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Side-scan sonar image (left) and the interpretation as a map of different types of seafloor (right), down to approx. 15-m water depth. The seafloor types are determined by the classification of side-scan sonar data and are verified by surface sediment samples-Insets A, B, C and D show details discussed in the text.
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The coastline of Khao Lak (Thailand) was heavily damaged by the 2004 Indian Ocean tsunami. Onshore
tsunami deposits and satellite images, which show large amounts of sediment transported offshore, indicate that the seafloor was impacted by tsunami run-up and backwash. In this study, high-resolution maps of sediment distribution patterns and the geo...
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... C) coarse sand, coral fragments and boulders, D) medium and coarse sands, E) medium sand, F) bedrock out- crops, G) boulders (reef platform), and H) fine and medium sands. Sediments composed of silt to fine sand (type A), are common north and south of the reef fringing Pakarang Cape. This sediment type extends down to a 15-m wa- ter depth (Fig. 2). The landward limit of seafloor type A is above the 5-m water depth contour. It is recognized by patches of low backscatter intensity, displayed in light- grey colors in the side-scan sonar mosaics. The offshore boundaries of the patches of sediment type A are irregular, and, in some places, extend seaward filling in shallow chan- ...
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... nels. The grain size analyses revealed these sediments to be poorly, to very poorly, sorted. Samples close to the coast- line are partly bimodal, with modes situated in the range of medium to coarse silt (4.6 to 5.4, Fig. 4(A)), and partly in the fine-sand fraction (3.7 to 4.0). The first mode gener- ally decreases with increasing water depth ( Fig. 2(C)). The modes of the poorly sorted sediments representing seafloor types B and C are within the coarse-sand fraction (−0.9 to 0.4). The landward limit of seafloor type B and C is situated above the 5 m depth contour. Offshore, a transitional boundary separates seafloor types B and C. A sharp boundary exists between seafloor type B and ...
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... seafloor types B and C are within the coarse-sand fraction (−0.9 to 0.4). The landward limit of seafloor type B and C is situated above the 5 m depth contour. Offshore, a transitional boundary separates seafloor types B and C. A sharp boundary exists between seafloor type B and seafloor type D sediments in a water depth of 16 to 18 m ( Fig. 2(A)). However, sediments classified as seafloor type B reappear further offshore and extend down to approximately a 27-m depth (Fig. 3). No correlation between the first mode and water depth is rec- ognized in samples retrieved from seafloor types B and C (Fig. 4(B)). Seafloor type D is composed of medium to coarse sand, which covers the ...
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... ately well sorted medium (first mode of 2.4) sand is found in water depths from 15 to 25 m. These sediments are clas- sified as seafloor type E (Figs. 2 and 3). They commonly oc- cur on the northwestern flanks of the sand ridges ( Fig. 3(B)). ...
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... with adjacent seafloor types and low backscatter intensity ( Fig. 3(B)). The mode of the grain size of samples retrieved from seafloor type E is inde- pendent of the water depth. Bedrock outcrops form parts of the seafloor. These ar- eas are classified as seafloor type F, which occurs south of Pakarang Cape in water depths less than 10 m (Fig. 2), and further offshore in depths greater than 25 m (Fig. 3). The in- terface with other sediment types is very sharp (Fig. 3(A)). The appearance of the bedrock outcrops is variable in side- scan sonar mosaics, depending on the inclination angle of the outcrop surfaces in relation to the position of the side- scan sonar ...
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... seaward boundary of the dead reef fringing Pakarang Cape is covered by a field of boulders (seafloor type G, Fig. 2(D)). This type of seafloor extends down to 10-12- m water depth. It is marked by a transitional boundary with other sediment types. The boulders are numerous and their diameter often exceeds 1 m. They are recognized as black dots with a bright acoustic shadow behind. The boulder density on the fringing reef is higher towards the offshore ...
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... is also an upward decrease in size and number of gravel- sized components, and an associated decrease in clay and silt content towards the top of the unit. The first and second modes in the analyzed samples range from 0.8 to 1.1, Fig. 7. Shallow-water reflection seismic profiles and side-scan sonar images in the area of seafloor types A, B and C (Fig. 2). Four seismic units are identified. The sediment of type A belongs to seismic unit 3, which forms a meter-thick cover. P.C.: Pakarang Cape. and 4.0 to 5.1, respectively. Unit 2 is separated from the underlying unit 3 by an erosional unconformity at 18-cm depth (Fig. 8). Small amounts of the older sediments are incorporated into the ...
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... fringing reef offshore Pakarang Cape is covered with boulders (sediment type G, Fig. 2). This reef terminates abruptly offshore (Fig. 2). Only a few coral boulders are ob- served seaward of the reef slope. In contrast, high amounts of coral debris, including boulders, are located north and south of the reef (seafloor types B and C, Fig. 2). Cob- bles from this area, which are covered by colonies of or- ganisms indicating ...
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... fringing reef offshore Pakarang Cape is covered with boulders (sediment type G, Fig. 2). This reef terminates abruptly offshore (Fig. 2). Only a few coral boulders are ob- served seaward of the reef slope. In contrast, high amounts of coral debris, including boulders, are located north and south of the reef (seafloor types B and C, Fig. 2). Cob- bles from this area, which are covered by colonies of or- ganisms indicating a stable environment, were recently re- ported by ...
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... fringing reef offshore Pakarang Cape is covered with boulders (sediment type G, Fig. 2). This reef terminates abruptly offshore (Fig. 2). Only a few coral boulders are ob- served seaward of the reef slope. In contrast, high amounts of coral debris, including boulders, are located north and south of the reef (seafloor types B and C, Fig. 2). Cob- bles from this area, which are covered by colonies of or- ganisms indicating a stable environment, were recently re- ported by Sanfilippo et al. (2010). Additionally, the acous- tic base is situated just beneath the surface north and south of Pakarang Cape (Fig. 7), which suggests the presence of hard rock and/or very coarse ...
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... Pakarang Cape (Fig. 7), which suggests the presence of hard rock and/or very coarse material close to the seafloor. This can be seen below seismic unit 4, which is composed of coarse sand ( Fig. 7(C)). It is likely that the areas di- rectly north and south of Pakarang Cape are remnants of old reef platforms. The channel system at a depth of 15 m (Fig. 2) is incised into the surface of this old platform. The elevated morphological platform found in an approximately 25-m water depth ( Fig. 1(A)) is interpreted as a paleoreef as well. This interpretation is based on its morphological ap- pearance, which is typical for a drowned reef platform (e.g. Finkl et al., ...
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... remarkable feature within the research area is the fine- grained (mainly silt) sediment cover observed close to the coastline (seafloor type A, Fig. 2). This material is de- posited in depressions of the inferred paleoreef platform and in the channel system north and south of Pakarang Cape (Fig. 7). The mechanisms responsible for the deposition and preservation of such fine-grained material in the inner shelf environment are still poorly understood (e.g. Hill et al., 2007). However, ...
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... thickness of the fine-grained sediments offshore Pakarang Cape (sediment type A, Fig. 2, seismic unit 3, Fig. 7) decrease from a local maximum of 2 m to only a few cm, with increasing distance from the shore, and increasing water depth. Dominating sediments grade also from fine sand to silt-sized particles with increasing distance from the shore. This suggests an onshore source of these sedi- ments. Available data indicate that most of these ...
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... et al., 2012) can be found in this unit. It is deposited above an erosional unconformity, contains abundant shell fragments, and is generally poorly sorted and embedded within ambient sediments. Material forming unit 2 derives partly from the nearby vicinity where similar sediments can be found both further onshore and offshore (seafloor type B, Fig. 2). The unit is composed of two subunits-2A and 2B-separated by a sharp, erosional boundary. The subunits may reflect various phases of the tsunami event, but further differentiation into run-up and/or backwash is still not possible. Macroscopic terrigenous material, which would point towards backwash influence, is not found, and the ...
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... types B and C. This supports the findings of Sanfilippo et al. (2010), who re- ported a minor influence on corals living on cobbles in these areas. These areas seem to be mostly unaffected by the tsunami event. No direct traces of tsunami impact below an 18-m water depth were found in hydroacoustic data or sediment samples. Seafloor type E (Fig. 2), deposited on the northwestern flanks of the sand ridge system, indicates a SW-NE directed current. In incisions at the base of the sand ridges, event deposits composed of silty material covered by coarse sand were found (Feldens et al., 2010). However, it is uncertain whether these features can be attributed to the 2004 Indian Ocean ...
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Information on palaeo-tsunami magnitude is scientifically and socially essential to mitigate tsunami risk. However, estimating palaeo-tsunami parameters (e.g., inundation distance) from sediments is not simple because tsunami deposits reflect complex transport processes. Here, we show a new approach to estimate tsunami inundation distance based on...
On November 8 th 2013, category 5 Supertyphoon Haiyan made landfall on the Philippines. During a post-typhoon survey in February 2014 Haiyan-related sand deposition and morphological changes were documented at four severely affected sites with different exposure to the typhoon track, and different geological and geomorphological settings. Extended...
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... This proposal is supported further by the physical and geochemical characteristics of sub-units e12 and e13. Analysis of grain size profiles can be used to support interpretations of tsunami deposits, with characteristics including increases in mean grain size (Costa et al., 2021;Feist et al., 2023;Smedile et al., 2020), upward fining of sediments Ikehara et al., 2021aIkehara et al., , 2021bYhasnara et al., 2023), and reductions in sorting capacity Feldens et al., 2012). ...
... Additionally, such flow velocities would be capable of creating the sharp basal erosional contact seen particularly in e13, which are observed in other offshore tsunami deposits (e.g. Feist et al., 2023;Feldens et al., 2012;Smedile et al., 2020). The gradual fining of sediment through sub-unit e13 (Fig. 6), is generated as finer particles settle from the suspension cloud on top of the coarse grains when currents have reduced (Costa et al., 2021). ...
The Shetland Islands (UK) are a seminal location for investigating palaeo-tsunami deposits. Onshore evidence suggests three tsunami have occurred during the Holocene: the Storegga tsunami ca. 8150 cal yr BP, the Garth tsunami ca. 5500 cal yr BP and the Dury Voe tsunami ca. 1500 cal yr BP. However, little research has been published on the impact of tsunami on the subtidal shelf where a large amount of North Sea hydrocarbon infrastructure is located. Here, we test the hypothesis that Holocene tsunami impacted shelf sediments, using radiocarbon dating and sedimentological characterization of cores recovered from the Fetlar Basin, offshore east Shetland. The cores contain distinct sand and shell lenses within a Holocene mud sequence, indicating a sudden change in hydrodynamic conditions. Radiocarbon dates bracketing the sand lenses overlap with the published dates for the Storegga event. Dates within the deposit are older (>9 cal. yr BP) which is consistent with reworking and redeposition of earlier sediments. Particle size analysis, ITRAX and MSCL data evidence increases in mean grain size, a reduction in sorting capacity, increased shell concentrations and peaks in associated elements (log(Ca/Fe), log(Ca/Ti) and Sr). These attributes indicate transport of allochthonous material from the inner shelf, and are typical of tsunami backwash-generated submarine debris flows. No evidence was found within the cores for any later Holocene tsunami, which may be due to either bioturbation, active currents, or lack of an initial deposit. The disturbance of sediments, and generation of a submarine debris flow within the Fetlar Basin by the Storegga event highlights the need to assess the potential impact of any future tsunami on planned and existing infrastructure at seabed. Erosion and deposition of allochthonous older marine sediment by the Storegga event also has consequence for interpretation of the coeval 8.2 ka cold event in marine sedimentary records in the tsunami affected region.
... Different local bathymetric conditions (e.g., basins, faults, gentle slopes), varying local sediment sources, and transport paths play an essential role in the offshore deposit configuration (e.g., Feldens et al., 2012). Only a few hundred meters apart, these offshore deposits can present different complexity as well as compositional, geophysical, and geochemical characteristics, like their onshore counterpart. ...
... 3600 cal yr BP tsunami deposits have substantial differences, even in the same shelf locations, but also similarities. Typical characteristics are erosional basal surfaces, reworked marine sediments, and the input of terrigenous materials, similar to observations in other studies Feldens et al., 2012;Ikehara et al., 2014;Tamura et al., 2015;Riou et al., 2020aRiou et al., , 2020bSmedile et al., 2020). Different sections of the deposit attributed to different hydrodynamic conditions could be identified where deposits are thick enough. ...
The well-known 1755 CE Lisbon tsunami caused widespread destruction along the Iberian and northern Moroccan
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... Additionally, there was little or no erosion associated with the deposits. In a related study of the same area, Feldens et al. (2012) found that the sea floor morphology and the distribution of sediment was largely unimpacted by the tsunami. Regarding terrestrial content, Van Den Bergh et al. (2003) found almost no land derived sediments beyond the immediate nearshore in tsunami deposits associated with the 1883 Krakatoa eruption while working in less than 20 m water depth offshore Java. ...
Until recently, the offshore effects of modern tsunamis were unknown. It is only within the past fifteen years that studies have started to examine the sedimentologic and stratigraphic signatures of modern tsunamis in the marine environment. Claims of paleo-tsunami deposits in Sweden, however, predate any knowledge of the offshore effects of tsunamis by more than ten years. These claims are based on lacustrine sediment sequences that would have been at outer shelf water depths during the proposed tsunamis. While numerous claims of paleo-tsunami deposits exist in Sweden, relatively few of these claims are reported in the peer-reviewed literature.
The current study re-examines proposed paleo-tsunamis from Stockholm, Iggesund, and northern Uppland and points out a lack of critical geographical, stratigraphical, and chronological data. Additionally, consideration of the Holocene stratigraphy of these locations in the context of what is now known about the effects of tsunamis in the offshore environment indicates that there is currently no credible evidence of paleo-tsunamis in Sweden.
... Transport of calcareous sand from the coral reef to deep sea through SC2-1 and SC2 submarine canyons has repeatedly occurred (Figures 2, 4). Large earthquakes and tsunamis are possible mechanisms for the transport of shallow marine sediments to the deep-sea (e.g., Feldens et al., 2009Feldens et al., , 2012Ikehara and Usami, 2018;Ikehara et al., 2021b). Especially in coral reefs, large tsunamis have enough potential to transport coral reef materials to both onshore (Goto et al., 2010;Kitamura et al., 2018;Fujita et al., 2020) and offshore (Feldens et al., 2009(Feldens et al., , 2012Paris et al., 2010;Sukuna-Schwartz et al., 2015). ...
... Large earthquakes and tsunamis are possible mechanisms for the transport of shallow marine sediments to the deep-sea (e.g., Feldens et al., 2009Feldens et al., , 2012Ikehara and Usami, 2018;Ikehara et al., 2021b). Especially in coral reefs, large tsunamis have enough potential to transport coral reef materials to both onshore (Goto et al., 2010;Kitamura et al., 2018;Fujita et al., 2020) and offshore (Feldens et al., 2009(Feldens et al., , 2012Paris et al., 2010;Sukuna-Schwartz et al., 2015). Recurrence intervals of calciturbidite depositions in cores YK15 PC01, PC05, PC07, and PC08 are calculated as several hundred-a few thousand years (average: 1,388 years) (Figure 4 and Supplementary Table S5). ...
Submarine canyons are efficient sediment transport pathways from shallow marine areas to deep sea. Along active margins, large tsunamis are a trigger for sediment transport to deep sea. However, sediment transport through submarine canyons by such extreme wave events in the carbonate depositional environments has not been fully understood. Large tsunamis have repeatedly struck the coral reef islands of the southern Ryukyu Islands and have transported large boulders composed of coral from the reef to shore. In this study, we examined sediment cores collected near the mouths of submarine canyons and basin floor on the southern Ryukyu arc’s forearc. The presence of coarse calciturbidites containing coral, molluskan fragments, and coral reef benthic foraminifera was limited on the submarine fan formed at the mouth of the reef-connected and shelf-incised submarine canyon. In cores collected near the mouth of shelf-incised submarine canyons with no reef connection and slope-confined canyons, no coarse calciturbidite is observed. Few calciturbidites were found in more downslope cores, implying that most calcareous grains derived from shallow marine areas were deposited on this fan. Depositional intervals of turbidites were calculated to be several hundred–a few thousand years, which agree with the recurrence intervals of large tsunamis estimated from onshore tsunami deposits, based on radiocarbon dates in hemipelagic mud intervals. No temporal change in the depositional intervals of calciturbidites in the cores from the submarine fan since the last glacial maximum. Therefore, the tsunamis may be an important mechanism for surface sediment reworking in coral reefs, and shelf-incised and reef-connected submarine canyons can play an important role in the efficient transport of shallow marine calcareous grains to the deep sea.
... Up to now, reconstructing the record of past tsunami events mainly focused in investigating deposits found in swale between beach ridges, such as at Phra Thong Island . While there have been many studies on tsunami deposits along the coastline of the Andaman Sea (e. g., Satake et al., 2006;Fagherazzi and Du, 2007;Hawkes et al., 2007;Choowong et al., 2007Choowong et al., , 2008Choowong et al., , 2009Jankaew et al., 2008;Feldens et al., 2009Feldens et al., , 2012, little work has focused on analyzing the sedimentological nature and occurrence of storm surges in this region. An example is Phra Thong Island where three sand sheets occurring interbedded with muddy swale deposits were attributed to tsunami events ( Fig. 1A; Jankaew et al., 2008;Monecke et al., 2008, Fujino et al., 2009Prendergast et al., 2012). ...
... Globally, high energy event sediments of both tsunami-induced and storm-induced deposits preserved inland have been extensively reported to include lamination, cross-bedding, mud rip-up clasts, normal and reverse grading, erosional and sharp contacts, thinning and finning landward, remains of marine organisms including corals (e.g., Atwater, 1987;Bryant et al., 1992;Liu and Fearn, 1993;Moore, 2000;Donnelly et al., 2001;Fujiwara et al., 2003;Gelfenbaum and Jaffe, 2003;Panegina et al., 2003;Liu, 2004Liu, , 2007Donnelly, 2005;Higman et al., 2006;Moore et al., , 2011Satake et al., 2006, Choowong et al., 2007, 2008. Elsner, 2007Fagherazzi and Du, 2007;Hawkes et al., 2007;Monecke et al., 2008;Morton et al., 2007;Jankaew et al., 2008;Monecke et al., 2008;Feldens et al., 2009Feldens et al., , 2012Williams, 2010Williams, , 2013Rhodes et al., 2011;Goff et al., 2012;Phantuwongraj and Choowong, 2012;Phantuwongraj et al., 2013;Williams et al., 2016;Kongsen et al., 2021a;b).). ...
Understanding the frequency of high energy storm and tsunami events is crucial for apprehending the vulnerability of coastal communities. Identifying and dating sedimentary evidence deposited by such high energy events can assist in the planning and installation of suitable protection measurements. The Andaman Sea coast of Thailand is particularly vulnerable to such events as illustrated by the 2004 Indian Ocean Tsunami. Here, three shore-perpendicular transects and ten sediment cores along a beach ridge in the northern part of Lam Son National Park, Ranong Province, Andaman Sea coast are investigated with respect to high energy deposits. A multi-proxy analysis was conducted including stratigraphical correlations between cores, detailed description of physical characteristics, sedimentary structure, grain size, organic matter and carbonate content as well as identification of the remains of marine organisms. Optically stimulated luminescence (OSL) dating was applied for determining the age of the sediments on this beach ridge plain. The stratigraphy and sediment characteristics, in particular composition, of the sands in the study area clearly allow us to distinguish between high energy deposits and normal beach sediments. Two high energy deposits were identified and attributed to result from the 2004 Indian Ocean Tsunami as well as a past storm, which, based on OSL dating occurred more than 340 ± 20 years ago.
... The coast is dominated by rocky cliffs alternating with sandy lowlands backed by beach ridges [29]. Offshore of Khao Lak, a fringing reef and an associated drowned paleoreef have been identified [30]. Previous hydroacoustic mapping (side scan sonar, shallow seismic with different boomer systems) revealed a small-scale channel system incised into a drowned, slightly inclined carbonate platform located approximately 10 m below sea level. ...
... Side scan sonar data offshore of Khao Lak were previously interpreted, discerning the facies "fringing reef and adjacent paleoreefs", "sandy inner shelf environment" and "shallow fine-grained sediments (flash flood deposits)" [30]. Here, we focus on the description of sand ridges striking oblique to the coastline in an ENE-SWS direction and associated elongated patches of finer sediments, characterized by low backscatter intensities, in the inner shelf environment above 28 m bsl. ...
... As no abrupt change of Ti/Ca ratios is observed in the upper 78 cm of the sediment core, we assume that stable conditions were encountered. Further indication for low accumulation rates is provided by the drowned paleo-reef platforms described by [30]. Since sedimentation at sea level maximum positions tends to occur first on the inner shelf before prograding into deeper waters [44], little sedimentation has taken place since the sea level highstand at approx. ...
Understanding the development of shallow seas is essential, as they provide a major environmental and economic resource. An investigation of the Holocene development and the present conditions of the Andaman Sea shelf was carried out based on hydroacoustic surveys and sedimentological sampling. The results show that the relative sea level in the offshore Phang Nga province (Thailand) was at a present-day water depth of approximately 63 m at 13 cal ka BP. This agrees with the sea level development of the Sunda Shelf. Since that time, the Andaman Sea continental shelf developed as a sediment-starved environment, with less than 2 m thickness of sediment deposited during the Holocene on the crystalline basement over large areas between 60 m and 20 m water depth. Between 28 and 17 m water depth, a series of moribund asymmetrical sand ridges exist. These ridges were formed around 9.0 ka cal BP. They strike oblique to the coastline. On the seaward side of the sand ridges, small NW-SE directed submarine dunefields developed, shaped by monsoon-induced currents.
Link tp pdf: https://www.mdpi.com/2673-964X/2/1/1/pdf
... After the 2004 Indian Ocean and 2011 Tohoku-oki tsunamis, offshore surveys were conducted to collect material, from the inner continental shelf (0 to 30 m below sea level (b.s.l.)) and over the shelf break (>100 m b.s.l.). These studies highlighted that tsunami outflow (backwash waves) had produced intense erosion, sediment transport and deposition offshore and that in the marine realm the tsunami-related units are subjected to the action of currents, waves and bioturbation immediately after their deposition (Paris et al., 2010;Feldens et al., 2012;Sakuna et al., 2012;Ikehara et al., 2014;Tamura et al., 2015;Yoshikawa et al., 2015;Seike et al., 2017). ...
After the 2004 Indian Ocean (IOT) and the 2011 Tohoku-oki tsunamis, new research in tsunami-related fields was strongly stimulated worldwide and also in the Mediterranean. This research growth yields substantial advancements in tsunami knowledge.
Among these advancements is the “Paleotsunami” research that has marked particular progress on the reconstruction of the tsunami history of a region. As an integration of the historical documentation available in the Mediterranean and the Gulf of Cadiz areas, geological and geoarchaeological records provide the insights to define the occurrence, characteristics, and impact of tsunamis of the past. Here, we present the recent advancements done for both the onshore and offshore realms.
As for the onshore, we discuss case studies dealing with recent high-resolution works based on: a) direct push in situ sensing techniques, applied to identification and characterization of typical paleotsunami deposits features; b) combined XRF-X-CT approach, implemented for the identification of fine-scale sedimentary structures useful for the definition of the causative flow dynamics; c) the geoarchaeological “new field” contribution, with the development of specific diagnostic criteria in search for tsunami impact traces in archaeological strata; d) comparison of multiple dating methods and of different modeling codes for the definition of the potential source for the displacement of boulders of exceptional dimension, identified by 3D size calculation.
As for the offshore advancements, we present case studies focusing on the recognition of tsunami deposits and their sedimentary traces in the geological record from the nearshore, thanks to diver-operated coring equipment, down to the continental slope, by means of vibracorer and long gravity core sampling in deeper areas. The examples provided show a multiproxy approach with a high potential of retrieving a complete record of paleotsunami traces at least during the Holocene. This is based on the combination of multidisciplinary approaches including X-ray imaging, high-resolution measurement of physical properties, X-ray fluorescence data, grain-size analysis, micropaleontology, palynological content, isotopic and optically stimulated luminescence dating methods.
... If coastal erosion rates are determined by the frequency and heights of strong waves (Masselink et al., 2016;Quartau et al., 2010). Tsunamis may also produce extensive coastal erosion (Paris et al., 2009;Ramalho et al., 2013) but they occur infrequently and are not thought generally important for long term coastal erosion Feldens et al., 2012;Milker et al., 2013). The largest database ever on cliff erosion concludes that rock resistance is the principal explanation of varied erosion rates and oddly other climatic and marine forces are not (Prémaillon et al., 2018). ...
Rates of coastal erosion are needed for planning purposes and to improve understanding of how the shelves of islands develop, ultimately becoming submerged banks. Near-shore submarine platforms created by erosion of lava deltas of known age provide an opportunity to quantify erosion rates, and to investigate how those rates vary between different types and ages of lava flows, as well as how they vary with wave climate. We have compiled data on deltas formed during historical and Holocene eruptions (age ≤ 6 ka), from both 'a'ā and pāhoehoe lava flows, and from diverse localities (Azores and Hawaiian islands and Ascension Island). Near-shore platforms were interpreted from multibeam sonar, bathymetric Light Detection And Ranging (LiDAR) and historical sounding data. From them, we estimated time-averaged shoreline retreat rates from the seaward distances between the modern coastlines and the submarine platform edges. The 35 interpreted platforms have rugged dipping surfaces left behind by erosion with an average gradient of 5° (range 2°–10°), eroded distance of 436 m (78–1119 m) and depth of platform edge of 25 m (5–56 m). To evaluate the latter depths, we computed seabed orbital speeds due to surface waves at the platform edges. Depths of platform edges correlate only modestly with those speeds, suggesting that platform surfaces have adjusted to wave conditions with a wide range of particle sizes and hence thresholds of motion. The largest retreat distances (≥800 m) are associated with platforms of the central Azores Islands, which are exposed to higher waves. However, retreat rates are highly varied (0.08–12.5 m/yr) and weakly correlated with measures of wave power, suggesting that waves are only a secondary influence. Given that lack of correlation, we suspect that the main factor contributing to variability arises from varied cliff resistance to erosion. Blocks between joints in lava flows, and hence resistance to erosion, vary greatly in size, from the thicker and less densely jointed 'a'ā flows to the thinner and shell-like layered lavas in pāhoehoe flows.
... Minerals 2020, 10, 731 2 of 30 to access, they lack the better preservation potential and spatial coverage of offshore deposits [5][6][7], recorded mainly by highly sediment-laden backwash flows. However, post-sedimentary processes, such as waves, currents, and bioturbation, can alter tsunami deposits in the offshore realm [8][9][10], making the selection of the study area of key importance. Although it might be less likely to have significant sedimentary imprints of tsunamis in deeper offshore areas (i.e., mid to outer shelf) [5], the controversial differentiation between tsunami and storm deposits [11] can be neglected when studying sediments beneath the storm wave base [12]. ...
Outer shelf sedimentary records are promising for determining the recurrence intervals of tsunamis. However, compared to onshore deposits, offshore deposits are more difficult to access, and so far, studies of outer shelf tsunami deposits are scarce. Here, an example of studying these deposits is presented to infer implications for tsunami-related signatures in similar environments and potentially contribute to pre-historic tsunami event detections. A multidisciplinary approach was performed to detect the sedimentary imprints left by the 1755 CE tsunami in two cores, located in the southern Portuguese continental shelf at water depths of 58 and 91 m. Age models based on 14C and 210Pbxs allowed a probable correspondence with the 1755 CE tsunami event. A multi-proxy approach, including sand composition, grain-size, inorganic geochemistry, magnetic susceptibility, and microtextural features on quartz grain surfaces, yielded evidence for a tsunami depositional signature, although only a subtle terrestrial signal is present. A low contribution of terrestrial material to outer shelf tsunami deposits calls for methodologies that reveal sedimentary structures linked to tsunami event hydrodynamics. Finally, a change in general sedimentation after the tsunami event might have influenced the signature of the 1755 CE tsunami in the outer shelf environment.
... Such an approach is much more challenging in offshore environments, where seafloor investigations and access to sediments is logistically difficult, expensive, and time consuming, especially in shallow waters, which are often influenced by waves, tides, and currents. Only a few studies so far document tsunami effects in offshore environments (e.g., Shanmugam, 2008;Feldens et al., 2010Feldens et al., , 2012Paris et al., 2010;Haraguchi et al., 2013;Ikehara et al., 2014;Sakuna-Schwartz et al., 2015). A comprehensive compilation of studies concerning backwash deposits in shallow water is given by Riou (2019). ...
... Reflection-seismic profile acquired with a boomer system showing two sub-surface channels filled with tsunami-laid deposits (blue) and covered by post-tsunami background sediment (orange). Results from investigations in the Andaman Sea offshore of Khao Lak four years after the 2004 Indian Ocean Tsunami.Modified afterFeldens et al. (2012). ...
The influence of tsunamis on the submarine environment was virtually unknown before the 2004 Indian Ocean Tsunami, which hit the coastlines of many countries around the Indian Ocean, and the 2011 Tohoku Tsunami, which mainly hit coastlines of Japan. When triggered by a tsunami, sediment can be redeposited over wide areas onshore, but especially offshore. Recent advances in seafloor mapping considerably improved the identification of offshore areas that were influenced by a tsunami and helped to find suitable locations for sampling and investigating recent and ancient subaquatic tsunami deposits. Tsunami-influenced deposits are observed from shallow marine environments to water depths below 1000 m. The application of a range of proxy methods to analyze sediment-surface samples and sediment cores in different case studies worldwide showed that tsunami deposits can be identified and distinguished from storm-generated and flash flood deposits in subaquatic sediment cores. One particular conclusion from this research is, that the preservation potential of tsunami deposits increases with water depth.
Keywords: High-resolution seafloor mapping; Offshore tsunami deposits; Sediment echo sounding; Sediment structures; Tsunami backflow.
