Kitamura et al. Progress in Earth and Planetary Science (2018) 5:41
https://doi.org/10.1186/s40645-018-0200-y

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open AccessUsing molluscan assemblages from
paleotsunami deposits to evaluate theinfluence of topography on the magnitudeof late Holocene mega-tsunamis on Ishigaki
Island, Japan
Akihisa Kitamura1,2*, Mami Ito3, Ryoya Ikuta1,2 and Masayuki Ikeda1,2

Abstract
Four ancient tsunami deposits were identified in a trench excavated on Ishigaki Island, Okinawa, Japan. Three of thetsunami deposits (T-I, T-II, and T-IV) consist of calcareous sand beds, whereas the other (T-III, located stratigraphicallybetween T-II and T-IV) consists of boulders. Deposit T-I was caused by a tsunami in 1771. 14C dating, together withthe elevations of the landward margins of these sandy tsunami deposits, suggests that tsunamis II and IV were similarin size to the 1771 tsunami, although the influence of local topographic features on the magnitudes of tsunamis has
not yet been examined. This study reconstructs the local topographic features by comparing the molluscan
assemblages incorporated within the tsunami deposits with those in recent beach deposits. The presence of species
that inhabit the intertidal zone in lagoonal settings in all the assemblages indicates that the present-day shallow
lagoon has been present off the study area since the occurrence of tsunami T-IV, which supports the previous
hypothesis that the magnitudes of the 1771 tsunami and tsunamis II and IV were similar. These molluscan assemblages
also suggest that a high relative abundance of large, heavy mollusc shells is a feature of the paleotsunami deposits inthe coastal lowlands found along the shallow coral lagoons.
Keywords: 1771 tsunami, Ancient tsunami deposit, Reef system, Southern Ryukyu Islands, Mollusc

Introduction
Paleotsunami research based on coastal geological records
is a useful approach to the detection of high-magnitude,
low-frequency tsunami events along subduction zones(e.g., Atwater 1987; Minoura et al. 2001; Nanayama et al.
2003; Cisternas et al. 2005; Monecke et al. 2008;
Shishikura et al. 2010; Ishimura and Miyauchi 2015;
Nelson et al. 2015; Kitamura 2016; Inoue et al. 2017).
Although previous paleotsunami research along the
Ryukyu Trench has examined coralline boulders (herein,“tsunami boulders”) (Kawana and Nakata 1994; Goto et al.
* Correspondence: kitamura.akihisa@shizuoka.ac.jp
1
Faculty of Science, Shizuoka University, 836 Ohya Suruga-ku, Shizuoka
422-8529, Japan
2
Center for Integrated Research and Education of Natural hazards, Shizuoka
University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan
Full list of author information is available at the end of the article

2010; Araoka et al. 2013), Ando et al. (2018) recently identified three sandy tsunami deposits (T-I, T-II, and T-IV)
and a layer of buried tsunami boulders (T-III) in a trenchon Ishigaki Island (Figs. 1, 2, and 3). Based on their 14C
ages and the elevations of the landward margins of thesesandy tsunami deposits, the authors estimated that tsunamis II and IV were similar in size to the 1771 tsunami(tsunami deposit T-I), assuming that these paleotsunamis
were generated at the same location and by the samemechanism.
It is widely accepted that on coral reefs, the magnitudeof tsunamis can be strongly influenced by the morphology of the reef crest and beach ridge, both of which reduce the tsunami’s energy. For example, on Taketomi
Island, located just 5 km south of Ishigaki Island, damage from the 1771 tsunami was reportedly slight(Makino 1968) (Fig. 1). Taketomi Island is protected by© The Author(s). 2018 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.

Kitamura et al. Progress in Earth and Planetary Science (2018) 5:41

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Fig. 1 Study location. a Plate boundaries around the southern Ryukyu Islands (after Ando et al. 2009). b Ishigaki Island, showing the location of
the study area (after Goto et al. 2010)

Fig. 2 a Photograph of the trench, showing sampling sites B1–B3. b Photograph of the trench, showing sampling sites B2–B6. c Photograph ofthe coastal area at site B2. The circle is around two persons. d Two Nerita squamulata individuals (arrows) at site B2

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Fig. 3 Columnar sections of the trench. Modified from Kitamura et al. (2018). The inset photograph shows tsunami deposits T-I and T-II

Sekisei Lagoon, which is 10–20 m deep and has formed
within the barrier reefs (Kan and Kawana 2006) between
the southwestern coast of Ishigaki Island and the eastern
coast of Iriomote Island, and includes Taketomi and Kuro
Islands (Machida et al. 2001). It is probable that this large
lagoon greatly reduced the tsunami’s energy before it
reached Taketomi Island (Goto et al. 2010). Therefore, theexamination of local geomorphological features is essential
when evaluating Ando et al.’s (2018) estimate, although, tothe best of our knowledge, there have been no previous
studies of the local geomorphology.
We collected molluscan shells from tsunami deposits
T-I, T-II, and T-IV during the survey reported by Andoet al. (2018), although these specimens have not been
analyzed until now. Here, we examine the molluscan assemblages preserved within these tsunami deposits andalso those present in recent beach deposits, with the aimof reconstructing the local geomorphological features.
Because molluscan species are strongly controlled by thesubstrate type, which is related in turn to the hydrodynamic conditions (e.g., Kondo et al. 1998), the species
compositions provide information on the shallow marine
environments in the study area. Our data provide a
unique opportunity to evaluate the magnitudes of tsunamis T-I, T-II, and T-IV and the utility of biologicalproxies in paleotsunami research.
Previous studies of molluscan assemblages within tsunami
deposits

Several studies have examined molluscan assemblages
within tsunami deposits in coastal lowlands (Morales etal. 2008; Reinhardt et al. 2012; Vött et al. 2011; Goff et
al. 2012; Engel et al. 2016; Mannen et al. 2018) andshallow-water inshore areas (Fujiwara et al. 2000;
Reinhardt et al. 2006; Donato et al. 2008). Many studies
have found that these molluscan assemblages are characterized by a mixture of mollusc shells from a wide rangeof habitats (e.g., Fujiwara et al. 2000; Donato et al. 2008;

Kitamura et al. Progress in Earth and Planetary Science (2018) 5:41

Morales et al. 2008; Richmond et al. 2011). This is also
commonly true of microfossils, such as foraminifera(e.g., Nanayama and Shigeno 2006; Sugawara et al. 2009;
Rubin et al. 2017) and diatoms (e.g., Sawai et al. 2009;
Goff et al. 2012). Kortekaas and Dawson (2007) andDonato et al. (2008) found that molluscan assemblages
are characterized by a high percentage of articulated bivalve shells if the sediment source includes a bivalve
habitat. However, Fujiwara et al. (2014) examined thespecies composition and occurrence of molluscan shells
in an onshore tsunami deposit formed by the 2011
Tohoku-oki tsunami, and reported a low percentage ofarticulated bivalve shells.
Recently, Kitamura et al. (2018) analyzed the oxygenisotope profiles of two articulated marine bivalves fromtsunami deposit T-II in the present study area. Theseshells were tightly closed and empty and yielded thesame calibrated ages of 920–620 cal. years BP, indicatingthat the shells were transported and buried alive by thetsunami (Ando et al. 2018). The isotopic analysis showedthat tsunami deposit II was caused by a tsunami during
the autumn (Kitamura et al. 2018). A combination ofradiometric dating and analysis of the δ18O profiles ofarticulated bivalves can provide important chronologicalconstraints when examining paleotsunami events.
Tsunami deposits from the 2011 Tohoku-oki tsunami
have been examined from many parts of the 900-km-longcoastline between southwestern Hokkaido and the northern Boso Peninsula. No marine mollusc shells were detected in most areas (Szczuciński et al. 2012; Takashimizu
et al. 2012; Kitamura 2016), except in one area on the Sendai Plain (Fujiwara et al. 2014). Sugawara et al. (2014) suggested that the absence of marine fossils can be explained
if the tsunami caused a significant amount of erosion on
the beach and in the coastal forest areas, but the erosion
was minor on the offshore seafloor.
Study area

The Ryukyu Islands are located along the Ryukyu Trench,
which marks the convergent boundary between theEurasian and Philippine Sea plates. They are divided into
three island groups, from north to south: the Amami,
Okinawa, and Sakishima Islands (Fig. 1). According to historical descriptions, the area has experienced nine tsunamis since 1644 AD (Watanabe 1985). The 1771
tsunami was an extremely large event, but historical documents show that it damaged only a restricted area, theSakishima Islands, including Ishigaki Island (Goto et al.
2010). Based on the clast sizes and spatial distributions ofboulders on the reefs of the Ryukyu Islands, Goto et al.
(2013) suggested that large tsunamis have struck locallybut repeatedly around the Sakishima Islands, but notaround the Amami and Okinawa Islands, at least during
the past 2300 years.

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Historical documents show that the coastal areas of
Ishigaki Island, located in the southern Ryukyu Islands,
Japan (Fig. 1), were inundated by a mega-tsunami (the1771 tsunami) at ~ 08:00 (Japan time zone) on 24 April
1771 after a large earthquake. The tsunami had a maximum wave height of ~ 30 m (Iwasaki 1927; Goto et al.
2010) and transported tsunami boulders of up to 10 m indiameter (Makino 1968; Shimajiri 1988; Goto et al. 2010).
The 14C dates of other tsunami boulders from the southern Ryukyu Islands were examined by Kawana and Nakata(1994) and Goto et al. (2010). Araoka et al. (2013) recently
estimated that large tsunamis have occurred at recurrence
intervals of 150–400 years since at least 2400 years ago.
More recently, Ando et al. (2018) identified threesandy tsunami deposits (T-I, T-II, and T-IV) and buried
tsunami boulders (T-III) in a trench extending inlandfrom the seaward-facing slopes behind a beach ridge on
Ishigaki Island. The slopes face a barrier reef lagoon located in the northeastern coastal area of Ishigaki Island.
Based on their 14C ages, deposit T-I was caused by the1771 tsunami and the three older tsunami deposits (T-II,
T-III, and T-IV) formed at 920–620, 1670–1250, and between 2700–2280 and 1670–1250 cal. years BP, respectively (Ando et al. 2018).
Ando et al. (2018) have inferred that the boulders of deposit T-III rest on a stratigraphic horizon lower than, but
projecting above, that of deposit T-II. Because the bases of
the two boulders could not be observed (they were submerged in groundwater), the stratigraphic relationship between these boulders and deposit T-IV is unknown.
However, based on the 14C ages, deposit T-III is younger
and stratigraphically higher than deposit T-IV (Fig. 3).
The elevations of the landward margins of the sandy
tsunami deposits T-I, T-II, and T-IV are up to 9, 6, and
8 m above mean sea level (MSL), respectively (Fig. 3).
Based on the ages of these deposits, the mean uplift rate
(Yokoyama et al. 2016), and the tidal range, Ando et al.
(2018) calculated that the elevations of the land margins
associated with deposits I, II, and IV during their deposition were 7.7–9.9 m, 3.9–6.7 m, and 3.9–8.5 m above
MSL, respectively. The authors concluded that tsunamis
II and IV were similar in size to the 1771 tsunami, assuming that these paleotsunamis were generated at thesame location and by the same mechanism.
The study trench was located on the seaward-facing
slopes behind a beach ridge (Figs. 1, 2, and 3). The beachridge faces a reef lagoon that is protected by a reef crest.
Hongo and Kayanne (2009) analyzed four drill cores(IB-1 to IB-4; Fig. 1) collected from the same lagoon anda reef crest located 4 km northeast of the study area andconcluded that the reef crest had reached sea level andthat sea level had stabilized at or around its present levelby ca. 7000–6000 years BP, with the burial of the lagoon
beginning after that time. On the basis of theKitamura et al. Progress in Earth and Planetary Science (2018) 5:41

stratigraphic distributions of the corals in core IB-3,
Hongo and Kayanne (2010) reported that the relative
sea-level peaked at ~ 3.0 ± 2.5 m above the present-day
MSL at ca. 5000 cal. years BP and gradually decreased
thereafter to the level of the present-day MSL.
The shoreface deposits in the study area consist ofcoarse calcareous sand containing fragments of corals
and molluscs. Many coralline and andesitic boulders arepresent on the shoreface and in the lagoon (Fig. 2c). A
4-m-high beach ridge has developed 30 m landward of
the shoreline at MSL and consists of fine calcareous
sand. Behind the beach ridge, the 150-m-wide lowland
connects to a 7.5% slope (Figs. 2a and 3). The lagoon is~ 1320 m wide and is less than 4.0 m deep (Hongo andKayanne 2009). The mean tidal range in the area around
Ishigaki Island is ~ 2 m (Goto et al. 2010). Typhoons
commonly affect the study area during the summer.

Methods/Experimental
We collected shell specimens by hand from a surface
area of 5 m2 on beach berms (~ 2 m above MSL) at six
sites (B1–B6) during the period 18–20 April, 2014
(Fig. 2a, b). We also collected molluscs from tsunami deposits T-I, T-II, and T-IV from a trench excavated in2014, using two methods. Method 1 involved the sampling of surface exposures in the walls of the trench. The
total sampling area was 43.9 m2 for the T-I deposits,
37.4 m2 for the T-II deposits, and 10.3 m2 for the T-IV
deposits. Method 2 involved sampling the molluscs fromthe > 2 mm fractions in blocks of sediment (1.6–6.5 kg)collected from four sites. Methods 1 and 2 are suitable
for recording rare and large specimens, and small specimens, respectively. The assemblages in tsunami deposits
T-I, T-II, and T-IV are hereafter referred to as assemblages T-I, T-II, and T-IV and were further subdivided
into T-I-1 and T-I-2, T-II-1 and T-II-2, and T-IV-1 andT-IV-2, indicating the use of method 1 or method 2 forsample collection, respectively. The taxonomic identification of molluscs was based on Okutani (2000). For thedisarticulated or fragmented shells that retained thehinge feature (the beak or umbo portion) on the shell, aseparate valve was counted as one individual. Each specimen from assemblages T-I, T-II, and T-IV was weighed,and we also measured the length, height, and width of theshells to the nearest 0.1 mm using digital calipers.
Results
A total of 627 individuals and 53 taxa were identified in
the recent assemblages (Additional file 1: Table S1 andAdditional file 2: Table S2). The dominant species in theassemblages at sites B1 to B5 were the gastropods Nerita(Theliostyla) squamulata and Nerita (Theliostyla) albicillaand the bivalves Barbatia (Abarbatia) lima, Fragum
fragum, and Atactodea striata, whereas the assemblage atPage 5 of 10site B6 was exclusively dominated by A. striata (66.1%)(Fig. 4). The percentage of articulated bivalve shells ranged
from 0% (sites B1, B3, B4, and B5) to 4% (B2; Fig. 4). The
assemblages consisted mainly of species that inhabit
rocky, sandy, or muddy bottoms in the intertidal zone andweighed less than 5 g (Fig. 4). The heaviest specimen was
the bivalve Tridacna maxima, with a shell weight of 90 g.
Many living individuals of N. squamulata and N. albicillawere observed on rocky substrates in the intertidal zone atsite B2 (Fig. 2c, d).
There were differences in the species compositions ofassemblages T-I, T-II, and T-IV collected with the two
sampling methods (Fig. 4; Additional file 3: Table S3,
Additional file 4: Table S4, Additional file 5: Table S5,
Additional file 6: Table S6). The dominant species in assemblage T-I-1 were F. fragum, F. unedo, and A. striata,
whereas the dominant species in assemblage T-I-2 were
F. fragum and F. loochooanum. The dominant species inassemblage T-II-1 were F. fragum, F. unedo, and Mactra
maculate, whereas the dominant species in assemblage
T-II-2 were Epicodakia bella, F. loochooanum, and F.
unedo. The dominant species in assemblage T-IV-1 were
F. fragum, F. unedo, T. maxima, A. striata, and Glycydonta marica, whereas the dominant species in assemblage T-IV-2 were F. loochooanum and F. unedo. The
percentages of articulated bivalve shells were less than
1%, except in assemblage T-II-1 (7%) (Fig. 4).
There were no differences in the relative abundancesof organisms from specific habitat types among assemblages T-I, T-II, and T-IV (Fig. 4). These habitats weremainly the rocky intertidal zone and the sandy or muddy
bottom of the intertidal zone (Fig. 4). The relative abundances of subtidal species were slightly higher in assemblages T-I-1, T-I-2, and T-IV-2 than in the otherassemblages. The shell weight of the dominant specimens collected with method 1 (mainly > 1.0 g) was significantly greater than that of specimens collected withmethod 2 (mainly < 0.5 g) (Fig. 4). We believe that therelatively high abundance of F. loochooanum in the assemblages obtained with method 2 can be explained bytheir smaller shells, and confirm that the combination ofthese two methods is essential to ensure that reliable
fossil data are obtained.
The shell-weight distributions of the specimens collected from the tsunami deposits with methods 1 and 2
are shown in Fig. 5. We observed no systematic landward
trends (such as landward reductions) in shell weight.
The molluscan abundances obtained with method 1(number/m2) are shown for each 10-m-long section of
the trench (Fig. 6). The abundances in deposit T-II weremuch lower than those in deposits T-I and T-IV. We
also observed no systematic landward trends (such as
landward reductions) in abundance. However, the molluscan abundances obtained with method 2 (number/kg)

Kitamura et al. Progress in Earth and Planetary Science (2018) 5:41

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Fig. 4 Species compositions, percentage of articulated bivalve shells, habitat types, and shell weights of molluscan assemblages from recent andtsunami deposits

showed that the abundances in deposit T-II were nearly
the same as those in deposit T-I and were significantly
greater than the abundances in deposit T-IV (Fig. 6).

Discussion
Influence of local topographic features on tsunami
magnitude

As noted above, the species compositions of mollusc assemblages provide information on the shallow marine
environments offshore from the study area. The bivalves
F. fragum, A. striata, and F. unedo were the dominant
species found in the assemblages from tsunami deposits
T-I, T-II, and T-IV and were also abundant in the recent
assemblage (Fig. 4). Both F. fragum and F. unedo areburrowing photosymbiotic bivalves (Ohno et al. 1995).
The bathymetric range of the former is the lower

intertidal zone to a depth of 20 m, whereas that of thelatter is the lower intertidal zone (Okutani 2000). Atactodea striata inhabits sandy substrata in the intertidal
zone of lagoons protected by reef crests (Kondo andYokogawa 2001). The species occurs over a wide depth
range within the intertidal zone, with peak abundancesnear the mean tide level (Takada et al. 2002). On thebasis of the ecology of these species, we conclude thatthe lagoonal conditions seaward of the study area havenot changed significantly since the occurrence of tsunami T-IV, which is consistent with the interpretation of
cored sediments near the study area by Hongo andKayanne (2009). Therefore, differences in the elevationsof the land extent of tsunami deposits I (7.7–9.9 m
above MSL), II (3.9–6.7 m above MSL), and IV(3.9–8.5 m above MSL) during their deposition, which

Kitamura et al. Progress in Earth and Planetary Science (2018) 5:41

Fig. 5 Histograms of shell weights of specimens collected from the tsunami deposits in a trench using methods 1 and 2

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Fig. 6 Molluscan abundances in the tsunami deposits obtained in a trench using methods 1 and 2

were estimated by Ando et al. (2018), were not influenced by the presence or absence of the reef crest. Thisconfirms Ando et al.’s (2018) interpretation that tsunamis II and IV were similar in size to the 1771 tsunami.
The present-day lagoonal conditions have persisted since
the occurrence of tsunami T-IV, allowing tsunami deposits T-I, T-II, and T-IV to be used as analogues formodeling future tsunamis in the study area.

Utility of biological proxies in the study of paleotsunami
deposits

We examined the features of the molluscan assemblages
associated with three sandy tsunami deposits by comparing them with a recent assemblage. As noted above, previous studies have identified two key features of themolluscan assemblages found in tsunami deposits: (1)the mixture of mollusc shells is from a wide range of
habitats; and (2) the percentage of articulated bivalve
shells is high if the sediment source area incorporates abivalve shell habitat. The former feature is inconsistent
with our data, insofar as there were no distinct differences in the patterns of habitat types among assemblages
T-II-1, T-II-2, and T-VI-1 and the recent assemblages.
However, the lagoon off the study area is less than 4 m
deep, so that the environmental heterogeneity in the sourcearea of the molluscan shells may be low. In this situation, it
is likely that a wide range of habitats of molluscan shells is
not a valid index for the molluscan assemblages in paleotsunami deposits. The very low percentage of articulated

bivalve shells in assemblages T-I, T-II, and T-IV (Fig. 4) indicates that the latter feature is not universal.
However, the relative abundance of heavy shells (mainly> 1.0 g) was significantly greater in assemblages T-I-1,
T-II-1, and T-IV-1 than in the recent assemblages (Fig. 4).
Because strong hydrodynamic flows are required to entrain large, heavy shells, we propose that the relative abundance of large, heavy shells in the molluscan assemblagesof paleotsunami deposits is a valid indicator of tsunami
strength in coastal lowlands inside shallow coral lagoons.

Conclusions
A comparison of the molluscan assemblages from threesandy tsunami deposits and from a recent beach deposit
on Ishigaki Island, southwest Japan, showed that theconditions in the reef systems seaward of the study area
have remained unchanged since the occurrence of tsunami T-IV. This confirms that the magnitudes of the1771 tsunami and tsunamis II and IV were similar. Ourdata indicate that the proportion of large, heavy shells in
the molluscan assemblages in paleotsunami deposits is avalid index of tsunami strength on coastal lowlands inside shallow coral reef lagoons.
Additional files
Additional file 1: Table S1. List of molluscan specimens collected atsites B1–B6. R: right valve; L: left valve. (XLSX 15 kb)
Additional file 2: Table S2. List of shell sizes and shell weights ofspecimens collected at sites B1–B6. (XLSX 41 kb)

Kitamura et al. Progress in Earth and Planetary Science (2018) 5:41

Additional file 3: Table S3. List of molluscan specimens collected fromthe tsunami deposits with method 1. (XLSX 19 kb)
Additional file 4: Table S4. List of shell sizes and shell weights ofspecimens collected from the tsunami deposits with method 1. (XLSX 27 kb)
Additional file 5 Table S5. List of molluscan specimens collected fromthe tsunami deposits with method 2. (XLSX 16 kb)
Additional file 6 Table S6. List of shell sizes and shell weights of specimens
collected from the tsunami deposits with method 2. (XLSX 30 kb)

Abbreviations
MSL: Mean sea level
Acknowledgements
We thank Dr. Masataka Ando, Dr. Toshiaki Masuda, Imai Takafumi, andToshiyuki Chinen for our survey on Ishigaki Island. We acknowledge constructive
reviews from anonymous reviewers for improvements. This study was supportedby the Integrated Center for Education and Research of Natural Hazards, Shizuoka
University, and Grants-in-Aid (26287126) awarded by the Japan Society for the
Promotion of Science.
Funding
This study was funded by the Integrated Center for Education and Researchof Natural Hazards, Shizuoka University, and Grants-in-Aid (26287126) awarded
by the Japan Society for the Promotion of Science.
Authors’ contributions
AK conceived designed, collected samples and identified specimens of molluscs
samples. MIt analyzed molluscs samples. RI and MIk collected samples
and examined evaluation of tsunami wave height. All authors read and 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 in publishedmaps and institutional affiliations.
Author details
1
Faculty of Science, Shizuoka University, 836 Ohya Suruga-ku, Shizuoka
422-8529, Japan. 2Center for Integrated Research and Education of Natural
hazards, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan.
3
Graduate School of Science, Shizuoka University, 836 Ohya Suruga-ku,
Shizuoka 422-8529, Japan.
Received: 17 November 2017 Accepted: 27 July 2018

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