南海北部海域北宋年间的古海啸记录及其潜在触发机制

Paleo-tsunami sedimentary records and potential triggering mechanism during the Earlier Song Dynasty at the northern South China Sea

鉴识南海北部历史上大规模海啸灾害事件及其指示特征与影响区域,对沿海地区海啸灾害的预测、评估和防范具有重要意义.现有南海北部古海啸灾害研究多集中于沿海陆地或海岛上零星保存的事件沉积物,而对沉积环境相对稳定且不易受后期改造破坏的浅海沉积物关注很少.本文以汕头海门镇和惠州三门岛外海B01和D02钻孔的浅海岩芯为研究对象,通过沉积物粒度分析、地球化学指标、沉积环境和加速器质谱(accelerator mass spectrometry,AMS)^(14)C年代测定等,分别在B01和D02岩芯的195~228和72~105 cm深度鉴别出“海啸事件沉积层”.两个岩芯的事件层有着相似的形成年龄,介于1048~1108年的北宋年间.该层平均粒径较上下层位显著增加,显示为正粒序单层沉积结构、未见平行或交错纹层.层内贝壳碎屑丰富、“新老”共存,其沉积物特征性地球化学指标明显不同于上下层位,且在界面附近地球化学元素含量或元素比值呈现突变规律.综合南海内部及其周缘同期海啸或疑似海啸记录与历史文献记载,我们认为南海北部上千公里沿岸一带可能发生了与菲律宾、西沙和南沙群岛近于同期的海洋巨灾事件.如此空间规模的极端灾害很可能代表了北宋年间的超级海啸事件,其触发机制应主要受控于该时期马尼拉俯冲带M_(w)>8.0的超强地震.

Identifying whether the northern South China Sea has historically experienced large-scale tsunami hazards and where these records have been preserved is of particular significance for the assessment and prevention of tsunami in the coastal zone.However,previous work has mostly focused on the study of event sediments from the onshore and isolated islands,less attention has been paid to the sediments in the northern shelf of South China Sea.This paper presents the particle size,elemental contents and ratios,and AMS^(14)C calibrated ages for the marine sediments from two onsite drillings at the offshore area of Haimen(Shantou,B01)and Sanmen(Huizhou,D02),respectively.In the B01 and D02 drillings,the“event sedimentary layers”are identified at the depth of~195-228 and 72-105 cm,respectively.The two layers,with abundant“old and new”organisms,were instantaneously deposited at the AD 1048-1108 during the Earlier Song Dynasty.They show the normal-grained sedimentary sequence characterized by the coarser particle size,more abundant shell fragments and more variable skewness and kurtosis values than those of the overlain and underlain layers.The geochemical signatures of the sediments in the event layers are distinct from those of the overlain and underlain lagers,in which Sr,Ba and Ca contents and Sr/Ba and Sr/Rb ratios sharply change.The geochemical abruptness is shown along either the interface or the vicinity of the“event sedimentary layers”.In combination with available data,the event sedimentary layers of the B01 and D02 drillings in northern Coastal South China Sea are believed to be resulted from the super-tsunami event occurred at the Earlier Song Dynasty(1048-1010).Such an event was temporally in accordance with those in the western Philippines,Penghu,Xisha and Nansha Islands,Nan’ao and Lantau(Hong Kong)islands,and the Pearl and Moyang River estuaries.Such a super-tsunami event extensively developed in the South China Sea might be mainly triggered by the M_(w)>8.0 earthquake along the Manila Subduction Zone or the synchronous earthquakes along with the Littoral Fault Zone of the northern South China Sea at the Earlier Song Dynasty.