table 1). It was not possible to identify
whether or not the contacts were erosive, because thematrix of the sand layer was mud and no rip-up-clast
was confirmed in CT images. The thickness of all sandlayers was measured carefully by visual observation and
CT images (see Additional file 1). The sand layer thickness varied among the cores (3–22 cm, Table 1, Fig. 2b,c). Furthermore, KBT3, KBT7, and KBT9 had no sandlayer (see Table 1, Fig. 2b, c). The KBT4 core exhibitedslightly different lithology because there seemed to be
three sand layers (Fig. 2a). The CT image of the KBT1
core showed a blurred whitish layer with a high CT
number above the mud-peat boundary (see Additional
file 1). This suggests that a sandy layer may exist above
the mud-peat boundary, although they are invisible byvisual observation.
Sand particles were detected by microscopic observationof the samples of the whitish layer in KBT1 core (Fig. 3b).
The CT images of other cores showed presence of thewhitish layer above the mud-peat boundary, similar to the
KBT1 core (see Additional file 1). The top and bottom
boundaries of the whitish layer are blurred in the CT images. Then, thickness of the whitish layer was determinedbased on the CT image, using the results of the KBT1 core

Higaki et al. Progress in Earth and Planetary Science(2021) 8:28

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Table 1 Sampling cores and sedimentary characteristics of each sand layerCore

Latitude(N)

Longitude(E)

Sand layer depth
(cm)

Grain
size

Sedimentary structures(visual observation)Basal
contact

Depth of the whitish
layer in the CT images (cm)P-1435.70985

140.85854

80 – 86

m-f 1)

No

Clear

155 – 165

278 – 287

m-f

No

Clear

35.70992

140.85861

77 – 88

m-f

No

Clear

289 – 295

m-f

No

Clear

35.70965

140.85862

78 – 83

m-f

No

Unclear266 – 271

m-f

No

Clear

KBT3

35.70941

140.85886

–

–

–

–

–

KBT4

35.70936

140.85855

113 – 131

P-14
KBT1
KBT1
KBT2
KBT2

53 – 62

m-f

No

Unclear

KBT4

175 – 178

m-f

No

Clear

KBT4

219 – 241m-f

normal grading and inverse
grading

Clear

KBT4

159 – 171
143 – 156KBT5

35.70983

140.85947

75 – 82

m-f

No

Clear

–

KBT6

35.70949

140.85916

286 – 292

m-f

No

Unclear109 – 116KBT7

35.70930

140.85906

–

–

–

–

–

KBT8

35.70954

140.85868

64 – 76

m-f

No

Clear

133 – 145

243 – 251

m-f

No

Unclear35.70977

140.85818

–

–

–

–

KBT8
KBT9

–

KBT10 35.70956

140.85893

221 – 233

m-f

No

Clear

131 – 144

KBT11 35.70935

140.85862

10 – 28

m-f

No

Clear

101 – 113

as a reference, with some degree of uncertainty (Table 1
and Fig. 2a). Furthermore, Fig. 3 suggests that the samples
from the upper and lower sand layers and the middle invisible sand layer (blurred whitish layer in the CT images)
have mineralogical compositions similar to the beach sand(Fig. 3). These sand layers consisted of predominantly
quartz, feldspar, and magnetite. The particles of the sandlayers are well rounded comparing with that of the beachsand and similar size particles are contained in the beachsand (Fig. 3a, b, c).

Fig. 3 Microscopic photo of sands contained in the a upper sandy layer (event 1), b above mud–peat boundary (event 2), c lower sandy layer(event 3), and d the Kimiga-hama beachHigaki et al. Progress in Earth and Planetary Science(2021) 8:28

4.2 Grain size distribution

The basal sample of sand layer (241 cm in depth) in the
KBT4 core shows poor sorting and lower (~30%) sandcontent (Fig. 4). However, the remainder of the sandlayer is generally well sorted with higher (approximately
80–90%) sand content and it consists mainly of fine sand
(2–3 phi). From 241 to 234 cm below surface, the sandlayer showed upward coarsening in grain size. Then, thegrain size distribution of the sand layer changed to the
upward fining trend (Fig. 4). Sand contents increases
synchronously to the upward coarsening trend. The
mud layer above the sand layer (214–219 cm below thesurface) also includes some sands together with silt or
clay size particles (Fig. 4).
4.3 Diatom analysis

First, we measured the diatom assemblage at the mudpeat boundary in the P-14 core because it may indicate acritical local environmental change. Generally, freshwater species were predominant at a core depth of 155
cm, whereas few (<5%) marine species and approximately 18% of marine-brackish species were also observed (Fig. 5). The proportion of marine-brackish andbrackish species was drastically higher immediately
above the mud-peat boundary, while the marine species
proportion decreased (Fig. 5). The total percentage ofmarine and brackish species exceeded 30% (Fig. 5). At
the mud-peat boundary (i.e., 165 cm), the percentages ofPage 7 of 19

marine, marine-brackish, and brackish species were approximately 0.5%, 1%, and 4%, respectively. Below the
mud-peat boundary, the percentage of marine, marinebrackish, and brackish species decreased.
At 272–292 cm, the freshwater species were generally
dominant, shown by the vertical changes in diatom assemblages (Fig. 5). At a core depth of 272 cm, marine,
marine-brackish, and brackish species percentages wereapproximately 1%, 0.5%, and 6%, respectively. In thesand layer located at depths of 278–287 cm in the P-14,
the percentages of marine, marine-brackish, and brackish species were approximately 0.5%, 0.5%, and 3%, respectively (284 cm in depth; Fig. 5). The percentages ofmarine and brackish species were approximately 0.5%
and 2%, respectively, below the sand layer (292 cm indepth; Fig. 5).
4.4 Radiocarbon dating

The results of radiocarbon dating revealed that the P-14
core presents the geological record from BC 1129 (Table 2,
Fig. 6). The age of plant fragments retrieved from the mudlayer, i.e., between the upper sand layer and mud-peatboundary at 86 and 165 cm below the surface, ranged approximately AD 750–1000 (1100–1200 yr BP). Old ages
(1520 and 1750 yr BP) were obtained from seeds and organic sediments in the mud layer. In addition, the age(1690 yr BP) of plant material from peat just below thelower sand layer was younger than the ages of peat above

Fig. 4 Columnar section of KBT4 core: a close-up photograph at 154–254 cm depth, CT images, columnar section, grain size, and sand content
are also shown. E1, E2, and E3 denote event sand layers. In the CT image, white parts denote sand layers with higher density, whereas gray partsdenote mud or peat. White parts represent the apparent direct top of the sand layer (green arrowheads show) in the CT image, indicating thatsands were contaminated into the mud layer. For the grain size diagram, the horizontal axis is grain size (phi). The volume percent (%) of eachgrain size is indicated by the color scale

Higaki et al. Progress in Earth and Planetary Science(2021) 8:28

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Fig. 5 Relative abundance of diatom assemblages (%) near the event horizons classified into four salinity groups: freshwater, freshwater-brackish,brackish-marine, and marine

the sand layer (Fig. 6). Because these reversed ages wereidentified as outliers by a general outlier model in the
OxCal program (Bronk Ramsey 2009b), they were not used
for estimation of the age ranges of the event deposits. The
depositional ages of the sand layer at depths of 80–86 cm,
the mud-peat boundary, and the sandy layer at depths of
278–287 cm could be constrained to AD 951–1906, AD
896–966, and BC 488–AD 215, respectively (2σ, Table 2).
4.5 Geochemical signatures

The elemental profiles of the P-14 and KBT4 cores obtained with the ITRAX core scanner are shown in Fig. 7aand b, respectively. The Mo inc/Coh ratio of the mudlayer was generally lower than that of the underlying peat,suggesting that sediments in the mud layer had a low organic matter (Fig. 7a, b).
In the P-14 core, although each elemental profile exhibited subtle variations with the mud and peat layer,
the Si, K, Fe, and Ti profiles showed a trend of lower
concentration in peaty sediment than in muddy sediment. Ca and Sr’s elemental profiles exhibited a distinct
and sharp increase in counts/kcps around the mud-peatboundary at 165 cm below the surface. Unlike the mud
and peat layers, the lower Mo inc/Coh ratios of the sandlayers at depths of 80–86 cm and 278–287 cm indicate alack of organic matter. In contrast, the Si and K profiles
showed a distinct increase in counts/kcps, and slight Ca
and Sr peaks in counts/kcps occurred in the sand layers.
Although Fe and Ti concentrations showed noremarkable change in the upper sand layer, a distinct increase was apparent in the lower sand layer. However,the Mn profile had few apparent changes in both sandlayers than the mud and peat deposits. The S profile
showed no characteristic peak in counts/kcps at theupper and lower sand layers. However, S and Ca profiles
exhibited a sharp increase in counts/kcps at 275 cm
below the surface, i.e., immediately above the lower sandlayer.
Elemental profiles for Si, K, Ti, Ca, Sr, and S exhibit
subtle variations with the mud and peat layer in the
KBT4 core, as similar to the P-14 core (Fig. 7b). Mn andFe profiles exhibit greater fluctuations than other elements, but also tend to be slightly low with the mud andpeat layer in the KBT4 core (Fig. 7b).
In the KBT4 core, the elements show different profiles
between the two sand layers at depths of 175–178 and
219–241 cm. The Mn, Fe, and Ti profiles exhibit a distinct increase in counts/kcps at 175–178 cm (Fig. 7b). In
addition, minor peaks in counts/kcps of Si, K, and Ca
occur at the same horizon. At depths of 219–241 cm, Si,K, Mn, Ti, Ca, Sr, and S profiles exhibit a distinct
increase, whereas the Fe profile also shows a slight increase in counts/kcps (Fig. 7b). By contrast, the profileof the Mo inc/Coh ratio exhibits a distinct decrease at
depths of 175–178 cm and 219–241 cm. Furthermore,
for Ca and S, peaks in counts/kcps occur around the
mud–peat boundary at 130 cm below the surface as they
do for the P-14 core (Fig. 7b).

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Table 2 Radiocarbon dating results
Core

Depth (cm)Material

Conventional Radiocarbon Age(BP)Calibrated Age (Cal)δ13C (‰)1)

Beta ID

P-14

45-47

plant material

105.6 ± 0.4 pMC

NA

-25.5

457714

P-14

55-57

plant material

130 ± 30

AD 1670–1780-24.4

457715

P-14

65-67

plant material

80 ± 30

AD 1685–1730-24.1

457716

AD 1800–post 1950

AD 1810–1925
AD 1950–
P-14

98-100plant material

1100 ± 30

AD 885–1015-18.3

457717

P-14

108-113

plant material

1110 ± 30

AD 885–995-15.9

452555P-14

125organic sediment

1750 ± 30

AD 230–380-22.6445711

P-14

139-141

plant material

1200 ± 30

AD 720–740-17.8457718P-14

149-151

seeds

1520 ± 30-19.4457719-27.9452556-27.1

457720-27.8457721

AD 765–895
AD 430–490
AD 510–515
AD 530–605
P-14

165-167

organic sediment1140 ± 30

AD 775–790
AD 800–980

P-14

171-173

organic sediment1060 ± 30

AD 900–925
AD 945–1020

P-14

181-183

organic sediment1140 ± 30

AD 775–790
AD 800–980

P-14

200

organic sediment

1280 ± 30

AD 665–775-27.2

445712P-14

250

organic sediment

1620 ± 30

AD 385–475-23.0445713

P-14

258-261

organic sediment

1770 ± 30-22.2452557P-14

271-273

organic sediment

1750 ± 30

AD 230–380-22.1

457722P-14

277.5-278.5

plant material

1910 ± 30

AD 30–40

-27.7

JGR2)

-26.5

JGR2)

AD 485–535
AD 180–190
AD 215–340

AD 50–135
P-14

287-288

plant material

1690 ± 30

AD 255–295
AD 320–415

P-14

292-294

organic sediment2470 ± 30

BC 770–415-23.0452558P-14

310

organic sediment2390 ± 30

BC 540–395-26.5445714

P-14

314-316organic sediment2330 ± 30

BC 410–375-25.6457723

P-14

378-380organic sediment2880 ± 30

BC 1131–973-24.7

478070BC 958–939
BC 1161–1144
BC 1192–1176
P-7

86.5–88.0

plant material

500±30AD 1405–1445

-22.8

JGR2)

KBT4-2132-134

organic sediment

1290 ± 30

AD 664–770-27.04872085 Discussion
5.1 Major environmental change in the Kobatake-ike pond

The lithostratigraphy, CT images and radiocarbon (14C)
results showed that general lithology is consistent among
the cores in the Kobatake-ike pond during the last 3000

years. The cores showed a lithological change from peat
to mud and this is a sole possible major environmental
change in this site. The radiocarbon dating indicatedthat the age of this boundary ranged from AD 896 to
966 (2σ). Formation of mud and peat layers is generally

Higaki et al. Progress in Earth and Planetary Science(2021) 8:28

Page 10 of 19

Fig. 6 Calibrated 14C ages for a probability density distribution. The respective dating results’ probability densities are shown in light gray; the black
ones show probability density distributions constrained using the sequence model. Dating results shown as a red probability density distribution are
not consistent with the stratigraphic order, according to the general outlier model in the OxCal program (Bronk Ramsey 2009b); it was thus eliminatedfrom estimation of the age ranges of the event deposits. Marks (red diamond, black diamond, and dot) show sampling points; red, results reported by
Yanagisawa et al. (2016); black, samples measured for this study; diamond, plant materials; circle, organic sediments. The vertical red line represents theAD 1677 tsunami

regulated by the balance of plant production and decomposition (Sakaguchi 1974). Peat layer is formed mainlyby the deposition of the plant residue that is broken
down imperfectly when the plant production is superior
to decomposition (Sakaguchi 1974). In the current
pond environment, no plants grow in the central part;hence, mud is deposited. Meanwhile, the pond’s outerrim is covered by reeds, although there is no marked
difference in water depth. Hence, there is a chancethat peat is developed in some parts of the present
pond. Therefore, it is likely that a slight change inwater depth might markedly affect mud or peat layer
formation.

The δ13C values of the dating materials retrieved fromthe peat and mud layers provide further evidence for theenvironmental change in the past Kobatake-ike pond. In
the case of aquatic plants in the lake, δ13C values ofapproximately 0 to −10‰ were reported (Watanabe
et al. 2010). These values are higher than the rangeof δ13C values (approximately −20 to −32‰, Schwarz andRedman 1988; Watanabe et al. 2010) reported for terrestrial C3 plants.
At the Kobatake-ike pond, dating materials obtained
from the peat layer and the top of the mud layer had δ13C
values corresponding to the isotopic compositions of terrestrial plants, i.e., approximately −22 to −28‰ (Table 2).

Higaki et al. Progress in Earth and Planetary Science(2021) 8:28

Page 11 of 19

Fig. 7 Normalized ITRAX XRF data of (a) P-14 core (55–380 cm depth) and of (b) KBT4 core (82–254 cm depth). Gray-shaded bars represent
depths of the respective sand layers

However, the plants obtained at 76–151 cm below thesurface showed δ13C values of approximately −15 to−19‰, with slightly heavier isotope compositions thanterrestrial plants (Table 2). These heavy isotope compositions suggest that these plant materials might contain not
only terrestrial but also aquatic plant fossils. This implies
that the pond’s water depth might have increased at
the time of mud-peat boundary, which resulted in achange in the depositional environment from a wetland to a pond and led to the vegetation change fromterrestrial plants to aquatic plants. To increase thepond’s water depth, either an increase in the influx ordecrease in the discharge is required. The following
two hypotheses can be deduced to explain environmental
changes.

1) Hydrological change in the water supply: A factor
to increase the water influx to the pond is aspring at Mt. Atago-yama (Yanagisawa et al. 2016),
although neither geological nor historical data exist
that imply a hydrological change in the spring.
Note that no significant river flows into the Kobatake-ike
pond.
2) Enclosure of the basin: Considering the historicalmaps and the present topography, the Kobatake
River, i.e., flowing out of the pond, might have been
dammed in the past by human activities. This is areasonable assumption, although there is no
historical or geological evidence. In Japan, manysmall storage reservoirs have been developed earlier
than the Edo Period in response to an increase in

Higaki et al. Progress in Earth and Planetary Science(2021) 8:28

the population and demand for agricultural
production (Shirai and Naruse 1983).
In summary, major environmental change from peat
to mud in the Kobatake-ike pond can be explained bysubtle deepening of water depth either by the natural or
artificial processes, although we cannot specify the actual
cause(s) and thus further research is required.
5.2 Identification of event deposits

Three unusual sandy layers were identified in the
Kobatake-ike pond; the upper sand interbedded within
the mud, the middle muddy sediment including sandparticles above the mud–peat boundary (the middle invisible sand layer) and the lower sand interbedded within
the peat, in descending order (Fig. 2). It is noteworthy
that the KBT4 core exhibits a different lithology withthree sand layers and an isolated black mud between thelower two sand layers (Fig. 2). In other cores, only one
sand layer is interbedded within the peat. Interpretationof the lower sand layers in the KBT4 core will be mentioned in the following section. Considering the absenceof terrestrial adjacent source of sands, the presence ofthe sand layers in the pond is unusual. Therefore, we
identified them as event deposits. Although the middle
invisible sand layer was only identified as a concentration of sand particles with blurred top and bottom
boundaries, they were commonly found from many sediment cores (Additional file 1). Therefore, we infer thatan unusual deposition of sand might have occurred here.
Based on the lithostratigraphy, CT images and radiocarbon ages, occurrence of the three sandy layers is consistent among the cores. These horizons were named events
1–3 in descending order. The results of the correlation ofeach event among the cores are shown in Fig. 2.
5.3 Identification of tsunami deposits

The Kobatake-ike pond is located on a hill 11 m above
the present sea level, but one must consider its past elevation and sea level when discussing the source of theevent deposits. In the Choshi Peninsula, even when considering tectonic uplift and sea-level changes, this pond
is regarded as having been located on a hill far from thesea for the last 3000 years (Kashima et al. 1990; Otaet al. 1985). Yanagisawa et al. (2016) reported that no
historical or instrumental records suggest pond inundation by storm surges or waves over the last 400 years.
They also excluded the possibility of storm wave deposition in the pond. Through numerical simulations of the1677 Enpo tsunami in the Kobatake-ike pond, they
revealed that an earthquake with a magnitude (Mw)
greater than 8.3 is necessary for tsunami waves to inundate the Kobatake-ike pond. Therefore, we inferred that
only large-scale tsunamis could inundate the Kobatake-

Page 12 of 19

ike pond. The pond is surrounded by the Late Pleistocene marine terraces, and terrestrial weathering might
have produced some amount of sandy sediments, which
can be a source of the event deposits. Nevertheless, there
is no cliff around the Kobatake-ike pond supplying the
sandy deposits into the pond. Furthermore, it is unlikelythat relatively frequent processes, such as typhoons or
floods, have supplied such weathered sediments to the
Kobatake-ike pond, because the deposition of the sand isquite infrequent; it is only three times during the period
from BC 1129 to AD 1950. Therefore, even if the sourceof part of sands was the Late Pleistocene terraces, theymight not have been eroded unless the infrequent andlarge event such as tsunamis.
The mineralogical composition of the sands is anotherimportant evidence to identify the source of the event deposits. All event deposits consisted of predominantly
quartz, feldspar, and magnetite (Fig. 3) and these minerals
are commonly found in the beach sand (Sudo 2006). Indeed, our microscopic observation showed that quartz,
feldspar, and magnetite are main contents of the present
beach sand of Kimiga-hama (Fig. 3). This suggests that thesource of the event sands is the beach, and it can be considered that a strong flow of seawater eroded the beachsands and transported them into the Kobatake-ike pond.
The grain-size analysis revealed that event 3 (e.g.,
KBT4 core) shows normal grading and inverse grading (see Fig. 4). A similar sedimentary structure istypically reported for deposits formed by recent
massive tsunamis such as the 2011 Tohoku-oki tsunami (Nakamura et al. 2012; Naruse et al. 2012) andthe 2004 Indian Ocean tsunami (Naruse et al. 2010).
Naruse et al. (2010) propose that inverse grading occurs during the waxing stage of oscillatory flows, suchas tsunami run-up or backwash flows; then, thegraded part is formed during the waning stage. Moreover, Sawai (2012) reported that inverse grading canbe interpreted as grain coarsening because of suspended sediment passage with flow acceleration. Furthermore, Moreira et al. (2017) report inverse or
massive sedimentary sequences in the basal section of
AD 1755 Lisbon tsunami deposits. They attributed
these sequences to the dominance of bedloaddominated transport and deposition regarded highflow velocities and high-density sediment loads during
the tsunami inundation (e.g., Jaffe et al. 2012).
Diatom and geochemical signatures are generally used
as one of proxies of seawater inundation for paleotsunami research (e.g., Chagué-Goff 2010; Hadler et al.
2015; Judd et al. 2017; Minoura and Nakaya 1991;
Ramírez-Herrera et al. 2012; Williams et al. 2011). For
example, higher Ca and Sr concentrations are useful asseawater indicators because Ca and Sr concentrations inseawater are generally higher than those in freshwater

(2021) 8:28

Higaki et al. Progress in Earth and Planetary Science(Wedepohl 1971). Moreover, higher sulfur (S) concentrations can be considered as a proxy of paleosalinity,
due to the higher availability of sulfate in seawater thanin freshwater (Casagrande et al. 1977; Chagué-Goff 2010;
Goff et al. 2012).
In this study, however, not all proxies of diatom andgeochemical signatures have been identified in each
event layer (Table 3). In the P-14 core, diatom analysis
showed that marine and brackish species are observed at
event 2 and also a few marine, marine-brackish, andbrackish species are all found in event 3. It is noteworthy
that marine and brackish species were also observed in
the normal sediments below events 2 and 3 (Fig. 5), although diatoms of marine and brackish species are notoriginally produced in the normal sediments in the pond
(Yanagisawa et al. 2016). Geochemical signatures indicating seawater, such as salinity, do not remain in the
porous sand layers and flow down to the bottom soil by
rainwater (e.g., Chagué-Goff et al. 2015). These indicators tend to be preserved in organic or fine sediments(e.g., Chagué-Goff 2010). Likewise, diatoms may migrate
downward in porous sediments. Chiba et al. (2015) demonstrated possible migration of diatoms from a porous
sandy deposit to underlying layer, based on constanthead permeability test. Yanagisawa et al. (2016) observed
marine-brackish species in the muddy sediment immediately below the AD1677 Enpo tsunami deposit, and argued that the downward migration of diatoms might
have occurred. Considering such observations, diatoms
originally included in event 3 might have migrated
downward through the porous peat layer. Furthermore,
the presence of marine-brackish diatoms in the muddy
sediment above event 3 indicates that the horizon must
be included in event 3, since the grain size data and CT
images suggested that the horizon indicated significant
sandy fractions (see Fig. 4).
Elemental profiles of the P-14 core showed higher Ca
and Sr concentrations in events 1 and 2, although no peakof S concentrations was observed (Fig. 7a). However, inthe profile of the KBT4 core, the higher Ca and S clear
peaks in counts/kcps were recognized in the bottom of
event layer 2, but Sr concentrations were not significantly
different from the normal sediments (Fig. 7b). For event 3,
all seawater indicators (Ca, Sr, and S) were found in bothPage 13 of 19

the P-14 and KBT4 cores. However, note that the higher
Ca and S clear peaks in counts/kcps are recognized immediately above event 3 than in sand layer (Fig. 7a). These results show that the mud immediately above event 3 isprobably a mud cap (e.g., Ishizawa et al. 2019). Although
the diatom and geochemical signatures from events 1-3
partially indicated that the Kobatake-ike pond was inundated by seawater, these indicators are not necessarily
consistent among the event layers. Various factors, suchas local variability of initial thickness and preservation
condition of the event layer and the downward migrationof diatom and elements, may be involved in the limited
applicability of the proxies in this case.
Considering the geomorphological, sedimentological,
and mineralogical evidences together with supportive
paleontological and geochemical evidences, we identified the deposits of events 1–3 as tsunami deposits
and the sediment source of them as the beach sandat Kimiga-hama.
5.4 Interpretation of a different lithology (i.e., 175–219cm) in the KBT4 core

The KBT4 core shows slightly different lithology. However, the thin sand layer (i.e., 175–178 cm) and an isolated black mud (i.e., 178–219 cm) were not observed in
the other cores. ITRAX analysis of the KBT4 core reveals that profiles of Ca and Sr (indicators of seawater),and Si and K (rock-forming minerals) exhibit a distinct
increase in this sand layer. Profiles of other elements
also show a similar trend to that of the lower sand layer(i.e., 219–241 cm, Fig. 7).
Three hypotheses for this sand layer can beconsidered.
(1) The sand layer is an independent event from events
1–3. This possibility is nevertheless unlikely because no
event layer that can be compared with this sand layer
was identified in other cores.
(2) The sand layer was formed by the tsunami that deposited the lower sand layer (i.e., 219–241 cm). Tsunami
waves bearing coastal sands invaded into the pond andstirred up muddy sediments to form a thick sand layer(event 3). Subsequently, the deposited sand was raised
again by the wave, which was reflected in the pond edge
and which was deposited again with original pond

Table 3 Proxy check list of event sand layers in the cores (P-14 and KBT4)
Core

Event

marinemarine-brackish

brackish

Ca

Sr

P-14

E1

-

-

-

✓

✓

E2

✓

✓

✓

✓

E3

✓

✓

✓

✓

KBT4

DiatomGeochemical signatures

✓

E2

-

-

-

✓

E3

-

-

-

✓

S

✓
✓

✓

✓

Higaki et al. Progress in Earth and Planetary Science(2021) 8:28

sediment. However, the environmental change in the
Kobatake-ike pond demonstrated by the mud-peatboundary suggests that the pond was a wetland at timeof event 3. Therefore, it is unlikely that tsunami waves
stirred up and redeposited approximately 20 cm thick
mud due to shallow water depth.
(3) KBT4 might have been located at a local topographic low in the wetland, such as a little channel. The
thickness of sand layer (event 3) at the KBT4 and the adjacent core P-21 (Yanagisawa et al. 2016) is muchthicker than that at other cores, suggesting that thicksediments may have been formed in the topographic
low. The black mud is a reworked sediment after event
3. The thin sand layer in the KBT4 core (i.e., 175–178cm), with indicators of seawater, can be considered as areworked sediment which was derived from the sandy
tsunami deposit (i.e., event 3) in the surrounding areas.
Considering the locality of the sites and thickness of thesand layer, this seems much more plausible among thethree hypotheses.
Therefore, we do not treat this thin sand layer in the
KBT4 core as an independent event. We inferred that
this sand layer was formed locally in association to theevent 3.
5.5 Depositional age of event deposits from age-depth
model

Depositional ages of event 1 (i.e., 80–86 cm), event 2(i.e., 155–165 cm; a whitish layer containing minor sandparticles in CT images), and event 3 (i.e., 278–287 cm)
were estimated to be AD 951–1906, AD 899–971, andBC 488–AD 215, respectively based on the age-depth
model (Fig. 6). Yanagisawa et al. (2016) identified thesand layer in the Kobatake-ike pond as the 1677 Enpo
tsunami deposit based on the multiple analyses oftephra, microfossil and historical documents, and radiocarbon ages of AD 1405–1445 (2σ) obtained below thelayer (Yanagisawa et al. 2016). Our results are consistent
with their interpretation.
The age of event 2 includes a large uncertainty. It
might be readily apparent from the age–depth model(Fig. 6) that the sedimentation rate changes suddenly at
the mud-peat boundary. The rate below the boundary isapproximately 1 m/kyr which is a common sedimentation rate in a wetland (Sakaguchi 1974), while above theboundary is 2.7 m/kyr. If this sedimentation rate is reflective of actual conditions, the deposition of the mud(approximately 80 cm) between events 1 and the mudpeat boundary occurred within approximately 100 years(Fig. 6). Furthermore, a large time gap separates thelayers below and above that mark the 1677 tsunami deposit (event 1). As reported for the 2011 Tohoku-oki
tsunami, a tsunami can erode the original sediment in
the coastal pond. Such significant erosion can result in a

Page 14 of 19

large time gap below the tsunami deposit (Shinozaki
et al. 2015). However, approximately 4 m thick sedimentis expected to have been eroded by the tsunami associated with event 1 (=1677 Enpo tsunami) if we consider
the sedimentation rate between events 1 and the mudpeat boundary. Such significant erosion is unlikely tohave occurred at the high elevation of the Kobatake-ike
pond, together with a historical account of the 1677
Enpo tsunami. In addition, the water depth of the pondis small, and it cannot accommodate such a thick sediment. Yanagisawa et al. (2016) reported that radiocarbon
dating of plant materials obtained from the muddy sediment below event 1 (core P-7; Yanagisawa et al. 2016)
ranged from AD 1405 to 1445. Therefore, erosion of thepond floor sediment by the 1677 tsunami should havebeen limited. Alternatively, the following two hypotheses
can be regarded as explaining this change in sedimentation rate and time gap beneath event 1.
5.5.1 Tsunami-induced sediment mixing related to event 2

The possibility exists that the age–depth model was affected by a mechanical depositional process to mix thesoft muddy sediment. Two possible depositional processes can be considered such as tsunami inundationand human activity. However, there is no historical evidence suggesting that the pond sediments have been disturbed by human activity. Therefore, this section
explains the possible depositional process due to the tsunami inundation. Shinozaki et al. (2015) reported for
Suijin-numa pond in Sendai Plain that the original
muddy sediment was eroded and suspended by the
Tohoku-oki tsunami to form an approximately 50 cm
thick reworked mud layer above sandy tsunami deposits.
According to radiocarbon dating of the reworked mud
layer, the ages obtained from the mud layer were almost
homogenized. They reported that the muddy tsunami
deposits contained a mixture of organic materials of
various ages during the last 1100 years due to tsunami
erosion. Such homogenized ages in the mud layer of apond are similar to our dating results obtained between
events 1 and the mud-peat boundary (Fig. 6). In
addition, our study revealed that the Kobatake-ike pond
was inundated by the tsunami at event 2 immediately
above the mud-peat boundary. Thus, erosion and deposition of reworked mud might have occurred due to thetsunami at event 2. If tsunami-induced reworking of the
mud has taken place, the mud layer above the boundary
represents an age older than the actual tsunami age. If
this model is correct, it is difficult to restrict the limiting
minimum age of event 2 using homogenized radiocarbon dating results, although the limiting maximum ageof AD 896–954 is unaffected because the age was obtained from the hard peat layer below the mud-peatboundary, which can be regarded as having not been

Higaki et al. Progress in Earth and Planetary Science(2021) 8:28

reworked. The homogenized mud should have been terminated below event 1 (1677 Enpo tsunami deposit) because radiocarbon age of plant materials obtained fromthe mud below event 1 was AD 1405-1445 (Yanagisawa
et al. 2016), which is much younger than the homogenized age. The radiocarbon dating result can also be
used as a reliable limiting minimum age of event 2 andalso as a limiting maximum age of event 1.
5.5.2 Reservoir effect

The dissolved inorganic carbon in water used for photosynthesis by aquatic plants has a lower 14C concentration than in the atmosphere because of seawater andfreshwater reservoir effects. Therefore, the radiocarbon
dating values of aquatic plant fossils are known to tend
to be older than the actual age (Kilian et al. 1995; Watanabe et al. 2010). For example, Kilian et al. (1995) performed radiocarbon dating using aquatic plants in raised
bog deposits. They reported that these results werestrongly affected by local reservoir effects and reported
dates hundreds of years older than the actual age. At the
Kobatake-ike pond, the δ13C values of dating materials
obtained from the mud layer between events 1 and the
mud-peat boundary show slightly heavier isotope compositions than those of terrestrial plants (Table 2). These heavy
isotope compositions suggest that these plant materials
might contain aquatic plant fossils not only terrestrial plants.
If this is the case, the radiocarbon ages obtained from the
mud layer between events 1 and the mud-peat boundary
might be affected by local reservoir effects and might
represent hundreds of years older than the actual age.
In either case, the age of event 2 cannot be sufficiently
constrained. Considering the uncertainty in radiocarbon
dating, the tsunami of event 2 might have occurred during the 10th to fifteenth centuries (AD 896–1445),
although no written record is known for such a tsunami
event. The relocation of the shrine at AD 976 (Choshi
Geopark Promotion Council 2016) can be related to thetsunami; however, the age of the relocation is based only
on the local traditional knowledge, for which the reliability has not yet been evaluated. Further investigation isneeded to constrain the age of the tsunami event
between the tenth and fifteenth centuries.
As for event 3, similar to the Suijin-numa case (Shinozaki et al. 2015), the tsunami-induced sediment mixing
might only occur in the softer mud layer: it might notoccur in the peat layer because they possess a higher
erosion threshold than mud (Jacobs et al. 2011; Otsubo
and Muraoka 1988; Tuukkanen et al. 2014). Homogenized dating results cannot be ascertained above andbelow event 3 (Fig. 6), which is intercalated within theharder peat layer. Therefore, it is unlikely that thetsunami-induced reworking affected the radiocarbon
ages of event 3 (BC 488–AD 215).

Page 15 of 19

5.6 Paleotsunami history along the southern and central
part of Japan Trench

Our results demonstrated that three massive tsunamis
struck the studied area during the past 3000 years: AD
1677, AD 896–1445, and BC 488–AD 215. Although age
restrictions of tsunami events in the southern part of theJapan Trench are poor, it is noteworthy that the timing of the paleo-tsunami events is overlap to the agesof known paleotsunamis in the central part of theJapan Trench, and hence there may be a possible
spatiotemporal relation of earthquake generations
between the central and southern parts of the Japan
Trench (Fig. 8).
Sawai et al. (2012) conducted a comprehensive survey
along the Pacific coast from Miyagi Prefecture to northern Ibaraki Prefecture. They identified five large tsunami
events based on historical and geological evidence andinferred that they were formed by the AD 1611 Keicho,
AD 1454 Kyotoku, and AD 869 Jogan tsunamis andolder tsunamis at AD 400–500 and BC 500–400. Saino(2012) and Matsumoto et al. (2013) reported that a tsunami struck the Sendai Plain around 2050 yr BP, which
was not reported by Sawai et al. (2012) (see also discussion by Sawai 2020).
The AD 1677 Enpo tsunami occurred in the southernpart of the Japan Trench approximately 60 years after
the AD 1611 Keicho tsunami (Fig. 8). The evidence forseawater inundation in event 2 suggests that a large tsunamigenic earthquake struck the Choshi region duringtenth–fifteenth centuries (AD 896–1445) and the possible known tsunami around AD 976 at Choshi City,
which seems close to the AD 869 Jogan tsunami, is one
candidate. In addition, since the age cannot be constrained well, the age range of event 2 is also close to theAD 1454 Kyotoku tsunami (Sawai et al. 2015). The
radiocarbon ages showed that event 3 was deposited
between BC 488 and AD 215. The age overlaps either tothe event in 2050 yr BP (Matsumoto et al. 2013; Saino2012) and in BC 500–400 (Sawai et al. 2012) along thecentral part of the Japan Trench. However, age ranges of
events 2 and 3 cannot be sufficiently constrained. Therefore, further age constraint for the events 2 and 3 isrequired.
No visible event deposit that can overlap to the AD
400–500 tsunami event reported in the central part of
the Japan Trench (Sawai et al. 2012) was identified in
the cores obtained from the Kobatake-ike pond (Fig. 8).
The ITRAX result showed no geochemical evidence
indicating seawater invasion and even suggests thatthe event without a sandy deposit is less likely toexist (Fig. 7). Therefore, no massive tsunami was generated during this period in the southern part of theJapan Trench. Alternative explanations are that thetsunami could not reach the elevation of theHigaki et al. Progress in Earth and Planetary Science(2021) 8:28

Page 16 of 19

Fig. 8 Paleotsunami histories in the Pacific coasts from Miyagi to Chiba Prefecture, which faces the central and southern parts of Japan Trench

Kobatake-ike pond (11 m) or the tsunami reached but
the deposits were eroded by later tsunamis. However,
the CT images and ITRAX profiles did not provide
useful information to assess whether or not the event
deposits had been eroded.
The discussion presented above related to the correlationof paleo-tsunami histories between the central and southern parts of the Japan Trench might indicate important
spatiotemporal relations of large earthquakes in these areas.
However, since the error ranges of the ages of both tsunami
deposits are large, further investigation is required to clarify
the relations of large earthquakes in both areas.

6 Conclusions
We identified three tsunami events occurring during thepast 3000 years from sediment cores obtained at the
Kobatake-ike pond. The uppermost tsunami deposit canbe correlated with the AD 1677 Enpo tsunami. Radiocarbon dating results further indicated that the older tsunamis might have occurred at AD 896–1445 and BC
488–AD 215. Additionally, the results show that the
paleo-tsunami histories in the central and southern partsof the Japan Trench may occur in a relatively short-time
interval, although the error ranges of both tsunami deposits at the southern and central parts of the Japan
Trench are large. This result implies the possibility thata spatiotemporal relation of large earthquake generations(strain accumulation and release) might exist between
the central and southern parts of the Japan Trench
areas. This possibility should be considered in tsunami
hazard assessments.
Abbreviations
yr BP: Years before present

Higaki et al. Progress in Earth and Planetary Science(2021) 8:28

7 Supplementary Information

Page 17 of 19

Received: 30 July 2020 Accepted: 3 March 2021

The online version contains supplementary material available at https://doi.
org/10.1186/s40645-021-00415-w.
Additional file 1: Figure S1. CT image of Core KBT1 visualized by 3D
Slicer, a free and open-source platform for analyzing and understanding
medical image data. Figure S2. CT image of Core KBT2 visualized by 3D
Slicer, a free and open-source platform for analyzing and understanding
medical image data. Figure S3. CT image of Core KBT4 visualized by 3D
Slicer, a free and open-source platform for analyzing and understanding
medical image data. Domain outlined by the white rectangle shows thatthe sampling area for grain size analyses (214–241 cm depth). Figure S4.
CT image of Core KBT6 visualized by 3D Slicer, a free and open-source
platform for analyzing and understanding medical image data. Figure
S5. CT image of Core KBT8 visualized by 3D Slicer, a free and opensource platform for analyzing and understanding medical image data.
Figure S6. CT image of Core KBT10 visualized by 3D Slicer, a free andopen-source platform for analyzing and understanding medical image
data. Figure S7. CT image of Core KBT11 visualized by 3D Slicer, a free
and open-source platform for analyzing and understanding medical
image data. Figure S8. CT image of Core P-14 visualized by 3D Slicer, afree and open-source platform for analyzing and understanding medical
image data. Figure S9. Grain size distribution of Core KBT4 (214–217 cm
depth). Figure S10. Grain size distribution of Core KBT4 (217–229 cm
depth). Figure S11. Grain size distribution of Core KBT4 (229–241 cm
depth). Table S1. Result of diatom assemblage analysis

Acknowledgements
We appreciate the landowner for giving us permission to perform thesurveys. CT imaging and non-destructive XRF analyses were conducted at
the Center for Advanced Marine Core Research at Kochi University (accept
no. 16A008, 16B008, 17A005, and 17B005). Dr. K. Ikehara, Dr. P. J. M. Costa,
and an anonymous reviewer provided valuable comments that improved
the manuscript. We thank all of these people for their support.
Authors’ contributions
HH, KG, HY, and DS conducted fieldwork and collected samples. KG proposed
the study topic and conceived and designed the study. HH and TI picked out
organic materials for radiocarbon dating and constrained event ages by OxCal.
HH led the writing of the text and preparation of the figures. All authors
participated in discussions during the research, later reading, and approval of
the final manuscript.
Funding
This research was supported by research funding from JSPS KAKENHI Grant
Number JP17H02971 and by FY2020 grant-in-aid for joint research from IRIDeS, Tohoku University, and was supported as a commissioned project of
the Nuclear Regulation Authority.
Availability of data and materials
Please contact author for data requests.

Declarations
Competing interests
The authors declare that they have no competing interests.
Author details
1
Department of Earth Sciences, Graduate School of Science, Tohoku
University, Sendai 980-8578, Japan. 2International Research Institute ofDisaster Science, Tohoku University, 468-1 Aza-Aoba, Aramaki, Aoba-ku,
Sendai 980-0845, Japan. 3Department of Earth and Planetary Science, The
University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
4
Department of Regional Management, Tohoku Gakuin University, 1-3-1
Tsuchitoi, Aoba-ku, Sendai 980-8511, Japan. 5Museum of Natural andEnvironmental History, Shizuoka 5762 Oya, Suruga-ku, Shizuoka, Shizuoka
Prefecture 422-8017, Japan.

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