Sudden slope failing triggered a tsunami, which struck the Japan Sea coast on 16 June 1026 AD

Sedimentationcaused by a tsunami

of non-earthquake source

16 June 1026 AD

Local histories document an episodeof ca ~1000 years BP tsunami disaster

Japan sea

Masuda

10 km

Flooded area of the 1026 tsunami estimated from histories and traditions

Masuda

Emerged islet before the 1026 tsunami

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 ± 80year BP

1026 AD

.

150

100

50

0

-50

-100

dune

undevelped dune

coast

estuarine�

lacustrine

flood plain

channel

�

cm

~2300 BP

sandy gravel

sand

flame structure

root impression

bioturbation

lithology structure

ground surface

pre

sent

sea level

mud

gravel

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 archivesof 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 originof the tsunami

interpretation

conclusion

experiments

Geographical and hydraulic characteristics1. 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 sandover 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.

10 cm dune sand

mud including fresh water benthic diatoms

inter-tidal estuary sand

including blackish benthic diatoms

abundant-channel fill mud

flames

5 cmmud deposited fromsuspended cloud

sand

mud

pointed tongues

Depositon of sand over a hydroplastic mud layer in a water flume (soft X-ray radiograph)

Sediment structure found at sand/mud interfaces in the trench walls of Masuda

balls and wisps of mud

syndepositonal mud balls and wisps

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 layersObservations 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 experimentsInterpretations: 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