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a Locations of survey points NM1 to NM20 and NM-A to NM-L. b Cross-sectional topography along the transect shown in Fig. 2a. Maximum run-up heights of the 2011 Tohoku-oki and 1896 Meiji Sanriku tsunamis are shown. Tsunami height at the coast of the 2011 Tohoku-oki tsunami is based on the 2011 Tohoku Earthquake Tsunami Joint Survey (TTJS) Group
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Throughout history, large tsunamis have frequently affected the Sanriku area of the Pacific coast of the Tohoku region, Japan, which faces the Japan Trench. Although a few studies have examined paleo-tsunami deposits along the Sanriku coast, additional studies of paleo-earthquakes and tsunamis are needed to improve our knowledge of the timing, recu...
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... are set about 50 m from the shoreline. The Sanriku railway runs north-south at ~ 300 m inland from the shoreline. These artificial objects were built well before the 2011 Tohoku-oki tsunami. The Maita River is ~ 5 m wide, and the banks are covered by revet- ments. The study site is used as a rice paddy field and contains agricultural terraces (Fig. ...
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... and 16 m in elevation, according to local people. No written re- cords of large tsunamis are available before the 1896 event. The 2011 Tohoku Earthquake Tsunami Joint Sur- vey (TTJS) Group reported that tsunami height was 22.84 m near the shoreline at this site and reached up to ~ 720 m inland and 12 m in elevation (http://www.coast- al.jp/ttjt/) (Fig. 3b). The inundation distances and run- up heights of the 1896 and 2011 tsunamis cannot be compared directly because the wave breakers were not constructed until after the 1896 ...
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... the coastal lowland of the study area. We set a survey transect perpen- dicular to the shoreline, 400-700 m from the sea. To re- trieve cores, we used a Handy Geoslicer sampling tool of 0.6 and 1 m long ( Takada et al. 2002) and a peat sampler. Coring sites were selected to evaluate the continuity of event deposits (NM1 to NM20 and NM-A to NM-L; Fig. 3a). Samples for analyses were taken from the cores using rectangular plastic cases (5 × 20 cm). In the field and laboratory, we conducted lithological observations, made sketches, and took photographs of the cores. As for some coring sites, also used a peat sampler that can obtain sam- ples from greater depths than the ones by Handy ...
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... on this evidence, it is unlikely that the event layers were formed by storm waves. The 2011 Tohoku- oki and 1896 Meiji Sanriku tsunamis inundated far be- yond the possible maximum limit of storm waves and fully covered the distribution area of the event layers (Fig. 3b). Therefore, while the event layers do not show the laminations or shell fragments that are apparent in the 2011 tsunami deposits, we infer that the event layers were probably formed by tsunami ...
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The 1601 CE earthquake (Mw ca. 5.9) in “Unterwalden,” Central Switzerland, triggered multiple subaqueous mass movements and a subaerial rockfall that generated tsunami waves with run‐up heights of up to 4 m and several hundred meters of inundation along the coastal lowlands of Lake Lucerne. In the shallow Lucerne Bay, historical chronicles reported...
Abstract Stratigraphic and paleontological investigations in Mugi Town, on the Pacific coast of Shikoku Island, revealed evidence of as many as five tsunami inundations from events along the Nankai Trough between 5581 and 3640 cal yr BP. Nine event deposits (E1–E9) were identified in cores ranging in length from 2 to 6 m, consisting of sandy and gr...
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... In addition to the discovery of new tsunami deposits, research on numerical modeling of tsunami and sediment transport (Sugawara 2021) and on tsunami size estimation (Namegaya and Satake 2014; Ishimura and Yamada 2019; Naruse and Abe 2017; Mitra et al. 2020Mitra et al. , 2021 has been conducted. In particular, geological surveys were performed at multiple sites along the Pacific Coast of the Tohoku region where the 2011 tsunami inundated provided thousands of years of tsunami history (e.g., Goto et al. 2015;Miyauchi 2015, 2017;Takada et al. 2016;Inoue et al. 2017). Although long-term tsunami-deposit history was obtained at multiple sites, correlation between tsunami deposits from site to site and between historical records and geological surveys and estimation of the probable sources of historical and paleotsunamis have been challenging (Takashimizu 2014;Goto et al. 2021;Ishizawa et al. 2022). ...
A decade after the 2011 Tohoku-oki earthquake (Mw 9.0), geological surveys were conducted at multiple sites along the Pacific Coast of the tsunami-inundated Tohoku region in Japan, providing thousands of years of tsunami history. However, the challenges of correlation between historical records and geological tsunami deposits and identifying sources of historical and paleotsunamis have newly surfaced. Particularly the simultaneity and source of the 1611 Keicho tsunami in the Tohoku region and the seventeenth-century tsunami in the Hokkaido region are problematic. To solve such major issues, we conducted a tsunami-deposit survey at Sekinehama on the north coast of Shimokita Peninsula, near the junction of the Japan and Kuril trenches. We performed nondestructive analyses (X-ray computed tomography and micro-X-ray-fluorescence core scanning), grain-size analysis, tephra analysis, and radiocarbon dating of sediments from two coastal outcrops and inland drill cores. We identified five tsunami deposits (TD1–TD5) during the last 6 kyr and correlated them at a 200–400 m distance from the coast. They also correlate with previously identified tsunami deposits around the Shimokita Peninsula. From our study on tsunami deposits, we found other washover deposits in the coastal outcrops that are not represented in the inland cores. These indicate minor washover events related to small tsunamis and infrequent storm surges. The modeled age of the latest tsunami deposit is 500–300 cal yr BP (1450–1650 cal CE). This either correlates with two known tsunamis (the 1611 Keicho tsunami and another seventeenth-century tsunami) or is a previously unknown tsunami that occurred in the fifteenth–seventeenth centuries. If the latest tsunami deposit is to be accurately correlated with tsunami deposits previously identified within a 50-km distance from the study site, we need to consider an unknown fifteenth-century tsunami. Our investigation yields insights regarding the tsunami source in the vicinity of the junction of the Japan and Kuril trenches.
... The magnitudes of these earthquakes have attracted the attention of the scientific community, particularly after the catastrophic 2011 Tohoku-oki tsunami (TOT), which clearly demonstrated that understanding tsunami hazard was not adequate to develop an effective disaster risk reduction strategy and prevent casualties due to tsunami episodes (Goto et al. 2021). To increase the knowledge regarding the frequency and magnitude of events from historical records, several studies evaluating the sedimentological evidence of tsunamis have been conducted along the Sanriku Coast (Minoura et al. 1994;Sugawara et al. 2012;Ishimura and Miyauchi 2015;Takada et al. 2016;Inoue et al. 2017;Ishizawa et al. 2018;Ishizawa et al. 2022) and the Pacific coast of Hokkaido (Nanayama et al. 2002(Nanayama et al. , 2003(Nanayama et al. , 2007Hirakawa et al. 2005;Ishizawa et al. 2017). ...
... Between Misawa in the Shimokita Peninsula (Tanigawa et al. 2014a) up to Hirono Town (Harashinai site), northern Sanriku Coast (Takada et al. 2016), there are no paleotsunami studies, rendering the region record incomplete (Fig. 1e). Southward, along with the Sanriku coast, several studies have been done (e.g., Imaizumi et al. 2010;Goto et al. 2015;Takada et al. 2016;Inoue et al. 2017;Ishimura 2017;Goto et al. 2019) (Fig. 1e), and the paleotsunami record is thorough. Due to the peculiar location and distance from the two neighboring subduction zones, the southern 1611, 1896, 1933. ...
... Previous paleotsunami studies have demonstrated that characteristics such as upward fining, landward thinning and finning, erosional sedimentary structures, and paleocurrent directions are relevant indicators of the nature of the flow and key points to link deposit events to a tsunami source (Sugawara et al. 2008;Richmond et al. 2011;Cisternas et al. 2018). However, in several cases, such characteristics can also be the result of river floods (Inoue et al. 2017;Shyu et al. 2019). By performing grain size analysis, it is possible to infer some of these textural characteristics and contribute toward the understanding of the sedimentary environment, energy, and transport media involved during its deposition (López 2017). ...
Paleotsunami studies along the Pacific coast of Tohoku, northern Japan, have been considerably developed recently, particularly after the massive impact of the 2011 Tohoku-oki tsunami. Nevertheless, in the southernmost Shimokita Peninsula, studies pertaining to paleotsunami are underdeveloped, leading to a vague understanding of the tsunamigenic sources northward of the Tohoku region, along with incomplete hazard evaluation. Paleotsunami deposits in Shimokita can be related not only to the Japan Trench along the Sanriku coast but also to the Kuril trench along the Pacific coast of Hokkaido. In this study, we unveiled the paleotsunami history of Hachinohe in northern Tohoku. Using a combination of sedimentological, geochemical, paleontological, and mineralogical proxies, we characterized seven sand layers that dated from ca. 2700 to ca. 5500 yr BP based on radiocarbon ( ¹⁴ C) ages as event deposits of marine origin. Sedimentological and paleontological evidence coupled with ground-penetrating radar imagery revealed a marsh environment comprising successive extinct ponds, controlling the depositional environment. Numerical modeling ruled out the possibility of storms as genetic sources, leading to the conclusion that the presence of event deposits with marine sediments in the study area would be associated with tsunami inundation episodes. Based on ¹⁴ C dating, the mean frequency of recurrence of tsunamis is estimated as 384 years (320–450 yr, 95% confidence interval) and a coefficient of variation of 0.78 (0.68–0.99, 95% confidence interval). The previously recorded limited paleotsunami evidence and absence of an estimated recurrence interval in the Shimokita Peninsula reaffirm the importance of Hachinohe as a tsunami record site for the activity of both trenches.
... The magnitudes of these earthquakes have attracted the attention of scienti c community, particularly after the catastrophic 2011 Tohoku-oki tsunami (TOT), which clearly demonstrated that understanding tsunami hazard was not adequate to develop an effective disaster risk reduction strategy and prevent casualties due to tsunami episodes (Goto et al., 2014). To increase the knowledge regarding the frequency and magnitude of events from historical records, several studies evaluating the sedimentological evidence of tsunamis have been conducted along the Sanriku Coast (Inoue et al. 2017; Ishimura and Miyauchi sedimentary sequence because it records the attenuation of X-rays on materials due to the Compton scattering effect, the magnitude of which depends primarily on density and consequently on the mineralogy (Mees et al. 2003). For instance, Paris et al. (2019) utilized high-resolution micro X-ray CT to characterize the sedimentary fabric, grading, and imbrication angles, inferring tsunami as the transport agent. ...
Paleotsunami studies along the Pacific coast of Tohoku, northern Japan, have been considerably developed recently, particuvclarly after the massive impact of the 2011 Tohoku-oki tsunami. Nevertheless, in the southernmost Shimokita Peninsula, studies pertaining to paleotsunami are underdeveloped, leading to vague understanding of the tsunamigenic sources northward of the Tohoku region, along with incomplete hazard evaluation. Paleotsunami deposits in Shimokita can be related not only to the Japan trench along the Sanriku coast, but also to the Kuril trench along the Pacific coast of Hokkaido. In this study, we unveiled the paleotsunami history of Hachinohe in northern Tohoku. Using a combination of sedimentological, geochemical, paleontological, and mineralogical proxies, we characterized seven sand layers that dated from ca. 2700 to ca. 5500 yr BP based on radiocarbon ( ¹⁴ C) ages as event deposits of marine origin. Sedimentological and paleontological evidence coupled with ground penetrating radar imagery revealed a marsh environment comprising successive extinct ponds, controlling the depositional environment. Numerical modeling ruled out the possibility of storms as genetic sources, leading to the conclusion that the presence of event deposits with marine sediments in the study area would be associated with tsunami inundation episodes. Based on ¹⁴ C dating, the average frequency of recurrence of tsunamis is estimated as 545–736 years with a 2σ error range. This interval is shorter than that estimated in Harashinai, northern Sanriku Coast, near our survey site, which suggests an average of 530 to 956 years (2σ) for the same time frame. The previously recorded limited paleotsunami evidence and absence of an estimated recurrence interval in the Shimokita Peninsula reaffirms the importance of Hachinohe as a tsunami record site for the activity of both trenches.
... For assessing the variety of tsunami or storm deposits, numerical computations can be powerful tool. In fact, many researchers have used numerical modeling to analyze distribution trends in onshore sand deposits left behind by tsunami or storm waves (e.g., Apotsos, Gelfenbaum, Jaffe, Watt, et al., 2011;Cheng & Weiss, 2013;Inoue et al., 2017;Watanabe, Bricker, et al., 2017;Watanabe, Goto, et al., 2018). Cheng and Weiss (2013) conducted numerical examinations using a simplified terrain approach to find a relation of inundation and distribution distance of sandy deposit based on topography, bathymetry, input wave, and grain size. ...
Tsunami and storm deposits can be utilized for estimating inundation zones and recurrence intervals of extreme waves in modern, historic, and prehistoric times. However, the distribution of these deposits is extremely complex and affected by various factors such as the size of the waves, topography, bathymetry, and the supply of sediment and its properties. Here, we use numerical computations to identify the key factors affecting the inundation extent of tsunamis and storm waves, and which subsequently govern the distribution of the corresponding coastal sand deposits. The results demonstrate that the overall topography slope has the most significant impact on the inundation extent of tsunamis and storm waves and subsequently the inland distribution distance of the transported deposits. The existence of onshore sediment sources is crucial for estimating the maximum extent of storm deposits because only a limited amount of sediment is carried inland by the storm waves. In contrast, the presence of onshore sediment sources is less critical for the delineation of the maximum distribution envelope of tsunami deposits compared to other parameters. The parameters that mainly control the sediment deposit volume over land under tsunami and storm wave conditions are the grain size and wave height, respectively. Our computed results are summarized using the Dean number, Shields number, and Iribarren number showing an inter‐connectivity between topography, input wave characteristic, and onshore distributions of both types of deposits. Despite some simplifications, this approach can efficiently lead to an identification and reconstruction of past catastrophic wave events.
... This is likely because most of our survey sites were protected by the inner bay. For that reason, storm wave impacts on boulder movements are expected to be extremely limited 8,18 . Therefore, through the satellite image analysis, we expect that large boulders that clearly changed their positions before and after the 2011 tsunami were moved during this event. ...
Around the world, numerous coastal boulders with weight of few thousand tons are suspected to have been transported by very large tsunamis, although their origins remain enigmatic. For clarifying origins of these boulders, the relation between the tsunami flow depth and the movement of meter-size boulders should be clarified but there is no proper field dataset. Here we collected first comprehensive dataset of both moved and unmoved boulders as well as the maximum flow depths along the Sanriku coast of Japan, where was affected by the 2011 Tohoku-oki tsunami based on satellite image analyses and field survey. The dataset revealed that up to ca. 1500 tons of boulders and concrete blocks were moved by the 2011 tsunami with approx. 28 m flow depth. We further revealed that most unmoved boulders were not moved because of the local setting rather than their heavy weights. The threshold of moved/unmoved boulders is estimated against the flow depth. The threshold predicted that approx. > 20 m flow depths are required to move approx. > 1000 tons boulders. The results imply that even a few thousand tons of enigmatic boulders in the world could have been moved by these sizes of the tsunami flow depths, although applicability of our results to other examples should be evaluated in the future work. We further tested the validity of an earlier proposed inverse model. Although the model result is consistent with the field observation, assumption of the appropriate parameters is problematic and further improvement of the model is required to estimate hydrodynamic features of the tsunami and to discriminate tsunami boulders from storm ones. Regarding such future work, our dataset is expected to be important for the evaluation of the improved numerical models.
... Moreover, the findings from the analyses of the deposits have been critical for the long-term risk assessment of huge tsunami events. Since the occurrence of Tohoku-oki event in 2011, many studies have been conducted along the coasts facing the central to northern parts of the Japan Trench to reevaluate paleo-tsunami histories (e.g., Goto et al. 2015Goto et al. , 2019Inoue et al. 2017;Ishimura and Miyauchi 2015;Ishizawa et al. 2018Ishizawa et al. , 2019Kusumoto et al. 2018;Minoura et al. 2013;Sawai et al. 2012Sawai et al. , 2015Takada et al. 2016;Takeda et al. 2018;Tanigawa et al. 2014aTanigawa et al. , 2014bWatanabe et al. 2014; see also Sawai 2017Sawai , 2020. For instance, Tanigawa et al. (2014b) reported two event layers of the past 6000 years in a meadow of Misawa City, Shimokita Peninsula, Aomori Prefecture (Fig. 1). ...
... For instance, Tanigawa et al. (2014b) reported two event layers of the past 6000 years in a meadow of Misawa City, Shimokita Peninsula, Aomori Prefecture (Fig. 1). The recurrence interval of the massive tsunamis in the Sanriku region was estimated to be 290-390 years (Ishimura and Miyauchi 2015), 500-750 years (Takada et al. 2016), or 400-600 years (Inoue et al. 2017). Around the Sendai and Ishinomaki Plains, near the central part of the Japan Trench, the paleo-tsunami history for the past 3000 years was obtained, including historically well-known tsunamis, i.e., the 869 Jogan, the 1611 Keicho, and possibly the 1454 Kyotoku tsunamis . ...
This study conducted a field survey and multiproxy analyses on sediment cores retrieved from the Kobatake-ike pond in Choshi City, Chiba Prefecture, Japan. Kobatake-ike pond is located at a high elevation (i.e., 11 m above present-day sea level) and faces the southern part of the Japan Trench. Three event sand layers were detected within the continuous mud and peat sequences of 3000 years. Based on the multi-proxy analyses, including mineralogical composition, diatom assemblages, and geochemical markers, these sedimentological events were associated with past tsunamis. The most recent event was a sandy layer and is attributed to the AD 1677 Enpo tsunami, which was reported by an earlier study conducted in the pond. Our results demonstrated that two older sand layers are associated with large tsunamis that struck the Choshi area in AD 896–1445 and in BC 488–AD 215. In addition, the age ranges of these events seem to overlap that of large earthquakes and tsunamis known from the central part of the Japan Trench. This implies a possible spatiotemporal relation of earthquake generations between the central and southern parts of the Japan Trench. However, since the error ranges of the ages of tsunami deposits at the southern and central parts of the Japan Trench are still large, further investigation is required to clarify the relations of large earthquakes in both areas.
... Some of the latest articles in this field analyzing past tsunamis simulation from sediment deposits are Sugawara et al. (2019), Inoue et al. (2017), Basavaiah et al. (2019). These contributions point to the scope in palaeotsunami studies, as already discussed in Sect. ...
In the last fifteen years, tsunami science has progressed at a rapid pace. Three major tsunamis: The Indian Ocean in 2004, the 2011 Tohoku tsunami, and the 2018 Palu tsunami were significant landmarks in the history of tsunami science. All the three tsunamis, as mentioned, suffered from either no warning or poor reception of the alerts issued. Various lessons learned, consequent numerical models proposed, post-2004 tsunami damage findings manifested into solutions. However, the misperceived solutions led to a disastrous impact of the 2011 Tohoku event. In the following years, numerous improvements in warning systems and community preparedness frameworks were proposed and implemented. The contributions and new findings have added multi-fold advancements to tsunami science progress. Later, the 2018 Palu tsunami happened and again led to a massive loss of life and property. The warning systems and community seemed un-prepared for this non-seismic tsunami. A significant change is to take place in tsunami science practices and solutions. The 2018 tsunami is one of the most discussed and researched events concerning the palaeotsunami records, damage assessment, and source findings. In the new era, using machine learning and deep learning prevails in all the fields related to tsunami science. This article presents a complete 15-year bibliometric analysis of tsunami research from Scopus and Web of Science (WoS). The review of majorly cited documents in the form of a progressing storyline has highlighted the need for multidisciplinary research to design and propose practical solutions.
... In the Pacific coast area of the Tohoku region, northeast Japan, tsunami deposit studies were well performed after the 2011 Tohoku-oki earthquake (Tanigawa et al., 2014;Ishimura and Miyauchi, 2015;2017;Goto et al., 2015;Takada et al., 2016;Inoue et al., 2017;Ishizawa et al., 2019). They discovered several tsunami deposits from the recent thousands of years, indicating that large tsunamis occur at a high frequency. ...
Tsunami deposit studies have progressed and provided extensive information on the presence of tsunami deposits, their ages, and physical parameters especially after the devastating tsunamis from the 2004 Sumatra earthquake and 2011 Tohoku-oki earthquake. However, the abundance of such information makes it challenging to assess its uncertainty and reliability. These issues associated with the tsunami deposit studies are significant and should be resolved for the benefit of people using tsunami deposit data. In this study, we used multisite drilling cores to elucidate the sediment distribution in the study area. Additionally, we conducted micro-X-ray fluorescence core scanning, roundness analysis, and radiocarbon dating to obtain a certain identification of tsunami deposits and a reliable lateral correlation of sediments. Herein, we present a long-term tsunami history and tsunami deposit distribution at Koyadori on the Sanriku Coast in northeast Japan. Adding to tsunami deposits including the 2011 CE tsunami deposits reported in our previous studies, three tsunami deposits widely distributed in the Koyadori lowland were newly identified, concluding 14 tsunami deposits during the last 6000 years. This continuous and long record of large tsunamis in the central part of the Sanriku Coast provides an insight into the mechanisms of earthquakes and tsunamis occurring in the Japan Trench and is also expected to contribute to improvement in hazard assessment of earthquakes and tsunamis. Quantitative data (geochemical signature and gravel roundness) are also useful for a lateral correlation of tsunami deposits. In addition, the correlated sediment distribution enables the identification of an area with high preservation potential of tsunami deposits and estimation of paleoenvironment since 6 ka. Therefore, detailed information on tsunami deposits in one area is as necessary as revealing tsunami deposits across a wider area for future studies.
... After the Tohoku-oki tsunami, numerous efforts have been dedicated to increasing the amount of data available on the spatial extent of the Jogan and earlier deposits to the north of the Sendai Plain (e.g., Sanriku Coast; e.g., Ishimura and Miyauchi, 2015;Takada et al., 2016;Ishimura, 2017;Inoue et al., 2017). Discoveries associated with the Jogan tsunami deposits from the northern Sanriku Coast (Inoue et al., 2017) suggest that the extent of the rupture may have been larger than the extent estimated before the 2011 Tohoku-oki tsunami. ...
... After the Tohoku-oki tsunami, numerous efforts have been dedicated to increasing the amount of data available on the spatial extent of the Jogan and earlier deposits to the north of the Sendai Plain (e.g., Sanriku Coast; e.g., Ishimura and Miyauchi, 2015;Takada et al., 2016;Ishimura, 2017;Inoue et al., 2017). Discoveries associated with the Jogan tsunami deposits from the northern Sanriku Coast (Inoue et al., 2017) suggest that the extent of the rupture may have been larger than the extent estimated before the 2011 Tohoku-oki tsunami. ...
Providing a wealth of geophysical and geological data, the 2011 Tohoku-oki tsunami offered a rare opportunity to advance research on tsunami sedimentology. In this chapter, six key findings, presented in the form of lessons learned, are identified based on a review of recent studies on the 2011 Tohoku-oki tsunami and other relevant literature:
1.limitations posed when using marine materials as evidence for tsunami inundation;
2.the larger extent and lower preservation potential associated with offshore tsunami deposits;
3.possible false dating of paleotsunami events due to tsunami-induced erosion;
4.uncertainty associated with tsunami inundation distances deduced from deposit extent;
5.spatial variability in deposit thickness and its relation to flow depth;
6.challenges associated with estimating the size and extent of earthquakes based on tsunami deposits.
In response to the extensive loss of life and property caused by the Tohoku-oki tsunami, as well as the fact that geological precursor evidence has not been effectively used in disaster management programs, the importance of tsunami deposit research has gained significant attention with the expectation that it will provide reliable information on the recurrence interval and size of devastating tsunamis. An improved understanding of the provenance, depositional processes, spatial distribution, and tsunami deposit variability will enhance our ability to identify, date, and quantify paleotsunamis.
... The hypothetical sources include (1) an interplate earthquake plus submarine landslide (Imamura et al. 2008), (2) tsunami earthquake along the Ryukyu trench (Nakamura 2009), and (3) a single massive landslide near the trench axis (Okamura et al. 2018). Although sandy tsunami deposits are used frequently to elucidate the paleotsunami histories along the coast of Japan (e.g., Minoura et al. 2001;Ishimura and Miyauchi 2015;Kitamura 2016;Inoue et al. 2017;Shimada et al. 2019), few have been reported from the Miyako and Yaeyama Islands because of the small number of surveys (Table 1). Detailed paleotsunami studies of the islands conducted by Ando et al. (2018) and by Kitamura et al. (2018a) have specifically examined sandy deposits. ...
Huge tsunami waves have repeatedly bombarded the southern end of the Ryukyu Islands (Miyako and Yaeyama Islands, southwestern Japan) at several-hundred-year intervals. Therefore, clarifying the islands’ paleotsunami history is important for risk assessment. Nevertheless, discrepancies of paleotsunami histories exist among regional studies of tsunami boulders and sandy tsunami deposits. Radiocarbon ages of tsunami boulders indicate that tsunami events of the last 2400 years have occurred every 150–400 years, most recently the historical 1771 Meiwa tsunami. Sandy tsunami deposits at Yaeyama Islands show that four tsunami events of the last 2000 years struck the islands at approximately 600-year intervals. Sandy tsunami deposits of the Miyako Islands have been studied only rarely. Therefore, studying sandy tsunami deposits in the Miyako Islands is crucially important for clarifying the paleotsunami history of this region. We conducted a trench survey on Minna Island, located among the westernmost Miyako Islands, which revealed two sandy tsunami deposits under a coral tsunami boulder transported by the 1771 tsunami. The upper tsunami deposit was likely deposited by the 1771 tsunami, as inferred from stratigraphic correlation to the tsunami boulder. However, the lower tsunami deposit was probably deposited 700–1000 years ago, which is consistent with the age range of the paleotsunami reported for Yaeyama Islands. Because sandy tsunami deposits found in this and earlier studies are thick and deposited at high elevation and far inland, these are useful markers of large tsunami events similar to the 1771 event. However, the reported tsunami boulders of various sizes are deposited along the coast and reefs: they can be formed not only by large tsunami events but also by small ones. It is noteworthy that each tsunami deposit is coarse and thick (40–48 cm) relative to the island elevation (about 12 m maximum, 7 m above the mean sea level at the study site). By assuming that tsunamis have affected this region repeatedly during the past few thousand years at around 600-year intervals, tsunamis might have been important geomorphic agents for building up small reef-surrounded islands such as Minna Island.
