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

Local histories document an episode of ca ~1000 years BP tsunami disaster

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.

Flame structures were found at the interface of dark gray mud and brown sand. Peeling off trench surface

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

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

abundant channel fill mud Sorted fine sand (tsunami layer)

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.

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 ?

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)

Depositional experiments of sand over hydroplastic mud in a water flume. mud sand

mud sand Soft X-ray radiograph of sediment structures formed by depositional experiments in a water flume.

Disappearance of flames and wisps, and mixing of sediments at sand-mud interfaces mud sand Mechanical vibrations

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