Tsunami Deposits Research Articles

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.

Discovery of a tsunami deposit from the Bronze Age Santorini eruption at Malia (Crete): impact, chronology, extension

A geomorphological survey immediately west of the Minoan town of Malia (Crete) shows that a tsunami resulting from the Bronze Age Santorini eruption reached the outskirts of the Palatial center. Sediment cores testify a unique erosional event during the Late Minoan period, followed locally by a high energy sand unit comprising marine fauna. This confirms that a tsunami impacted northern Crete and caused an inundation up to 400 m inland at Malia. We obtained a radiocarbon range of 1744–1544 BCE for the secure pre-tsunami context and an interval 1509–1430 BCE for the post-event layer. Examination of tsunami deposits was used to constrain run-up not exceeding 8 m asl. The results open the field for new research on the Bronze Age Santorini tsunami regarding both impact and consequences for the Minoan civilization.

Stratigraphic evidence of two historical tsunamis on the semi-arid coast of north-central Chile

On September 16, 2015, a Mw 8.3 earthquake struck the north-central Chile coast, triggering a tsunami observed along 500 km of coastline, between Huasco (28.5°S) and San Antonio (33.5°S). This tsunami provided a unique opportunity to examine the nature of tsunami deposits in a semi-arid, siliciclastic environment where stratigraphic and sedimentological records of past tsunamis are difficult to distinguish. To improve our ability to identify such evidence, we targeted one of the few low-energy, organic-rich depositional environments in north-central Chile: Pachingo marsh in Tongoy Bay (30.3°S).We found sedimentary evidence of the 2015 and one previous tsunami as tabular sand sheets. Both deposits are composed of poorly to moderately sorted, gray-brown, fine-to medium-grained sand and are distinct from underlying and overlying organic-rich silt. Both sand beds thin (from ∼20 cm to <1 cm) and fine landward, and show normal grading. The older sand bed is thicker and extends over 125 m further inland than the 2015 tsunami deposit. To model the relative size of the tsunamis that deposited each sand bed, we employed tsunami flow inversion. Our results show that the older sand bed was produced by higher flow speeds and depths than those in 2015. Anthropogenic evidence along with 137Cs and 210Pb dating constrains the age of the older tsunami to the last ∼110 years. We suggest that the older sand bed was deposited by the large tsunami in 1922 CE sourced to the north of our study site. This deposit represents the first geologic evidence of a pre-2015 tsunami along the semi-arid north-central Chile coast and highlights the current and continuing tsunami hazard in the region.

Paleotsunamis on the Southern Hikurangi Subduction Zone, New Zealand, Show Regular Recurrence of Large Subduction Earthquakes

Abstract Geological records of subduction earthquakes, essential for seismic and tsunami hazard assessment, are difficult to obtain at transitional plate boundaries, because upper-plate fault earthquake deformation can mask the subduction zone signal. Here, we examine unusual shell layers within a paleolagoon at Lake Grassmere, at the transition zone between the Hikurangi subduction zone and the Marlborough fault system. Based on biostratigraphic and sedimentological analyses, we interpret the shell layers as tsunami deposits. These are dated at 2145–1837 and 1505–1283 yr B.P., and the most likely source of these tsunamis was ruptures of the southern Hikurangi subduction interface. Identification of these two large earthquakes brings the total record of southern Hikurangi subduction earthquakes to four in the past 2000 yr. For the first time, it is possible to obtain a geologically constrained recurrence interval for the southern Hikurangi subduction zone. We calculate a recurrence interval of 500 yr (335–655 yr, 95% confidence interval) and a coefficient of variation of 0.27 (0.0–0.47, 95% confidence interval). The probability of a large subduction earthquake on the southern Hikurangi subduction zone is 26% within the next 50 yr. We find no consistent temporal relationship between subduction earthquakes and large earthquakes on upper-plate faults.

Identification of Coastal Sand Deposits From Tsunamis and Storm Waves Based on Numerical Computations

AbstractTsunami 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.

Core-CT: A MATLAB application for the quantitative analysis of sediment and coral cores from X-ray computed tomography (CT)

X-ray computed tomography (CT) is a non-destructive imaging technique that provides three-dimensional (3D) visualisation and high-resolution quantitative data in the form of CT numbers. CT numbers are derived from the X-ray energy, effective atomic number and density of the analysed material. The sensitivity of the CT number to changes in material density means that it can be used to identify facies changes within sediment cores by detecting downcore shifts in sediment properties, and to quantify skeletal linear extension rates and internal biological erosion from coral cores. Here we present two algorithms to analyse CT scan images for geoscience research and package them within an open-source MATLAB application (Core-CT) that is freely available at GitHub (https://github.com/yuting-yan/Core-CT). The first algorithm computes CT numbers from a user-defined region of interest to identify boundaries of density change (e.g., sedimentary facies, laminations, coral growth bands). The second algorithm segments regions with major density contrast (e.g., internal void space or biogenic material) and provides 3D geometrical measurements of these irregularities. The versatility of Core-CT for geoscience is demonstrated for a range of environmental settings using both lake and marine sediment cores (Chile and Singapore) and coral cores (Red Sea). Core-CT analysis of sediment cores shows the application is able to: (1) distinguish episodic tsunami deposits from lacustrine sediments, (2) generate rapid and detailed measurement of varved sediments, and (3) identify sedimentary facies from an unsplit marine sediment core. When applied to coral cores, Core-CT can measure skeletal linear extension of annual growth bands and provide 3D visualisation and volumetric quantification of internal bioerosion cavities. Other potential applications of Core-CT include the identification of cryptotephra in sediment cores, or laminations in speleothem samples.

LOCAL MARINE RESERVOIR AGE (ΔR) RECONSTRUCTED BASED ON THE TSUNAMI DEPOSIT FROM PANGANI BAY (TANZANIA)

ABSTRACTQuantifying the local marine reservoir age (ΔR) and its change over time is critical for precise radiocarbon calibration of marine samples and for the study of the ocean carbon cycle. ΔR values are scarce for the African coast facing the Indian Ocean, and the few values available were obtained from pre-bomb shells collected during the 19th century. Here, the ΔR value for calibrated year 1110 ± 25 (1σ) CE was reconstructed from radiocarbon dating and Bayesian analysis of marine and terrestrial materials coexisting in a tsunami deposit discovered in Pangani Bay (Tanzania, western Indian Ocean coast). The reconstructed ΔR of –8 ± 40 (1σ, n = 3) is similar to pre-bomb regional estimates and provides new information to investigate regional ΔR change over time. The Bayesian analysis of the dated samples revises the age of the tsunami event found in Pangani Bay to 1064–1157 cal CE (95.4% confidence level) or 1110 ± 25 (1σ) cal CE, about one century younger compared to the previous estimate. Our results indicate that the new ΔR value and the proposed calibration approach can be used to refine existing chronologies in the region, with implications for paleo-environmental reconstructions and archaeological studies of Early Swahili societies.

Environmental DNA signatures distinguish between tsunami and storm deposition in overwash sand

Sandy onshore deposits from tsunamis are difficult to distinguish from storm deposits, which makes it difficult to assess coastal hazards from the geological record. Here we analyse environmental DNA from microbial communities preserved in known tsunami and storm-deposited sediments and intercalating soils and non-marine sediments near Cuddalore, India, and Phra Thong Island, Thailand. Both sites were impacted by the 2004 Indian Ocean Tsunami and a subsequent storm flooding event (2011 Cyclone Thane at Cuddalore and a 2007 storm at Phra Thong Island). We show that the microbial communities in the overwash deposits are significantly different from soil and sediments that are not derived by overwash processes at both locations. Our method also successfully discriminates between modern tsunami deposits and storm deposits. We suggest molecular techniques have the potential to accurately discriminate overwash deposits from catastrophic natural events.

Sedimentological evidence of washover deposits from extreme wave events in Zamboanga del Sur, Mindanao, southern Philippines

The Philippines' active tectonic setting and vulnerability to tsunami hazards underscore the necessity to understand tsunami sources and their impacts. Several tsunamigenic earthquakes have devastated coastal communities in the past but geological evidence of these infrequent extreme wave events (EWEs) have yet to be described and analyzed in detail. This study documents and establishes evidence of potential tsunami deposits preserved in a mangrove environment in western Mindanao Island, which was inundated by the 1976 Moro Gulf tsunami. The 1976 Moro Gulf tsunami, by far the worst tsunami disaster in the Philippines, is associated with a Mw 8.1 earthquake along the Cotabato subduction zone. The sedimentological characteristics of potential tsunami deposits in three coastal sites bordering Pagadian and Illana Bays in Zamboanga del Sur, Mindanao, southern Philippines were analyzed and compared to describe the preservation of washover deposits in different coastal systems. The potential washover deposits of the 1976 tsunami were identified based on their sedimentary characteristics and features that contrast with the background (pre-tsunami event) sediments. The potential 1976 tsunami deposits are predominantly sand-sized and coarser than the background sediments. They also contain mud rip-up clasts, magnetite lamina, and an erosive base, similar to reported washover deposits elsewhere. The thickness of the washover deposits ranges from 7 to 12 cm. The thickest sand layer was observed in a mangrove swamp, implying that local topography influenced the distribution and preservation of the deposit. A potentially older EWE deposit was also observed in Pagadian City. The background sediments in both the mangrove swamps and coastal plains are mud to fine sand. While scarcity in historical data and rapidly changing environmental conditions pose challenges in studying washover deposits in a tropical setting, this study highlights essential information and lessons for future researchers of washover deposits in the Philippines. Sedimentological data on the potential EWE washover deposits in western Mindanao Island serve as fundamental information in understanding the processes and mechanisms of extreme wave events in this region. This study further establishes geological evidence of the country's worst historical tsunami, the 1976 Moro Gulf tsunami, which contributes to our knowledge of tsunami deposits in tropical settings.

Detailing the impact of the Storegga Tsunami at Montrose, Scotland

The Storegga tsunami, dated in Norway to 8150±30 cal. years BP, hit many countries bordering the North Sea. Run‐ups of &gt;30 m occurred and 1000s of kilometres of coast were impacted. Whilst recent modelling successfully generated a tsunami wave train, the wave heights and velocities, it under‐estimated wave run‐ups. Work presented here used luminescence to directly date the Storegga tsunami deposits at the type site of Maryton, Aberdeenshire in Scotland. It also undertook sedimentological characterization to establish provenance, and number and relative power of the tsunami waves. Tsunami model refinement used this to better understand coastal inundation. Luminescence ages successfully date Scottish Storegga tsunami deposits to 8100±250 years. Sedimentology showed that at Montrose, three tsunami waves came from the northeast or east, over‐ran pre‐existing marine sands and weathered igneous bedrock on the coastal plain. Incorporation of an inundation model predicts well a tsunami impacting on the Montrose Basin in terms of replicate direction and sediment size. However, under‐estimation of run‐up persisted requiring further consideration of palaeotopography and palaeo‐near‐shore bathymetry for it to agree with sedimentary evidence. Future model evolution incorporating this will be better able to inform on the hazard risk and potential impacts for future high‐magnitude submarine generated tsunami events.

Texture and mineralogy as proxies of late Holocene tsunami deposits from the Doñana National Park (SW Spain)

The multidisciplinary analysis (dating, texture, total mineralogy) of two deep cores collected in the Donana National Park (SW Spain) allows to distinguish the main features of three tsunami deposits found in three different paleoenvironmental scenarios (TSU-1: inner area of an open lagoon; TSU-2: inner area of a restricted lagoon; TSU- 3: outer area of a restricted lagoon). The main textural feature of the inner lagoonal tsunami deposits is the increase in bioclastic sands, while the outer TSU-3 is composed almost entirely of medium and fine sands without bioclasts. The mineralogical analysis of TSU-1 and TSU-2 reveals a decrease in phyllosilicates in the basal layers of these tsunami deposits and a parallel increase of quartz, feldspars, dolomite and sometimes calcite. Silica is the main component of TSU-3, whose mineralogical composition changes progressively due to the action of tidal currents in the inlet lagoon on which it is deposited. This rich- SiO2 outer tsunami deposit is derived from the erosion of the adjacent aeolian systems, while the inner TSU-1 and TSU-2 were generated by the transport of bioclastic sands from the outermost parts through the lagoon (TSU-1), or through a nearby tidal channel (TSU-2).

Reconstruction of flow conditions from 2004 Indian Ocean tsunami deposits at the Phra Thong island using a deep neural network inverse model

Abstract. The 2004 Indian Ocean tsunami caused significant economic losses and a large number of fatalities in the coastal areas. The estimation of tsunami flow conditions using inverse models has become a fundamental aspect of disaster mitigation and management. Here, a case study involving the Phra Thong island, which was affected by the 2004 Indian Ocean tsunami, in Thailand was conducted using inverse modeling that incorporates a deep neural network (DNN). The DNN inverse analysis reconstructed the values of flow conditions such as maximum inundation distance, flow velocity and maximum flow depth, as well as the sediment concentration of five grain-size classes using the thickness and grain-size distribution of the tsunami deposit from the post-tsunami survey around Phra Thong island. The quantification of uncertainty was also reported using the jackknife method. Using other previous models applied to areas in and around Phra Thong island, the predicted flow conditions were compared with the reported observed values and simulated results. The estimated depositional characteristics such as volume per unit area and grain-size distribution were in line with the measured values from the field survey. These qualitative and quantitative comparisons demonstrated that the DNN inverse model is a potential tool for estimating the physical characteristics of modern tsunamis.

Incorporating Universal Design into Tsunami Modeling Results for Cascadia Subduction Zone Faults to Create an Inundation Map and Universally Designed Evacuation Map for Port Angeles, WA

Current tsunami hazard inundation and evacuation maps in the Puget Sound are based primarily on Cascadia and Seattle fault tsunamis. The standard evaluation process for tsunami impacts focuses on elevation and hypothetical fault rupture of known and predicted earthquakes. However, there are several known tsunami deposits in the Puget Sound that are not from Cascadia or Seattle fault tsunamis, potentially from other faults within the region, that could affect tsunami mitigation. Work to understand newly discovered crustal deformation and faults in Puget Sound is ongoing, therefore evacuation and inundation maps need to be updated to include these new faults and integrate universal design more broadly. Methods involved using GeoClaw software to map tsunamis from the Cascadia Subduction Zone (CSZ), Leech River fault (LRF), and Utsalady Point fault (UPF). Modeled tsunamis determined the overall inundation of Port Angeles, Washington through a wide range of earthquake inputs of magnitude, proximity, and recurrence. The output simulations were evaluated with key components of universal design to create a new tsunami hazard map. Comparison between the universal design-based map to current the tsunami hazard map allowed for an evaluation of the current evacuation map. This evaluation can improve the assessment of bridges and other evacuation mechanisms. This research can contribute to future tsunami hazard map revisions saving lives, can help with emergency management planning, and spur reevaluating evacuation plans within the tsunami impact area.

Pristine soil property and mineralogy as the strategic rehabilitation basis in post-earthquake-induced liquefaction, tsunami and landslide in Palu, Indonesia

Unprecedented terrible and catastrophic disasters on September 28, 2018 triggered by earthquake-induced liquefaction, tsunami and landslide hit Palu, Central Sulawesi, Indonesia. The objective of the study was to assess and evaluate soil mineralogical compositions, properties, heavy metals, and water quality in the areas of post-earthquake-induced liquefaction, tsunami and landslide to provide the basis for agricultural and ecosystem rehabilitation. The soil samples were collected by digging pits after 12-days of disaster at liquefied soils, tsunami sediments, and soil mass motion/landslide for analyses. Water samples for analysis were collected from soil pits, surface flow and new spring. Results showed that the earthquake-induced liquefaction, tsunami and landslide completely reshape the ecosystem landscape and reset the initial soil formation as revealed by the formation of sand boil microrelief, accumulation of tsunami sediment on initial soil surfaces, and redistribution of soil through mass movement on landscapes. Liquefaction increased the number of weatherable minerals in the sand fraction, varying from 28 to 58% as compared to natural soils (17–42%), indicating the high nutrient reserve of pristine liquefied soils in the long-term. The clay minerals were dominated by smectite followed by illite and kaolinite and were not affected by earthquake impacts. The liquefaction increased soil pH and sand fraction and decreased all soil nutrients, which collectively became severe limiting factors for crop establishment. The tsunami resulted in new material deposits of 5–10 cm thick overlain the natural soils in coastal areas. The deposits did not increase the number of weatherable minerals but resulted in the highest electrical conductivity of soil and water, and high concentrations of Ca, Na, Mg and K ions. The concentration of total and availability of all types of heavy metals is below the threshold limits, indicating no potential heavy metal risks in post-earthquake-induced disaster areas. Information of high weatherable minerals and pristine soil properties, water and heavy metals from this study may provide a strong effective basis for rehabilitation and management of agriculture and the environment in post-disaster areas in the long-term period of time.

Tracing woody-organic tsunami deposits of the 2011 Tohoku-oki event in Misawa (Japan)

With a minimum of three reported waves, the 2011 Tohoku-oki tsunami’s destructive force caused massive damage along the northern Japanese Aomori coast. At Misawa the coastal control area was inundated up to 550 m inland and sandy sediment remnants can be traced to c. 350 m (c. 61–63% of the maximum inundation) from the shoreline. Linking the discovery of floatable plastic objects within a woody and organic layer to our analytical data lead to the detection of a yet undocumented woody-organic tsunami deposit first appearing on top of the sandy deposit but then reaching even further inland (approx. 69–72% of the max. inundation). By this observation our understanding of the documented part of the tsunami inundation may be improved. As a consequence, sand sheets of historic and paleo-tsunamis represent minimum estimates for the coastal inundation and underestimation may be reduced by addressing the woody and organic fraction of a tsunami’s inundation.

Three thousand year paleo-tsunami history of the southern part of the Japan Trench

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.

Rapid Assessment of Tsunami Offshore Propagation and Inundation with D-FLOW Flexible Mesh and SFINCS for the 2011 Tōhoku Tsunami in Japan

This study demonstrates the skills of D-FLOW Flexible Mesh (FM) and SFINCS (Super-Fast INundation of CoastS) in combination with the Delft Dashboard Tsunami Toolbox to numerically simulate tsunami offshore propagation and inundation based on the example of the 2011 Tōhoku tsunami in Japan. Caused by a megathrust earthquake, this is one of the most severe tsunami events in recent history, resulting in vast inundation and devastation of the Japanese coast. The comparison of the simulated with the measured offshore water levels at four DART buoys located in the north-western Pacific Ocean shows that especially the FM but also the SFINCS model accurately reproduce the observed tsunami propagation. The inundation observed at the Sendai coast is well reproduced by both models. All in all, the model outcomes are consistent with the findings gained in earlier simulation studies. Depending on the specific needs of future tsunami simulations, different possibilities for the application of both models are conceivable: (i) the exclusive use of FM to achieve high accuracy of the tsunami offshore propagation, with the option to use an all-in-one model domain (no nesting required) and to add tsunami sediment dynamics, (ii) the combined use of FM for the accurate simulation of the tsunami propagation and of SFINCS for the accurate and time efficient simulation of the onshore inundation and (iii) the exclusive use of SFINCS to get a reliable picture of the tsunami propagation and accurate results for the onshore inundation within seconds of computational time. This manuscript demonstrates the suitability of FM and SFINCS for the rapid and reliable assessment of tsunami propagation and inundation and discusses use cases of the three model combinations that form an important base for tsunami risk management.

The 2011 Tohoku-oki tsunami-induced sediment remobilization on the Sendai shelf, Japan, from a comparison of pre- and post-tsunami surface sediments

Tsunamis are generally considered to disturb the seafloor, rework surface sediments, and change seafloor environments. However, the response of the seafloor to such extreme wave events has not been fully elucidated. Herein, we compare the surface sediments before and after the 2011 Tohoku-oki tsunami on the Sendai shelf and demonstrate that both sandy and muddy sediments were significantly reworked on the shelf. Muddy sediments (> 10 cm thick) were redeposited as graded mud with no or little bioturbation, characterizing the offshore muddy tsunami deposit, while well-sorted sand was found as the sandy tsunami deposit. This redeposited layer could also be retained in the shelf mud sequence. The results imply that the high friction velocity of the tsunami wave and its long-term effect on Sendai Bay might contribute to the large sediment reworking. Part of the resuspended mud moved offshore to the slope area as turbidity currents. Thus, the tsunami is an important mechanism not only for shelf sedimentation but also for deep-sea sedimentation along active plate margins. The detection of 134Cs derived from the Fukushima Daiichi Nuclear Power Plant accident in the redeposited mud indicates that the suspended shelf water state was maintained for some days after the tsunami.

Recurrence of intraplate earthquakes inferred from tsunami deposits during the past 7300 years in Beppu Bay, southwest Japan

We studied onshore sediment cores obtained from a coastal marsh on the south coast of Beppu Bay, northern Kyushu Island, Japan, to ascertain whether prehistoric intraplate earthquakes in the bay had generated tsunamis, and to establish a chronology and recurrence intervals for these earthquakes. The latter had been inferred from investigations on shallow offshore faults, but the timing of the fault movements has large uncertainties. We identified five sand layers in the up to 8.8 m long sedimentary sequence, which is mainly composed of non-marine organic-rich mud and marine organic-poor mud. Most sand layers, 1–20 cm thick, exhibit sharp upper and lower contacts with the surrounding muds, implying that they were deposited by sudden events. The sand layers are geochemically distinct, with peaks in Si, K, Ca, Ti, Mn, Fe, and Sr, and diatom assemblages dominated by brackish–marine and marine species. This suggests that the sand came from the sea bottom and/or shore rather than from the freshwater marsh. Based on radiocarbon age modeling, the depositional ages of these sand layers are: 3270–3450 cal yr BP, 4250–4510 cal yr BP, 4970–5280 cal yr BP, 5750–6750 cal yr BP, and 6430–7070 cal yr BP. The oldest deposit is inferred to record a tsunami associated with the 7.3 ka caldera-forming eruption of the Kikai volcano, southern Japan. Thus, we exclude it from the calculation of the recurrence interval of tsunamigenic intraplate earthquakes in the bay. Recurrence is approximately 460–1850 years, including the 1596 CE Keicho Bungo earthquake and a fault rupture 1700–2200 years ago, which have been reported in studies on ruptures of shallow offshore strike-slip and normal faults. Brackish–marine and marine species dominate the fossil diatom assemblages in the upper muddy sand layer (younger than 2750–2870 cal yr BP) above the non-marine organic-rich mud, implying that the muddy sand was deposited in an inner bay or tidal flat environment. The facies change from non-marine to marine sediments after 2750–2870 cal yr BP cannot be explained by a change in the regional sea level because the latter has fallen in the last 6000 years. Therefore, the most likely interpretation for this environmental change is a local coseismic subsidence of the marsh, probably attributed to a rupture of the active fault located south of the marsh. This study is the first to report on recurrent tsunamigenic intraplate earthquakes based on research of onshore tsunami deposits, and suggests that investigating tsunami deposits can contribute to an assessment of tsunami risks for intraplate earthquakes elsewhere.

An application of aromatic compounds as alternative tracers of tsunami backwash deposits

This manuscript provides some comprehensive technical insights regarding the application of polycyclic aromatic hydrocarbons (PAHs) characterized by using Gas-Chromatography Mass Spectrometry. Although numerous chemical species such as water soluble ionic species (e.g. Na+, K+, Cl−, Ca2+, Mg2+) and acid leachable heavy metal fractions (e.g. Fe, Cd, Al, Mo, Sb, As, Cu, Zn, Pb, and Mn) can be used to characterize tsunami deposits, the knowledge of PAH congeners as alternative chemical species for identifying tsunami backwash deposits is strictly limited. This manuscript is exclusive because it aims to find some alternative chemical proxies in order to distinguish tsunami backwash deposits from typical marine sediments. A wide range of diagnostic binary ratios of PAH congeners have been selected in order to characterize Typical Marine Sediments (TMS), Tsunami backwash deposits (TBD), Onshore Tsunami Deposits (OTD) and Coastal Zone Soils (CZS). The state of the art and future perspectives coupled with both advantages and disadvantages of above mentioned chemical tracers will be critically reviewed and further discussed.