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Sudden slope failing triggered a tsunami, which struck the Japan Sea coast on 16 June 1026 AD Sedimentation caused by a tsunami of non-earthquake source
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Local histories document an episode of ca ~1000 years BP tsunami disaster
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What is the truth about the tsunami invasion? What is the cause of the disappearance of islet? Local histories: Invasion of a gigantic tsunami at midnight on 16 June 1026 AD. Submergence of a rocky islet in the offshore just before tsunami occurrence. Local tradition: Serious damages by catastrophic seawater flooding. Official documentations: No description of earthquake and tsunami occurrence in every 1026 AD official archive of Japan and Korea.
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Flame structures were found at the interface of dark gray mud and brown sand. Peeling off trench surface
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Flames, wisps, and a film of organic mud. These structures were syndepositional and preserved from any later disturbance. Lacquer-resin peel of trench walls. Continuous film of mud flames and wisps of mud mud sand
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Vertical sequence from channel to dune. The radiocarbon date of 940±80 BP corresponds with the historical age of the tsunami event (1026 AD). 940 ± 80 year BP 1026 AD Flame structure
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abundant channel fill mud Sorted fine sand (tsunami layer)
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Paleontology: Moderate occurrence of estuary planktonic diatoms Cyclotella striata and Thalassiosira bramaputrae and brackish benthonic diatom Amphola sp. indicate the origin of sand in inter-tidal estuaries. Reduced occurrence of fresh-water planktonic diatoms, as well as moderate benthonic species occurrence, suggest the mud being a product of a shallow water pond with restricted water circulation.
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No other tsunami in official archives of this time period. + No earthquake occurrence in and around the Japan Sea. Discovery of a tsunami deposit + Synchronous ages of history and geology -geology- -history- A tsunami of local and non-earthquake source ?
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Slope failure origin of the tsunami interpretation conclusion experiments Geographical and hydraulic characteristics 1. Submergence of a rocky islet in the offshore just before tsunami invasion (local histories). 2. Submerged reefs in the present offshore (scuba observation). 3. Large outflux of riverine sediments (hydrologic estimate)
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Depositional experiments of sand over hydroplastic mud in a water flume. mud sand
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mud sand Soft X-ray radiograph of sediment structures formed by depositional experiments in a water flume.
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Disappearance of flames and wisps, and mixing of sediments at sand-mud interfaces mud sand Mechanical vibrations
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Geological results: Catastrophic invasion of fast-flowing streams High concentration of Ca carbonates Transport of coastal materials into lacustrine environments Landward thinning of sediment layers Observations and experiments: Present storm surges to be agents of erosion without production of extensive deposits on land areas Syndepositonal origin of sediment structures indicated by water flume experiments Interpretations: Abrupt landward transport of voluminous seawater and coastal materials No seismic shocks before and after sediment deposition Conclusions: Deposition of tsunamigenic sand Tsunami of non-earthquake source Slope instability cause of localized tsunami
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