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