Browsing by Author "Sugawara, D"
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- ItemAnisotropy of anhysteretic remanent magnetization (AARM) reveals cryptic flow fabric of tsunami(American Geophysical Union, 2013-12-09) Kon, S; Nakamura, N; Sugawara, D; Goto, K; Chagué-Goff, C; Goff, JRSandy tsunami deposits may provide valuable information on tsunami inundation as well as hydrodynamics, such as flow speed. However, if the layer does not have sedimentary structures such as cross laminations, it is difficult to infer the flow direction, which is important to interpret the behavior of the tsunami, such as inflow and outflow as well as repetition of waves. Anisotropy of magnetic susceptibility (AMS), in combination with grain size data, can provide information about the hydrodynamic conditions prevailing during the emplacement of tsunami sequences. It might also allow the reconstruction of transport directions because it provides a cryptic alignment of ferromagnetic and paramagnetic minerals, such as coarse-grained magnetite or platy phyllosilicate minerals (e.g. biotite). These minerals behave differently in different hydrodynamic conditions: for example, platy biotite may deposit in a cryptic micro-ripple. This therefore suggests that the usefulness of bulk AMS together with optical observations is limited in the study of flow fabric in tsunami deposits. The anisotropy of anhysteretic remanent magnetization (AARM) on the other hand isolates the fine-grained magnetite subfabric of needle-shaped inclusions exsolved in silicate minerals. Samples (18) from tsunami deposits, believed to have been laid down by the Jogan event (869 AD), were collected from a section on the Sendai Plain, east Japan. The transport direction in these deposits could not be determined by AMS analysis due to large declination and inclination errors. The AARM technique was thus used to determine the cryptic subfabric of magnetite exsolutions along cleavages in biotite and amphibole. Our scanning electron microscopy (SEM) observations confirmed that the maximum AARM orientation is parallel to the needle-shaped magnetite microexsolutions in biotite and amphibole. We therefore infer that the large error of AMS is caused by the alteration of these paramagnetic minerals, and AARM provides a cryptic alignment of fine-grained magnetite microexsolutions. In order to apply this method to ancient historical Tsunami events, we also collected 40 samples from consecutive sand layers of possible tsunami deposits at 7 sites using 2 m long geoslicers in Rikuzen-Takata, northeast Japan. The AARM and SEM confirmed the tendency of same flow direction of sand layers at each site, suggesting a tsunami origin.
- ItemEnvironmental impact assessment of the 2011 Tohoku-oki tsunami on the Sendai Plain(Elsevier B.V., 2012-12-30) Chagué-Goff, C; Niedzielski, P; Wong, HKY; Szczuciński, W; Sugawara, D; Goff, JRLarge areas of farmland in the Sendai Plain, Japan, were inundated by the 11 March 2011 Tohoku-oki tsunami and covered by a discontinuous 30–0.2 cm thick sediment layer consisting of sand and/or mud and generally thinning and fining inland. Two months after the tsunami, numerous rice paddy fields and depressions remained ponded with brackish or saline water. A series of field surveys in May, August and October 2011 were carried out north of Sendai airport, in order to assess the environmental impact of the tsunami. While evaporation had resulted in elevated conductivity in ponded water in May (up to 68.2 mS cm− 1), rainfall over the next five months led to dilution, although brackish water was still recorded in depressions and on paddy fields. Tsunami sediments, underlying soil and soil beyond the tsunami inundation limit were collected at 43 sites along and near a transect extending over 5 km inland, and analysed for grain size, organic content, water leachable ions, acid leachable metals and exchangeable metalloids. Water leachable anion and cation concentrations were elevated in sandy and muddy tsunami deposits and soils particularly in areas, where seawater had stagnated for a longer period of time after the tsunami, with up to 10.5% Cl, 6.6% Na, 2.8% SO4, 440 mg kg− 1 Br measured in surface sediments (< 0.5 cm depth). Vertical variations were also recorded, with higher concentrations often measured in the surface samples. A similar trend could be observed for some of the metalloids (As) and metals (Zn, Cu and Ni), although in general, maximum concentrations of metals and metalloids were not much higher than in soils not inundated by the tsunami and were within background levels for uncontaminated Japanese soils. The impact of saltwater inundation was documented in the chemistry of soils underlying tsunami sediments, which were affected by salt contamination down to ~ 15 cm depth, and soils not covered by tsunami deposits. The latter implies that the extent of tsunami inundation may successfully be determined using geochemical markers in absence of any sedimentological evidence. Water leachable ions mostly decreased over time, however, they remained high enough to impact on rice farming, which was completely halted in 2011. Although further work is required to assess the longer term impact of tsunami inundation, flushing of salt with freshwater, as well as the possible removal of sandy/muddy sediments and underlying soil are recommended to allow crop production to resume. © 2020 Elsevier B.V.
- ItemErosion, deposition and landscape change on the Sendai coastal plain, Japan, resulting from the March 11, 2011 Tohoku-oki tsunami(Elsevier B.V., 2020-12-12) Richmond, B; Szczuciński, W; Chagué-Goff, C; Goto, K; Sugawara, D; Witter, R; Tappin, DR; Jaffe, BE; Fujino, S; Nishimura, Y; Goff, JRCase studies of recent tsunami impacts have proven to be extremely useful in understanding the geologic processes involved during inundation and return flow, and refining the criteria used to identify paleotsunami deposits in the geologic record. Here, we report on erosion, deposition and associated landscape change resulting from the March 11, 2011 Tohoku-oki tsunami along a nearly 4.5 km shore-normal transect on the coastal plain near Sendai, Japan. The study area on the broad, low-relief prograding coastal Sendai plain comprised a sand beach backed by ~ 3 m high sand dunes and a forest, a wetland, the Teizan canal, agricultural rice fields, buildings and roads. Field observations focused on measurements of tsunami flow characteristics (height and direction), mapping of erosion features and assessing sediment deposition based on shallow trenches at 50–100 m spacing. Recorded tsunami inundation heights reached up to about 11 m above mean sea level within the first 500 m from the shoreline and then ranged between 3 and 5 m for the next 2 km, gradually decreasing to about 3 m close to the inundation limit. The tsunami deposit generally thinned landward from an average maximum ~ 30 cm thick sand deposit in the coastal forest to a thin mud drape several mm thick near the inundation limit. A discontinuous sand-dominated sheet was prevalent to about 2800 m from the shoreline where mud content then gradually increased further landward eventually resulting in a mud-dominated deposit ranging from 3.5 cm to a few mm thickness. The overall thinning and fining of the deposit was often interrupted by localized features that led to complex sedimentological relationships over short distances. Satellite imagery taken on 14 March 2011, 3 days after the Tohoku-oki Tsunami shows prominent foreshore incisions with 100 s + meters spacing alongshore, a foredune ridge that underwent severe erosion and development of a prominent shore-parallel elongated scour depression. Our field survey in early May 2011 revealed that the foreshore recovered quickly with rapid post-tsunami sediment deposition from incident waves, whereas the dune–ridge complex had undergone only minor re-working from eolian processes. © 2020 Elsevier B.V
- ItemField observations of erosion, deposition, and tsunami flow characteristics on the Sendai Coastal Plain after the March 2011, Tohoku-oki Tsunami, Japan(American Geophysical Union, 2011-11-05) Richmond, BM; Goto, K; Fujino, S; Nishimura, Y; Sugawara, D; Tappin, DR; Witter, RC; Jaffe, BE; Chagué-Goff, C; Szczuciński, W; Yulianto, E; Goff, JRHistorical and instrumental data show that the Pacific coast of Mexico has been exposed to destructive tsunamis over at least the past 500 years. This coast is also affected by hurricanes generated in the eastern Pacific. The great 1985 Mexico earthquake and its aftershock generated tsunamis that affected the Ixtapa-Zihuatanejo and Michoacán coast. The purpose of our study was two-fold, a) to determine whether we could distinguish storm from tsunami deposits, and b) whether tsunami deposits from historical events are preserved in the tropical environments of the Ixtapa-Zihuatanejo coast. Two anomalous sand units in the Ixtapa estuary are interpreted as the result of high-energy marine inundation events that occurred in the last century. Several lines of evidence using a multi-proxy approach (historical studies, interviews with local witnesses, geomorphological and geological surveys, coring and trenching, and laboratory analyses including grain size, micropaleontology, geochemistry, magnetic susceptibility and radiometric dating) indicate the occurrence of two tsunamis that we link to local events: the 1985 Mexico and possibly the 1979 Petatlan earthquakes. We thereby provide the first onshore geological evidence of historical tsunamis on the Pacific coast of Mexico. © American Geophysical Union
- ItemGeological evidence and sediment transport modelling for the 1946 and 1960 tsunamis in Shinmachi, Hilo, Hawaii(Elsevier, 2018-02-01) Chagué, C; Sugawara, D; Goto, K; Goff, JR; Dudley, WC; Gadd, PSThe Japanese community of Shinmachi, established on low-lying land between downtown Hilo and Waiakea, Hawaii, was obliterated by the 1946 Aleutian tsunami but was rebuilt, only to be destroyed again by the 1960 Chilean tsunami. The aim of this study was to find out if any geological evidence of these well documented events had been preserved in the sedimentary record in Wailoa River State Park, which replaced Shinmachi after the 1960 tsunami. This was achieved by collecting cores in the park and performing sedimentological, chronological and geochemical analyses, the latter also processed by principal component analysis. Sediment transport modelling was carried out for both tsunamis, to infer the source of the sediment and areas of deposition on land. The field survey revealed two distinct units within peat and soil, a thin lower unit composed of weathered basalt fragments within mud (Unit 1) and an upper unit dominated by fine volcanic sand within fine silt exhibiting subtle upward fining and coarsening (Unit 2, consisting of Unit 2A and Unit 2B), although these two anomalous units only occur on the western shore of Waiakea Mill Pond. Analysis with an ITRAX core scanner shows that Unit 1 is characterised by high Mn, Fe, Rb, La and Ce counts, combined with elevated magnetic susceptibility. Based on its chemical and sedimentological characteristics, Unit 1 is attributed to a flood event in Wailoa River that occurred around 1520–1660 CE, most probably as a result of a tropical storm. The sharp lower contact of Unit 2 coincides with the appearance of arsenic, contemporaneous with an increase in Ca, Sr, Si, Ti, K, Zr, Mn, Fe, La and Ce. In this study, As is used as a chronological and source material marker, as it is known to have been released into Wailoa River Estuary and Waiakea Mill Pond by the Canec factory between 1932 and 1963. Thus, not only the chemical and sedimentological evidence but also sediment transport modelling, corroborating the historical record, suggest that Unit 2A was deposited by the 1946 tsunami, and the sediment most likely originated from Wailoa River Estuary, beach and nearshore seafloor. The upper part of this unit, Unit 2B, is believed to have been deposited by the 1960 tsunami, as suggested by sediment transport modelling, although limited accommodation space is likely to have resulted in the thin deposit (3 cm thickness) present at that site. Limited accommodation space on the island of Hawaii has led to only rare locations where tsunami deposits are preserved, despite the repeated occurrence of tsunamis affecting the island. © 2017 Elsevier B.V.
- ItemGeological evidence for the destruction of Shinmachi, Hawaii, by the 1946 Aleutian and 1960 Chile Tsunamis?(American Geophysical Union, 2016-12-12) Chagué-Goff, C; Goto, K; Goff, JR; Gadd, PS; Sugawara, D; Nakamura, NAround 1900, Japanese workers brought to Big Island, Hawaii, to work on the sugar plantations, established the Shinmachi (`New Town') community on low-lying land on Hilo's waterfront. Although Shinmachi was obliterated by the 1946 Aleutian tsunami, it was rebuilt, only to be totally destroyed again by the 1960 Chile tsunami. Shinmachi was never rebuilt and the site is now part of the Wailoa State Park. Short cores were collected throughout the park in an attempt to recover the geological evidence of these two tsunamis. Two anomalous layers, a lower sand (Unit 1) and an upper fining upward fine sand to silt (Unit 2) intercalated within soil and peat and exhibiting sharp lower and upper contacts, were recorded at only a few locations, probably reflecting, at least partly, the effect of anthropogenic disturbance and a limited amount of accommodation space on recent Holocene lava flows. One core was analysed by ITRAX core scanner equipped with a magnetic susceptibility (MS) meter. Unit 1 exhibits high MS associated with high Fe, Mn and Rb counts, but low Si and K counts, reflecting the volcanic composition of the material, and probably an older flooding event from the river. Unit 2 on the other hand is characterised by a different suite of elements, including Si, K, Ti, Mn, Fe, Ca, Sr, Zr and As. These most likely represent the mineralogical and chemical composition of shallow marine sediments from Hilo Bay and/or brackish sediments from Wailoa River estuary. High concentrations of As in particular have previously been reported in sediments from Hilo Bay and Wailoa River estuary and attributed to the release of arsenic trioxide by a canec manufacturing plant between 1932-1963. In this study, As was absent below Unit 2, and can thus be used as chronological control. Unit 2 therefore most likely represents the 1946 tsunami deposit. There was no clear evidence for the 1960 tsunami, probably reflecting the limited amount of accommodation space in the area.
- ItemHeavy minerals in the 2011 Tohoku-oki tsunami deposits—insights into sediment sources and hydrodynamics(Elsevier B.V., 2012-12-30) Jagodziński, R; Sternal, B; Szczuciński, W; Chagué-Goff, C; Sugawara, DThe 2011 Tohoku-oki tsunami left sand and mud deposits more than 4 km inland on the coastal plain of Sendai, Japan. The tsunami deposits, pre-tsunami soils and beach sediments were analysed for grain size, and heavy mineral content and assemblages to test the applicability of heavy mineral analyses in the identification of tsunami deposits and interpretation of associated sedimentation processes. Heavy minerals comprised on average 35% of the tsunami deposit in the 0.125–0.25 mm grain size fraction. The most common were orthopyroxenes, clinopyroxenes, amphiboles, limonites and opaque minerals. Heavy mineral concentrations and assemblages were similar in the tsunami deposits, beach and pre-tsunami soils and sediments and thus tsunami deposits could not simply be identified based on their heavy minerals. Sediment provenance analysis revealed that tsunami deposits left within 1.5 km of the shoreline were mostly eroded from the beach, dune and local soils, while deposits farther inland (> 1.5 km) were mostly derived from local soil erosion. No evidence was found for a significant contribution of offshore sediments. Detailed analyses revealed that the lowermost portion of tsunami deposits was mostly of local origin, while the sediment source of the upper portion was variable. A comparison with a previous study of heavy minerals in 2004 IOT deposits confirms that heavy minerals in tsunami deposits are mostly source-dependent and may represent a useful supplementary tool in studies of tsunami deposits. However, the interpretation must always be placed in the local geological context and corroborated with other “tsunami proxies”. © 2020 Elsevier B.V.
- ItemImpact of tsunami inundation on soil salinsaltion - up to one year after the 2011 Tohoku-okitsunami(Springer, 2013-09-27) Chagué-Goff, C; Wong, HKY; Sugawara, D; Goff, JR; Nishimura, Y; Beer, J; Szczuciński, W; Goto, KThe long-term effect of tsunami inundation on soil salinisation was assessed following the 2011 Tohoku-oki tsunami in two areas on the Sendai Plain, near Sendai airport in the Miyagi Prefecture and Matsukawa-ura near Soma in the Fukushima Prefecture. Data gathered over four sampling seasons 2, 5, 9 and 11 months after the tsunami near Sendai airport show that the salt content generally decreased with time. Concentrations were nevertheless higher in February 2012 than in October 2011, probably due to capillary action and evaporation following long periods with little precipitation in the winter, while the lower concentrations in October were attributed to dilution due to intense rainfall prior to the sampling period. In February 2012, the area with chloride concentrations over the guidelines for the establishment of rice seedlings still extended for nearly 1 km between 2.45 and 3.33 km inland. Chloride concentrations also reached the guideline values at the land surface 1.71 km inland. This corresponded to the limit of the area deemed not suitable for rice production by local rice farmers. However, recent observations revealed that rice crops were not only halted in 2011 but also in 2012, probably due to high salinisation of soil and/or surface and groundwater. Our study shows that soil salinisation was still recorded to nearly 15 cm depth in areas with fine-grained organic-rich soil ~2.5 km from the shoreline 11 months after the tsunami, and that water-leachable ions were preferentially retained in organic-rich muddy sediment and soil, reflecting the long-term impact of tsunami inundation. In Matsukawa-ura, salt crusts still covered the area flooded by the tsunami in February 2012 and both the soil and muddy tsunami deposit were characterised by high chloride and sulphate concentrations. The latter might also lead to sulphide toxicity. Remediation measures have been implemented in certain areas, but further research needs to be carried out to test the effectiveness of the measures being used to allow rice production to resume. © 2014, Springer Science+Business Media Dordrecht.
- ItemInitial field survey report of the 2011 East Japan Tsunami in Sendai, Natori adn Iwanuma Cities(UNESCO-IOC, 2011-07-10) Sugawara, D; Goto, K; Chagué-Goff, C; Fujino, S; Goff, JR; Jaffe, BE; Nishimura, Y; Richmond, B; Szczuciński, W; Tappin, DR; Witter, R; Yuliento, EThe East Japan Earthquake (Mw 9.0) and associated tsunami struck the Pacific coast of eastern Japan on March 11th, 2011 at 2:46 p.m. (Japan Standard Time). Maximum run-up heights reached about 40 m along the Sanriku region’s coast and around 10 m on the Sendai coastline. The coasts of Iwate, Miyagi and Fukushima Prefectures in particular were badly damaged, and a considerable amount of time and money will be required to restore these areas. Tsunami inundation up to 5 km inland occurred across the Sendai Plain, which remained partly flooded for several weeks after the event. Some areas were still under water 2 months after the earthquake, and are likely to remain flooded for some time as a result of subsidence. The Disaster Control Research Center at Tohoku University conducted scientific research of the inundated coastal plains of Sendai, Natori and Iwanuma Cities in collaboration with other research institutes from Japan and overseas with the support of UNESCO-IOC (Intergovernmental Oceanographic Commission). Data and samples collected during the field survey are now being analyzed by various institutes around the World. This report is a summary of our field survey that focused on an area to the north of Sendai Airport. Radioactivity data recorded at the time of survey are also provided.
- ItemMagnetic anisotropies for tsunami deposits: application to the 3.11(Japan Geoscience Union, 2013-05-24) Kon, S; Nakamura, N; Goto, K; Sugawara, D; Iijima, Y; Chagué-Goff, C; Goff, JRTsunami deposits consist of well-sorted fine sand intercalating with non-marine black organic mud. It is difficult to reveal a transport direction of the deposit if the deposit showed no sedimentary fabrics,such as ripples. The proxy of anisotropy of magnetic susceptibility (AMS) appears to be a promising tool for the study of flow fabrics in recent-tsunami deposits such as Sumatra tsunami (Wassmer et al. 2010). The AMS fabric might allow us to reconstruct transport directions of unconsolidated tsunami sediments during emplacement because AMS provides a cryptic alignment of ferromagnetic and paramagnetic minerals. Such cryptic minerals, such as magnetite or phyllosilicate minerals, would behave as a different emplacement mode in a different hydrodynamic condition. In the AMS fabrics of volcanic rocks, there are large discrepancies between the magnetic lineation and the framework-forming silicate linear fabric. This suggests that the uncorroborated use of bulk AMS to detect flow fabric in tsunami deposits has risks. In this article, we show that the anisotropy of anhysteretic remanent magnetization (AARM) may resolve the difficulties. The combination of inundation eye-witness, SEM, AMS, and AARM confirms the flow pattern of recentand paleo-tsunami deposits from the geoslicer sampleing at Rikuzen-Takata city, Japan during 2011, 11th March Tohoku tsunami. We determined if the sandy deposits are of tsunami from these magnetic anisotropies. © 2013, Japan Geoscience Union.
- ItemModern and possible paleotsunami deposits in Samenoura, Sanriku Coast, and their relation to tsunami source mechanisms(Japan Geoscience Union Meeting, 2014-05-02) Sugawara, D; Nishimura, Y; Goto, K; Goff, JR; Jaffe, BE; Richmond, B; Chagué-Goff, C; Szczuciński, W; Yokoyama, Y; Miyairi, Y; Sawada, CSamenoura is situated in the bay head of a small inlet on the Pacific coast of Oshika Peninsula, one of the nearest places to the epicenter of the 2011 Tohoku-oki Earthquake. According to the Joint Survey Group, wave heights were measured at more than 20 m near the coastline. This area was severely damaged as a result of both co-seismic subsidence and tsunami inundation. We carried out field surveys of the Tohoku-oki and paleotsunami deposits at Samenoura in March, May and October 2013. Sandy deposits laid down by the Tohoku-oki tsunami were up to 20 cm thick at locations with an elevation greater than 10 m, and were several cm thick within the forest higher up. The tsunami deposit also contained numerous shell fragments and foraminifera. Although some possible sources of the tsunami deposits can be attributed to narrow sandy beaches near the study area, the deposition of such a thick sandy deposit is more or less enigmatic, considering the steep Ria-type coastal topography.Using a gouge auger and geoslicer, we found at least two sand layers intercalated within muddy sediments. A volcanic ash layer, which corresponds to the AD 915 Towada-a tephra, was also identified from a horizon between these sand layers. The underlying sand layer was most probably laid down by the 869 Jogan earthquake tsunami, one of the large-scale events known to have affected the region. Previous studies of the Jogan tsunami have proposed several possible source models that involve an interplate thrust earthquake. Given that the local bathymetry and topography of Samenoura Bay may be sensitive to the waveform of a large-scale tsunami, paleotsunami deposits found from this area may be the key to determining the source mechanisms of events on the Sanriku Coast.In this presentation, the possible correlation of the sandy deposits with known paleotsunami events based on detailed radiocarbon dating is discussed. The hydrodynamic character and processes of tsunami sediment erosion and deposition in Samenoura Bay are analyzed using numerical modeling of both interplate and outer-rise earthquake scenarios. © Japan Geoscience Union Meeting, 2014.
- ItemNew insights of tsunami hazard from the 2011 Tohoku-oki event(Elsevier, 2011-12-01) Goto, K; Chagué-Goff, C; Fujino, S; Goff, JR; Jaffe, BE; Nishimura, Y; Richmond, B; Sugawara, D; Szczuciński, W; Tappin, DR; Witter, R; Yulianto, EWe report initial results from our recent field survey documenting the inundation and resultant deposits of the 2011 Tohoku-oki tsunami from Sendai Plain, Japan. The tsunami inundated up to 4.5 km inland but the >0.5 cm-thick sand deposit extended only 2.8 km (62% of the inundation distance). The deposit however continued as a mud layer to the inundation limit. The mud deposit contained high concentrations of water-leachable chloride and we conclude that geochemical markers and microfossil data may prove to be useful in identifying the maximum inundation limit of paleotsunamis that could extend well beyond any preserved sand layer. Our newly acquired data on the 2011 event suggest that previous estimates of paleotsunamis (e.g. 869 AD Jogan earthquake and tsunami) in this area have probably been underestimated. If the 2011 and 869 AD events are indeed comparable, the risk from these natural hazards in Japan is much greater than previously recognized. (c) 2011 Elsevier
- ItemPossible paleo-tsunami deposits at Rikuzentakata City, Japan(National Committee of Japan for IGU, 2013-08-05) Iijima, Y; Sugawara, D; Goto, K; Chagué-Goff, C; Hayase, R; Hashimoto, K; Kon, S; Nakamura, N; Goff, JRRikuzentakata City, NE Japan, has been repeatedly suffered by tsunami inundations including 1896 Meiji-Sanriku, 1933 Showa-Sanriku, 1960 Chilean Tsunami, and 2011 Tohoku-Oki Tsunami. Up to 30 cm thick sand layer was deposited by the 2011 tsunami in this city (Naruse et al., 2012). Our study indicates that historical and prehistoric tsunamis also left deposits in this area. Nevertheless, previous studies of paleo-tsunami deposits in this area are limited (Haraguchi et al., 2006, Imaizumi et al., 2007), because of the difficulty of finding paleo-tsunami deposits along this ""ria"" coast. We conducted a field survey using a geoslicer to acquire sediment cores in order to explore the magnitude and history of tsunamis in this area. Overall 10 cores, each 2 m long and 12 cm wide were acquired during the survey. The sedimentary sequences were mostly composed of peaty soil, which was thought to have been deposited in a marsh environment, however these soils units were inter-fingered by numerous 1-15 cm thick sand layers. According to initial work including grain size analysis, some of the sand layers deposited 1.4 km from the present shoreline are identified as having a possible tsunami origin because they tend to show upward fining characteristics, indicating rapid sedimentation from suspended load. We will also present the preliminary results of tephra chronology, radiocarbon and 210Pb dating, and diatom analysis.
- ItemSediment sources and sedimentation processes of 2011 Tohoku-oki tsunami deposits on the Sendai Plain, Japan — insights from diatoms, nannoliths and grain size distribution(Elsevier B.V., 2012-12-30) Szczuciński, W; Kokociński, M; Chagué-Goff, C; Cachão, M; Goto, K; Sugawara, DThe 11th March 2011 Tohoku-oki tsunami inundated the low-lying Sendai Plain (Japan) more than 5 km 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 2 km 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 1 km 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 ~ 1 km of the coastline were mostly derived from beach and dune erosion. From 1 to 2 km 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. © 2020 Elsevier B.V.
- ItemUsing magnetic fabric to reconstruct the dynamics of tsunami deposition on the Sendai Plain, Japan — the 2011 Tohoku-oki tsunami(Elsevier B.V., 2014-12-01) Schneider, JL; Chagué-Goff, C; Bouchez, JL; Goff, JR; Sugawara, D; Goto, K; Jaffe, BE; Richmond, BThe magnetic fabric and grain size of sand deposits emplaced during the 2011 Tohoku-oki tsunami were studied in five trenches along a 1800 m long shore normal transect on the Sendai plain as well as in a near shore sedimentary infill of a scour depression. The magnetic susceptibility in all deposits is due to ferromagnetic minerals (mainly magnetite) making the anisotropy of magnetic susceptibility (magnetic fabric) suitable for fabric analyses. The dominant magnetic fabric is planar in all trenches and stronger in finer-grained inland deposits than in the coarser sediments. This planar fabric is related to tractive shearing of the bedload basal portion of the tsunami flow that led to the deposition of traction carpet layers. Among the various fabric parameters used for this study, the vertical evolution of the shape factor (q) of the magnetic ellipsoid in each trench follows the evolution of the magnetic lineation (L) and foliation (F). These parameters provide information on the hydrodynamic energy (flow speed) fluctuations during the emplacement of the tsunami deposit. For the most proximal deposits, characterized by well-sorted reworked beach sand with minor fluctuations in grain-size distribution, the magnetic fabric is sensitive to hydrodynamic energy variations during sedimentation. Reconstruction of tsunami flow orientation in the sediments, based on the orientation of the mean Kmax calculated for each trench, appears to be unambiguous only for the sandy infills of small topographic depressions. The variations in flow direction indicators elsewhere could be related to local variation of the flow and to micro-topographic effects. These findings are encouraging for the use of the magnetic fabric proxy in the study of paleotsunami deposits. © 2014 Elsevier B.V.