Tsunami deposits refine great earthquake rupture extent and recurrence over the past 1300 years along the Nankai and Tokai fault segments of the Nankai Trough, Japan

Quaternary Science Reviews xxx (xxxx) xxx

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Tsunami deposits refine great earthquake rupture extent and recurrence over the past 1300 years along the Nankai and Tokai fault segments of the Nankai Trough, Japan Osamu Fujiwara a, *, Akira Aoshima b, Toshiaki Irizuki c, Eisuke Ono d, 1, Stephen P. Obrochta e, Yoshikazu Sampei c, Yoshiki Sato a, Ayumi Takahashi f, 2 a

Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, AIST Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan Iwata Minami High School, 3084 Mitsuke, Iwata, Shizuoka 438-8686, Japan c Institute of Environmental Systems Science, Academic Assembly, Shimane University, 1060 Nishikawatsu-cho, Matsue, Shimane 690-8504, Japan d Faculty of Education, Niigata University, 8050 Ikarashi 2-no-cho, Nishi-ku, Niigata 950-2181, Japan e Graduate School of International Resource Science, Akita University, 1-1 Tegata Gakuen-machi, Akita 010-8502, Japan f Interdisciplinary Faculty of Science and Engineering, Shimane University, 1060 Nishikawatsu-cho, Matsue, Shimane 690-8504, Japan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 May 2019 Received in revised form 23 September 2019 Accepted 13 October 2019 Available online xxx

Study of prehistoric to medieval-age tsunami deposits along a riverbank site near the eastern Nankai Trough, central Japan, show that, not only did Tokai earthquakes occur with a higher frequency than previously thought, but that contemporaneous ruptures of the Tokai and Nankai fault segments were also more common. The site revealed a ~1-km long coast-normal cross section of the strand plain and exposed four sandy tsunami deposits, each of which indicates inundation over 2 km inland of the coast. Radiocarbon dating of previously studied and newly discovered deposits in the region indicates a shorter recurrence time for Tokai earthquakes and clarifies their linkage with Nankai earthquakes. We attribute the younger two tsunami deposits to the 1498 and 1096 CE Tokai earthquakes. The older two deposits confirm the occurrence of the Tokai earthquakes in 887 CE and in the latest 7th century. These events are not reliably recorded in historical documents in the Tokai region but were noted in the Nankai area. The 887 CE earthquake likely represents a full-length rupture of the Tokai and Nankai segments, as was the case for the 1707 CE earthquake. Integrated with the previous studies, these new results show that nine Tokai earthquakes occurred over the last 1300 years, the oldest in the latest 7th century, and in 887, 1096, 1361, 1498, 1614, 1707, 1854 and 1944 CE. Recalculated recurrence intervals range from 90 to 265 years. Except for the 1498 Meio Tokai earthquake, the Tokai earthquakes occurred simultaneously with Nankai earthquakes. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Nankai Trough Historical earthquake Tokai earthquake Tsunami deposit Recurrence mode

1. Introduction A future great (magnitude
8,
M8) earthquake on the Nankai Trough subduction zone (Fig. 1), and the accompanying large tsunami, have been of great concern in Japan for more than four decades. As a part of the response to the 2011 Tohoku-oki

* Corresponding author. E-mail address: [email protected] (O. Fujiwara). 1 present address: Faculty of Letters, Komazawa University, 1-23-1 Komazawa, Setagaya-ku, Tokyo, 154-8525, Japan. 2 present address: Himeji Medical CO-OP, 10 Futaba-cho, Himeji, 670-0832, Japan. https://doi.org/10.1016/j.quascirev.2019.105999 0277-3791/© 2019 Elsevier Ltd. All rights reserved.

earthquake (M9.0), the Japanese government formally announced that the “largest possible earthquake” rupturing the entire length of the Nankai Trough could be up to magnitude 9.0 to 9.1, with a maximum tsunami run-up height of ~30 m. Although such a worstcase scenario predicts massive fatalities and economic loss that would shake the foundations of modern Japan and impact many Pacific coasts (e.g., Central Disaster Management Council, 2013), the scenario does not consider historical or geological evidence of earthquakes rupturing the full 700 km length of the Nankai Trough prior to the 1707 Hoei earthquake (M8.6). Historical records show that many Nankai Trough earthquakes rupture only a few hundred kilometers of one or the other of two segments of the subduction zone, the western “Nankai” segment

Fig. 1. Temporal and spatial distribution of historical Nankai Trough earthquakes. Upper panel: Tsunami source areas for the 1707 CE (green line) and two 1854 CE earthquakes (orange solid and dashed lines), as modified from Hatori (1974, 1976). Earthquake rupture areas of the 1944 CE Tonankai and 1946 CE Nankai earthquakes (blue shaded areas) are modified from Tanioka and Satake (2001a, b). Black arrow shows plate movement vector from Zang et al. (2002) and Loveless and Meade (2010). Base map modified from Geospatial Information Authority of Japan (GSI): http://maps.gsi.go.jp/development/ichiran.html#relief (Land and submarine topographic data from GSI and Japan Coastal Guard, respectively). Lower panel: Horizontal solid and dotted lines show the estimated rupture areas for each earthquake by historical analyses (modified from Ishibashi, 2014). Liquefaction features at archaeological sites are modified from Sangawa (2007). Uplift around Cape Omaezaki interpreted as evidence for the 1361 CE Tokai earthquake, quoted from Fujiwara et al. (2010) and Kitamura et al. (2018). Possible tsunami deposit in the Kosai area from the 1361CE earthquake, from Garrett et al. (2018) and Riedesel et al. (2018). Coastal uplift around the Shiono-misaki during the 1361CE earthquake(s) as reported by Shishikura et al. (2008).

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and the eastern “Tokai” segment, with a boundary in the Cape Shiono-misaki region (Fig. 1). Every 90e150 years over the past 400 years, great earthquakes have occurred on the Nankai and Tokai segments less than several years apart. Examples of closely spaced earthquakes are the 1854 CE Ansei Tokai (M8.4) and Ansei Nakai (M8.4; only 32 h apart) and the 1944 CE Tonankai (M7.9) and 1946 CE Nankai (M8.0) earthquakes. Although the 1707 CE Hoei earthquake ruptured the Tokai and Nankai segments simultaneously, which earthquakes ruptured one or both segments is unconfirmed prior to the 17th century (Fig. 1). This is primarily because historical records from the rural Tokai region were incomplete compared to those from the Nankai area near to the Kyoto and Nara regions, where the capital of Japan was located. Reliably-documented earthquakes with descriptions of ground shaking and tsunami, and in some cases coseismic coastal uplift and subsidence, include the 1498 CE Meio (M8.2e8.4), 1096 CE Eicho Tokai (M8.0e8.5), 1361 CE Koan Nankai (M8.2e8.5), 887 CE Nin-na (M8.0e8.5), and 684 CE Hakuho earthquakes (M8.2?). Despite the incompleteness of available records, historical analyses suggest relatively long and irregular recurrence intervals in older times (the longest more than 400 years, between the 1096 CE earthquake and the previous one) (Fig. 1). The occurrence of a 1099 CE Kowa Nankai earthquake continues to be debated (Ishibashi, 2016), and a Nankai Trough earthquake formerly believed to have occurred in 1605 CE may date from 1614 CE (Ishibashi, 2014). This conclusion is also supported by results from an extensive network of sediment cores obtained from the Shima Peninsula facing the eastern Nanakai Trough (Fujino et al., 2018). The cores provide evidence for an additional ten possible tsunami deposits ranging in age from 4500 to 500 years ago that occurred with similarly long recurrence times and large variance (100e600 years). The youngest three tsunamis may have accompanied the 1498, 1096 and 684 CE Nankai Trough earthquakes (Fujino et al., 2018). Based on the spatial distribution of seismic intensity, tsunami height, and coastal deformation associated with historical Nankai Trough earthquakes, Seno (2012) proposed that earthquakes there be classified into two types with respect to earthquake rupture mode and recurrence pattern. He defined the “Hoei-type” (887, 1361, 1707 and 1944 þ 1946 CE) earthquake and the “Ansei-type” (684, 1096 þ 1099, 1498, 1854 þ 1854 CE) earthquake, with estimated recurrence intervals of ~350 and ~400 years, respectively. The most recent examples of the Hoei-type are the 1944 and 1946 earthquakes, and the two 1854 events are of the Ansei-type. Seno (2012) predicts that the next Tokai earthquake will be of the Ansei-type and will not occur for at least 200 years. Assessment of such analyses of the historical record, which are of the utmost importance for the disaster management, can only be made through study of the much longer records of prehistoric as well as historic events. Paleoseismological studies that rely on archaeological and coastal stratigraphic evidence, such as liquefaction features that record strong ground shaking, shorelines that show coastal uplift or subsidence, and the deposits of high tsunamis, have improved the chronology of great earthquakes back to 6e7 ka (e.g., Garrett et al., 2016). Further, evidence of coseismic uplift at Cape Omaezaki (Fig. 1) (Fujiwara et al., 2010; Kitamura et al., 2018) integrated with the discovery of a likely tsunami deposit in the Kosai region, 60 km west of the cape (Garrett et al., 2018; Riedesel et al., 2018), and analysis of historical documents describing the strong ground shaking around Kyoto, Nara and Kumano (north of Shiono-misaki) (Ishibashi, 1998, 2014) suggest a possible Tokai earthquake coincident with the 1361 CE Koan Nankai earthquake (Fig. 1). The coseismic uplift around Cape Omaezaki is a common characteristic with the 1854 CE Ansei Tokai earthquake and suggest westward propagation of a fault rupture in this area. Historical document suggests that the1361CE Nankai earthquake

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occurred two days after the Tokai earthquake (Ishibashi, 1998, 2014). Analyses of archaeological evidence are critical for deciphering the history of past Nankai Trough earthquakes and their tsunamis, partly because of the difficulties of precisely dating events with radiocarbon and archaeological chronology during this period (e.g., Ishibashi, 2014; Garrett et al., 2016). Archaeological evidence documents the prosperity and decline of coastal settlements associated with the Tokai earthquakes. The Motojima (元島 in Japanese) ruins (Fig. 2) at our study site provide a good example (Shizuoka prefectural archaeological research institute, 1998, 1999). The name of the ruins means the former (元; moto) location of two medieval villages, Ohshima and Kojima, which were relocated due to river improvement work beginning in 1604 CE. The villages were important bases for the water transportation industry in the medieval period, reaching a peak in the late 15th century and then declining rapidly. Liquefaction features discovered from the Motojima ruins suggest that the decline was a result of the 1498 CE Meio earthquake and tsunami, for which historical records indicate heavy damage along the Enshu-nada coast (Fig. 1) (Shizuoka prefectural archaeological research institute, 1998, 1999; Yata, 2009). Occurrence of a Nankai earthquake around 1498CE and of Tokai earthquakes at about 887 and 684 CE is suggested by the strong ground shaking recorded as liquefaction features at archaeological sites and historical descriptions (Ishibashi, 1999; 2014; Sangawa, 2001, 2007) (Fig. 1). This raises the possibility of two full-length Nankai Trough earthquake ruptures in 887 and 684CE (Ishibashi, 1999, 2014). However, given the lack of evidence for coincident tsunamis, the possibility remains that the ground shaking was caused by inland earthquakes. Determining coastal run-up distance is key to differentiating tsunami from storm deposits (e.g., Fujiwara, 2015; Fujino et al., 2018), but much of the coastal plains along the Nankai Trough coast are not wide enough to distinguish short-distance inland washover during large storms from long-distance inland inundation by large tsunami. Sandy tsunami deposits are most confidently identified when emplaced on finer sediment, but much coarse sediment of similar grain size deposited by rivers along Nankai Trough coasts makes it difficult to distinguish the tsunami deposits from “normal” river deposits in the lowlands. Additionally, widespread agricultural activity in this area since prehistoric times has obliterated much evidence of tsunami. Here we report the results of a study of tsunami deposits in the Otagawa Lowland, an exceptionally wide strand plain overlying a muddy sedimentary basin that faces the Tokai segment of the Nankai Trough (Figs. 1 and 2). We mapped the deposits of four large tsunami along a 1-km long, 4-m high coast-normal cross section in recent excavations at a river-improvement site. The area has had little anthropogenic disturbance because, prior to modern reclamation, it was largely a wetland unfit for farming and human habitation (e.g., Shizuoka prefectural archaeological research institute, 1998). This wide strand plain site is ideal for measuring inundation distance in order to distinguish tsunami from storm deposits, and the muddy tidal sediment beneath the plain contains abundant material for radiocarbon dating. 2. Study area The Otagawa Lowland is located to the east of the Tenryu river fan delta (Fig. 2A). The Tenryu river (catchment area of 5050 km2) is the main sediment source of the Enshu-nada coast (e.g., Hattori et al., 1974), which is dominated by quartz- and feldspar-rich sand derived from the granitic rocks widely distributed further inland (Fig. 2A) (Yoshii and Sato, 2010). The Otagawa river

Fig. 2. Overview map and imagery of the study area. A: Primary surface geology modified from Geological Survey of Japan (2015) and surface sample locations. The Shimanto Group lange matrix of the Late Cretaceous-Paleogene accretionary complex. The Ryoke granitoids are Early to Late Cretaceous felsic plutonic rocks. Late Pleistocene terraces (light is a me green patches in central part of the figure) and the alluvial lowland (pale blue parts) consist mainly of sand, gravel and mud. Shaded map from GSI (https://maps.gsi.go.jp/

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development/ichiran.html) B: Aerial photograph showing sample locations and excavation areas. Photograph taken by Fukuroi public works office in May 2010. C: Topographic classification map of the Otagawa Lowland showing sample locations. Topographic classification determined using aerial photographs (scale 1/20,000) taken in 1970 by the GSI. Water areas during the 15-16th century based on Shizuoka prefectural archaeological research institute (1999) and Fujiwara et al. (2007).

Fig. 2. (continued).

catchment (488 km2) is underlain by rocks of mainly the Mesozoic and Cenozoic accretionary prism of the Shimanto Group (Fig. 2A). Due to its distance from the fan delta, mainly fine-grained sediment has been deposited in the Otagawa Lowland. Rapid tectonic subsidence of ~0.8 m/1000 years (Fujiwara et al., 2015) has led to the accumulation of a thick, muddy back-marsh sequence, in which tsunami deposits are well preserved. The Enshu-nada coast is a microtidal environment (about 1.3 m during spring tide). The Otagawa Lowland has prograded seaward over the last 6000 years (Watanabe, 1995), as evidenced by preservation of three rows of sandy beach ridges, referred to herein as rows I, II, and III from landward to seaward (Fig. 2C). The beach ridges are ~3e4 m in height, except for sea-side portions of row III that represent artificially enhanced coastal dunes with a relief of ~10 m. Archaeological remains and the radiocarbon ages obtained from the backmarsh deposits are key to dating the ridges and their development. Previous work suggests that beach ridge row I, on which the Motojima ruins lie, marks the Middle Holocene coastline (Watanabe, 1995). Later the coast line prograded to the position of beach ridge row II, on which the 5th century or earlier Hamanbe ruins are located (Board of Education, Iwata City, 1987; Fujiwara et al., 2008). Beach ridge row III, to the south of the Maekawa river, began to form by the 13 to 14th century (Fujiwara et al., 2007), providing a barrier that allowed the river to be used as an inland waterway by the 15th century (Shizuoka prefectural archaeological research institute, 1999). The positions of the beach ridges indicate that the coast was 1.3 km, 0.8 km, and 0.5-0.7 km inland of its present position during the 5th, 13th to 14th, and 15th centuries, respectively. According to the historical maps, the coastline has further prograded several hundred meters to its present position

since the 17th century (Fujiwara et al., 2007; Ebara, 2012). Previous work on swales in Sumatra (Monecke et al., 2008), Thailand (Jankaew et al., 2008), Hokkaido (Nanayama et al., 2003), and Sendai (Sawai et al., 2012) has demonstrated their utility in preserving tsunami deposits. Swales of the Otagawa Lowland are filled by the fine sediment deposited in marshes, swamps, and small meandering rivers. These areas have been drained since the 16th century (Shizuoka prefectural archaeological research institute, 1999; Ebara, 2012). Old Ota River flowed westward separately from the Haranoya River, which flowed southward and connected to the Maekawa River before the replacement of the Ota River to the modern watercourse in 1604CE (Fig. 2C). The Benzaiten River is a relic of a former lagoon, the mouth of which was narrowed by beach ridge rows II and III. A storm surge generated by the 1680 CE typhoon, one of the largest in the Edo period of Japan, inundated 4e5 km inland along this lagoon (History of Asaba town editing committee, 2000). Probable cause of this extreme inundation was the synergistic effect of heavy rain, coastal set up during strong winds, and poor drainage due to the damming effect of the narrow lagoon mouth between the beach ridges. A ~10 cm thick muddy sand bed near the lagoon mouth was likely deposited with this storm surge and flooding (Fujiwara et al., 2007). In contrast, the central to western part of the lowland, including our study area, is well protected from the Pacific Ocean. The tsunami that destroyed the Motojima settlement appears to have reached a height of 8 m along the Enshu-nada coast (Hatori, 1975). While the 1096, 1605 CE tsunami caused severe damage along the eastern Nankai Trough coast (e.g., Ishibashi, 2014; Yata, 2009), there is no clear evidence of damage in the Otagawa Lowland, and construction of artificial banks after the 1680 CE typhoon

6 O. Fujiwara et al. / Quaternary Science Reviews xxx (xxxx) xxx Fig. 3. Schematic coast-normal cross-section of the study site. Former coastlines located ~1.3 km (5th century), ~0.8 km (13th century) and ~0.5 km (15th century) from the present coastline. Historical maps from the 17th century indicate that the coastline was located several hundred meters inland from its present position. Detailed geologic column sections are shown in Fig. A.1.

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Fig. 4. Photograph of tsunami deposits at the southern site A) Overview of southern and western part of the 2010e2102 CE excavation site in the Motojima Ruins. Differential erosion of the excavated walls highlights the Ts2 deposit, consisting of an unconsolidated, relatively course-grained bed, as a horizontal groove in back-marsh mud. Base and thickness of Ts2 are shown by dotted line and arrow, respectively. Yayoi Period (~2nd century BCE) surface (buried beach ridge) is exposed on the floor of the excavated site. B) Close up of deposit Ts2. Peeled face of the section shows ripple lamination in a very fine sand bed. Thick and thin sand layers (white bars) alternate with mud drapes (black bars). C) Southern side of the southern site showing the tsunami deposits (from top) Ts3, Ts2, and Ts1, which are expressed as dark bands in the excavation walls. These sandy beds serve as an aquifer. Base and thickness of Ts3, Ts2, and Ts1 are shown by dotted lines and arrows, respectively. D) Landward view of the Ts1 deposit from site S1. Concentration of mud balls and wood debris characterizes the upper and lower part of Ts1, respectively (See Fig. A.2). Base and thickness of Ts1 are shown by dotted line and arrow, respectively.

disaster and coastal dunes protected the area from the ~5e6 m high 1707 and 1854 tsunamis (Fujiwara, 2013). Tsunami from the 1944 CE Tonankai earthquake reached about 2 m high around the Cape Omaezaki area but was only ~0.9 m around the Otagawa Lowland (Watanabe, 1998). Our investigation of tsunami deposits (from summer of 2011 CE to summer of 2014 CE) was conducted at the southern and northern sites, which are separated by the area disturbed by the 1994e2001CE archaeological excavation and pre-2011 CE river

improvement works (Fig. 2B). At the southern site, we first researched a flood plain sequence covering the buried beach ridge row I (Figs. 3 and 4A) at the archaeological excavation site, which was excavated in 2010e2012 CE, and subsequently studied the flood plain sequence on both sides of the buried beach ridge row I along the excavation walls during the river improvement works exhuming the Motojima ruins (Figs. 3 and 4C). The trench walls at the archaeological excavation site (3365 m2) extended continuously ~60 m and ~120 m coast-normal and coast-parallel directions,

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respectively. The height of the studied walls was ~2 m-high, except for the western wall which reached a height of 3.5 m. Spoil from the excavation along the northeastern wall (between site S5 and S7 in Fig. 2B) made it unsuitable for study. Tombs ranging from the 2nd BCE to 3rd century CE (from later Middle Yayoi period to the early Kofun period) were constructed on the beach ridge (Shizuoka prefectural archaeological center. 2013). River improvement works exposed a ~4 m deep trench extending ~300 m and 120 m coast-normal and coast-parallel directions, respectively. At the northern site, the river improvement works exposed a ~4 m-deep trench, which was ~500 m and ~65 m in coast-normal and coastparallel directions, respectively. 3. Materials and methods 3.1. Description of outcrops The excavated walls, mainly composed of clay and humic clay beds intercalated with occasional beds and layers of silt, sand and gravel, were described using standard methods by visual observation and touch/feel (e.g., Nelson et al., 1996). Peels of sedimentary structures at some locations on the excavated walls were made using a hydrophilic polyurethane resin (HYCEL SAC-100). Elevation of the excavated walls is mainly based on the leveling data by the construction work office and Shizuoka prefectural archaeological center using a total station, supplemented by the authors with a Virtual Reference Station (VRS) using a real-time kinematic global positioning system. Identification of the potential tsunami deposits from the excavated walls was based mainly on three sedimentary properties, landward current direction, long inundation distance, and multiple layered structures (e.g., Fujiwara and Kamataki, 2008; Naruse et al., 2010; Fujiwara, 2015). Landward current direction was reconstructed from the current ripples and landward fining and thinning trend of the sand beds. We tracked each potential tsunami deposit along the excavated walls and measured their inundation distances from the estimated coast lines at the times that the tsunami occurred. Structures consisting of interbedded sand and mud layers are formed by intermittent flows, sand layers are formed by sediment flows, and mud layers are the result of suspension fallout in stagnant muddy water (e.g., Martin, 2000). During a large tsunami, runup flow forms a normally graded sand layer in its waning stages, and the subsequent stagnant muddy water leaves a mud drape on the sand layer. The same depositional process applies during tsunami back wash, where a sand layer and mud drape couplet represents a run-up or backwash flow. In such cases, large tsunami consisting of a train of large waves sometimes leaves behind multiple layered deposits (e.g., Nanayama and Shigeno, 1993; Fujiwara and Kamataki, 2008; Naruse et al., 2010; Fujiwara et al., 2012; Fujiwara and Tanigawa, 2014). Since the size of each wave decreases toward the end of a tsunami wave train, ideally layer thickness and grain size also decrease upward, creating a fining upward sequence. Such tsunami deposits are clearly distinct from deposits that consist of an amalgamation of multiple flood and storm deposits, in which each layer has arrhythmic thickness (Fujiwara, 2008, 2015). In addition to above-mentioned three features, we also consider the horizontal distribution pattern to distinguish the potential tsunami deposits from the abandoned channel fill deposits. In cross-section, the appearances of the meandering, abandoned channels varies with the crossing angle of the channels and excavated walls. They exhibit a symmetric perpendicular u-shape when orthogonal to the flow axis, but an asymmetric u-shape or horizontal beds in other cases. On the other hand, tsunami deposits

exhibit a flat, tabular shape regardless of viewing angle. 3.2. Chemistry and grain-size analyses Carbon, nitrogen and sulfur (CNS) elemental measurements, along with grain-size analysis, aid in identification of sedimentary environments and structures, respectively. Sulfur is a commonly used indicator for sea water incursion into coastal lowland marshes. A high C/S ratio is suggestive of an oxygenated environment and/or decreasing salinity, with values above five indicative of freshwater, values near three typical of fine seafloor sediment deposited in an oxic environment, and values near one commonly exhibited by sediment deposited in poorly-oxygenated bays and estuaries (Berner, 1984; Berner and Raiswell, 1984; Lyons and Berner, 1992; Sampei et al., 1997). Thus, variation of C/S ratios is useful in detecting relative sea-level changes caused by coastal uplift or subsidence. The origin of plant organic matter is reflected in C/N ratios, with freshly deposited organic matter derived mainly from plankton in the range of 6e9 (Bordowskiy, 1965a; Prahl et al., 1980; Biggs et al., 1983) and terrestrial plants generally above 15 (Bordowskiy, 1965b; Ertel and Hedges, 1984; Post et al., 1985; Ertel et al., 1986; Hedges et al., 1986; Orem et al., 1991). Geochemical signatures of marine origin are also useful in identifying sediment deposited by tsunami that lack marine microfossils (e.g., Chague Goff et al., 2012). The vertical variation in grain size can also differentiate tsunami deposits by revealing beds with multiple graded structures (e.g. Fujiwara and Kamataki, 2008; Naruse et al., 2010). CNS elemental analysis and grain-size analysis were conducted on continuous sediment cores collected from Site N6, in the central area of the northern site (Fig. 3). Eleven short cores were collected from the excavated wall using a 2  2 cm diameter, 50-cm-long, uchannel sampler and then spliced into a composite 383-cm-long core. The sediment cores were sub-sampled into 1-cm thick slices. Each slice was subdivided into two sub-samples: one for CNS elemental analysis and the other for grain-size analysis. CNS elemental analysis was conducted on 90 sub-samples at 4 cm intervals along the composite core. Approximately 2 g of dried samples were powdered using an agate mortar and pestle, and approximately 10 mg of each powdered sample was placed in a thin Ag film cup and weighed. 1M-HCl was then added several times at ~110  C to remove the carbonate fraction. The dried samples were wrapped in a thin Sn film cup for combustion. The total organic carbon (TOC), total nitrogen (TN), and total sulfur (TS) contents were measured by using a CHNS elemental analyzer (EA1108; FISON Co. Ltd.). Grain size was measured for 192 sub-samples at 1e2 cm intervals along the composite core. A portion of each dried sample was soaked in a 6% hydrogen peroxide solution for several days to remove organic matter, and then ultrasonicated prior to analyzing with a laser diffraction particle-size analyzer (SALD3000S; Shimadzu Co., Ltd.). 3.3. Mineralogy We expected differing mineral compositions that reflect differences in bedrock geology in the catchment areas between the Enshu-nada beach sand and Otagawa river bed sand. Because of the general absence of marine microfossils in sand beds from the excavated walls, unique mineral suites in the beach sediment can be an index of washover deposits. Garnet in particular is a good indicator of marine incursion because it is common in Enshu-nada coast sediment and derived from the Ryoke granitoids and metamorphic rocks distributed in the Tenryu River catchment (Aoshima et al., 2011) but not in the Otagawa River catchment (Fig. 2A).

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Table 1 Results of radiocarbon dating with sample number, ID number, measured material, d13C ratio, conventional 14C age (radiocarbon years), and calibrated 1 and 2s calendar year ranges. All samples were measured at Paleo Labo. Co., Ltd., Japan, by the AMS method. Sample No.

ID No.

Material

d13C (‰)

14C Age (yrBP±1s)

1

PLD-26109

Charred piece of tree trunk or branch

27.69 ± 0.49

956 ± 23

2

PLD-26108

Piece of tree trunk

29.41 ± 0.24

1030 ± 20

3

PLD-23986

Charred piece of tree trunk or branch

28.91 ± 0.21

1173 ± 18

4

PLD-23982

Charred piece of tree branche

27.90 ± 0.22

1173 ± 19

5

PLD-23985

Charred piece of tree trunk or branch

28.19 ± 0.24

1173 ± 19

6

PLD-23984

Charred piece of tree trunk or branch

27.00 ± 0.17

1228 ± 18

7

PLD-23983

Charred piece of tree trunk 2  12 cm

28.78 ± 0.21

1330 ± 20

8

PLD-26111

27.93 ± 0.75

1204 ± 28

9

PLD-26110

Charred piece of tree trunk or branch ~3 cm long Piece of tree trunk or branch

28.26 ± 0.21

2447 ± 21

10

PLD-26851

Piece of tree trunk or branch 0.8  5 cm

30.51 ± 0.16

1234 ± 18

11

PLD-19520

Plant remains (stem of herbs)

26.13 ± 0.15

981 ± 18

12

PLD-19519

Plant remains (stem or leaf of herbs)

28.05 ± 0.12

937 ± 19

13

PLD-20052

Piece of tree trunk or branch 216 cm

22.22 ± 0.17

1822 ± 19

14

PLD-20051

Charred material (piece of tree?)

11.54 ± 0.15

1683 ± 19

15

PLD-19521

Tree trunk or branch

26.70 ± 0.12

935 ± 20

16

PLD-20885

Charred material (piece of tree?)

27.22 ± 0.17

387 ± 19

17

PLD-26846

Tree trunk or branch

28.60 ± 0.20

947 ± 17

18

PLD-26847

Tree trunk or branch

26.29 ± 0.16

1851 ± 18

19

PLD-26848

Tree trunk or branch

27.38 ± 0.15

2030 ± 19

20

PLD-26849

Tree trunk or branch

24.63 ± 0.15

3034 ± 20

21

PLD-26850

Charred tree trunk or branch

24.57 ± 0.16

3268 ± 20

22

PLD-19525

Plant remains (stem or leaf of herbs)

26.50 ± 0.12

923 ± 20

23

PLD-19530

Tree trunk or branch

22.50 ± 0.12

1010 ± 19

Calibrated Age 1s

2s

1028 - 1047 CE (21.6%) 1088 - 1122 CE (36.3%) 1138 - 1148 CE (10.4%) 995 - 1009 CE (41.9%) 1011 - 1019 CE (27.1%) 778 - 791 CE (14.1%) 805 - 819 CE (11%) 822 - 841 CE (15.7%) 861 - 889 CE (27.8%) 778 - 791 CE (13.7%) 805 - 819 CE (11%) 821 - 841 CE (16.3%) 860 - 889 CE (27.8%) 778 - 791 CE (13.7%) 805 - 819 CE (11%) 821 - 841 CE (16.3%) 860 - 889 CE (27.8%) 719 - 741 CE (22.6%) 767 - 777 CE (13.4%) 792 - 804 CE (9.96%) 812 - 825 CE (7.94%) 841 - 862 CE (14.5%) 659 - 681 CE (69.1%)

1023 - 1058 CE (29.2%) 1067 - 1073 CE (1.89%) 1076 - 1153 CE (64.4%) 984 - 1026 CE (95.8%)

773 - 779 CE (5.52%) 789 - 869 CE (63.4%) 736 - 687 BCE (26%) 662 - 646 BCE (7.9%) 547 - 480 BCE (31.6%) 440 - 432 BCE (2.81%) 712 - 744 CE (36.5%) 765 - 777 CE (15%) 792 - 803 CE (6.81%) 819 - 821 CE (1.26%) 842 - 859 CE (8.83%) 1020 - 1041 CE (56.7%) 1107 - 1116 CE (12.1%) 1040 - 1050 CE (9.96%) 1082 - 1128 CE (42.4%) 1134 - 1151 CE (16.6%) 140 - 157 CE (16.3%) 166 - 195 CE (28.5%) 208 - 231 CE (23.6%) 346 - 373 CE (41.3%) 376 - 392 CE (27%) 1040 - 1052 CE (11.2%) 1081 - 1109 CE (25.9%) 1116 - 1151 CE (31.6%) 1451 - 1488 CE (63%) 1604 - 1608 CE (6.26%) 1033 - 1047 CE (16%) 1087 - 1123 CE (40.5%) 1138 - 1149 CE (12.5%) 129 - 176 CE (46.7%) 190 - 212 CE (22.3%) 47 - 16 BCE (46.9%) 13 BCE - 1 CE (21.7%) 1372 - 1354 BCE (15.2%) 1300 - 1258 BCE (44.8%) 1244 - 1233 BCE (8.76%) 1604 - 1583 BCE (23.5%) 1556 - 1554 BCE (2.52%) 1544 - 1536 BCE (7.94%) 1534 - 1505 BCE (35.1%) 1046 - 1093 CE (41.9%) 1121 - 1141 CE (19%) 1147 - 1155 CE (7.86%) 998 - 1004 CE (12.9%) 1012 - 1028 CE (56.8%)

775 - 894 CE (92.5%) 930 - 938 CE (3.02%)

775 - 894 CE (91.6%) 929 - 940 CE (3.96%)

775 - 894 CE (91.6%) 929 - 940 CE (3.96%)

695 708 764 787

-

703 745 781 878

CE CE CE CE

652 701 747 715 766 749 666 620 589 691 762 789

-

695 709 762 743 892 683 637 619 411 747 779 873

CE (85.5%) CE (2.23%) CE (7.93%) CE (6.72%) CE (88.7%) BCE (29.4%) BCE (10.2%) BCE (0.2%) BCE (55.7%) CE (44.7%) CE (16.4%) CE (34.4%)

1016 1086 1137 1034 1061

-

1048 1124 1149 1059 1154

(1.87%) (27.2%) (15.5%) (50.9%)

CE CE CE CE CE

(62.5%) (27.3%) (5.73%) (19.4%) (76%)

133 - 239 CE (95.8%)

264 - 274 CE (3.15%) 331 - 408 CE (92.4%) 1034 - 1154 CE (95.5%)

1446 - 1516 CE (80%) 1596 - 1617 CE (15.6%) 1027 - 1058 CE (24.3%) 1068 - 1072 CE (1.29%) 1076 - 1153 CE (70%) 89 - 104 CE (4.51%) 122 - 231 CE (91%) 91 - 66 BCE (7.88%) 63 BCE - 23 CE (87.6%) 1386 - 1339 BCE (26.7%) 1315 - 1314 BCE (0.3%) 1309 - 1219 BCE (68.5%) 1609 - 1502 BCE (95.5%)

1038 - 1159 CE (95.5%)

989 - 1034 CE (95.5%) (continued on next page)

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Table 1 (continued ) Sample No.

ID No.

Material

d13C (‰)

14C Age (yrBP±1s)

24

PLD-20055

Tree trunk or brancha 110 cm

23.38 ± 0.18

1201 ± 19

25

PLD-19527

Plant remains (stem or leaf of herbs)a

23.52 ± 0.12

1240 ± 20

26

PLD-19526

Tree trunk (outermost grouth ring)a

23.51 ± 0.13

1257 ± 19

27

PLD-20053

Plant remains (stem or leaf of herbs)a

27.13 ± 0.20

1233 ± 19

28

PLD-20056

Ttree trunk or brancha 0.64 cm

22.38 ± 0.16

1865 ± 22

29

PLD-19528

Tree trunk (outermost grouth ring)a

23.82 ± 0.18

1874 ± 21

30

PLD-19524

Tree trunk (outermost grouth ring)a

24.80 ± 0.13

1687 ± 20

31

PLD-19529

Charred material (piece of tree?)a

29.33 ± 0.12

2144 ± 20

32

PLD-20054

Plant remains (stem or leaf of herbs)a

24.78 ± 0.15

1751 ± 19

a

Calibrated Age 1s

2s

775 - 778 CE (3.75%) 789 - 829 CE (37.1%) 837 - 867 CE (27.9%) 693 - 747 CE (52.9%) 763 - 775 CE (13.9%) 795 - 797 CE (1.43%) 695 - 703 CE (9.72%) 707 - 746 CE (50.8%) 764 - 769 CE (7.82%) 713 - 744 CE (33.1%) 765 - 777 CE (14.3%) 792 - 804 CE (7.75%) 816 - 823 CE (3.6%) 87 - 106 CE (15.7%) 121 - 171 CE (40.2%) 194 - 210 CE (12.4%) 80 - 139 CE (62.2%) 160 - 160 CE (0.487%) 197 - 207 CE (5.92%) 344 - 390 CE (68.7%)

771 - 885 CE (95.7%)

340 - 326 BCE (9.75%) 203 - 163 BCE (55.7%) 126 - 122 BCE (3.01%) 250 - 261 CE (13.3%) 278 - 327 CE (55.9%) 1621 - 1601 BCE (22.1%) 1583 - 1543 BCE (43.1%) 1537 - 1534 BCE (3.65%) 1523 - 1573 CE (54.2%) 1628 - 1641 CE (14.1%)

33

PLD-19523

Tree trunk or branch

28.07 ± 0.12

3310 ± 22

34

PLD-20706

Charred tree trunk or branch

26.85 ± 0.14

311 ± 18

35

PLD-20705

Charred material (piece of tree?)

29.29 ± 0.21

273 ± 20

1530 - 1541 CE (15.4%) 1635 - 1656 CE (53.2%)

36

PLD-20702

Charred material (piece of tree?)

27.05 ± 0.14

317 ± 18

37

PLD-20701

Plant remains (stem or leaf of herbs)

26.62 ± 0.18

353 ± 18

38

PLD-20703

Plant remains (stem or leaf of herbs)

25.95 ± 0.15

950 ± 18

1522 1585 1624 1479 1578 1591 1031 1088 1138

39

PLD-20704

Plant remains (stem or leaf of herbs)

27.89 ± 0.15

1135 ± 18

a

40 41

PLD-20700 PLD-20699

Charred material (piece of tree?) Charred material (piece of tree?)a

27.80 ± 0.22 22.36 ± 0.36

1425 ± 21 4189 ± 24

42

PLD-21535

Stem of reed

28.93 ± 0.18

907 ± 18

43

PLD-21534

Piece of tree trunk or branch

27.50 ± 0.23

1124 ± 19

44

PLD-20886

Plant remains (stem or leaf of herbs)

28.64 ± 0.21

866 ± 20

45

PLD-20887

Charred material (piece of tree?)

23.14 ± 0.17

893 ± 19

46

PLD-21532

Charred material (piece of tree?)

27.94 ± 0.19

1080 ± 19

47

PLD-21536

Charred material (piece of tree?)

27.16 ± 0.23

2692 ± 20

48 49

PLD-21533 PLD-24571

Charred material (piece of tree?) Leaf of trees

28.55 ± 0.17 29.10 ± 0.23

2585 ± 20 1837 ± 22

50

PLD-26845

Ostrea gigas (Attached on the boulder)

0.30 ± 0.22

1370 ± 19

The samples are generally smaller than 1e2 cm long, except as otherwise noted. a Quoted from Fujiwara et al. (2015).

-

1575 1590 1638 1521 1582 1621 1047 1122 1148

CE CE CE CE CE CE CE CE CE

(51.1%) (4.66%) (13.2%) (37.7%) (3.24%) (27.6%) (18.5%) (39%) (11.4%)

889-901 CE (17.1%) 921-961 CE (51.8%) 616 - 647 CE (69.2%) 2878 - 2861 BCE (15.8%) 2806 - 2757 BCE (44.7%) 2716 - 2706 BCE (7.98%) 1049 - 1085 CE (42.7%) 1124 - 1137 CE (12.7%) 1150 - 1161 CE (13.2%) 894 - 903 CE (12.4%) 918 - 933 CE (19.2%) 936 - 966 CE (37.2%) 1164 - 1205 CE (68.7%) 1051 - 1081 CE (31.7%) 1129 - 1132 CE (2.62%) 1152 - 1185 CE (34.3%) 902 - 919 CE (19.3%) 964 - 994 CE (49.1%) 888 - 880 BCE (8.53%) 842 - 810 BCE (60%) 799 - 784 BCE (70.2%) 134 - 179 CE (43.3%) 187 - 213 CE (25.6%) Marine13: 1008 - 1054 CE (68.9%)

688 757 791 678

-

754 778 867 774

CE CE CE CE

(56.5%) (16.5%) (22.5%) (95.5%)

691 - 748 CE (41.5%) 762 - 779 CE (15.7%) 789 - 875 CE (38.2%) 81 - 220 CE (95.7%)

77 - 214 CE (95.7%)

262 328 349 208

-

277 407 304 105

CE (5.62%) CE (89.9%) BCE (18.5%) BCE (77%)

237 - 342 CE (95.5%) 1640 - 1519 BCE (95.6%)

1498 1513 1617 1523 1629 1788 1495 1512 1616 1463 1556

-

1503 1600 1644 1572 1664 1791 1507 1600 1643 1525 1632

CE CE CE CE CE CE CE CE CE CE CE

(1.24%) (72.6%) (21.6%) (34.8%) (60.1%) (0.6%) (3.6%) (71.5%) (20.3%) (45.6%) (50%)

1025 - 1058 CE (26.3%) 1069 - 1070 CE (0.4%) 1076 - 1153 CE (68.7%) 781 - 786 CE (0.9%) 875 - 977 CE (94.5%) 597 - 655 CE (95.7%) 2886 - 2848 BCE (22.2%) 2812 - 2736 BCE (53.3%) 2734 - 2691 BCE (18.3%) 2688 - 2678 BCE (1.78%) 1040 - 1108 CE (55.5%) 1116 - 1169 CE (37.8%) 1172 - 1183 CE (2.31%) 887 - 978 CE (95.6%)

1055 - 1077 CE (4.84%) 1154 - 1221 CE (90.6%) 1045 - 1095 CE (38.5%) 1120 - 1141 CE (10.2%) 1147 - 1211 CE (46.8%) 898 - 924 CE (23.9%) 944 - 1015 CE (71.7%) 893 - 861 BCE (23.7%) 859 - 807 BCE (72%) 802 - 771 BCE (96%) 94 - 96 CE (0.5%) 126 - 239 CE (95%) Marine13: 976 - 1096 CE (95.5%)

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Eight samples were collected from potential tsunami deposits (Ts1, Ts2, Tn1 and Tn4), modern (Sample M1) and former beach deposits (buried beach ridge sand, Sample M2), and modern (Sample M5) and former Ota River floor deposits (sand filling the abandoned river channel; Sample M3) (Figs. 2, 3 and 6A and Fig. A.1). Approximately 50 g were wet-sieved to separate the 250e355 mm size fraction. Because the grain size of the sample obtained from Tn4 was finer than 250 mm, we used panning to separate the ~100 mm size fraction from the silt and clay fractions. Each sample was ultrasonically cleaned for 10e20 min with dilute hydrochloric acid to remove iron oxide coating the grains. After drying, we used a binocular stereomicroscope (20e200 magnification) to categorize at least 200 grains by mineral species. Percentages of quartz, feldspar, lithic fragments and opaque minerals (mainly mafic minerals) were calculated. Heavy minerals were condensed using tribromomethane (specific gravity, 2.9) from four samples, Ts1, Tn1, modern beach sand (Sample M1) and modern Ota River floor (M5). Other samples were too fine and too poor in heavy minerals for effective separation. After washing with acetone, dried samples were examined with a binocular stereomicroscope (20e200 magnification), and the minerals of at least 200 grains were identified. Percentages of muscovite, biotite, magnetite, amphibole, pyroxene, olivine, garnet

11

and epidote were also calculated. In the same way, we tried to identify garnet crystals in an additional four samples (M4, M6, M7 and M8) from different localities along the Otagawa River (Fig. 2). 3.4. Chronology Fifty samples obtained from the excavated walls were radiocarbon-dated using accelerator mass spectrometry (AMS) to create an age framework for the back-mash sequence and date potential tsunami deposits. (Twelve ages were previously reported by Fujiwara et al., 2015). All samples are detrital above-ground portions of terrestrial plants with little abrasion or weathering, excluding sample 50 which is Ostrea gigas, a brackish water bivalve that attaches to channel gravel (Table 1, Fig. A.1). Radiocarbon years (yr BP) were converted to calendar years by using MatCal (Lougheed and Obrochta, 2016) and the IntCal13 data set (Reimer et al., 2013). In the case of the marine bivalve, the Marine13 calibration curve was used, with no local marine reservoir effect (DR) specified, as it is not defined in the study area. Sample materials for dating potential tsunami deposits were systematically collected from three horizons, below, within and above the deposits wherever possible. Stratigraphically overlying and underlying samples were generally collected no more than

Fig. 5. Results of mineralogical analyses. A) Basic mineralogy of selected deposits compared to modern beach sand, buried beach ridge sand, present-day Ota River bed sand, and sand within an abandoned channel. Tsunami deposits (Tn1, Tn4, Ts1 and Ts2), beach sand and buried beach ridge sand are dominated by quartz and feldspar with some accessory mafic minerals. Sand from the abandoned channel and modern river bed are comparatively rich in lithic fragments with little mafic minerals. B) Composition of mineral fractions concentrated using tribromomethane. The Ts1 and Tn1 deposits and beach sand are marked by biotite and garnet derived from the Enshu-nada coast, whereas the river bed is rich in magnetite from the Ota River catchment.

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Fig. 6. Tsunami deposits at the northern site A) Overview of the excavated walls from site N8. Height of the excavated wall is about 4 m. Unconsolidated relatively coarse-grained beds are expressed as grooves along the excavated walls over hundreds of meters. Bed Tn2 was not identified at this location due to distance from the coast. B) At Site N1, photograph showing beds Tn1 and Tn2. C) Close up of lower part of bed Tn1 at Site N1 showing rounded pebbles and marine shell fragments.

3 cm from potential tsunami deposits with few exceptions. Sample 35 was collected ~50 cm above a potential tsunami deposit. Samples collected within the deposit may be shrubs and trees uprooted by the tsunami. We attempted to limit samples from within the

deposit to relatively unabraded portions of tree trunks, branches, and plant remains. The underlying and overlying samples generally represent maximum and minimum ages for the potential tsunami deposits, respectively. However, erosion by the tsunami may

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Fig. 7. Results of geochemical and grain size analyses. From left to right are median and mean grain size (phi units), sorting, total organic carbon, TN, TS, C/N and C/S ratios, shown adjacent to a generalized stratigraphic column. Results of grain-size analyses show that each tsunami deposit (Tn1 to Tn4) has a multiple layered structure. Results of geochemical analysis indicates a general freshening trend, the cause of which is coastal progradation with variations in redox state and salinity: wetland with marine influence (below 1.5 m), wetland connecting with a tidal river (~0 m), and a wetland with few vascular plants (above 0 m). Low total nitrogen and total sulfur values in beds Tn1 and Tn2 reflect the dilution by rapid supply of coarse sediment. No detectable changes in the total sulfur values and the C/S ratios are observed in beds Tn3 and Tn4.

complicate the interpretation of the estimated maximum ages, biasing them towards older ages. We avoided using large wood fragments with diameters exceeding 3 cm, because they may be resistant to weathering and have a long residence time. We present all calibrated ages and modelled posterior distributions as 2s ranges in calibrated years BCE/CE (Table 1). Even though the samples were collected along nearly identical horizons in the excavated walls, the age ranges differ between samples. To determine the most likely median age and range for each potential tsunami deposit, we created cumulative calendar age probability density functions (PDFs) for radiocarbon ages below, within, and above each tsunami deposit. We limited this analysis to samples with calibrated ages that are consistent with the stratigraphic and archaeological order and two sigma age ranges. This was done separately for the northern and southern site deposits using the method by which duplicates are combined in the age modeling software Undatable (Lougheed and Obrochta, 2019). Specifically, this is accomplished by summing all probabilities associated with each calendar year. For example, if three calibrated radiocarbon calendar age PDFs have a probabilities of 0.01, 0.02, and 0.03 for a calendar age of 1000 y BP, then the cumulative PDF initially has a probability of 0.06 for this age. By definition, the sum of a PDF is 1, requiring that each cumulative PDF be normalized by the sum of all elements prior to highest posterior density analysis to calculate the discontinuous 68.2 (1s) and 95.4 (2s) percentile ranges (Bronk Ramsey, 2009). The end result is a posterior calendar age PDF

that contains characteristics of each of the combined PDFs. The PDFs are, in essence, mathematically averaged. 4. Results 4.1. Surface geology and potential tsunami deposits 4.1.1. Southern site 4.1.1.1. Motojima ruins. Excavated walls show the cross-section of the buried beach ridges, which are primarily comprised of quartzand feldspar-rich medium sand beds and overlying back-marsh deposits consisting primarily of humic mud (Figs. 3 and 4 and Fig. A.1A). Abandoned river channels with u-shaped cross-sections, several meters wide and filled with sand and rounded gravel, are also apparent in the excavated walls. Two potential tsunami deposits exhibiting a multiple layered structure and continuous distribution are recognized and are denoted as Ts2 and Ts3 in ascending order (Fig. 3, and Fig. A.1A). Bed Ts2 consists of about three sets, depending on location, of normal-graded, rippled, very fine sand layers and mud drapes with a maximum thickness of 18 cm that is continuously distributed at an elevation of 0.4 m (Figs. 3 and 4A). It shows alternating thick and thin rippled sand layers (Fig. 4 B) and looks just like the alternating beds of sand and clay formed by the 2011 Tohoku-oki tsunami on the Kujukuri coast (about 1 km inland; Fujiwara et al., 2012) and Yamamoto Town (about 1.3 km inland; Fujiwara and Tanigawa,

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Fig. 8. Photographs of bed Tn4 at the northern site A) Overview of the excavated walls including bed Tn4 between sample sites N3 and N4. Dip of the excavated wall is ~46 . B) B) Closeup of bed Tn4 at site N4 showing alternating layers of rippled sand (dark color) and mud drapes (bright color). Some sand layers indicate a landward current direction. Scale in cm. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

2014), Kanto region and south of Sendai, respectively. An asymmetrical stacking pattern, shown by alternate deposition of thick and thin sand layers, is similar to that reported from the 2004 Indian Ocean tsunami deposits (Naruse et al., 2010) and the 2011 Tohoku-oki tsunami deposits (Oota et al., 2017); it reflects the asymmetric hydraulic properties of tsunami run-up and backwash flows over flat areas. In such cases, weak backwash flow forms thin, fine-grained deposits that cover thick, coarse, strong-current deposits during run-up. Bed Ts2 has an erosional base and grades into the overlying back-marsh deposits. In places, it is erosionally covered by an abandoned river channel with gravelly sand beds and gravelly flood deposits (Fig. A.1). Bed Ts3 is composed of at least three sets of very fine sand and mud drapes, 8 cm thick in total, distributed near an elevation of 1.4 m along the western wall. However, Ts3 is not observed on the other walls because of artificial disturbance. Ts3 has a sharp overlying contact with the underlying back-marsh mud and grades upward into overlying back-marsh mud. 4.1.1.2. River improvement site. Excavated walls reveal the crosssection of the buried beach ridge and overlying back-marsh muds on either side of the ridgeline (Fig. 3, and Fig. A.1A). The back-marsh muds consist mainly of humic clay overlain by clay and silt beds. Abandoned river channels, up to 20 m wide in cross-section and several meters deep, are also present. Three potential tsunami deposits characterized by multiple layered structure and continuous distribution are recognized within back-marsh muds covering the seaward slope of the buried beach ridge (Figs. 3 and 4C). Taking their stratigraphic positions into account, the upper two beds are correlated with Ts2 and Ts3 at the Motojima ruins site, respectively (Fig. 3, Fig. A.1). The lowest potential tsunami deposit is referred to as Bed Ts1.

Table 2 Sample numbers in Table 1 for the ages that were used to calculate the cumulative calendar year probability density functions in Fig. 9.

above within below

Ts1

Ts2

3 4, 5 6, 7, 8, 10

11 1, 15 2, 12

Ts3

Tn1

Tn2

Tn3

Tn4

23, 39 24, 25, 43 26, 27

22

16 1, 11, 15

26, 27 30 32, 40

35 36 37

38, 42, 44, 45

Bed Ts1, 20 cm in thickness, is comprised of two separate layers, each of which consists of medium to very fine sand. The lower layer includes abundant wood fragments and plant debris (Fig. 4 D, Fig. A.2). The upper layer erosionally covers the lower one and is rich in pebble-sized rounded mud clasts (mud balls). Similar tsunami deposits were observed after the 2011 Tohoku tsunami along the Sendai coast where the tsunami destroyed muddy lagoon banks (Fujiwara, 2015). Bed Ts2, up to 25 cm thick, exhibits generally similar features to same bed at the Motojima ruins. Bed Ts3, ~8 cm thick, consists of at least two sets of very fine sand layers with mud drapes. It either grades into the overlying back marsh deposits or is erosionally covered by abandoned channel deposits. High contents of quartz, feldspar, and biotite, as well as heavy minerals such as garnet, characterize beds Ts1 and Ts2, similar to samples of the modern beach sand and the buried beach ridge (Fig. 5). Deposits of the modern river bed and abandoned river channels, on the other hand, are characterized by magnetite and high lithic fragment contents of low quartz, feldspar, and heavy minerals, and the absence of garnet (Fig. 5). Garnet is also absent in four samples of the modern river bed. Mafic metavolcanic rocks comprising the accretionary complex in the Shimanto Group is the main source of magnetite (Fig. 2A).

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Fig. 9. Cumulative age probability density functions for ages of dated samples (Table 2) taken from above, within, and below each tsunami deposit at the northern (top) and southern (bottom) sites. Dark and light shading indicates the discontinuous 68.2 and 95.4 highest posterior density percentile ranges, respectively. Dotted lines indicate the age of historical earthquakes; 684 CE Hakuho Nankai, 887 CE Nin-na Nankai, 1096CE Eicho Tokai and 1498 CE Meio Tokai earthquakes, respectively.

4.1.2. Northern site From bottom to top, excavated walls are comprised mainly of muddy peat, humic clay beds and clay beds (Fig. 3 and Fig. A.1B) with abundant silt above 0.5 m. A brief, local sea level rise around 100 BCE to 300 CE is suggested by the deposition of an intertidal molluscan fossil-bearing sandy silt bed at the southern end of the excavation site (Fujiwara et al., 2015). Some abandoned river channels with u-shape cross-sections, up to 5 m wide and several meters deep, are recognized in the excavated walls (Fig. 6A). These are filled by rounded gravels and mud clasts, as well as sand beds with abundant lithic fragments. Four potential sandy tsunami deposits are recognized from the excavation site and referred to in ascending order as beds Tn1 to Tn4 (Figs. 3 and 6). These are comprised of multiple layered structures and taper landward with erosional base and abundant rip-up clasts. Distribution heights of beds Tn1, Tn3 and Tn4 increase landward reflecting the antecedent topography. Bed Tn1 is continuously distributed along the excavated walls and generally decreases landward in both grain size and thickness from 70 cm in the south to 45 cm in the north (Figs. 3 and 6). Mean grain size and gravel content gradually decreases from Site N1 (muddy medium sand bed with some rounded pebbles and marine shell fragments; Fig. 6B and C) to Site N8 (very fine sand to silt beds; Fig. 6A). Bed Tn1 consists of at least six sand layers, each of which exhibits generally normal grading (though reverse is also occasionally observed). Mud drapes separating these layers are sometimes preserved despite erosion during deposition of the overlying sand layers and later bioturbation. Multiple layered structure and general fining upward trends are also apparent from grain-size analyses (Fig. 7). Cross-laminated sand, wood debris, and late 4th century pottery shards (potsherds) are also characterize bed Tn1 (Fujiwara et al., 2015). Bed Tn2 is a normally-graded, muddy medium to fine sand bed that is typically covered by a dense layer of wood fragments and

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plant debris. It is about 20 cm thick at Site N1, but pinches out landward near Site N6 (Fig. 3, Fig. A.1B). Mean grain size and gravel content gradually decreases from Site N1 (muddy medium to fine sand with some rounded pebbles) to Site N6 (sandy silt to silt bed with granule-size pumice). Multiple layered structures are indicated by the results of grain-size analyses (Fig. 7). Tn3 is a silt bed that is intermittently observed at the excavation site depending on the condition of the excavated walls. The thickness of Tn3 decreases from 12 cm at Site N1 to 4 cm at Site N6, except at Site N8 (Fig. 3). Mean grain size gradually decreases from Site N1 (fine alternation of very fine sandy silt and clay) to Site N8 (laminated silt bed). The results of field survey and grain-size analyses indicate a multiple layered structure (Fig. 7). Bed Tn4 is characterized by an alternation of rippled sand beds and mud drapes (Fig. 8); it is very similar to the 2011 Tohoku-oki tsunami deposit on the Kujukuri coast (Fujiwara et al., 2012) and Yamamoto Town (Fujiwara and Tanigawa, 2014). It is continuously distributed throughout the excavation site with sedimentary features indicative of landward flow. First, it gradually thins from Site N3 (25 cm) to Site N8 (8 cm), with some fluctuation (Fig. 3, Fig. A.1B). Second, current ripples in several sand beds indicate a landward paleocurrent direction (Fig. 8B). Third, the mud/sand ratio decreases landward, with mainly sand-dominated wavy bedding at seaward Sites N3 and N4 and mud-dominated lenticular bedding at landward Sites N5, N6, N7 and N8 (Fig. A.1B). Field observations and grain-size analyses show multiple layered structures, at least four sets of sand layers with mud drapes, and a generally fining upward sequence (Figs. 7 and 8B). Some thick sand layers show inverse grading suggesting strong traction current flow. The high quartz, feldspar and heavy mineral content of beds Tn1 (which is particularly rich in garnet) and Tn4 (Fig. 5), reflective of beach sediment, suggests that the sediment deposited in these beds was derived from the Enshu-nada coast or beach ridge in the downstream area of the study site. Below an elevation of 1.5 m, sulfur content is relatively high and present in detectable levels to 0 m (Fig. 7). C/S values vary between 2 and 10 below 1.5 m, above which it is generally lower than 2, with the exception of a peak at 0.4 m. C/N values exhibit a rough upward decreasing trend with the lowest values at ~0 m. 4.2. Dating results 4.2.1. Southern site Dating results are shown in Table 1 and Fig. A.1. Samples from the lower back-marsh deposits (No.13, 14, and 18e21) generally show stratigraphically consistent ages (from ~1600 BCE to ~400 CE), excluding sample No. 17, which has an exceptionally young age of 1027e1153 CE. Samples from the abandoned river channel deposits at the northern (No. 50) and southern end of the excavation site (No. 6, 7, 8,10) indicate age ranges of 976e1096 CE and 650e890 CE, respectively. Sample No. 9 from the abandoned river channel has much older, stratigraphically inconsistent age of 589e683 BCE and is probably reworked old wood. Samples from the upper backmarsh deposits (No.1e5, 11, 12, 15, 16) generally show consistent ages with their stratigraphic positions (from 775 to 894 CE to 1446e1617 CE). 4.2.2. Northern site Samples from the lower part of the excavation walls, muddy peat and humic clay beds, show ages of 2800e2700 BCE (No. 41) at the lowest horizon and 597e657 CE (No. 40) at the uppermost horizon, respectively. Sample No. 46 (898e1015 CE) may have included a younger root that penetrated Bed Tn1 from above. Variable ages (from 77 to 214 CE to 328e407 CE) were obtained

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from four samples in Bed Tn1 (No. 28e30, 49). Many of these samples show ages older than the Tn1 deposits, which include archaeologically-interpreted potsherds, and so are interpreted as reworked samples. We obtained mostly stratigraphically consistent ages, ranging from the 678e774 CE to 1513e1644 CE, for samples from the middle humic clay beds (No.23e27, 38, 39, 42e45) and upper clay/silt beds (No. 22, 34e37). 5. Discussion 5.1. Age and correlation of potential tsunami deposits between the southern and northern sites We created cumulative calendar-age PDFs using the radiocarbon ages obtained from below, within, and above each potential tsunami deposit to estimate their depositional ages (Table 2, Fig. 9). The ages of the deposits directly overlying and underlying the tsunami deposit roughly limit the minimum and maximum ages for Tn1 and Tn3. To refine the depositional ages of Ts1, Ts2, Tn2 and Tn4, we used the radiocarbon ages obtained from the samples within these deposits, which provide an estimate of the age of material uprooted by tsunamis. The ages obtained from material within the deposit is stratigraphically consistent with those obtained from the directly underlying and overlying beds, excluding the samples from Tn1, which are older than the age suggested by pottery shards in the Tn1 (later 4th century). The most probable ages for deposits that can be reliably constrained are as follows: end of 7th century for Tn1, end of 9th to the end of 10th century for Tn2, late 11th to late 12th century for Tn3, end of the 15th to the early 17th century for Tn4, late 8th to the end of 9th century for Ts1, and early 11th to the early 12th century for Ts2. Ts3 lacks information constraining its minimum age and is at least younger than 1450 CE. Ages and stratigraphic positions allow us to correlate the Ts1 and Ts2 deposits to Tn2 and Tn3, respectively (Fig. 3, Fig. A.1). Correlation of Ts3 to Tn4 is also consistent with the available stratigraphic information. 5.2. Estimation of inundation distances and identification of tsunami deposits 5.2.1. Depositional environments The environment inundated by each tsunami can be reconstructed from the results of elemental analysis. The high sulfur and low C/S values below 1.5 m elevation are indicative of marine influence in a shallow wetland, with variations in redox state and salinity. The presence of sulfur down to ~0 m reflects a marine influence limited to that depth. Considering the local topography, this was likely a wetland connected with a tidal river with varying seawater influence. Above 0 m, low C/N values suggest a wetland with few vascular plants. Overall, the data indicate a freshening trend (Fig. 7), the cause of which is coastal progradation. This is consistent with a history of the study area (Section 2), and suggests that beds Tn1 and Tn2 invaded a coastal marsh whereas beds Tn3 and Tn4 infiltrated into a wetland. The C/S peak at 0.4 m is consistent with increased TOC due to poor oxygenation and low sulfur, suggesting a decrease in salinity (Fig. 7). This occurs between beds Tn2 and Tn3 but is not attributable to a known earthquake. It is probably unrelated to a Tokai earthquake and is more likely due to a temporary change in coastal morphology or the meandering of a river channel. Considering the coastal deformation pattern during the 1854 CE Ansei Tokai earthquake, a Tokai earthquake is not expected to generate coastal uplift large enough to explain the salinity decrease in the Otagawa Lowland. The 1854 CE Ansei Tokai earthquake generated a remarkable westward tilting along the eastern Enshu-nada coast,

with ~1.2 m uplift around the Cape Omaezaki and 0.5 m subsidence in the Lake Hamana area (Ishibashi, 1981; Sato and Fujiwara, 2017). However, the Otagawa Lowland is located near the “hinge line” of this tilting and probably experienced little vertical deformation. In the Tn1 and Tn2 deposits, low TOC, TN and TS values reflect dilution by a rapid supply of coarse sediment (Fig. 7). The variability of C/S values probably suggests a mixture of materials derived from both marine and fresh water environments, perhaps indicating large washover events. 5.2.2. Estimation of inundation distances All the sedimentary and mineralogical features suggest that beds Tn1 to Tn4 are washover deposits. In particular, the multiple layered structure with a general fining upward sequence strongly suggests that these deposits are of tsunami origin. In addition, we confirmed that these deposits have much larger inundation distances than is typical in the area for storm surge deposits. The coastline of the Otagawa Lowland has prograded at least 1.3 km since the 5th century. The coastline during 13e14th and 15th - 17th centuries was located at least 0.8 to 0.7 and 0.3e0.5 km inland from present. This indicates coastline progradation rate ~60 m per 100 years between the 5th and 13th centuries and ~170 m per 100 year between the 14th and 17th centuries. Assuming uniform coastline progradation during each of the above-mentioned periods, the coastline would have been located ~1.1, 1.0, 0.9, and 0.5 km inland of its present position during the deposition of Tn1 (ca. 700 CE), Tn2 (ca. 900 CE), Tn3 (ca. 1100 CE) and Tn4 (ca. 1500 CE). Roughly estimated inundation distances are, over 2.2 km (Tn1), over 2.0 km (Tn2), over 2.4 km (Tn3) and over 2.8 km (Tn4). These are minimum estimates because tsunami inundate much further inland from the landward limit of identifiable sandy tsunami deposits (e.g., Abe et al., 2012). Such large inundation distances are inconsistent with storm deposits, indicating that the most plausible source of these deposits were large tsunamis. Bed Tn1 is 70 cm and 45 cm thick at a respective distance of about 1.7 and 2.2 km from the former coastline, and is much thicker than the 2011 sandy tsunami deposits on the Sendai plain, where maximum thicknesses range from ~22 to 10 cm at locations between 1.8 and 2.4 km from the coastline (Richmond et al., 2012). A topographic low is a possible reason for the thickness of Bed Tn1. The asymmetrical distribution of a buried beach ridge, which is a western extension of beach ridge row I, the main channels of the old and modern Otagawa River (Fig. 2 C), and the intertidal mud (Fig. 3) suggest that central to western parts of the lowland, including the study site, have been a persistent topographic low. 5.3. Comparison with historical earthquakes Comparison with historically documented destructive earthquakes and tsunamis along the eastern Nankai Trough region suggests that our correlated tsunami deposits Tn4 and Ts3, as well as the paired Tn3 and Ts2 deposits were produced by the 1498 CE Meio and 1096 CE Eicho Tokai earthquakes, respectively. The timing of deposition of beds Tn2 and Ts1 is most consistent with the 887 CE Nin-na Nankai earthquake. Historical descriptions of the 887 CE earthquake suggest synchronous rupture of the Nankai and Tokai segments during this earthquake (Fig. 1). Strong ground shaking over a wide area of the Nankai and Tokai regions with severe damage around Kyoto was recorded, with long-lasting aftershocks and tsunami that were especially severe in the Osaka region (Ishibashi, 1999, 2014). Our discovery of the Tn2 and Ts1 deposits is the first evidence for tsunami in the Tokai region at the end of 9th century. This was most likely locally-generated tsunami because, in general, the highest waves result from tsunami generated directly offshore

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(Watanabe, 1998). The 1946 CE Nankai tsunami provides the only historical record of wave heights in the Nankai and Tokai regions. While tsunami height was 3e5 m high along the western Nankai Trough coast, it was only 1.2 m around the Otagawa Lowland (Watanabe, 1998). These tsunami propagation characteristics mean that the most likely source of the thick tsunami deposits in the Otagawa Lowland is a large tsunami produced by rupture along the Tokai fault segment. The above data suggest rupture of a 887 CE Tokai earthquake synchronous with the 887 CE Nankai earthquake. The earliest tsunami deposit, Tn1 occurs around the time of the 684 CE Hakuho Nankai earthquake. Possible tsunami deposits attributed to this earthquake were reported from localities along the western Nankai Trough coast (e.g., Garrett et al., 2016) as well as from the Shima peninsula facing the eastern Nankai Trough (Fujino et al., 2018). Liquefaction features archaeologically placed between the later 7th and 8th centuries have been described in the Tokai region and tentatively attributed to the Tokai earthquake (e.g., Sangawa, 2001). However, the possibility that the earthquake(s) on inland active fault(s) caused the liquefaction features cannot be ruled out. Discovery of the Tn1 tsunami deposit represents the first reliable evidence for tsunami in the Tokai region at the end of 7th century, and it suggests the Tokai rupture was contemporaneous with the 684 CE Nankai earthquake. Trans-Pacific tsunamis generated at far-field locations, such as the Chilean and Cascadian subduction zones, have also struck the Enshu-nada coast (Watanabe, 1998), but they would not have been high enough to create such thick deposits. For example, measured wave heights along the Enshu-nada coast were 1.1 m (vicinity of Lake Hamana) for the 1960 CE magnitude 9.5 Chilean earthquake tsunami (Watanabe, 1998). Estimated height is less than 2 m for tsunami produced by the 1700 CE magnitude 8.7e9.2 Cascadia earthquake (Satake et al., 2003). More work, however, is needed to rule out the Izu-Ogasawara Trench, to the east of the Nankai Trough, as the source of tsunami deposits along the Enshu-nada coast. For example, the 1605 Keicho earthquake caused a large tsunami along the Pacific coast of eastern and western Japan, as evidenced by tsunami deposits in the Lake Hamana region (Kumagai, 1999; Komatsubara et al., 2008). Because the earthquake intensity was low, it has been considered a “tsunami earthquake” originating from the Nankai Trough subduction zone (e.g., Ishibashi and Satake, 1998). However, Harada et al. (2013) suggest that it may have been generated along the Izu-Ogasawara Trench. 5.4. Recurrence pattern of Tokai earthquakes If we attribute the Tn1 and Tn2/Ts1 tsunami deposits to ruptures of the Tokai fault segment, the recent history of Tokai earthquakes includes the following nine events: end of 7th century, 887, 1096, 1361, 1498, 1614, 1707, 1854 and 1944 CE (Fig. 1). Based on these data, recurrence intervals of Tokai earthquakes range from 90 to 265 years over the last 1300 years, and the range in recurrence intervals is much smaller than the 100e600 years estimated by Fujino et al. (2018). In addition, there exists no firm evidence of Nankai earthquake corresponding to the 1498 CE Meio Tokai earthquake (Fig. 1). Our results also suggest that tsunami size was different for each event. In spite of the intensive investigations along long continuous excavation walls, no trace of the tsunami attributed to 1361 CE Tokai earthquake was found. This likely indicates that the 1361 CE tsunami was too low to inundate the study area, or that, if the tsunami reached the area, it was too shallow to deposit an identifiable tsunami deposit. This is consistent with the pattern of the 1944 CE Tonankai earthquake being smaller than the preceding 1854 CE Ansei Tokai and 1707 CE Hoei earthquakes.

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6. Conclusions Our two major conclusions are: 1) four tsunami deposits of prehistoric and medieval age are preserved in the Otagawa Lowland facing the eastern Nankai Trough, and 2) comparison of ages on new and previously studied deposits refines the earthquake history of the Tokai region. Large excavation walls along the Otagawa River record four tsunami deposits showing a landward fining trend and multiple layered structures. The two younger tsunami deposits are attributed to the 1498 and 1096 CE Tokai earthquakes. The third deposit confirms a 887 CE Tokai earthquake, which has been inferred from analyses of historical documents, but for which no direct evidence has been previously found. Our evidence suggests that the 887 CE earthquake was a full-length rupture of Tokai and Nankai fault segments. The oldest deposit from the latest 7th century also records a Tokai earthquake paired with the 684 CE Nankai event, which was previously inferred from liquefaction features at archaeological sites. Integration of historical, archaeological and geological evidence reveals nine Tokai earthquakes during the past 1300 years. Eight of these were simultaneous with Nankai earthquakes, excluding the 1498 CE Meio Tokai earthquake. Estimated recurrence intervals of the nine Tokai earthquakes range from 90 to 265 years, a recurrence interval shorter and more regular than previously inferred intervals. Funding A.A. was able to conduct mineralogical analyses by the support of a research grant from Nakatani Foundation for Advancement of Measuring Technologies in Biomedical Engineering. Acknowledgements Our gratitude goes to the Fukuroi public work office and the Shizuoka prefectural archaeological center for allowing us access to the excavation sites and leveling data. The Fukuroi public work office also provided us with aerial photographs around the river improvement site. We also thank Prof. A. Kitamura of Shizuoka Univ. and Dr. K. Tanigawa of GSJ, AIST for their help at field survey and Dr. B.C. Lougheed for advice on radiocarbon dating. This manuscript was greatly improved by comments from two anonymous reviewers and the journal editor. O.F led the fieldwork and prepared the text, figures and tables. A.A. conducted fieldwork and mineralogical analyses. T.I. led the laboratory analyses of the samples and interpreted the depositional environment. E.O. and Y.S. conducted the fieldwork and geomorphological analyses. S. O. performed radiocarbon analyses and English proofreading of the manuscript. Y.S. led the geochemical analyses and interpretation of depositional environments. A.T. carried out the geochemical and grain-size analyses as part of her graduation thesis. The authors are listed alphabetically except for O.F. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.quascirev.2019.105999. References Abe, T., Goto, K., Sugawara, D., 2012. Relationship between the maximum extent of tsunami sand and the inundation limit of the 2011 Tohoku-oki tsunami on the Sendai Plain, Japan. Sediment. Geol. 282, 142e150. https://doi.org/10.1016/ j.sedgeo.2012.05.004. Aoshima, A., Sato, T., Suzuki, R., Shimotani, G., 2011. Characters and origins of garnet

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