A Study Of Palaeotsunamis Of China In The Late Holocene

A muddy sediment core,T08-A,with a time span covering the past two millennia was collected from the mud belt on the inner shelf of the East China Sea.The mean grain size distribution revealed that the sedimentary sequence was formed by two components,the<57.8 ?m fraction,constituting about 90%of the sediments,and the>57.8 ?m fraction.Two abrupt changes around 850-800 yr BP and 1200-1100 yr BP in the grain size profiles of the<57.8 ?m fraction and silt were responses to the South China Sea palaeotsunami and a cold event in the Northern Hemisphere.Surface sediments in the East China Sea tend to be coarse from the inner shelf to the outer shelf.As the tsunami propagated in the East China Sea,coarse grains were carried westward and deposited at the coring site,forming a coarse layer.To understand the impact of the SCS tsunami,numerical simulations of Mw 8.0 earthquake-induced tsunamis were performed.The results suggested that islands and coasts of the South China Sea were the most affected regions,while the East China Sea experienced little impact.The variation of the tsunami-affected layer was the most significant in the grain size sequence,indicating that this region had not been struck by event greater than the SCS palaeotsunami.Though tsunamigenic earthquakes might occur along the Ryukyu subduction zone,such possibility seems slight due to the sparse distribution of great earthquakes along this zone.Therefore,the East China Sea has not been hit by hazardous tsunamis directly over the past two millennia.Geological studies and historical documents have confirmed a tsunami hit the South China Sea 1000 years ago,likely triggered by an earthquake along the Manila Trench.However,the destructions this tsunami brought to the coastal regions remain unknown.Here we report the tsunami's devastating impacts on Nan'ao,an island 10 km to the east coast of Guangdong province.Sand layers characteristic of tsunami are buried in the coastal deposits in the southeast part of the island,they are dated back to 1000 years ago(the approximate time of the tsunami hit),and shells and ceramics shards of Song Dynasty are scattered in the sand layers.The ancient town of Nan'ao was totally destroyed by this tsunami,and historical relics of mid-and late-Song,Yuan and early-Ming Dynasty were nearly nonexistent.The town prospered again after about AD 1575,more than 500 years after tsunami,as indicated by the establishment of the formal governance.The wreckage of the ancient town is still buried under the southeast part of the modern city.