MIS25-01

Room:201B

Time:May 24 09:00-09:15

The foraminifera assemblages in tsunami sediments on Ishigaki Island, southwesternJapan
TU, Yoko1∗ ; CHIEN, Chih-wei2 ; ANDO, Masataka3 ; KITAMURA, Akihisa4 ; SHISHIKURA, Masanobu5 ;NAKAMURA, Mamoru6
1

Department of Natural History Sciences, Hokkaido University, Japan, 2 Department of Earth Sciences, National Central University, Taiwan, 3 Center of Integrated Research and Education of Natural Hazards, Shizuoka University, 4 Institute of Geosciences,
Faculty of Science, Shizuoka University, 5 Institute of Earthquake and Volcano Geology, National Institute of Advanced Industrial Science and T, 6 Faculty of Science, University of the Ryukyus
Foraminifera, single celled protists, are widely applied to paleoenvironmental, paleooceanic, stratigraphic and sedimentological investigations due to their abundant populations. Moreover, they are sensitive to living environment and have high preservation potential within the sediments (Mamo et. al, 2009). Although foraminifera assemblage analysis is remarkable tools forgeoscience researchers to understand the variation of paleoenvironment, however, the use of foraminifera assemblage has notbeen developed well in palotsunami research. Up to the present, not only investigation methods but also analytic procedures havenot been established.
In 1771, a destructive tsunami struck Ishigaki Island, southwestern Japan, with the maximum run-up height of 30 meters tocause about 9000 fatalities (half populations of Ishigaki Island), and the mortality rates of 25 villages on the eastern and southerncoasts reached 10- 90%. For the mechanism of this tsunami, Nakamura (2009) proposed the source should be a large subduction
thrust earthquake that occurred near Ryukyu trench axis, while Goto et al. (2010) suggested a model of extensive submarine
landslides triggered by an intra-plate earthquake. Furthermore, Ando et al. (2015) identified four tsunami evens from tsunami
sediments for the last 2000-2500 years.
In this study, we tried to identify tsunami events based on foraminifera assemblage in soil deposits. In order to approach
this aim, we analyzed forty-seven soil samples obtained from five excavation sites on both eastern and western coasts in thisisland. A total of 117 foraminifera species from 36 genus were recognized through our analysis. Subsequently, we classified
all foraminifera tests into three clusters based on the result of Hatta and Ujiie (1992): shallow-water foraminifera (<15m),
inter-mediate depth foraminifera (15m to 50m) and deep-water foraminifera (>50m). After this procedure, we took the ratio
of individual numbers in deep-water + inter mediate-depth to total amount for each soil sample. This analysis yields that samples with high ratios (>0.2) were identified as tsunami deposits, conversely those with low ratios (<0.15) as beach sand or
terrigenous sediments. This suggests that foraminifera assemblage analysis can provide significant tool to identify paleotsunami
deposits from other sedimentary processes. Our analysis of foraminifera assemblages provides at least three tsunami events in
1771, AD 1000 to 1300, and BC 300 to 600 that struck Ishigaki Island.

Keywords: foraminifera assemblages, tsunami sediments, Ishigaki Island, 1771 tsunami, Ryukyu subduction zone

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MIS25-02

Room:201B

Time:May 24 09:15-09:30

Evaluating the magnitude of late Holocene mega-tsunamis on Ishigaki Island based on an
analysis of molluscan assemblages
KITAMURA, Akihisa1∗ ; ITO, Mami2 ; ANDO, Masataka3 ; IKUTA, Ryoya1 ; IKEDA, Masayuki1 ;IMAI, Takafumi2
1

Faculty of Science, Shizuoka University, 2 Graduate School of Science, Shizuoka University, 3 Center for Integrated Researchand Education of Natural hazards, Shizuoka University
Four ancient tsunami deposits have been identified on Ishigaki Island, Okinawa, Japan. Three tsunami deposits (T-I, T-II,
and T-IV) are calcareous sand beds which have sharp erosional bases and normal graded structure, while one tsunami deposit(T-III) is buried tsunami boulders between tsunami deposits T-II and T-IV. The youngest tsunami deposit T-I, was caused by theAD 1771 Meiwa tsunami, which had a wave height of at least 10 m in the study area. The depositional ages of the three older
tsunami deposits (T-II, T-III, and T-IV) are 790-610 cal. yrs BP, 1494-1258 cal. yrs BP and 2502-2287 to 1494-1258 cal. yrs
BP, respectively. The elevations of the landward margins of sandy tsunami deposits T-I, T-II, and T-IV are up to 9, 6, and 8 m,
respectively. This study examines the influence of the local topography, such as beach ridge and reef system, on the runup of theseancient tsunamis based on a comparison of molluscan assemblages in the tsunami deposits T-I, T-II, and T-IV and those found inrecent beach deposits. The results show that the conditions in the reef system off the study area have remained largely unchanged
since the occurrence of tsunami T-IV. Beach ridge during the occurrence of tsunami T-IV did not become well developed relative
to those during the occurrence of tsunami T-I and T-II. These findings suggest that the magnitude of tsunami T-II were smaller
than those of tsunamis T-I and T-IV, while the magnitude of tsunami T-IV did not reach that of tsunami T-I (AD 1771 Meiwa
tsunami).
Keywords: large tsunami, Ishigaki Island, late Holocene, analysis of molluscan assemblages

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MIS25-03

Room:201B

Time:May 24 09:30-09:45

Paleo environment changes and tsunami deposits in Susami city, Wakayama prefectureduring the Holocene
CHIBA, Takashi1∗ ; GOTO, Kazuhisa2 ; FUJINO, Shigehiro1 ; YOKOYAMA, Yusuke3 ; MIYAIRI, Yosuke4 ;SAWADA, Chikako4 ; ISHIZAWA, Takashi5 ; NISHIMURA, Yuichi6 ; SUGAWARA, Daisuke2 ; GOFF, James7
1

Faculty of Life and Environmental Sciences, Univ. of Tsukuba, 2 IRIDeS Tohoku Univ., 3 ACES Univ. of Tokyo, 4 AORI, Univ.
of Tokyo, 5 Department of Earth Science, Graduate School of Science, Tohoku Univ., 6 Graduate School of Science, Hokkaido
Univ., 7 School of Biological, Earth and Environmental Sciences, Univ. of New South Wales
Kii peninsula was attacked by many tsunamis accompanied by the previous Nankai earthquakes. There is little geologicalevidence supporting past tsunamis in western coast of Kii peninsula (Komatsubara et al. 2007, Fujino et al. 2008), although there
are some geological evidences recognized as tsunami deposits in eastern coast. On the other hand, it is suggested that some greatearthquakes brought about co-seismic uplift larger than the usual great earthquake such as Showa Nankai earthquake in southernpart of Kii peninsula generated (Shishikura et al. 2008). However, information of tectonic movements by past earthquakes
is limited. Therefore, we need to obtain the geological evidence of the past tsunamis and tectonic movements due to Nankai
earthquakes in western part of Kii peninsula to reveal the recurrence intervals and co-seismic or inter-seismic crustal movements.
Against this background, we conducted drilling survey in lowland probably be not affected directly from sea waves at Susami
city, Wakayama Prefecture, central Japan and obtained about 800cm long core sample, and also analyzed fossil diatoms.
The core is composed of organic rich mud from 150cm to 300cm, organic-organic poor mud from 300cm to 680cm and gravel
deposit of basement rock fragments from 680cm to 760cm depth mainly. Basement rock sedimented in Miocene is recognized
below 760cm depth. Also, at least 7 sand or gravel layers are included in organic-rich muddy sediment from 150cm to 300cm
depth in the core. Meanwhile, K-Ah volcanic ash (erupted in 7200 cal. BP) layer is recognized at 560cm to 630cm depth. And
also we obtained accelerator mass spectrometry (AMS) radiocarbon dates covering during 2000BC-6000BC.
The diatom assemblages included in the mud deposits to 300cm from 760cm depth were dominated by marine and marinebrackish species such as Planothidium delicatulum and Cocconeis scutellu. These species decreased and freshwater diatom suchas Eunotia and Pinnlaria increased gradually from lower to upper of the core. Especially, the organic rich mud above 300cm depth
contained no marine species. The changes of diatom assemblages in the muddy sediments from 300cm to 760cm depth show
the tendency of increase of freshwater diatoms and decrease of marine diatoms, suggesting effects of paleo-sea level changes socalled Jomon transgression and its regression. These diatom assemblage changes also suggest that tidal flat was formed in thisarea during 3500BC-6000BC, and changed to salt marsh after that. On the other hand, the sand and gravel layers from 150cm to300cm depth contained very small amounts of marine diatoms. It is suggests that these sand and gravel layers were transported
by strong current from seaside such as tsunami during 2000BC-3500BC.
Keywords: Nankain trough, Kii peninsula, Tsunami deposit, Fossil diatom assemblage, Holocene

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MIS25-04

Room:201B

Time:May 24 09:45-10:00

Trench and coring survey of the tsunami deposits in the coastal area along the Wakasa
Bay, Takahama, Fukui Prefecture.
YAMAMOTO, Hirofumi1∗ ; URABE, Atsushi2 ; SASAKI, Naohiro1 ; TAKASHIMIZU, Yasuhiro2 ;KATAOKA, Kyoko S.2
1

University of Fukui, 2 Niigata University

Some sand layers which might be formed by the tsunami event were found in the peaty deposits under the coastal lowland in
Sonobe area, Takahama-cho, Fukui Prefecture. This time, we mainly investigate the shallow sand layers than 1m.
The coring and trench survey in this area shows that the sand layers are distributed more than 500m to the inland form theshore. The sand include grains of the well-rounded rock fragments and organic remains such as shell, foraminifera and spine of
the sea urchin, which are a lot included in the coastal sand of this area. In addition, the sand layers eroded the peaty sediments
at the base, and include the rip-up clast of them. The radiocarbon dates of the peaty sediments below and above the sand layer
show that the layers deposited between the 14th and the 16th century.
These data shows that the sand layers might be formed by ancient tsunami event of 16th century recorded in the historical
documents in this area.

Keywords: tsunami deposits, Wakasa Bay area, Takahama, coastal plain

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MIS25-05

Room:201B

Time:May 24 10:00-10:15

Event deposits discovered on the mouth area of Arakawa River, Niigata, Japan
TAKASHIMIZU, Yasuhiro1∗ ; ONO, Katsunori1 ; URABE, Atsushi1 ; TSURUMAKI, Shunsuke1
1

Niigata University

A characteristic sandy layer probably resulting from past tsunami event was discovered on the mouth area of Arakawa River,
Niigata, Japan. The event layer shows following features. The deposit is 1) distributed at least approximately 1 km from thecoast; 2) shows a fining- and thinning- landward trend, 3) indicates increasingly poor sorting landward, and 4) includes mud
clasts eroded from the muddy layer below the event deposits.
Keywords: Tsunami deposits

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MIS25-06

Room:201B

Time:May 24 10:15-10:30

Geologic evidence of tsunamis in Kujukuri
PILARCZYK, Jessica1∗ ; SAWAI, Yuki2 ; HORTON, Benjamin1 ; NAMEGAYA, Yuichi2 ; SHINOZAKI, Tetsuya2 ;TANIGAWA, Koichiro2 ; FUJIWARA, Osamu2 ; SHISHIKURA, Masanobu2 ; MATSUMOTO, Dan2 ; DURA, Tina1
1

Rutgers University, 2 National Institute of Advanced Industrial Science and Technology (AIST)

Along the northern part of the Japan Trench, the subduction of the Pacific plate under the North American plate has frequently
generated tsunamigenic-earthquakes up to ˜M 8.0. In contrast, the middle and southern parts of the Japan Trench were consideredrelatively inactive until the 2011 Tohoku-oki (M 9.0) event generated one of the largest tsunamis in recorded history. Geologic
evidence from the Sendai plain revealed an event in A.D. 869 that could have forecast the severity of the Tohoku-oki tsunami
in 2011. Seismic models indicate that the Tohoku-oki earthquake may have transferred stress southwards down the fault to thepotentially locked southern segment of the Japan Trench (Simons et al., 2011 Nature). This scenario could produce an earthquake
in the near future that would be comparable in magnitude to the Tohoku-oki event. Reconstructing the history of individual greatearthquakes and accompanying tsunamis from the coastal zone adjacent to the southern trench provides an assessment of theseismic hazard for several metropolitan areas.
We have found two anomalous marine sand layers preserved in low-energy freshwater environments where they would notnormally occur (i.e., present day rice paddies). The medium to coarse sand layers range in thickness from 3-10 cm, are intercalated with muddy peat, and the two upper layers can be traced 3.8 km inland and ˜5 km along the present Kujukuri coastline near
Sanmu City. The sand layers have features consistent with tsunami deposits found elsewhere, such as a distinct erosional base,marine geochemical signature, offshore foraminifera, rip-up clasts, normal grading, and a mud drape. Preliminary radiocarbon
dating of seeds, charcoal and insect cuticles constrain the age of the upper sand to A.D. 1613 ? 1651. Possible candidates for theupper sand are the Genroku tsunami of A.D. 1703 and the Empo tsunami of A.D. 1677. The age of the bottom sand is A.D. 971? 1047, an age that coincides with an 11th century gap in the historical record.

Keywords: Japan Trench, Tsunami deposit, Kujukuri

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MIS25-07

Room:201B

Time:May 24 10:30-10:45

Was the 2011 Tohoku tsunami once-a-millennium disaster in the Sendai Plain?
SAWAI, Yuki1∗ ; NAMEGAYA, Yuichi1 ; TAMURA, Toru1 ; NAKASHIMA, Rei1 ; TANIGAWA, Koichiro1
1

National Institute of Advanced Industrial Science and Technology (AIST)

Geological evidences show that recurrence intervals of outsize tsunamis along the Japan Trench may be shorter than previously
thought. In the Sendai Plain, A sand layer was found in peat above a historical volcanic ash (AD 915), and is dominated bybrackish-marine diatoms. Radiocarbon dating obtained from plant macrofossils permits correlation of the sand layer for the 1454
Kyotoku tsunami. The distribution of the layer shows that the run-up distance of the 1454 tsunami was at least 1 km, longer
than any tsunamis that occurred during the last 200 years except for the 2011 Tohoku tsunami. Numerical simulation for tsunami
inundation suggests that such an inundation distance is accounted for a rupture similar to that of the Jogan earthquake. The
1454 Kyotoku tsunami is thus considered as an unusually large tsunami comparable with the AD 869 Jogan and 2014 Tohoku
tsunamis, indicating that such tsunami inundated on the Sendai Plain more often than previously thought.
Keywords: Japan Trench, Tsunami deposit, Recurrence interval

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MIS25-08

Room:201B

Time:May 24 11:00-11:15

Quartz optically-stimulated luminescence dating of AD 869 Jogan tsunami deposit
TAMURA, Toru1∗ ; SAWAI, Yuki1 ; ITO, Kazumi1
1Geological Survey of Japan, AIST

Optically-stimulated luminescence (OSL) dating, as applicable to the sediment grain directly, has a potential to provide aneffective way to establish the chronology of tsunami deposits. AD 869 Jogan tsunami is one of the oldest tsunamis recorded in ahistorical document in Japan. Deposit of the Jogan tsunami thus offers an opportunity to check the reliability of OSL dating toa deposit older than 1,000 years, which may be prehistoric elsewhere. We applied the quartz OSL dating to a sand layer formedby the Jogan tsunami found in a geoslice sample obtained from the Sendai plain, Japan, to compare resultant OSL ages with thedepositional age. The sediment succession consists of beach-dune sand, lower peat, the Jogan tsunami deposit, upper peat, pre2011 paddy soil, and the 2011 tsunami deposit, in ascending order. A standard Single Aliquot Regenerative (SAR) protocol wasapplied to large aliquots of 180-250 µm fraction of two samples from the beach-dune sand, and four samples from differing levelsof the Jogan tsunami deposit to obtain equivalent dose (De,bulk ). OSL decay curves are dominated by the medium component,
and thus for two samples from the Jogan deposit the fast OSL component was isolated to be used for determining the equivalentdose (De,f ast ). All samples show unimodal distribution of De,bulk with relatively low overdispersion 11-22 %, suggesting that
sediment grains were well bleached before burial. Using De,bulk , OSL ages of the tsunami deposit was underestimated by 30-40%, and even the beach-dune sand was dated younger than AD 869. De,f ast in contrast resulted in a robust age estimate witha slight underestimation. Pulsed annealing test shows that the bulk OSL signal is thermally unstable. The medium component
OSL, isolated from the bulk signal, is clearly truncated in the natural OSL decay curve compared to regenerated ones, which isconsidered as the main source of the underestimation. Such unfavourable influence of the medium OSL component has beencommonly reported in tectonically active regions, which are also prone to tsunami, and thus should be considered with caution
in dating tsunami deposits.
Keywords: tsunami, luminescence dating, Jogan earthquake, tsunami deposit, Sendai Plain, chronology

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MIS25-09

Room:201B

Time:May 24 11:15-11:30

Highly precise radiocarbon dating of tsunami deposits
MIYAIRI, Yosuke1∗ ; YOKOYAMA, Yusuke1 ; SAWADA, Chikako1 ; ISHIZAWA, Takashi2 ; NISHIMURA, Yuichi3 ;SUGAWARA, Daisuke4 ; KAWAMATA, Takahisa5 ; GOTO, Kazuhisa4
1

Atmosphere and Ocean Research Institute,The University of Tokyo, 2 Department of EARTH SCIENCE, Graduate School of
Science, Tohoku University, 3 Graduate School of Science, Hokkaido University, 4 International Research Institute of Disaster
Science, Tohoku University, 5 Board of Education, Iwanuma City
Highly precise dating of tsunami deposit will be a useful tool for elucidation the frequencies of occurrence of the paleotsunami.
Radiocarbon dating is used as dating tool of tsunami deposit. However, conventional radiocarbon dating does not have the enoughprecision to distinguish historical tsunami deposits.We have developed a highly precise radiocarbon dating method using a highdensity sampling method.We will present about a high precision radiocarbon dating method of tsunami deposit using data from
Iwanuma investigation site.
Keywords: radiocarbon dating, tsunami deposit, peat, wiggle matching, 14C, AMS

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MIS25-10

Room:201B

Time:May 24 11:30-11:45

Spatial variation of sediment observed by reflecting surface and columnar core at Hirota
bay
YOKOYAMA, Yuka1∗ ; SAKAMOTO, Izumi1 ; YAGI, Masatoshi1 ; INOUE, Tomohito1 ; IIJIMA, Satsuki1 ;NEMOTO, Kenji1 ; FUJIMAKI, Mikio2
1

School of marine science and technology, Tokai University, 2 COR

The study of onshore features for tsunami impact is well researched, but offshore is only a few researches. In this presentation,
we will show about characteristic of tsunami deposit left by 2011Tohoku earthquake and spatial change of sediment in Hirota
bay using by Sub Bottom Profiler (SBP) and Vibration Core Sampler (VCS).
We took the columnar core at water depth from 8 to 30 m. The columnar cores were able to sectionalize into mainly two units
by lithofacies, Unit-1(sand layer) and Unit-2(muddy layer) from the top. And, we sectionalize U2 into six more subunits, called
U2-I to U2-Ⅵ from the top.
Therefore, layer of U2 is U2-I: sandy silt with shell piece, U2-Ⅱ: silt with bioturbtaion, U2-Ⅲ: sandy silt with wood chip,
U2-Ⅳ: sandy silt with sand at the base, U2-Ⅴ: silty sand with granule and very coarse sand at the base and U2-Ⅵ: silt layer.
Sagayama et al. (2014) assume that U2-I have a possibility of paleo event unit by diatom analysis.
SBP data were able to sectionalize to some reflection surfaces (R1-R7). R1 and R6 distribute widely, but other reflection
surfaces clear away toward offshore.
We estimate distribute of each unit using by comparison of lithofacies and reflection surface. Reflection surface accord withsedimentary units. Then, U1 distribute widely and number of U2 combination were decrease toward offshore from coastal area.
These data shows paleo event sediment mechanism and it also show spatial changes of sub bottom environment.

Keywords: Tsunami deposit, Sanriku coast

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MIS25-11

Room:201B

Time:May 24 11:45-12:00

Hydraulic experiment and numerical modeling on tsunami deposit aiming tsunami source
estimation
TAKAHASHI, Tomoyuki1∗
1

Faculty of Safety Science, Kansai University

Tsunamis generate large amounts of bed load and suspended load because of strong tractive force and turbulence in shallow
seas. The sediment transport due to tsunamis have caused various damages such as collapsing coastal structures, shoaling navigation channels in ports and blocking water intakes in power plants. A risk assessment of the sediment transport is important fortsunami disaster mitigation, and a numerical model to analyze the phenomena has been required. Many hydraulic experiments
and field investigations on the sediment transport and the bathymetry change caused by tsunamis have been carried out, and themeasured data have contributed to constructing the numerical model. However, because such damages occur in the water, almost
all of the researches on the tsunami sediment transport have assumed an inundated condition. They focused on the sedimenttransport and the bathymetry change when tsunamis arrive in the shallow seas. On the other hand, the sediment transport canmake tsunami deposit on land, lakes and marshes. Some analysis of the tsunami deposit have obtained invaluable knowledge ofhistorical and paleo-tsunamis. Therefore, the hydraulic experiments on the tsunami sediment transport on land are also carriedout and the numerical model is attempted to extend to tsunami run-ups recently.
The present numerical model can cover tsunami source, propagation and sediment transport in the sea. The model can predictbathymetry changes due to tsunamis in coastal areas if fault models of earthquakes are given. A small fishing port, for example,
has complicated coastal structures like breakwaters, and generates numerical instability easily. However, the practical and detailed simulation on tsunami sediment transport using a two meters grid was conducted stably. Therefore, if the numerical modelis upgraded to simulate the sediment transport and the tsunami deposit by tsunami run-up, the model is expected to enable toanalyze tsunami sources using the tsunami deposit distribution on land, lakes and marshes.
Based on the background described above, the hydraulic experiments and the numerical modeling on sediment transport under
the inundated condition are explained firstly. Besides, the hydraulic experiments on land aiming to extend the numerical model
to tsunami run-up are introduced. From simulation on the tsunami sediment transport in actual sea areas, present state and issuesof the numerical model are organized. Finally, a method to estimate tsunami sources using tsunami deposit distribution is investigated.
Keywords: tsunami run-up, sediment transport, bed load, suspended load, topography change, inverse analysis

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MIS25-P01

Room:Convention Hall

Time:May 24 18:15-19:30

Field evidence of tsunami sediments along the east coast of Taiwan
NISHIKAWA, Yuka1∗ ; CHEN, Wen-shan1 ; YEN, Jiun-yee2 ; MATSUTA, Nobuhisa3 ; YU, Neng-ti4
1

National Taiwan University, 2 National Dong Hwa University, 3 Okayama University, 4 National Hsinchu University of Education

Study on paleo tsunami deposits in geologic stratum is one of valuable tools for identification of paleo earthquake in thetectonically active coast, in Taiwan. After the 2011 Tohoku tsunami, demand increased greatly for geologists with expertise in
the geology of tsunami deposits, who could study prehistoric tsunami deposits.In Taiwan, east coast is very close to the Pacific
Ocean and the Ryukyu Trench. In order to confirm the tsunami attacked the east coast of Taiwan, we are investigate the East
Coast from Hualien to Taitung coast.

Keywords: the east coast of Taiwan, tsunami sediment

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MIS25-P02

Room:Convention Hall

Time:May 24 18:15-19:30

Source of tsunami sediments estimated using foraminifera in the east coast of Ishigaki
IslandSHIGA, Shota1 ; NAKAMURA, Mamoru1∗ ; FUJITA, Kazuhiko1 ; MATSUMOTO, Takeshi1 ; YAMASHIRO, Sakaki1 ;ANDO, Masataka2 ; KITAMURA, Akihisa2 ; IKUTA, Ryoya2 ; TU, Yoko3
1

Faclty of Science, University of the Ryukyus, 2 Shizuoka University, 3 Hokkaido University

Investigations of old literatures and tsunami boulders revealed that large historical and paleo tsunamis had been attacked the
Sakishima Islands for several thousands years. The 1771 Yaeyama tsunami (Meiwa tsunami) was the biggest tsunami in thehistorically recorded ones in the Yaeyama Islands. The 14C dating of the coral fossils over the tsunami boulders also showed therepeating large paleo tsunamis in the Sakishima Islands.
The sand layers were frequently observed at the excavation sites of the archeological surveys. However, the source of suchsands using the foraminifera had not been surveyed in the Sakishima Islands.
We investigated the source of the sands of the tsunami sediments using the analysis of the foraminifera. We performed
the trench-survey at Tanaka cattle farm, Ibaruma, the east coast of the Ishigaki Islands on November 5-8, 2013. Five trench,
whose elevations were from 2.65 m to 8.13 m, had been excavated along the survey line from the shore to inland. We sampled
the sediments at the interval of 20 cm to downward direction. The mud contents, particle-size analysis, and the foraminiferal
analysis of the sediments were done. We picked the foraminifera whose number is over 150 from the sample of the grain sizeof 2-0.5 mm and analyzed them. The sampled was decimated when the quantity of the sample was large. Then we sampled thebeach sands and analyzed them to investigate the source of the tsunami sediments. We performed the community analysis using
cluster analysis and multidimensional scaling method. For the community analysis, we used the recent foraminifera group data
sampled at beach, sea grass beds, reef crest, and moat in the north of Ishigaki Island (Fujita, 2006), and compared them withthose of tsunami sediments.
Almost trench sites consists of first (cultivated soil), second (sand layer including rip-up crusts), third (silt layer), and fourth(sand layer with grading and coral fossils) layers. The most inland trench consists of first (cultivated soil), second (grading sandlayer with rip-up crusts), third (mud layer including many gravel), and fourth (Shimajiri mud stone) layers.
The age of second and fourth layers, which were possibly the tsunami sediments, were estimated as 200-300 years and 500-800
years from the dating the C14 age of the in-situ bivalves.
The community analysis of the foraminifera showed that the compositions of the foraminifera at the seaward and landward
sites had changed gradually through the same layers. The group which lived near the reef crest was dominant in the seaward site,whereas the group near the shore was dominant near the inland site.
This suggests that the sediment of the reef crest and shelf was deposited near the shore, although the sand near the shore wasdeposited in the inland by the tsunami of 200-300 years ago. Moreover, the foraminifera (Amphistrgina spp. and Elphidium spp.),which could dissolve at fresh water condition, was concentrated at fourth layer. The ground water had leaked near the bottomof the trench sites. The above-mentioned foraminifera would have remained selectively whereas others solved in the fresh-watercondition. This suggests that the effect of the dissolution would be important factor for the preservation of foraminifera in thetsunami sediments.

Keywords: tsunami sediments, foraminifera, Ishigaki Island

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MIS25-P03

Room:Convention Hall

Time:May 24 18:15-19:30

Four Holocene uplifted benches and two historical tsunami gravels around Koseda coast,
Yakushima Island
NAKAGAWA, Shojiro1 ; SASAKI, Hiroyuki1 ; OMOTE, Masamichi2 ; NANAYAMA, Futoshi3∗ ; GESHI, Nobuo3 ;WATANABE, Kazuaki3 ; NARUO, Hidetoshi4 ; MAENO, Fukashi5 ; KOBAYASHI, Tetsuo6
1
5

Yakushima Earth-Science Club, 2 Yakushima Town Office, 3 Geological Survey of Japan, AIST, 4 Takeokadai Highschool,
Tokyo University, 6 Kagoshima University

Many researchers have noted that Yakushima may have been struck by a huge earthquake and tsunami along Ryukyu subduction zone during historical age, but there is currently no clear evidence of this. We described four Holocene wave-cut benches
uplifted by subduction earthquakes and two historical tsunami gravel beds around Koseda coast in northeastern Yakushima Island.
Keywords: Holocene wave-cut bench, huge tsunami trace, huge earthquake, Ryukyu subduction zone, Koseda coast, YakushimaIsland1/1

MIS25-P04

Room:Convention Hall

Time:May 24 18:15-19:30

Tsunami deposit survey on the Chichi Is. and Haha Is. in the Bonin Islands (Preliminary
report)
HARADA, Tomoya1∗ ; GOTO, Tomoko2 ; ISHIBE, Takeo1 ; NARUHASHI, Ryutaro1 ; FURUMURA, Takashi1
1

Earthquake Research Institute, Univ. of Tokyo, 2 Graduate School of Frontier Science, Univ. of Tokyo

The Izu-Bonin subduction zone has been regarded as so-called ”Mariana-type subduction zone” where M>7 interplate earthquakes do not occur. Ishibashi and Harada (2013) proposed a working hypothesis that the 1605 Keicho earthquake which has
been considered a great tsunami earthquake along the Nankai trough was a giant/great earthquake along the Izu-Bonin trench
based on the similarity of the distributions of ground shaking and tsunami of this event and the 2010 Bonin earthquake. Haradaet al. (2013) carried out the tsunami numerical simulations from the fault models along the Izu-Bonin trench and their results
support the Ishibashi and Harada’s hypothesis. However, it has not discovered that the evidences of the giant/great earthquake
along the Izu-Bonin trench yet. Therefore, in order to discover the tsunami traces of the earthquake such as tsunami deposit ortsunami boulders, we started to tsunami deposit survey on the Chichi Is. and Haha Is. in the Bonin Islands last year.
We have conducted two tsunami deposit surveys on the Chichi Is., Haha Is. and some territorial islands around the two islands.
Total number of survey points is 23. We found tsunami deposits at three survey areas in the Chichi Is. (Sakai-ura beach, Yatsusegawa valley and Minami-fukurozawa valley).
Some preliminary results are as follows. We found at least three event layers on the outcrops in the Sakai-ura beach in Chichi
Is. Upper two layers were possibly deposited by the tsunamis or storms because of their typical features of tsunami deposit.
The third layer (thickness is 50˜100 cm) has the sharp basal contact with under soil and consists of large amount of corals
which suggest an unidirectional flow. At the two survey points in Yatsuse-gawa valley, some event layers were identified. In the
Minami-fukurozawa valley, two thick layers include large gravels and four or five thin layers on the outcrops. However, originof the thick layers has not been revealed yet. More tsunami deposit surveys are necessary in the islands to confirm these layers
were tsunami deposits or not.
We thank Mr. Ichiro Hosoda, other teachers, and students of the Ogasawara high school for supporting our survey. This study
was supported by JSPS KAKENHI Grant-in-Aid for Young Scientists (B) (Grant Number 26750129).

Keywords: tsunami deposit, Chichi Is. and Haha Is., Izu-Bonin subduction zone, 1605 Keicho earthquake

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MIS25-P05

Room:Convention Hall

Time:May 24 18:15-19:30

Tsunami sediment immediately after K-Ah ash fall in Miyazaki Plain
ICHIHARA, Toshihiko1∗ ; AKAZAKI, Hiroshi3 ; MATSUDA, Kiyotaka2 ; HAMADA, Mari2
1

Fukken Co.Ltd, 2 Miyazaki Prefecture Archaeological Center, 3 Miyazaki Prefectural Museum of Nature and History

Miyazaki Prefectural Museum of Nature and History, along with the Ishizaki river renovation of Miyazaki Sadowara Kuroda
in 1996, have created peeling specimen of outcrops.The peeling specimens contains Akahoya volcanic ash of mollusk shells andJomon transgression period.It is possible to observe the formation of up to 7.5 m depth from the surface. Result of study insedimentological deposition environments of the stripping specimen, it was possible to find a tsunami deposits.
The peeling specimens include Crassostrea gigas, Tegillarca granosa, Thalassinoides isp..From these, Akahoya volcanic ash
was deposited under inner bay environment close to the tidal flats or tidal flats.
Akahoya ash, exhibited a white tinged with pale yellow to pale pink in the specimen, the layer thickness is about 1.5m.The
boundary between the lower strata exhibits a clear erosion.Bottom is a massive volcanic ash containing a large amount ofwood.The top, HCS-mimics, parallel lamina, is observed climbing ripple laminae.The upper is covered with a non-structureof volcanic ash.Paleocurrent which is determined from the form of climbing ripple is a west direction from the east.Akahoya
ash facies, such as erosion surface can not be seen until the climbing ripple from the bottom, i is judged to be continuously
deposited.HCS-mimics are formed in the inner bay, we observed flow to the landward, there is a possibility of tsunami deposits.
Fujiwara et al. (2010) reported the tsunami deposits with characteristics similar in Oita, and the like be covered with ash fall of
Akahoya, and are to be tsunami deposits caused by eruption.On the other hand, the tsunami deposits reported this time, not thatit can be determined that volcanic ash.Also, the thickness of the secondary sediment is developed thick and 1.5m, while climbing
ripple formed in sediment supply is often on even the naked eye observation, and the like to become only Akahoya ash, at least
drop Akahoya ash is considered to be those that occurred after sufficiently deposited.In view of the above, this time the observed
tsunami deposits, I believed to be due to the tsunami that occurred immediately after ash. Because it is information from a single
stripping specimen, it is enough assurance not be obtained, it is necessary to consider the tsunami occurrence factors other thanthe Akahoya eruption, information of tsunami research is scarce Miyazaki coast of tsunami than other regions in performing thestudies, it is very valuable sample.
Keywords: tsunami deposits, Kikai-Akahoya-ash (K-Ah), Miyazaki Prefecture, Peels of Outcrops

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MIS25-P06

Room:Convention Hall

Time:May 24 18:15-19:30

Preliminary report on paleotsunami study in Nankoku City, Kochi Prefecture, westernJapan
TANIGAWA, Koichiro1∗ ; SHISHIKURA, Masanobu1 ; FUJIWARA, Osamu1 ; NAMEGAYA, Yuichi1 ;MATSUMOTO, Dan1
1

Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST)

Kochi Prefecture has been struck repeatedly by large tsunamis generated by subduction zone earthquakes along the Nankai
Trough. Historical records through approximately 1,300 years indicate great earthquakes (M ˜8) basically occurred at intervalsof 100-200 years along the Nankai Trough. However, because historical documents are relatively sparse before the 16th century,
it is difficult to evaluate magnitudes and rupture areas of earthquakes during this period. Tsunami deposits provide basic datafor reconstructing not only the long-term earthquake history but also the magnitudes and the rupture areas. Therefore we studied
tsunami deposits in Kochi Prefecture.
We drilled in coastal lowlands of Toyo Town, Nankoku City, Shimanto Town and Kuroshio Town. We obtained cores andgeoslices at 9 sites in a coastal lowland up to 3 m in elevation in Nankoku City. Deposits above the Kikai-Akahoya tephra(erupted ca. 7,200 cal BP) consist mainly of mud, peat and include event sand sheets. We present initial findings following thepreliminary results of radiocarbon dating and fossil diatom analysis for some cores.
Keywords: tsunami deposit, fossil diatom analysis, Nankai Trough, Nankoku City, Kochi Prefecture

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MIS25-P07

Room:Convention Hall

Time:May 24 18:15-19:30

Nankai Earthquake events recorded in lacustrine sediment along the eastern coast of theKii Peninsula, southwest Japan.
OKAMURA, Makoto1∗ ; MATSUOKA, Hiromi1 ; TSUZUKI, Mitsuo2 ; TORAYA, Kenji2
1kochi University, 2 Disaster Mitigation Research Center, Nagoya University

We research on tsunami sediment from lacustrine deposits along the Nankai Trough for prehistoric earthquake reconstruction.
Total 21 cores are collected from the two lakes named Zasa-ike and Ashihama-ike, which locate behind coastal ridge along thesoutheastern coast off Kii Peninsula. Eighty C14 radioactive carbon dating are carried on plant remains containing the cores
deposited under non-marine condition.
As a result, Ashihama-ike records two major events, 2000yBP to 2300yBP and 1000yBP to 1100yBP through 4500 years
interval. The other Zasa-ike has six events are detected, which are 7000yBP, 6500yBP, 3500yBP, 2000-2300yBP, 1300yBP, and1100yBP through the interval of 7500 years. The oldest event in these cores is conglomeratic coarse sediment covered by K-Ah
tephra obtained from the lowermost part of cores. Both two events, 2000yBP to 2300yBP and 1000yBP to 1100yBP can becorrelated among the ponds Ashihama-ike and Zasa-ike which has two kilometers in distance each other.

Keywords: Nankai Earthquakes, tsunami sediment

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MIS25-P08

Room:Convention Hall

Time:May 24 18:15-19:30

Assessing modern sediment distributions as tsunami indicators for coastlines facing theJapan Trench
PILARCZYK, Jessica1∗ ; SAWAI, Yuki2 ; HORTON, Benjamin1 ; NAMEGAYA, Yuichi2 ; SHINOZAKI, Tetsuya2 ;TANIGAWA, Koichiro2 ; FUJIWARA, Osamu2 ; MATSUMOTO, Dan2 ; DURA, Tina1
1

Rutgers University, 2 National Institute of Advanced Industrial Science and Technology (AIST)

Geologic studies conducted in coastal areas of Japan have revealed evidence of earthquakes and tsunamis that predate thehistorical record. These studies assess the long-term seismic trends along subduction zones, and provide improved hazard assessment by constraining the possible magnitudes of future events. However, one of the obstacles facing the proper identificationof paleo-tsunami deposits is the lack of a modern analogue.
In 2013 we collected modern surface samples along two coastal transects located on Hasunuma Beach (Kujukuri), a region
with a history of tsunamis, and documented their foraminiferal (taxonomy and taphonomy) and grain size distributions. Highest
concentrations of foraminifera were found in swash and foreshore zones and markedly decreased landward towards the backshore and dune. Swash and foreshore assemblages were dominated by Pararotalia nipponica, Quinqueloculina sp., and planktics,
whereas the backshore and dune contained only species with robust tests (e.g., Pararotalia nipponica, Ammonia parkinsoniana,
and Lenticulina sp.).
Taphonomic analysis (surface condition of individual tests) revealed that swash and foreshore samples contained higher abundances of unaltered and fragmented foraminifera, whereas the majority of foraminifera in dry beach samples were corroded due
to subaerial exposure. Results of particle size analysis show a marked decrease in size from the swash zone (coarse sand) to thedunes (fine to very fine sand).
Partitioning Around Medoid (PAM) cluster analysis of the modern surface data indicated that foraminiferal taphonomy, aproxy that is not commonly applied to overwash deposits, discriminated coastal zones more effectively than taxonomy or grain
size. Although a multi-proxy approach is necessary to properly assess overwash deposits, foraminiferal taphonomy will be mostuseful in determining sediment provenance and aiding in the interpretation of anomalous sand layers previously identified at thislocation.

Keywords: Tsunami deposit, Foraminifera, Kujukuri

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MIS25-P09

Room:Convention Hall

Time:May 24 18:15-19:30

Field Survey of Tsunami Deposits in a Reclaimed Lagoon in Minami-Soma City, Fukushima
Prefecture
GOTO, Tomoko1∗ ; KUSUMOTO, Satoshi2 ; ISHIBE, Takeo2 ; SATAKE, Kenji2 ; SUGAI, Toshihiko1
1GSFS, University of Tokyo, 2 ERI, University of Tokyo

For the purpose of reconstructing paleo-tsunami histories, we conducted field survey of tsunami deposits at the Idagawa drained
land in Odaka district, Minami-Soma City, Fukushima Prefecture in June and November 2014. We set a survey line perpendicular to the coast and obtained 11 core samples of 1.4-2.7 m length at distance ranging 0.6-2.7 km by using handy geo-slicer.
We sketched the facies, analyzed grain sizes, and measured the 14 C ages. Tsunami trace of the 2011 Tohoku-oki earthquake
covers the rice paddy soil in the drained land with the thickness of 3-30 cm, and it can be observed at the all sampling sites. Six
sandy layers including the 2011 tsunami trace were identified in the 2.7 m long core sample obtained at the central part of thedrained land. The 14 C age at the lowermost part of the sample was 2,310±20 yr BP, indicating that there are possibly six or moretsunamis attacked the survey area.
Tsunami deposit survey on the Fukushima coast is important to constrain the southern limits of the 869 Jogan and 1611 Keicho tsunami deposits, and northern limit of the 1677 Enpo tsunami deposit. Along the coasts of Sendai plain to Fukushima,
paleo-tsunami deposits were reported by previous studies (Sawai et al. , 2012, GRL). At the present survey site, three event layers which deposited after 2,820 yrBP were identified in 1.1-1.6 m long core samples by a previous survey (Goto and Aoyama,
2005, Abstracts for the JpGU). The sedimentary features of the 2011 Tohoku-oki tsunami deposit were reported by Oota andHoyanagi (Proceedings of the 2014 annual meeting of the Geological Society of Japan). The survey site is a former lagoon wherea reclamation project started in 1919, and has been utilized for rice paddies until the 2011 Tohoku-oki earthquake. Old maps
published in 1910, 1888 and 1821 indicate a lagoon or inner bay facing a coast, which is affected by the tide. It is expected thatthe paleo-tsunamis transported sandy deposit from coast to the inner part of the bay, which can be distinguished with inner-bay
muddy deposit.
The facies of the core sample obtained at the central part of drained land (1.8 km from the coast) can be divided into 11 units
from the top to the bottom as follows: 1. Very fine to coarse sand including mud drape and normal grading (thickness ~ 30 cm);
2. Soil for rice paddies including undecomposed plants; 3. Alternative layers of medium sand with lamination and mud clasts; 4.
Muddy layer including shell fragment; 5. Fine to medium sand with normal grading and lamination; 6. Muddy layer; 7. Medium
sand with normal grading and lamination; 8. Reductive muddy layer with olive yellowish color; 9. Medium sandy layer withnormal grading and rich mud clasts; 10. Reductive muddy layer; and 11. Medium to coarse sand layer with normal grading,lamination, and mud clasts. Six sandy layers (1, 3, 5, 7, 9, 11) have sharp basal contacts with the lower units, and can be widely
identified at the survey area: hence, they are considered to be event deposits. To estimate the depositional ages, we obtained 14 C
ages for wood/shell fragments and charred materials as follows: 1000±20 yrBP (calendar year 2σ: AD1310-1420) from a shellfragment below unit 3, 1660±30 yrBP (AD270-430) from a charred material above unit 5, 1960±20 yrBP (40BC-AD90) from acharred material above unit 9, and 2310±20 yrBP (410BC-260BC) from wood fragment at the lower part of unit 11. Inner-bay
muddy deposits below unit 8 contain rich oyster assemblage and shell fragments of Trapezium clam in core samples with the
distance <1.2 km from the coast, while their productivity abruptly decreased above unit 5. This indicates that the sedimentary
environment changed at the border of unit 5. In future, we plan to reveal the characteristics of deposit and reconstruct the sedimentary environment through the microfossil and elementary analyses as well as careful investigation of samples by increasing
the number of samples.
Keywords: Tsunami deposit, Fukushima, Paleo-tsunami, Historical earthquakes

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MIS25-P10

Room:Convention Hall

Time:May 24 18:15-19:30

Sedimentary features of the 2011 Tohoku-Oki tsunami deposit and paleo-tsunami history
in Numanohama, Sanriku Coast
GOTO, Tomoko1∗ ; SATAKE, Kenji2 ; SUGAI, Toshihiko1 ; HARADA, Tomoya2 ; ISHIBE, Takeo2 ;GUSMAN, Aditya2
1

GSFS, University of Tokyo, 2 ERI, University of Tokyo

We have conducted tsunami deposit surveys in Numanohama marsh in Miyako City, Iwate Prefecture since the summer of2012. Tsunami deposit from the 2011 Tohoku-Oki earthquake at this site has characteristic facies that contain both marineand land-origin particles. This facies represents a sequence of tsunami inflow transporting marine sand/pebble and return flowtransporting land soil, riverbed pebble and talus deposits. We identified at least 10 event layers from the 5.8 m long column
sample; we named these layers as S1 to S10 from the top to the bottom. S10 has a characteristic facies with mixed marine
and land particles, similar to the 2011 deposit. Radiocarbon dating (14 C) result shows that S10 deposited from 1800±20 yrBP
(calendar year within 2σ range: AD130-320) to 340±30 yrBP (AD1470-1640).
V-shaped valleys are distributed along the Sanriku coast from Miyako to Kuji City. At the survey site, the tsunami heightfrom the 2011 Tohoku-Oki earthquake was 34.1 m (Tsuji et al. , 2011). Oguchi et al. (2013, Abstracts for the fall meeting of the
Association of Japanese Geographers) made detailed geomorphological survey using a laser scanner in Aneyoshi, Miyako City,and showed that tsunami wave eroded land soil and vegetation in valley plain, and lead collapse of rock wall or slope. Sediments
eroded by tsunami re-deposit in a valley plain as ‘tsunami deposit’. Understanding the characteristic features of tsunami deposit
from the 2011 Tohoku-Oki earthquake in a V-shaped valley is useful for identifying paleo-tsunami deposits along the Sanriku
coast.
The characteristic feature of the 2011 Tohoku-Oki tsunami deposit could be identified up to 300 m inland from the shoreline.
The content ratio of marine pebbles gradually decreases toward inland. On the contrary, riverbed pebbles become rich at upstreamsites with a distance of >560 m from the coast. The event layers could be divided into five kinds of particles; A (marine
sands/pebbles), B (rock clast from the rock wall), C (riverbed pebbles), D (talus deposits), and E (land soil). Particles A-E are
considered to be transported toward inland by tsunami inflow, while a part of these particles were re-transported by return flow.
Particles classified into B and D are strongly influenced by return flow and transported by mud or debris flow from the top ofsmall valley. After such a flow joins the main river stream, riverbed pebble were transported further seaward.
We conducted a drilling survey by using handy geo-slicer, observed outcrops and pit at the valley plain to investigate the
distribution of each paleo-event layer. In addition to the six layers in our previous study, we identified four additional event
layers S7-S10. The 14 C dating revealed that those events occurred during the period from 1950±20yrBP (AD0-AD120) to
410±20yrBP (AD1440-1610). We plan to estimate the depositional ages from more dating samples and to reconstruct paleo
environment in this marsh.
Keywords: Tsunami deposit, Sanriku coast, Paleo-tsunami, Historical earthquakes

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MIS25-P11

Room:Convention Hall

Time:May 24 18:15-19:30

Tsunami deposits observed with rice paddy development at Noda village, Iwate Prefecture
TAKADA, Keita1∗ ; YOSHIDA, Kenichi2 ; ODASHIMA, Atsushi2 ; IMAI, Kentaro3 ; EBINA, Yuichi3 ;GOTO, Kazuhisa3 ; KOSHIYA, Shin4 ; YAMAMOTO, Hidekazu4 ; SHISHIKURA, Masanobu5 ; IGARASHI, Atsuo1 ;ICHIHARA, Toshihiko1 ; KINOSHITA, Hirohisa1 ; IKEDA, Tetsuya1
1
4

Fukken Co.Ltd, 2 River Division,Department of prefectural Land Development,Iwate Pref Govt, 3 IRIDeS Tohoku University,
Faculty of Engineering, Iwate University, 5 Institute of Earthquake and Volcano Geology, AIST

Tsunami-like layers were appeared on drainage channel wall that excavated for rice paddy development at Noda village, Iwate
Prefecture. This site located behind the tide embankment (elevation of the embankment top is TP+12m) along the northern partof Sanriku coast. The 2011 Tohoku-oki tsunami was inundated to about 1km inland going over the embankment.
We identified four sand and gravel layers that were distributed continuously in peat buried by surface paddy soil (Event I to
Event IV from the top). These layers show a tendency becoming thin and shallow toward inland. Therefore upper layers weremixed in surface soil and disappear. Each layer has an erosional base and consist of one massive unit, however it has no distinct
laminae, rip-up mud clasts and grain size grading. We consider each event layer was probably formed by tsunamis because of
the distribution and sedimentary structures are similar to tsunami deposits that have been reported in previous studies.
Event II sand is deposited below tephra layers. The distinct tephra layer is identified as Baitoushan-Tomakomai tephra (B-Tm)
that was deposited in early to middle 10th Century. Other one can be identified as Towada-a tephra (To-a) of AD915 is observed
only in a few points just below the B-Tm tephra layer. Radiocarbon dating results just above and below Event II sand layer isconsistent with the tephra chronology. These analytical results suggest Event II sand deposited early to middle 10th Century,
therefore Event II is potentially corresponding to 869 Jogan tsunami. Radiocarbon dating results just above and below Event I
gravel layer shows 14th Century to early 17th Century. 1611 Keicho Oushu (Sanriku) tsunami is known as one of the historicallarge tsunamis which recorded damage along the Sanriku coast at this time. It is pointed out that tsunami might have also occurred in 1454 along the Pacific coast of Tohoku district, although there is no definite record.
Radiocarbon dates of lower event layers suggest that each layer deposited 5th Century to 6th Century (EventIII) and 1st Century to 3rd Century (Event IV). Our survey result mean four (five including 2011 Tohoku-oki) inundation of potentially large
tsunamis occurred past 2000 years in this site.
Keywords: tsunami deposit, historical tsunami, Jogan tsunami, Keicho Oushu (Sanriku) tsunami, Noda village Iwate Prefecture

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MIS25-P12

Room:Convention Hall

Time:May 24 18:15-19:30

Estimation of formative mechanism and classification of surface facies of tsunami origin
sediment at Hirota bay
YOKOYAMA, Yuka1∗ ; SAKAMOTO, Izumi1 ; YAGI, Masatoshi1 ; INOUE, Tomohito1 ; IIJIMA, Satsuki1 ;NEMOTO, Kenji1 ; FUJIMAKI, Mikio2
1

School of marine science and technology, Tokai University, 2 COR

The recent 2011Tohoku tsunami strongly affected the coastal area of the Pacific coast of Tohoku. The study of onshore features
for tsunami impact is well researched, but offshore is only a few researches. In this presentation, we will show about classificationof surface facies, and estimate the formative mechanism of tsunami origin sediment.
We took the columnar core at water depth 8-30 m. The columnar cores were able to sectionalize to mainly two units fromlithofacies. Unit-1(U1) consists of sand layer and Unit-2(U2) consists of muddy layer. Yokoyama et al. (2014) assume that
U1 were 2011 tsunami origin sediment, and U2 were normal sediment in this bay. We estimate U1 distribution with thickness
approximately 7-80 cm. U1 thickness have lateral change around coastal area (water depth about 8 m), but homogenize athickness around water depth about 13-16 m. Therefore, we estimate some sediment lobe were unified toward offshore.
U1 were able to sectionalize to some sub units from grading. Sub unit number was decrease from coastal area to offshore.
Sub unit-1a (bottom layer of U1) have granule to fine pebble at base, and grain size gradually become finer from coastal area tooffshore.
13HV8 (water depth about 13 m) was able to sectionalize to sub unit-1a and 1b, sub unit-1a consists of grading section(medium-very coarse sand), parallel lamina section (medium sand) and fine sand section (very fine sand-fine sand) from thebottom, it is similar to the character of Bouma sequence -a, -b, -d or -e, and sub unit-1b have same characteristics, too.

Keywords: Tsunami deposit, Sanriku coast

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MIS25-P13

Room:Convention Hall

Time:May 24 18:15-19:30

Tsunami origin sediments observed from the Ohtsuchi bay, Iwate Pref., Japan.
SAKAMOTO, Izumi1∗ ; NAMBA, Makoto1 ; YOKOYAMA, Yuka1 ; YAGI, Masatoshi1 ; NEMOTO, Kenji1 ;FUJIMAKI, Mikio2
1

School of Marine Science and Technology, Tokai Univ., 2 COR

The recent 2011 Tohoku tsunami strongly affected the coastal area of the Pacific coast of Tohoku. Tokai University and
JAMSTEC team investigated the Tohoku coastal area as a part of Tohoku Ecosystem-Associated Marine Sciences (TEAMS).
We researched using acoustic equipments (Multi narrow echo sounder : MNB and Sub bottom profiler : SBP), and bottom
sampler.
SBP data was seen signature reflecting (20-50cm down from seabed), and able to estimate the reflecting surface to depthof approximately 20 m at Ohtsuchi bay. The most strong reflector (R1) near the sea-bottom (20-70cm in depth) have rich inunevenness characters and have large lateral change around the shallow area (5-15m). However, R1 changes flat and smoothreflector to the offing (over -30m in depth).
Characteristic of columnar core sample at Ohtsuchi bay consists by 2 (shallow area) to 3 (deep area) sedimentogical units, suchas upper sandy unit (30-110cm), middle reddish brown silt unit (60-130) and lower sandy unit. This coarse sand and gravel layerfrom Unit-1 eroded out the underlying reddish brown silt (Unit-2) zone. Geological facies (Unit-1) of the cores from the Ohtsuchi
correspond with reflector from SBP such as R-1. It is estimated that the upper sandy sediment unit with grading structure above
the erosion layer, which observed Ohtsuchi bay, is assumed to be a layer of sediment gravity flow caused by the tsunami activity.

Keywords: tsunami sediment, Ohtsuchi bay

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MIS25-P14

Room:Convention Hall

Time:May 24 18:15-19:30

Effectiveness of luminescence dating of tsunami deposits examined from the March 2011
Tohoku-oki Tsunami deposits
HAYASHIZAKI, Ryo1∗ ; SHIRAI, Masaaki1
1

Tokyo Metropolitan University

Optically Stimulated Luminescence (OSL), which determines depositional age of sediments using minerals such as quartz andfeldspar, may be useful for dating tsunami deposits. Furthermore, OSL dating is occasionally used for transport-depositional
processes of sediments in terms of bleaching achievements, which might be able to apply tsunami deposits. In spite of thesepotentials, OSL dating studies on tsunami deposits have been hardly conducted. The current study was undertaken in order toconfirm these effectiveness of OSL dating on tsunami deposits using the 2011 Tohoku-oki tsunami deposits at Soma city andMinamisoma city, Fukushima Prefecture, northeastern Japan.
Single grain OSL dating was able to extract the grains indicating true depositional age of the 2011 Tohoku-oki tsunami deposits at 11 locations (26 samples), which shows nearly zero OSL intensities. It is interpreted from the results that single grain
OSL dating is a feasible method to estimate appreciate depositional age of tsunami deposits. Meanwhile, some grains were
overestimated depositional age remarkably. Therefore general OSL dating, which measures a number of grains on one aliquot,
is probably difficult to estimate accurate depositional age of tsunami deposits. The results of OSL dating indicated most of sandgrains in tsunami deposits had not been exposed to daylight during tsunami transport processes. This means that sand grains intsunami deposits preserving bleaching achievements of sediment sources. OSL dating is able to calculate bleaching achievementsof minerals, therefore, it might be able to estimate sediment sources of tsunami deposits.
Keywords: OSL dating, transport and depositional processes, tsunami deposits, the 2011 off the Pacific coast of Tohoku Earthquake, feldspar

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MIS25-P15

Room:Convention Hall

Time:May 24 18:15-19:30

Correlations of tsunami deposits based on high-dense array drilling survey at Koyadori,
Iwate Prefecture
ISHIMURA, Daisuke1∗ ; ICHIHARA, Toshihiko2 ; SAKATA, Tomohiro3 ; OHATA, Masahiko3 ; TAKADA, Yuya3
1

IRIDeS, Tohoku Univ., 2 Fukken Co., Ltd., 3 Dept. Earth Science, Tohoku Univ.

Tsunami deposits study greatly increased after The 2011 off the Pacific coast of Tohoku earthquake tsunami. However, tsunami
deposits are covered and sometimes disturbed, and thus we generally conducted coring survey and correlate tsunami deposits
between each core based on lithology and age. Especially, the correlation of tsunami deposits is very significant for assessmentof tsunami and earthquake risk. In this study, we carried out high-density array drilling survey to confirm continuity of tsunami
deposits. Additionally, we tried to compare the correlation of each tsunami deposits when we change the interval of coring sites.
Consequently, we correlated each tsunami deposits confidently in 2.5 m, 5 m, and 10 m interval. In 20 m and 50 m interval,
we can correlate some tsunami deposits, however accuracy of the correlation is much lower than that of 2.5 m and 5 m interval.
Although this study is one case, we need to confirm the accuracy of correlation in the future.
Keywords: tsunami deposits, high-dense array drilling survey, Sanriku Coast, correlation of tsunami deposits

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MIS25-P16

Room:Convention Hall

Time:May 24 18:15-19:30

Local correlation of sandy paleo-tsunami layers and estimation of inundation distance ineastern Hokkaido
NAKAMURA, Yugo1∗ ; NISHIMURA, Yuichi1
1

ISV, Hokkaido Univ.

We found multiple paleo-tsunami sand layers of the last 3000 years in seven marshes (Urahoro, Kinashibetsu, Onbetsu,
Kushiro, Akkeshi, Nemuro, and Betsukai) on a 200-km-long coastline of eastern Hokkaido, Japan. In the previous studies,
up to six paleo-tsunami layers were described in these regions, but inundation area of only few events were discussed. The
purpose of the present study is to evaluate the inundation area of every paleo-tsunami events along each transect based on thelocal correlation of paleo-tsunami layers. Along the Urahoro transect (370 to 625 m from the sea), at most ten sand layers wereobserved and five layers of them were successfully correlated along the whole transect by comparing their particle sizes andmineral compositions. For the uppermost layer we correlated, the mean grain size smoothly decreases landward from 0.24 to
0.74 phi, and its heavy mineral ratio decreases from 40 to 34 %. The other four layers also show individual decreasing trends
both in particle size and heavy mineral ratio. In other regions, there are two sand layers in Kinashibetsu, six in Onbetsu, two in
Kushiro, two in Akkeshi, eight in Nemuro, and two in Betsukai. Four layers in Onbetsu can be correlated and traced to 590-670
m from sea, and four in Nemuro are traced to 260-300 m. These layers are presumed to be formed by relatively large events.
Along the Betsukai transect, the far side of Nemuro peninsula, two fine-sand layers were observed and presumed to be evidencesof large paleo-tsunami events which formed sand layers along the Pacific coast. Methods for wide correlation of paleo-tsunami
layers, however, have not been developed and should be discussed by future works.

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MIS25-P17

Room:Convention Hall

Time:May 24 18:15-19:30

Sedimentary process and distribution of terrestrial tsunami deposit: a flume experiment
YAMAGUCHI, Naofumi1∗ ; SEKIGUCHI, Tomohiro2
1

CWES, Ibaraki University, 2 CRiED, University of Tsukuba

Flume experiments were conducted to examine the effects of the magnitude of tsunami and existence of a lake in coastal
lowland on sedimentary process and resulting sediment distribution regarding the setting that run-up tsunami transports sediments
in coastal zone (e.g. sandy beach or dune) to coastal lowland. We prepared three topographical models of the terrestrial area: flatmodels with a shallow or deep pool filled with water (9 mm and 45 mm depth, respectively, and 1.0 m long) at their center, andan entirely flat topographical model without a pool. In each run, only one run-up tsunami flow passed through the terrestrial area
without generating strong backwash. The tsunami flow was supercritical throughout the terrestrial area in the experimental series
without a pool, while it transformed from supercritical into subcritical generating a hydraulic jump in the pool for the runs with apool. The tsunami magnitude, terrestrial tsunami deposit, and terrestrial topography setting showed the following relationships:(i) The total amount of tsunami sediment depended on the magnitude of tsunami.
(ii) In the experimental series with a pool, most of sediments deposited in the pool, but little sediment was in the area where
the hydraulic jump occurred.
(iii) The amount of the deposit at a given site did not always depend on the magnitude of the tsunami even in the same
topography setting.
Although further investigation is needed, the present experiments showed some possible clues for future field surveys tounderstand tsunami deposits.
Acknowledgements: This work was partly supported by the Sasakawa Scientific Research Grant from The Japan Science
Society.
Keywords: terrestrial tsunami deposit, coastal lake

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MIS25-P18

Room:Convention Hall

Time:May 24 18:15-19:30

An attempt to estimate high-precision 14C dating of tsunami deposits based on the Bayes
theorem
ISHIZAWA, Takashi1∗ ; GOTO, Kazuhisa2 ; YOKOYAMA, Yusuke3 ; MIYAIRI, Yosuke3 ; SAWADA, Chikako3 ;NISHIMURA, Yuichi4 ; SUGAWARA, Daisuke2
1

Graduate School of Earth Science, Tohoku University, 2 International Research Institute of Disaster Science (IRIDeS),Tohoku
University, 3 Atmosphere and Ocean Research Institute, University of Tokyo, 4 Graduate School of Science, Hokkaido University
One of the most important issues to estimate recurrence interval of subduction zone earthquakes and tsunamis is precise
determination of depositional ages of tsunami deposits (Komatsubara et al., 2006). However, a common technique, which isusually dating organic materials from above and below the candidate deposits, often has large statistical errors ranging from few
tens to few hundreds of years. Furthermore, erosion of bottom soil during the tsunami inundation may lead large bracketing ageof the event (Goto et al., 2014).
These statistical errors arise during calibration of 14 C date. Because of the fluctuations in the calibration curve, calibration willoften result in widened and multi-peaked calendar age uncertainties (Blaauw, 2010). To solve this problem, the Bayes theorem
is useful to constrain the calibration results. The purpose of this study is to conduct millimeter-scale high resolution radiocarbon
dating and to estimate precise determination of the depositional age of tsunami deposits based on the Bayes theorem. In thisstudy, we use deposition model of Oxcal ver. 4.2.4 (Ramsey, 2009). An age-depth model can be made using this model based on
the information of the stratigraphic order of deposition and depth information (Ramsey, 2008).
As examples, here we show results of high-resolution radiocarbon dating of soil samples below the paleo tsunami deposits
at Urahoro, Hokkaido (Nanayama et al., 2002) and those at Noda Village, Iwate. For example, tsunami deposit at Urahoro isconsidered to have been formed in the 17th century (Nishimura and Nakamura, 2010), because it lies just below the Ta-b tephrathat erupted A.D. 1677. However, it is difficult to estimate the age using 14 C dating method, because this age has wiggles
in calibration curve. In spite of these wiggles, high resolution measurement allows us to make age-depth model using Bayes
theorem and to estimate the precise age of this deposit as early to middle 17th century.

Keywords: tsunami deposits, 14C dating, Bayes theorem

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MIS25-P19

Room:Convention Hall

Time:May 24 18:15-19:30

Laboratory experiment of tsunami deposit using a large wave flume
YOSHII, Takumi1∗ ; MATSUYAMA, Masafumi1
1

Central Research Institute of Electric Power Industry

The laboratory experiment was conducted to study the relationship between the hydraulic condition and sedimentological features of tsunami deposit. The large wave flume in CRIEPI which can generate incoming wave with up to 3.0 m/s on the shoreline
was used in this study. This large scale experiment allows us to use the actual sand and the reproducibility of sedimentologicalfeatures of tsunami deposit is expected. In the presentation, we will show the preliminary results of this laboratory experiments.
Keywords: tsunami deposit, laboratory experiment

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MIS25-P20

Room:Convention Hall

Time:May 24 18:15-19:30

Identification criteria of the tsunami deposits based on the Japanese paleotsunami researches
GOTO, Kazuhisa1∗ ; NISHIMURA, Yuichi2 ; SUGAWARA, Daisuke1 ; FUJINO, Shigehiro3
1

Tohoku University, 2 Hokkaido University, 3 Tsukuba University

Identification criteria of the tsunami deposits have frequently been discussed by many researchers (e.g., Morton et al., 2007;
Goff et al., 2012). As Goto et al. (2014) noted, it is generally agreed that there is no single criterion that can be used to identify atsunami deposit but rather that multi-proxy analyses of criteria, such as sedimentology, micropaleontology and geochemistry are
required.
Identification criteria may be different in countries and local regions because of the difference of geological, historical, andcultural backgrounds. For example, Japan has ˜1,300 years historical records of earthquake and tsunami. Therefore, manytsunami deposits up to 1,300 years can be correlated to the historically described tsunami events and this is used as one of themost important identification criteria in Japan (e.g., Komatsubara et al., 2006).
This study investigated previously published research papers that discussed about candidate tsunami deposits along the Japanese
coast to explore the identification criteria of the tsunami deposits specifically for Japan. We found that the identification criteriathat were used in the previous papers can be classified into 40 to 50 categories. Some of them are used to specify whether the
candidate deposit is the event deposit and thus those are not straightforward ones to identify tsunami deposit. On the other hand,
there are some categories that would be useful to identify the tsunami deposit. These information would be valuable to assign
the validity to the tsunami deposit identification for the future researches.
Keywords: tsunami, tsunami deposit

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