Historical and paleo-tsunami deposits during the last 4000 years and their correlations with historical tsunami events in Koyadori on the Sanriku Coast, northeastern Japan
© Ishimura and Miyauchi. 2015
Received: 11 November 2014
Accepted: 21 May 2015
Published: 27 June 2015
Large tsunamis occurring throughout the past several hundred years along the Sanriku Coast on the Pacific coast of northeastern Japan have been documented and observed. However, the risk of large tsunamis like the tsunami generated by the 2011 off the Pacific coast of Tohoku earthquake could not be evaluated from previous studies, because these studies lacked evidence of historical and paleo-tsunami deposits on the coastline. Thus, we first identified event deposits, which are candidates for tsunami deposits, from excavating surveys conducted on the coastal marsh in Koyadori on the Sanriku Coast, northeastern Japan. Second, we determined the physicochemical sediment properties of the deposits (roundness of grains, color, wet and dry densities, and loss on ignition) and established their geochronology by radiocarbon dating and tephra analysis. Third, we identified event deposits as tsunami deposits, based on their sedimentary features and origin, sedimentary environment, paleo-shoreline, and landowner interviews. In this study, we report 11 tsunami deposits (E1–E11) during the past 4000 years, of which E1, E2, E3, and E4 were correlated with the 2011 Tohoku-oki tsunami, the 1896 Meiji Sanriku tsunami, the 1611 Keicho Sanriku tsunami, and the 869 Jogan tsunami, respectively. From age data and the number of tsunami deposits in the trench, we estimated that tsunamis larger than the 1896 Meiji Sanriku tsunami occur and hit the study area on average every 290–390 years. However, historical tsunami correlations revealed variable tsunami occurrence, indicating diverse tsunami generation and/or the combination of several types of large earthquakes from different sources around the Japan Trench.
KeywordsHistorical and paleo-tsunami deposits Sanriku Coast 2011 Tohoku-oki tsunami Historical tsunami correlation Geochronology
Historical and paleo-tsunami research and its application to geophysical study
The best sources of precise long-term tsunami data are coastal lowlands, in particular, marshes (Minoura and Nakaya 1991; Witter et al. 2003; Sawai et al. 2009; Shennan et al. 2014). In some regions (Hokkaido of Japan, Alaska of USA, and the North Island of New Zealand), coseismic and post-seismic crustal movements are recorded in sediments as lithological and biological changes (Witter et al. 2003; Sawai and Nasu 2005; Hamilton and Shennan 2005; Hayward et al. 2005). Thus, in this study, we excavated trenches at a coastal marsh in Koyadori, in the middle part of the Sanriku Coast (Fig. 1). The aim was to provide new geological evidence of historical and paleo-tsunami deposits.
Tsunami history and previous study of tsunami deposits
Historical tsunamis along the Sanriku Coast during AD 1611–2011
Earthquakes in observed records (after 1896)
11 March 2011
26 September 2003
16 May 1968
23 May 1960
4 March 1952
3 November 1936
3 March 1933
1 November 1915
5 August 1897
15 June 1896
Earthquakes in documented records (before 1896)
23 August 1856
17 February 1793
29 January 1763
13 April 1677
2 December 1611
Historical tsunamis’ runup height around Koyadori
Measured runup height
Haraguchi and Iwamatsu (2011)
Iwate Prefecture (1969)
Earthquake Research Institute, the University of Tokyo (1934)
Unohana and Ota (1988)
Estimated runup height based on historical documents
Historical and paleo-tsunami deposits along the Sanriku Coast have been studied at several sites by The Headquarters for Earthquake Research Promotion (2006; 2007; 2008; 2009; and 2010), Haraguchi et al. (2006a, b; 2007), Haraguchi and Goya (2007), Imaizumi et al. (2007), Torii et al. (2007), and Haraguchi and Ishibe (2009) before the 2011 event. However, historical tsunami deposits were identified at only one of the onshore sites at Rikuzentakata (Fig. 1). This site has revealed four historical tsunami deposits during the past 700 years, the latest of which was correlated with the 1960 Chile tsunami (The Headquarters for Earthquake Research Promotion 2007). The other events are not well correlated with historical tsunamis. Recent sediments, in particular, those deposited during the past 1000 to 2000 years, are badly preserved at other onshore sites (Torii et al. 2007; The Headquarters for Earthquake Research Promotion 2010). Therefore, historical tsunami deposits are poorly understood along the Sanriku Coast.
Initial mapping of geomorphic surfaces around Koyadori was based on interpretation of 1:8000- and 1:10,000-scale aerial photographs taken by the Geospatial Information Authority of Japan before and after the 2011 Tohoku-oki tsunami, and anaglyph images prepared from 1 m and 5 m mesh DEM (Digital Elevation Model) provided by the Geospatial Information Authority of Japan and the Iwate Prefecture.
Dry and wet density, color, and loss on ignition measurement
We sampled the entire 7 cm3 cube (each side = 2.2 cm) from the block samples and measured their wet and dry bulk densities. The color of wet sediments in cubic samples is quantified by the L*, a*, and b* parameters measured by the Soil Color Reader SPAD-503 instrument (Konica Minolta Sensing, Inc.). The a* and b* parameters specify the red (+) to green (−) and yellow (+) to blue (−) content, respectively, while L* represents lightness (0 = black, 100 = white). The loss on ignition (LOI) was conducted in each block sample following Bos et al. (2012) at 3–6 cm intervals, although this sampling was restricted to peat and peaty silt.
Furthermore, to reveal the origins of the event deposits and to confirm tsunami deposits, we sampled fluvial and beach sediments (Locations 1–8) (Fig. 4) for particle roundness analysis in 2012 and 2013. Samples were washed and dry-sieved through 2 mm mesh and the gravels were divided into six roundness categories (very angular, angular, sub-angular, sub-rounded, rounded, and well-rounded).
Radiocarbon dating (30 samples) was conducted by accelerator mass spectroscopy (AMS) at the Institute of Accelerator Analysis Ltd. and Geo Science Laboratory. The obtained age data were calibrated using the OxCal 4.2 program (Ramsey 2009) and the calibration curve IntCal13 (Reimer et al. 2013).
The Towada-a tephra (To-a) (AD915; Machida and Arai 2003) is a useful indicator of the 869 Jogan tsunami deposits in the Sendai and Ishinomaki Plains (Minoura and Nakaya 1991; Sawai et al. 2007; Shishikura et al. 2007). From the presence and distribution of To-a along the southern Sanriku Coast, Ishimura et al. (2014) suggested that To-a had also been deposited at the central Sanriku Coast. Therefore, we conducted a cryptotephra analysis to identify the invisible To-a horizon.
Each block sample was sampled at 3–6 cm intervals. These samples were washed using 60 μm nylon mesh and dry-sieved using 124 μm nylon mesh. Thin sections made with the 60–124 μm fractions revealed volcanic glass contents. The refractive index of volcanic glass shards, which is useful for identifying widespread tephras in Japan (Machida and Arai 2003), was measured with a refractive index measuring system (RIMS 2000: Kyoto Fission Track Co., Ltd.). The RIMS system measures volcanic glass shards to an accuracy of ±0.0002 (Danhara et al. 1992). The major element compositions were analyzed by energy-dispersive spectrometry using an electron probe microanalysis (EPMA) system (Horiba Emax Energy EX-250) at the FURUSAWA Geological Survey. The major elements were measured by scanning a 4 μm grid of the targeted grain under a counting time of 150 s and accelerating voltage of 15 kV. The beam current and diameter were 0.3 nA and 150 nm, respectively. The atomic number effect was corrected by the ZAF procedure.
2011 Tohoku-oki tsunami and its deposits
The inundation and runup heights of the 2011 Tohoku-oki tsunami at Koyadori ranged from 13 to 18 m a.s.l. and from 26 to 29 m a.s.l., respectively (Table 2; Haraguchi and Iwamatsu 2011). Figure 3 shows the landform changes before and after the 2011 event. Immediately following the 2011 Tohoku-oki tsunami (April 2011), the beach was not yet re-established and the beach ridges may have been shortened by the tsunami backwash (Fig. 3a, b). After the beach was restored in June 2011, the shortcut channel was filled with beach deposits (Fig. 3c). The poor drainage area remained until October 2012 (Fig. 3d). The Tohoku-oki tsunami hit the coastal levee originating from beach ridges, destroying it and the trees on it, and eroding it to a depth of 1–1.5 m (Fig. 5d). The eroded materials were transported landward and deposited as tsunami deposits. Approximately 9 m a.s.l. and 600 m landward, a boulder was recognizable as a tsunami deposit because of the attached oyster shells (Fig. 5e). Tsunami deposits composed of sand and gravel sourced from the beach and beach ridges were found up to 600 m landward in December 2012 (Fig. 5f).
Description of the KYD-trench
Deposits in the trench wall were divided into five facies (event deposits, marsh deposits, channel fill deposits, artificial fill deposits, and cultivation soil), based on their sediment structure, continuity, and composition (Fig. 6). All the walls contained marsh deposits and interbedded event deposits.
Characteristics of event deposits in the KYD-trench
Comparison of grain size among event deposits
General grain size
Measured thickness [cm]
Gravel content [wt%]
Roundness (well-rounded + rounded) [%]
Upper: granule to medium sand, Lower: pebble to coarse sand
Granule to coarse sand
Granule to coarse sand
Granule to fine sand
Granule to coarse sand
Granule to coarse sand
Granule to coarse sand
Coarse to medium sand
Granule to medium sand
Coarse to medium sand
Coarse to medium sand
The marsh deposits are composed of plant remains and organic sediments. Their densities are inversely correlated with their LOIs and indirectly indicate their organic carbon content and degree of decomposition (Fig. 7). Color, in particular, the L* and b* parameters, is correlated with density, whereas the LOI fluctuates between event deposits. Macroscopically, the LOI decreases from the trench bottom to the E4 deposits and increases from the E4 deposits to the E3 deposits.
The channel fill deposits exhibit two cross-sectional geometries and compositions, categorized as Channel 1 and Channel 2 (Fig. 6). Channel 1 is distributed from grid N–5 to N–10 and from grid W–5.5 to W–7 (Fig. 6). From the altitude of the channel bottom in both walls, the flow direction of Channel 1 was determined as east to west. Sediments are finer in Channel 1 than in Channel 2, comprising coarse sand to fine pebbles, and interbedded with peaty silt. Channel 2 is distributed from grid N–1 to W–10 and from grid E–5.5 to E–11.5 and flows from northwest to southeast (Fig. 6). The composition is poorly sorted pebble to cobble.
The artificial fill deposits with a buried PVC pipe, distributed from grid W–7 to W–11 and from grid N–1.5 to N–2.5 (Fig. 6), were identified from interviews with landowners as underdrains constructed 40–50 years ago.
The cultivation soil is distinguished from marsh deposits by its different particle composition, color, and texture. This soil type is interbedded between the E3 and E1 deposits (Fig. 6). Event deposits, marsh deposits, and cultivation soil are also easily distinguishable by their density and color (Fig. 7). The dry bulk density of cultivation soil is intermediate between low-density marsh deposits and high-density event deposits.
Radiocarbon ages and calibrated ages
Conventional 14C age [yrBP]
Calibrated age (2σ) [calBP]
110 ± 20
270–210 (27.4 %), 150–20 (67.9 %)
190 ± 20
290–260 (19.6 %), 220–140 (52.9 %), 20– (22.9 %)
340 ± 30
490–310 (95.4 %)
790 ± 20
740–670 (95.4 %)
1420 ± 20
1350–1290 (95.4 %)
1570 ± 30
1540–1390 (95.4 %)
2320 ± 30
2380–2300 (90.1 %), 2240–2180 (5.3 %)
2360 ± 30
2490–2330 (95.4 %)
150 ± 20
290–250 (15.7 %), 230–130 (48.1 %), 120–70 (13.1 %), 40–0 (18.5 %)
370 ± 30
510–420 (55.0 %), 400–310 (40.4 %)
1090 ± 30
1060–930 (95.4 %)
990 ± 30
970–890 (57.4 %), 880–790 (38.0 %)
1030 ± 30
1050–1030 (2.8 %), 990–900 (91.9 %), 850–830 (0.7 %)
700 ± 30
690–640 (77.5 %), 590–560 (17.9 %)
1100 ± 30
1070–930 (95.4 %)
1420 ± 30
1370–1280 (95.4 %)
1680 ± 30
1700–1650 (10.2 %), 1630–1520 (85.2 %)
2410 ± 30
2690–2630 (11.2 %), 2620–2590 (2.9 %), 2500–2340 (81.3 %)
2530 ± 30
2750–2680 (35.8 %), 2640–2490 (59.6 %)
2810 ± 30
3000–2840 (95.4 %)
3500 ± 30
3860–3690 (95.4 %)
2780 ± 30
2960–2790 (95.4 %)
2870 ± 30
3080–2880 (95.4 %)
3500 ± 30
3860–3690 (95.4 %)
3020 ± 30
3340–3140 (92.0 %), 3130–3110 (1.4 %), 3100–3080 (2.0 %)
500 ± 30
620–610 (0.7 %), 560–500 (94.7 %)
1190 ± 30
1230–1210 (2.9 %), 1190–1050 (89.0 %), 1030–1000 (3.5 %)
Seed (Juglans sp.)
1240 ± 30
1270–1070 (95.4 %)
1090 ± 30
1060–930 (95.4 %)
1040 ± 30
1050–1020 (5.2 %), 1000–910 (90.2 %)
Description of the canal-trench
From the radiocarbon dating, we determined that the invisible To-a (AD915 (1035 cal. BP)) lies between the E3 and E5 deposits. The volcanic glass contents in each trench wall sample increase after the E4 deposition (Fig. 7). In particular, in the east and west walls, the volcanic glass content suddenly increases and gradually decreases from the lower to upper parts, indicating an invisible tephra horizon. However, this trend is absent in the south wall, probably because it has been eroded by the E3 deposits.
Major element compositions of volcanic glass shards
Ishimura et al. (2014)
Ishimura et al. (2014)
Aoki and Machida (2006)
Aoki and Machida (2006)
Aoki and Machida (2006)
KYD-TrW sec. 2 20–25 cm
Identification of tsunami deposits
The roundness similarities between the event and beach deposits (Fig. 10) indicate that event deposits were transported from beach and beach ridges to the inland trench sites. Landward transport from the sea is expected in tsunami and storm events. The general characteristics of tsunami and storm deposits have been reported by many researchers (Morton et al. 2007; Kortekaas and Dawson 2007; Switzer and Jones 2008; Goff et al. 2012; Phantuwongraj and Choowong 2012). On average, tsunami deposits are generally thinner than storm deposits (Morton et al. 2007; Phantuwongraj and Choowong 2012), and sedimentary structure is less common in tsunami deposits than in storm deposits (Morton et al. 2007; Kortekaas and Dawson 2007; Switzer and Jones 2008; Goff et al. 2012). The basal contact of both sediments is unconformable or erosional (Morton et al. 2007; Kortekaas and Dawson 2007; Switzer and Jones 2008; Goff et al. 2012; Phantuwongraj and Choowong 2012), although tsunami deposits sometimes show a loading structure (Goff et al. 2012). On transect scales (several hundred meters), the cross-shore geometries of tsunami and storm deposits are characterized by “broad thin drapes with tabular or landward thinning” and “narrow thick deposits with abrupt landward thinning,” respectively (Morton et al. 2007). These characteristics of tsunami deposits are recognized in the event deposits in the trenches. In the KYD-trench (length = 12 m), all event deposits are generally thinner than 20 cm and appear as draped or eroded paleo-surfaces. Some of them exhibit a loading structure. In the canal-trench (length = 150 m), the E1 and E3 deposits appear as draped deposits, with landward thinning in the E3 deposits. Furthermore, the KYD-trench is located 300 m inland from the beach, and landowners reported no storm deposits in the trench sites during the past 40–50 years. In contrast, the paleo-shoreline after the To–Cu deposition (about 6 ka) is estimated to be at least on the seaside of the KYD-Br1 site (Fig. 4). The elevations of primary To–Cu tephra within the KYD-Br1 to KYD-Br3 cores are −1.60, −1.82, and −3.08 m a.s.l., respectively, showing landward deepening, and sediments deposited after the To–Cu deposition are consistent with non-marine environments such as marsh (Ishimura et al. 2014). From these data, we consider that the present depositional setting (beach ridge and behind marsh) around the trench sites was already established by 6 ka. Therefore, prior to the To–Cu fall, the paleo-surface topography places the paleo-shoreline on the seaside of the KYD-Br1 core site. The features of the event deposits and the geomorphological settings from 6 ka to the present, together with the responses of interviewed landowners regarding recent events, indicate that all event deposits in the KYD-trench are sourced from tsunamis rather than storms.
Ages of tsunami deposits and correlation to historical tsunami event
Radiocarbon dating (Fig. 8, Table 4) suggests that the deposits in the KYD-trench are relatively close in age with no large age gap. Radiocarbon dating of event deposits is performed on plant fragments (such as reeds), because these constitute the youngest material in a sampled horizon. Plant materials in the trench are likely to be fragments of in situ plants, but downward invasion of roots and underground stems should not be ruled out. Thus, the ages of the plant material were assumed to represent the youngest ages of the sampled horizons. In contrast, a charcoal and a hard-shell plant seed (Juglans sp.) (Samples No. 21 and 28; Table 4) are assumed to be transported materials, whose ages mark the older age limit of the sampled horizons. Organic sediments (Samples No. 5 and 24; Table 4) are older than plant fragments, consistent with our radiocarbon dating interpretations. The true age of the sediment is expected to lie between the ages of the plant and other materials. Ishimura et al. (2014) identified To–Cu (6 ka) tephra and Oguni Pumice (7.3 to 7.4 ka) in the KYD-Br3 core drilled next to the KYD-trench at depths of 4.41–5.98 m (total thickness of primary and secondary tephra) and 8.55–8.60 m, respectively. The horizons and ages of these sediments are consistent with the radiocarbon-dated geochronology of the KYD-trench determined in this study.
Estimated ages of tsunami deposits and their correlation with historical tsunami events
Most probable historical event
2011 Tohoku-oki Earthquake Tsunami
1896 Meiji Sanriku Tsunami
1611 Keicho Sanriku Tsunami
To-a (AD 915)
869 Jogan Tsunami?
(no historical document around Koyadori)
The ages of the E2 deposits range from modern times to 290 cal. BP (i.e., they are younger than AD 1660). Certainly, the E2 deposits can be correlated to one event among the 1611, 1677, 1793, 1856, 1896, and 1933 events (Table 2). The runup heights (Table 2) and stratigraphic position of the E2 deposits suggest a correlation with the 1933 Showa Sanriku tsunami and the 1896 Meiji Sanriku tsunami, because the height of the beach ridge at Koyadori was approximately 5 m a.s.l. in both events. Although both tsunamis inundated up to the trench sites, only single-event deposits were identified from AD 2011 to 1660. Tsunami deposits can be absent for several reasons stated as follows: 1) disturbance and/or removal by cultivation, 2) erosion by succeeding tsunami events, 3) sediment availability, and 4) tsunami size. The first cause is easily explained. If sediments were deposited by the 1896 Meiji Sanriku and the 1933 Showa Sanriku tsunamis, the latter deposits would first be disturbed and removed by cultivation processes. In this case, we would correlate the preserved tsunami deposits to the 1896 Meiji Sanriku tsunami. Regarding the second cause, the 1896 deposits might have been eroded by the 1933 deposits. However, the E2 deposits show no clear base erosion in either the KYD- or canal-trenches, and no remnants of eroded tsunami deposits are evident between the E2 and E3 deposits. Thus, the second cause is inconsistent with the observations. Meanwhile, the third cause is inconsistent with the study site setting. If a large tsunami, with height exceeding that of the beach ridge, hits Koyadori, sediments of beach and beach ridge must be transported landward because there is much sediment in the coast and the beach was re-established a few months after the 2011 event. The forth cause, tsunami size, relates to the transportation and preservation of tsunami deposits. The inundation heights were larger in the 1896 event than in the 1933 event (Table 2). Thus, we can easily expect that the volume of the 1896 tsunami deposits exceeded that of the 1933 deposits. This also suggests that the 1896 tsunami deposits were better preserved than the 1933 tsunami deposits. From these considerations, we inferred that the E2 deposits are correlated to the 1896 Meiji Sanriku tsunami.
Considering the above correlation of the E2 deposits, the age of the E3 deposits was estimated as 54–620 cal. BP (AD 1896–1330). Thus, the E3 deposits can be correlated to one event among the 1454, 1611, 1677, 1793, and 1856 events (Table 2). Based on the tsunami runup height of these events (Table 2), the E3 deposits are most probably associated with the 1611 Keicho Sanriku tsunami. The E3 deposits are thick and composed of coarse materials (Table 3), and are traceable in the canal-trench (Fig. 9). Assuming a similar depositional setting from about 6 ka onward, we considered that the feature differences (thickness and grain size) among event deposits roughly indicate the tsunami size. The features of the E3 deposits (Table 3) suggest a large, very energetic tsunami. According to a local legend (Imamura 1934), the 1611 Keicho Sanriku tsunami inundated and surged through the Koyadori–Oura pass (Fig. 2). This indicates that the 1611 Keicho Sanriku tsunami was at least as high as the 2011 Tohoku-oki tsunami, since the latter failed to reach the geomorphic pass.
According to the radiocarbon dates of the channel deposits, the E4 deposits are aged 1000–1350 cal. BP (AD 950–600), and possibly correlate with the 869 Jogan tsunami. By targeting our tephra analysis at the To-a (AD 915) horizon, we determined an absolute timing for the E4 deposits. The increased content of volcanic glass above the E4 deposits (Fig. 7) suggests a tephra fall after the E4 sedimentation. The refractive index of volcanic glass shards above the E4 deposits ranged from 1.504 to 1.511 (mode: 1.507–1.508), which includes the To-a tephra range (Machida and Arai 2003; Ishimura et al. 2014). Similarly, the chemical compositions of volcanic glass shards were consistent with previously reported To-a compositions (Aoki and Machida 2006; Ishimura et al. 2014). From these data, we inferred that the To-a tephra fell between the E4 and E3 deposits, and we assigned the E4 deposits to the 869 Jogan tsunami. This identification based on radiocarbon dating and tephra provides significant information on the size and source of the 869 Jogan tsunami and earthquake, indicating that this tsunami reached the middle part of the Sanriku Coast and its inundation area was possibly as large as the 2011 event. Since the Jogan tsunami is not reported in historical records around Koyadori and insufficient information is available for regionally and chronologically identifying the tsunami deposits along the Sanriku Coast, this finding requires confirmation in paleographical and geological researches.
Tsunami ages and their intervals
Conclusive age estimates and correlations of historical tsunami events are summarized in Table 6. Although some ambiguity of the ages remains, we calculated the average interval of tsunami occurrence as 290–390 years. Before considering the approximate age intervals of tsunami events, we need to discuss the preservation potential of tsunami deposits at this site. Szczucinski (2012) and Spiske et al. (2013) mentioned the preservation potential of tsunami deposits in tropical and temperate climate regions, respectively, and showed that the characteristics of tsunami deposits (thickness and sedimentary structure) degrade over time. Spiske et al. (2013) emphasized the significance of the preservation potential in assessing the intervals and frequencies of tsunamis, because tsunami deposits are not necessarily preserved in whole inundated areas. They identified five determining factors of preservation potential as follows: 1) composition and genetic type of the tsunami deposits, 2) coastal topography and depositional environment, 3) co- and post-seismic uplift or subsidence, 4) climate, and 5) anthropogenic modification. In Koyadori, tsunami deposits originated from beach and beach ridge deposits and are coarser than those reported in Szczucinski (2012) and Spiske et al. (2013), indicating larger resistance to post-tsunami surface processes. As mentioned above, the sedimentary environment has remained largely unchanged since 6 ka, and the beach ridge and behind-marsh environment have maintained accommodation space for tsunami and marsh deposits. The 2011 event was followed by co- and post-seismic subsidence (Ozawa et al. 2011), enhancing the preservation environment of tsunami deposits. The Sanriku Coast has a temperate climate and experiences fewer and weaker storms and high tide events (such as typhoons) than the western part of Japan. According to interviews with landowners, no storm deposits have settled in the trench sites during the past 40–50 years. Artificial modification is limited to deposits younger than E3 at this site. Moreover, 2011 tsunami deposits were found in pits and coring surveys conducted around the KYD-trench in 2013 and 2014. These deposits were clearly identifiable, despite being partially bioturbated by grass and reed. Such vegetation covered the tsunami deposits, preventing erosion and removal by post-tsunami surface processes. Even in the event of dense bioturbation, tsunami deposits are easily identified by their grain composition, size, and roundness, which widely differ from those of background deposits (e.g., peat and debris flow deposits). Therefore, we conclude that the preservation potential of tsunami deposits is very high in Koyadori. Consequently, the calculated average interval probably truly reflects the interval and frequency of large tsunamis.
The calculated average interval (290–390 years) is shorter than that obtained for the Sendai and Ishinomaki Plains (Sawai et al. 2007; 2012; Shishikura et al. 2007), reflecting the high frequency of large tsunamis causing destructive damage along the Sanriku Coast. However, if we have correctly correlated the historical deposits to the historical tsunami events, we can state the age intervals from the E1 to E4 deposits as 115, 285, and 742 years, respectively. This variability probably indicates the diversity of the tsunami generation mechanism (e.g., large earthquake, tsunami earthquake, submarine mass failure, and tsunami of distant origin) and/or the combination of several types of large earthquakes from different sources around the Japan Trench.
On the other hand, the size of historical and paleo-tsunamis can be estimated from our results because the 1896 Meiji Sanriku tsunami inundated the KYD-trench site and transported tsunami deposits there. In contrast, neither the 1968 Tokachi-oki tsunami (nearby source, runup height approximately 3 m around Koyadori; Table 2) nor the 1960 Chile tsunami (distant source, runup height approximately 4 m around Koyadori; Table 2) inundated, perhaps because they were blocked by beach ridges (height approximately 5 m a.s.l.). Furthermore, the environmental setting at the study site has been established since approximately 6 ka. These observations preliminarily suggest that tsunamis larger than the 1896 Meiji Sanriku tsunami occur at the calculated average interval, providing a first step for assessing the risk and size of tsunamis along the Sanriku Coast. To understand the tsunami generation mechanism and earthquakes along the Japan Trench, we require detailed information of ages, intervals, and sizes of historical and paleo-tsunamis at multiple sites.
We identified eleven tsunami deposits, including the 2011 tsunami deposits, based on sedimentary structure and continuity in two trenches and comparisons of the roundness of the gravel composing the event deposits. Radiocarbon dating and tephra analysis allowed us to establish the geochronology in the KYD-trench wall sediments and to correlate tsunami deposits with historical tsunami events. The four younger tsunami deposits (the E1–E4 deposits) are correlated with the 2011 Tohoku-oki tsunami, the 1896 Meiji Sanriku tsunami, the 1611 Keicho Sanriku tsunami, and the 869 Jogan tsunami events, respectively. The average interval of tsunami occurrence at Koyadori is estimated at 290–390 years based on continuous records in the KYD-trench. However, the age intervals between the E1 to E4 deposits are variable (E1/E2: 115 years, E2/E3: 285 years, E3/E4: 742 years), likely reflecting the diversity of the tsunami generation mechanism and/or different earthquake sources around the Japan Trench. By correlating the historical tsunami runup height data with extant tsunami deposits, we could preliminarily estimate the sizes of paleo-tsunamis at the study site. In the future study, we need to confirm our tsunami correlations by correcting many geological data along the Sanriku Coast. Ultimately, we aim to assess tsunami risk and understand the earthquake phenomena around the Japan Trench.
We are grateful to Kazuomi Hirakawa, Toshifumi Imaizumi, Shuji Yoshida, Heitaro Kaneda, Tomoo Echigo, and Shinsuke Okada for their comments and helps in the fieldwork. We thank Hiroyuki Tustsumi for permission of using a soil color meter. The landowners of the KYD-trench site are also thanked for allowing us to conduct our surveys on their properties. The editor Ken Ikehara and two anonymous reviewers provided constructive comments that improved the manuscript. This study was a part of “Geophysical and geological studies of earthquakes and tsunamis for off-Tohoku district, Japan” and supported by the Ministry of Education, Culture, Sports, Science, and Technology, Japan (MEXT). This work was supported by Intramural Research Grant for Special Project Researches from International Research Institute of Disaster Science, Tohoku University.
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