Sandy Tsunami Deposits Research Articles

Testing Megathrust Rupture Models Using Tsunami Deposits

AbstractThe 26 January 1700 CE Cascadia subduction zone earthquake ruptured much of the plate boundary and generated a tsunami that deposited sand in coastal marshes from northern California to Vancouver Island. Although the depositional record of tsunami inundation is extensive in some of these marshes, few sites have been investigated in enough detail to map the inland extent of sand deposition and depict variability in tsunami deposit thickness and grain size. We collected 129 cores in marshes of the Salmon River estuary in Oregon and reanalyzed 114 core logs from a 1987–88 study that mapped the inland extent of circa 1700 CE sandy tsunami deposits. The ca. 1700 CE tsunami deposit in the Salmon River estuary is easily recognized in cores ≤1 m deep in which a buried marsh peat is overlain by a well sorted sand bed with a sharp lower contact that thins and fines inland. We use tsunami deposit data and models of sandy tsunami sediment transport (using Delft3D‐FLOW) to test 15 rupture models that could represent a ca. 1700 CE earthquake. At least 12–16 m of slip offshore of the Salmon River, which results in 0.8–1.0 m of coastal coseismic subsidence, is required to match the ca. 1700 CE sand deposit's inland extent, which is consistent with models of heterogeneous megathrust slip in ca. 1700 CE. Our methods of detailed tsunami deposit mapping, combined with sediment transport modeling, can be used to test models of megathrust ruptures and their tsunamis to potentially improve earthquake and tsunami hazard assessments.

Identifying tsunami traces beyond sandy tsunami deposits using terrigenous biomarkers: a case study of the 2011 Tohoku-oki tsunami in a coastal pine forest, northern Japan

The distributions of sandy tsunami deposits do not reflect the true extents of tsunami inundation areas, leading to underestimates of inundation by past tsunamis and thus the magnitudes of their associated tsunamigenic earthquakes. To archive the sedimentological and geochemical features of the 2011 Tohoku-oki tsunami deposit, we performed visual observations and computed tomography, grain-size, water content, and organic geochemical analyses of sediments from a coastal forest at Oirase Town, northern Japan. Stratigraphic observations revealed the 2011 tsunami deposit to be a landward-thinning interbedded sand and soil layer that became ambiguous in landward locations. The sediment samples from the inundated area did not contain marine-sourced biomarkers; instead, peak concentrations of isolongifolene, an organic compound derived from Pinus in the forest, were observed within or just above the sandy tsunami deposits in sediment sections. Peak isolongifolene concentrations were also detected in landward soils inundated by the tsunami in which no sand layer was observable, but were not observed beyond the inundation limit. Although this characteristic biomarker is unique to this and similar depositional environments, these results suggest that lateral changes of the concentrations of environment-specific biological proxies in the sedimentary column may record tsunami inundation.

Numerical estimation of maximum possible sizes of paleo-earthquakes and tsunamis from storm-derived boulders

Estimating the magnitudes of potential large earthquakes and associated tsunami hazards along a subduction zone is important for risk evaluation. Coastal boulder deposits can serve as valuable geological indicators in intertidal zones with high wave velocity, even though finer sediments are generally washed away. This report presents a new method to constrain the maximum possible sizes of paleo-earthquakes and tsunamis using boulders of storm wave origin as a constraint in numerical simulations. The fundamental assumption is that presently observed storm-derived boulders have not moved a great distance by past tsunami waves. This illustrative case study uses the sizes and spatial distributions of storm wave boulders on the reef at Kudaka Island, Okinawa Prefecture, which faces the Ryukyu Trench. In all, 47 tsunami scenarios with different locations and fault parameters were assumed. Results show that tsunami waves with a maximum water level of 2.2 m and maximum flow speed of 1.5 m/s at a beach of Kudaka Island did not strike the island. Under this condition, earthquakes of Mw≥8.3 are unlikely to have occurred in the central Ryukyu Trench if one obeys an empirical scaling law for subduction earthquakes or ≥2 m of maximum possible slip near the trench if the fault size is fixed as 100 km × 50 km. Those tsunami and earthquake sizes could not have been generated during this period if one assumes that the reef of Kudaka Island and emplacement of storm boulders began more than 3500 years ago. This finding implies that the seismogenic mechanism of the Ryukyu Trench and the risk of a giant earthquake differ from those of other trenches such as the Japan Trench and the Chilean Trench, where giant earthquakes of Mw≥9.0 have occurred repeatedly during the past. The proposed method would enable researchers to estimate the maximum possible sizes of the paleo-earthquakes and tsunami waves in any region, even in regions with a vague or unknown history of past events because of a lack of sandy tsunami deposits or seismic records.

Effect of artificial structures on the formation process of the 2011 Tohoku-oki tsunami deposits

The interaction between tsunami hydrodynamics and sediment behavior results in a complex depositional process for onshore tsunami deposits. Furthermore, the formation of modern tsunami deposits can be affected by artificial structures, which can make the depositional processes difficult to understand. In the present study, we focus on artificial structures to reveal the relationship between sedimentary structures and hydrodynamics of the 2011 Tohoku-oki tsunami. Field observations indicated that a coastal dike and three shore-parallel roadways significantly affected the source, thickness, and sedimentary structures of the tsunami deposits. The sandy tsunami deposits were mainly sourced from the backshore, which was severely eroded at the landward side of the dike by tsunami overflow. The thickness of the tsunami deposits increased seaward of roadways, while the deposit became thinner landward of roadways. A one-dimensional numerical simulation indicated a local decrease in the flow velocity at the seaward side of roadways, which elucidated the increase in the deposit thickness. Most tsunami deposits are characterized by normal graing, inverse grading, and ungraded units. Inverse grading was observed 0.05 km from the shoreline and at the seaward side of the roadways, except at the most landward roadway. Changes in the grain-size distribution of the deposits and variations in the simulated flow velocity across the roadways suggested that the inverse grading was formed because of waxing and depletive flow conditions with high concentrations of suspended sediments. Our study elucidated the key role that artificial structures play in the formation of tsunami deposits and the resultant sedimentary structures, in developed coastal areas. Artificial structures, which have a somewhat extreme topography compared to the natural geomorphology, also contributed to demonstrate the concept of the acceleration matrix. This is because of the remarkable change in the steadiness and uniformity of the tsunami flow and the resultant sedimentary structures.

Millennial paleotsunami history at Minna Island, southern Ryukyu Islands, Japan

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.

Tsunamis and submarine landslides in Suruga Bay, central Japan, caused by Nankai–Suruga Trough megathrust earthquakes during the last 5000 years

Magnitude 8 megathrust earthquakes that have generated large tsunamis with wave heights of >5 m in coastal areas of southwestern Japan have occurred every 100–150 years along the Nankai–Suruga Trough, which marks the convergent boundary between the Eurasian and Philippine Sea plates. Using sedimentological, geochemical, and paleontological analyses, as well as 14C dating, we examined tsunami deposits in 12 sediment cores collected from the Hamatome lowland, Yaizu Plain, Shizuoka, which faces the Suruga Trough. Four sandy tsunami deposits (Sands 1–4) are identified in sediments deposited during the last 5000 years. Sand 1 was deposited between 805 and 405 BC, suggesting that only one tsunami left a sedimentological imprint in the Hamatome lowland lagoon during the period of ∼4000 years between 3090 and 2913 BC and the AD 1096 Eicho-Tokai earthquake. This very low frequency of tsunami inundation may suggest that this tsunami was larger than those typically associated with megathrust earthquakes. Sands 2–4 are correlated with tsunamis associated with the Eicho-Tokai, AD 1361 Shohei-Koan, and AD 1498 Meio-Tokai earthquakes, respectively. Our data indicate that a sand spit, which had previously protected the lagoon from oceanic waves, was abruptly destroyed following the AD 1096 Eicho-Tokai earthquake and did not re-form at the same location. This disappearance of the sand spit is inferred to have involved an earthquake-induced submarine landslide. The data presented in this paper, together with information obtained from historical documents, indicates that M8 megathrust earthquakes in AD 1096 and AD 1498 caused submarine landslides in the study area, resulting in coastal subsidence and damage to coastal areas additional to that caused by the tsunamis.

Sand deposits reveal great earthquakes and tsunamis at Mexican Pacific Coast

Globally, instrumentally based assessments of tsunamigenic potential of subduction zones have underestimated the magnitude and frequency of great events because of their short time record. Historical and sediment records of large earthquakes and tsunamis have expanded the temporal data and estimated size of these events. Instrumental records suggests that the Mexican Subduction earthquakes produce relatively small tsunamis, however historical records and now geologic evidence suggest that great earthquakes and tsunamis have whipped the Pacific coast of Mexico in the past. The sediment marks of centuries old-tsunamis validate historical records and indicate that large tsunamigenic earthquakes have shaken the Guerrero-Oaxaca region in southern Mexico and had an impact on a bigger stretch of the coast than previously suspected. We present the first geologic evidence of great tsunamis near the trench of a subduction zone previously underestimated as potential source for great earthquakes and tsunamis. Two sandy tsunami deposits extend over 1.5 km inland of the coast. The youngest tsunami deposit is associated with the 1787 great earthquake, M 8.6, producing a giant tsunami that poured over the coast flooding 500 km alongshore the Mexican Pacific coast and up to 6 km inland. The oldest event from a less historically documented event occurred in 1537. The 1787 earthquake, and tsunami and a probable predecessor in 1537, suggest a plausible recurrence interval of 250 years. We prove that the common believe that great tsunamis do not occur on the Mexican Pacific coast cannot be sustained.

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

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.

Organic geochemical investigation of far‐field tsunami deposits of the Kahana Valley, O‘ahu, Hawai‘i

AbstractFar‐field tsunami deposits observed in the Kahana Valley, O‘ahu, Hawai‘i (USA), were investigated for their organic‐geochemical content. During short high‐energy events, (tsunamis and storms) organic and chemical components are transported with sediment from marine to terrestrial areas. This study investigates the use of anthropogenic based organic geochemical compounds (such as polycyclic aromatic hydrocarbons, pesticides and organochlorides) as a means to identify tsunami deposits. Samples were processed by solid–liquid extraction and analyzed using gas chromatography–mass spectrometry. A total of 21 anthropogenic marker compounds were identified, of which 11 compounds were selected for detailed analysis. Although the tsunami deposits pre‐date industrial activity in Hawai‘i by several hundred years, distinct changes were found in the concentrations of anthropogenic marker compounds between sandy tsunami deposits and the surrounding mud/peat layers, which may help in identifying tsunami deposits within cores. As expected, low overall concentrations of anthropogenic markers and pollutants were observed due to the lack of industrial input‐sources and little anthropogenic environmental impact at the study site. This geochemical characterization of tsunami deposits shows that anthropogenic markers have significant potential as another high‐resolution, multi‐proxy method for identifying tsunamis in the sedimentary record.

Using molluscan assemblages from paleotsunami deposits to evaluate the influence of topography on the magnitude of late Holocene mega-tsunamis on Ishigaki Island, Japan

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

Effects of topography on particle composition of 2011 tsunami deposits on the ria-type Sanriku coast, Japan

A geological survey was conducted in a small valley along the Sanriku coast, and the grain compositions of both autochthonous gravels and the 2011 tsunami deposit from the Tohoku-oki earthquake were investigated to reveal the depositional form of the tsunami deposits on the ria-type coast. The features of autochthonous gravels in the beach, riverbed, and slope areas are characterized by a combination of their rock type and roundness. The roundness values of the beach and slope gravels are large (0.8–0.9) and small (0.1–0.2), respectively, regardless of rock type, and the riverbed gravels are typically granite rocks with an intermediate roundness (0.4–0.5). The 2011 tsunami deposits along the survey transect include the above autochthonous gravels and show several characteristic features, such as a sharp erosional contact with the underlying peaty soil, normal and/or inverse grading structures, mud sheets, and richness in gravels in the lower unit, while the thickness shows no systematic changes with the distance from the coast. The presence of slope gravels suggests high tsunami inundation and flux, which eroded bare rock and talus and transported them to the valley plain by backwash current. The mixture of beach and riverbed deposits implies the formation of both an upstream current from the beach, which transported beach pebbles inland, and a backwash current along the channel, which accumulated sub-angular riverbed gravels around the river mouth. The characteristics of the 2011 tsunami deposit on the Sanriku coast, where narrow valleys are surrounded by high cliffs with steep slopes, are significantly different from those of sandy tsunami deposits in Sendai Plain, where a flat plain develops within several kilometers of the coastline. The differences among sedimentary facies are affected by the topography and autochthonous deposits.

Stratigraphic records of tsunamis along the Japan Sea, southwest Hokkaido, northern Japan

AbstractThe stratigraphy of tsunami deposits along the Japan Sea, southwest Hokkaido, northern Japan, reveals tsunami recurrences in this particular area. Sandy tsunami deposits are preserved in small valley plains, whereas gravelly deposits of possible tsunami origin are identified in surficial soils covering a Holocene marine terrace and a slope talus. At least five horizons of tsunami events can be defined in the Okushiri Island, the youngest of which immediately overlies the Ko‐d tephra layer (1640 AD) and was likely formed by the historical Oshima‐Ohshima tsunami in 1741 AD. The four older tsunami deposits, dated using accelerator mass spectrometry 14C, were formed at around the 12th century, 1.5–1.6, 2.4–2.6, and 2.8–3.1 ka, respectively. Tsunami sand beds of the 1741 AD and circa 12th century events are recognized in the Hiyama District of Hokkaido Island, but the older tsunami deposits are missing. The deposits of these two tsunamis are found together at the same sites and distributed in regions where wave heights of the 1993 tsunami (Hokkaido Nansei‐oki earthquake, Mw = 7.7) were less than 3 m. Thus, the 12th century tsunami waves were possibly generated near the south of Okushiri Island, whereas the 1993 tsunami was generated towards the north of the island. The estimated recurrence intervals of paleotsunamis, 200–1100 years with an average of 500 years, likely represents the recurrence interval of large earthquakes which would have occurred along several active faults offshore of southwest Hokkaido.

A record of local storms and trans-Pacific tsunamis, eastern Banks Peninsula, New Zealand

This study of five sand units at Lavericks Bay, New Zealand, reports on the sedimentary evidence for three trans-Pacific tsunamis and two local storms. The 1868 Arica, 1877 Iquique and 1960 Valdivia tsunamis from Chile were the largest distantly generated events in New Zealand’s history but have never before been identified at the same location. It is also the first time that the 1877 Iquique tsunami deposit has been found in New Zealand. Two further sand units were identified as local storm deposits laid down in 1869/1870 and 1929. The identification and chronology of these events were established through the use of geochemistry, palynology, diatoms, charcoal abundance and historical documents. Their relative magnitudes were estimated through the use of grain size parameters and lateral extent of the recognisable sand layers. The recognisable sandy tsunami deposits extend about 60% of the inundation distance, while the storm sediments are finer and less extensive. There were two notable geochemical differences between the storm and tsunami deposits. Both storm deposits had lower concentrations of marine proxy elements associated with lower Ca–Ti and Sr–Ba ratios. Other differences were noted between some of the tsunami and storm deposits such as rip-up clasts and sediment characteristics, but these were by no means unequivocal. It is possible that geochemistry may prove to be the only proxy capable of not only differentiating effectively between storm and tsunami sediments but also identifying the maximum inland extent of a deposit and of inundation. It is the ability to better understand the nature and extent of such catastrophic events through these subtle differences in event characteristics that will help improve risk management for coastlines around the world.

Geochemical characteristics of deposits from the 2011 Tohoku‐oki tsunami at Hasunuma, Kujukuri coastal plain, Japan

AbstractWe examined the geochemical characteristics and temporal changes of deposits associated with the 2011 Tohoku‐oki tsunami. Stable carbon isotope ratios, biomarkers, and water‐leachable ions were measured in a sandy tsunami deposit and associated soils sampled at Hasunuma, Kujukuri coastal plain, Japan, in 2011 and 2014. At this site, the 2011 tsunami formed a 10–30 cm ‐thick layer of very fine to medium sand. The tsunami deposit was organic‐poor, and no samples contained any detectable biomarkers of either terrigenous or marine origin. In the underlying soil, we identified hydrocarbons and sterols derived from terrestrial plants, but detected no biomarkers of marine origin. In the samples collected in 2011, concentrations of tsunami‐derived water‐leachable ions were highest in the soil immediately beneath the tsunami deposit and then decreased gradually with depth. Because of its finer texture and higher organic content, the soil has a higher water‐holding capacity than the sandy tsunami deposit. This distribution suggests that ions derived from the tsunami quickly penetrated the sand layer and became concentrated in the underlying soil. In the samples collected in 2014, concentrations of water‐leachable ions were very low in both soil and sand. We attribute the decrease in ion concentrations to post‐tsunami rainfall, seepage, and seasonal changes in groundwater level. Although water‐leachable ions derived from seawater were concentrated in the soil beneath the tsunami deposit following the tsunami inundation, they were not retained for more than a few years. To elucidate the behavior of geochemical characteristics associated with tsunamis, further research on organic‐rich muddy deposits (muddy tsunami deposits and soils beneath sandy tsunami deposits) as well as sandy tsunami deposits is required.

Ground penetrating radar examination of thin tsunami beds — A case study from Phra Thong Island, Thailand

Coastal overwash deposits from tsunamis and storms have been identified and characterised from many coastal environments. To date, these investigations have utilised ad-hoc time, energy and cost intensive invasive techniques, such as, pits and trenches or taking core samples. Here, we present the application of high-frequency ground penetrating radar (GPR) to identify and characterise the 2004 Indian Ocean Tsunami (IOT) and palaeotsunami deposits from Phra Thong Island, Thailand. This site is one of the most intensively studied palaeotsunami sites globally and preserves a series of late-Holocene stacked sandy tsunami deposits within an organic, muddy low-energy backbeach environment.Using 100, 500 and 1000MHz GPR antennas, 29 reflection profiles were collected from two swales (X and Y) inland of the modern beach, and two common mid-point (CMP) profiles using the 200MHz antennas were collected from Swale Y. Detailed examination of the CMPs allowed accurate velocity estimates to be applied to each profile. The reflection profiles included across-swale profiles and a high-resolution grid in Swale X, and were collected to investigate the feasibility of GPR to image the palaeotsunami deposits, and two profiles from Swale Y where the tsunami deposits are poorly known. The 500MHz antennas provided the best stratigraphic resolution which was independently validated from the stratigraphy and sedimentology recovered from 17 auger cores collected along the profiles. It is clear from the augers and GPR data, that the different dielectric properties of the individual layers allow the identification of the IOT and earlier tsunami deposits on Phra Thong Island. Although applied in a coastal setting here, this technique can be applied to other environments where thin sand beds are preserved, in order to prioritise sites for detailed examination.

Reexamination of the A.D. 869 Jogan earthquake size from tsunami deposit distribution, simulated flow depth, and velocity

The rupture parameters and magnitude of the A.D. 869 Jogan earthquake, a predecessor of the 2011 Tohoku earthquake, were previously estimated by matching tsunami deposit distributions with simulated inundation areas. The tsunami inundation associated with the 2011 Tohoku earthquake, however, extended farther inland than the sandy tsunami deposits. Numerical simulation of the 2011 tsunami indicated that flow depths and velocities were approximately 1 m and 0.6 m/s, respectively, at the most inland sand deposit sites on the Ishinomaki and Sendai plains. While these values depend on the assumed bottom roughness, we used these values to compare tsunami deposits and inundation simulation of the 869 Jogan earthquake from both uniform-slip and 2011-type variable-slip fault models. The results showed that the rupture length of the 869 Jogan earthquake was at least 200 km and its minimum moment magnitude was 8.6.

Tsunami recurrence revealed by Porites coral boulders in the southern Ryukyu Islands, Japan

Information about past tsunami hazards, such as their recurrence interval and magnitude, is needed for future disaster prediction and mitigation. We examined radiocarbon ages of the surfaces of massive coral boulders cast ashore by past tsunamis in the southern Ryukyu Islands, Japan, where few historical and geological records of past tsunamis are available. We selected only non-eroded Porites coral boulders along the shoreline, because their characteristics make it possible to determine the probable timing of their deposition by tsunamis, and we applied a dating method that uses the cumulative probability distributions of large numbers of radiocarbon measurements of those boulders to determine the timing of past tsunamis. The results demonstrate that the southern Ryukyu Islands have repeatedly experienced tsunami events since at least 2400 yr ago, with a recurrence interval of ∼150–400 yr. The largest Porites tsunami boulder that we studied (long axis, 9 m), which is probably the largest single-colony tsunami boulder in the world, was displaced by the A.D. 1771 Meiwa tsunami. Although the 1771 Meiwa tsunami was likely the largest event in at least the past 700 yr, calculations of current velocity show that all identified tsunamis occurring before 1771 were probably large enough to cause considerable damage to human-built structures and loss of life. This study demonstrates that by reliably dating large numbers of selected coastal boulders it is possible to ascertain the timing, recurrence interval, and magnitude of past tsunamis in a location where few adequate survey sites of sandy tsunami deposits exist.

Flow speed estimated by inverse modeling of sandy tsunami deposits: results from the 11 March 2011 tsunami on the coastal plain near the Sendai Airport, Honshu, Japan

Tsunami deposits contain information about the flow that created them which can be interpreted to estimate tsunami magnitude. Sandy deposits near Sendai Airport are modeled using an inverse sediment transport model to explore the spatial and temporal variation of tsunami flow speed in the 11 March 2011 Tohoku-oki tsunami. The 2011 deposits contain grain-size signatures of both sediment transport convergences and sediment settling out of suspension. Sediment transport convergences form massive or inversely graded intervals. Sediment falling out of suspension forms a specific type of normal grading, termed suspension grading. The Jaffe and Gelfenbaum (2007) inverse sediment transport model, which relates the thickness and bulk grain size of suspension-graded intervals to tsunami flow speed, was applied to 24 suspension-graded intervals, ranging in thickness from 2 to 4cm, at 7 trenches located from about 250 to 1350m inland from the shoreline. Modeled tsunami flow speeds range from 2.2 to 9.0m/s, and were strongly dependent on the choice of Manning's n roughness parameterization. Flow speeds were highest from about 75–300m landward of the forested sand dunes where the tsunami encountered lower roughness in a low-lying area as it traveled downslope, and decreased by 16% as the tsunami moved inland over the subsequent 596m. As many as 5 suspension-graded intervals, interpreted as forming during the onshore flow of up to 5 waves, were identified at each location, with a tendency for an inland decrease in the number of suspension-graded intervals suggesting fewer waves inland. Tsunami flows tended to be fastest in the lower two intervals (earlier waves) and slowest in the uppermost interval (last wave) of the deposits. Modeled reconstructions of the suspension-graded intervals at a vertical resolution of 1-cm reproduced observed upward fining of the distributions reasonably well, supporting the use of the inverse model. Estimates of the time required to form 99% of the thickness of the suspension-graded intervals ranged 2 to 5min, allowing that sediment falling from suspension could have formed these portions of the deposits in the time between waves. Application of the inverse model to determine paleotsunami flow speeds from deposits, although now possible, is complicated by uncertainty in roughness. As inverse approaches to estimating roughness and tsunami flow speed improve, the magnitude of paleotsunamis will be better understood and the ability to assess tsunami hazard from paleotsunami deposits will improve.

Sediment sources and sedimentation processes of 2011 Tohoku-oki tsunami deposits on the Sendai Plain, Japan — Insights from diatoms, nannoliths and grain size distribution

The 11th March 2011 Tohoku-oki tsunami inundated the low-lying Sendai Plain (Japan) more than 5km inland leaving sand and mud deposits over most of the area. In order to establish the sources of the tsunami deposits and interpret processes of their sedimentation, samples were collected from the deposits, underlying soils and the beach along a shore-perpendicular transect and analysed for grain size, diatom assemblages and nannoliths. The fining-inland tsunami deposits consisted of poorly to moderately sorted medium to coarse sand within 2km of the coastline and very poorly to poorly sorted mud farther inland. More specifically, there was a slight fining of the coarse to medium sand mode within the sandy deposits and an increased contribution of the coarse and very coarse silt fraction in the mud deposits. The tsunami deposits also exhibited vertical changes including fining upward and coupled coarsening-fining upward trends. Few diatoms were present in beach sediments, soils and tsunami deposits within 1km of the coastline, while diatoms were more abundant farther inland. Diatom assemblages in the soil and tsunami deposits were similar and dominated by species typical of freshwater-brackish habitats, while no typically marine species were encountered. Nannoliths were generally absent in the studied sediments, except for few specimens. Our data indicate that there was probably no or only a very minor component of marine sediments transported onland by the tsunami. The sandy tsunami deposits within ~1km of the coastline were mostly derived from beach and dune erosion. From 1 to 2km landward the contribution of these sources decreased, while sources comprising local soil erosion and the entrainment of sediments from the Teizan-bori canal increased. Farther inland, local soil erosion was the major sediment source for the mud deposits. The tsunami deposits were most likely deposited during at least two inundations, mostly out from suspension resulting in an upward grain size fining trend. However, bed load deposition was also important in the sandy deposits as suggested by a common grain size upward coarsening, position on C–M diagrams and frequent horizontal lamination. The present study reveals that even very large tsunamis may not transport marine sediments onland and thus many commonly applied indicators of tsunami deposits based on the assumption of their offshore origin (marine diatoms, foraminifera, nannoliths, marine sediments) may be of limited use.

Relationship between the maximum extent of tsunami sand and the inundation limit of the 2011 Tohoku-oki tsunami on the Sendai Plain, Japan

This paper focuses on the relationship between the maximum extent of sandy tsunami deposits and inundation distance of the 2011 Tohoku-oki tsunami. Inundation limits of the Tohoku-oki tsunami were assessed over seven shore-normal transects along the Sendai Coastal Plain. Inundation distances were found to range from 0.6 to 4.0km. At the maximum extent of the tsunami deposit, the sand unit ranged in thicknesses from 0.1 to 4.0cm. The maximum limit of the ≥0.5cm thick sand layer extended to over 90% of the inundation distance where the inundation distance was less than 2.5km. This observation is consistent with previous studies that examined characteristics of recent tsunami events that had inundation distances of at least 2km. The maximum limit of the ≥0.5cm thick sand layer extended to 3km (57–76% of the inundation distance) where the inundation distance was more than 2.5km. Our results are important for estimating paleo-tsunami inundation distances based on the landward extent of sand deposits and valuable for the assessment of future tsunami risks in other coastal areas in the world, which have been devastated by huge tsunamis in the past.