南海古海啸重建与海啸沉积研究进展

doi:10.13745/j.esf.sf.2020.9.6

摘要/Abstract

摘要：

中国东海、南海等近海临近琉球海沟、马尼拉海沟等俯冲带,地震频发。过去的海啸研究主要关注历史文献分析、海啸数值模拟等,据此评估中国近岸海啸灾害的历史和风险。历史时期是否引发了海啸,特别是具有特大致灾风险的大海啸记录,目前还不明确。近年来,本课题组通过对海岛、海洋沉积和海岸带及其岛屿的沉积过程、海啸遗迹和历史记录研究,阐述了确定古海啸的系列研究方法。首先通过对南海西沙群岛东岛湖泊沉积序列、大量砗磲和珊瑚块在海岛分布的特征分析,识别出距今千年的一次海啸事件。以此为标志,根据湖泊沉积结构作为识别海岛海啸沉积的特征。同时提出了确定海岛海啸发生时代的样品采集和定年方法,其中包括根据事件沉积层顶部和底部植物残体¹⁴C年龄定年和历史文献记录的印证。首次确定在过去1 300年中,南海发生过一次海啸,其发生时间为公元1076年。为了寻找更古老的海啸记录,结合对东海闽浙沿岸过去两千年海洋泥质沉积的分析,发现南海海啸在沉积序列中留下记录,但除此之外沉积记录中并无更强的扰动,因此东海在过去两千年中受到海啸的影响较小。1076年的海啸同时冲击了南海沿岸,通过对广东南澳岛考察发现,岛屿东南海岸保存着距今约1 000年的海啸沉积层,其中夹杂着宋代陶器瓷器残片。对遗迹数量变化的分析显示,岛上的文化受海啸破坏出现了长达500年的文化中断,直至明代中后期设镇之后才逐渐恢复。根据海啸层植物残体、贝壳¹⁴C测年、覆盖海啸层的海砂光释光定年以及瓷器碎片的年代鉴定了海啸的发生时代,并据此提出了海岸带古海啸沉积的定年方法。此外,不同环境下海啸沉积的特征也存在较大区别,需要结合地形、沉积物来源以及地球化学特征等多种指标进行识别。有迹象表明海南岛东侧海岸带有海啸破坏的明显证据,需要进行深入的研究。

关键词: 古海啸, 海啸沉积, 古海啸定年, 南海, 东海, 海岸灾害

Abstract:

The East and South China Seas are adjacent to the Ryukyu and Manila Trenches, where earthquakes occur frequently. Tsunami researches in the past were focused on analyzing historical documents and numerical simulation to assess the risk and history of tsunamis striking the coastline, but the knowledge of historical tsunami events was uncertain. In the past decade, by investigating coastal, marine sediments and their depositional processes, tsunami relics, and historical documents, we have developed research approaches to reconstruct paleotsunami. We first recognized a 1000-year-old tsunami based on sedimentary evidences from Dongdao and Xisha Islands and the South China Sea, and proposed using lake sediment characteristics as a criterion for identifying paleotsunami. Sampling and dating methods, including validation from both the age of the event layer and historical literature, were also proposed. We suggest that a tsunami occurred in the South China Sea in the past 1300 years, and the year of this event is AD 1076.To expand the age span of the reconstruction, a 2200-year-long sediment record from the East China Sea was studied. Disturbance caused by the South China Sea tsunami could be seen in the record, whereas the absence of a stronger abrupt change indicated the East China Sea has not been hit by catastrophic tsunamis in the past 2200 years. The 1076 tsunami hit the coastal regions of South China. Tsunami deposits containing shards of Song ceramics were preserved on the coast of Nan’ao Island; the culture of Nan’ao experienced a 500-year decline after the South China Sea tsunami and did not recover until the late Ming Dynasty. We used radiocarbon ages of plant remains and shell carbonates, OSL analysis of sand, and ceramic dating to determine the age of the paleotsunami; based on the dating results, we put forth the methods for tsunami deposit dating. In addition to these dating methods, proxies of geomorphology, geochemistry, and sedimentary sources can also be used under different depositional environments. The east coast of Hainan Island is also vulnerable to tsunamis, and future studies are needed to reconstruct its paleotsunami records.

Key words: paleotsunami, tsunami deposit, paleotsunami dating, the South China Sea, the East China Sea, coastal tsunami hazard

中图分类号:

杨文卿, 谢周清, 孙立广. 南海古海啸重建与海啸沉积研究进展[J]. 地学前缘, 2021, 28(2): 246-257.

YANG Wenqing, XIE Zhouqing, SUN Liguang. Research progress in the reconstruction of paleotsunami in the South China Sea and the tsunami deposit characteristics[J]. Earth Science Frontiers, 2021, 28(2): 246-257.

图/表 6

参考文献 66

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指标	海啸	风暴	说明
沉积厚度	几厘米至几十厘米,可能缺失	几厘米至几十厘米,可能缺失	厚度上两类沉积重叠较大,且受物质来源、地形等因素影响变化范围很大,可能更厚或更薄甚至缺失
内陆分布距离	几百米至几千米	几十米至几百米	与海浪淹没距离有关,不一定呈连续分布,某些区域可能出现间断
沉积构造	没有或很少有层理	可能有平行层理	西沙群岛和广东的海啸沉积内部未见层理构造,均为单层沉积,与理论模型相符
粒序	多为正粒序或无粒序构造	正粒序或逆粒序	西沙群岛和广东南澳的海啸沉积未找到明显的粒序构造,随深度变化较小,事件类型根据其粒径分布进行判断
粒径分布	多是单峰分布,以砂、粉砂粒径为主	单峰或双峰分布	粒径分布受沉积物源、海岸地形等因素影响很大,可能会出现多组分、多峰混杂的情况
物质组成	砂、粉砂、卵石、贝壳碎片	砂、粉砂、卵石、贝壳碎片	二者区别不大,海啸的粗颗粒的成分比例可能多一些
地球化学指标	应用较少,西沙的海啸沉积中的有机质(TOC)含量下降明显,显著区别于上下层位	Cl、Sr及Cl/Br对古风暴有较好的指示	目前的研究中指示意义相对较小,用得不多,其意义和应用仍有待进一步发展
有孔虫等微型生物壳体	受保存环境影响很大,热带地区湿热的海岸平原可能使古海啸层中的有孔虫发生溶解	同样存在溶解的可能	南澳的海啸层中未观察到有孔虫,可能是事件发生时间太长溶解了,西沙湖泊中保存着有孔虫,可能是湖水和上层沉积物对雨水的隔离作用令溶解过程不易发生;海底沉积中未见相关报道
砾石沉积	可将砾石搬运至距海岸线近1 km处	最远约搬运至二三百米处	在南澳岛距海岸线500多米的山坡处找到了磨圆较好、岩性不同于周围基岩的砾石,西沙东岛的砗磲、珊瑚也类似
人工产物或遗迹	南澳海啸层中发现了宋代瓷陶器残片,以色列凯塞利亚港海啸沉积中存在古代港口的建筑遗迹,日本2011年海啸层中存在福岛核事故造成的人工放射性核素含量异常	未见报道	较少见,目前的研究中均指示海啸事件,可能是海啸灾害的有力证据,但特大风暴的破坏作用可能也会形成人工产物遗迹,未见报道并不表示没有这种可能
洞穴沉积	可在海岸十余米高的洞穴深处形成过去几千年连续的沉积,保存好	未见报道	风暴沉积难以到达并深入洞穴内部,通过沉积环境避开了风暴沉积的干扰
深海沉积	日本海沟7 500 m水深处发现2011年海啸和过去一千多年的两次特大海啸形成的浊流沉积,沉积构造层理清晰、保存完好	未见报道,理论上不会形成深海沉积	深海沉积指示性更好,且很少受环境变化的扰动,可能会提供完整的长时间尺度记录,包括末次冰期的古事件,但采样困难、不易获取,故相关研究很少,未来可能是重建古海啸的方向之一

指标	海啸	风暴	说明
沉积厚度	几厘米至几十厘米,可能缺失	几厘米至几十厘米,可能缺失	厚度上两类沉积重叠较大,且受物质来源、地形等因素影响变化范围很大,可能更厚或更薄甚至缺失
内陆分布距离	几百米至几千米	几十米至几百米	与海浪淹没距离有关,不一定呈连续分布,某些区域可能出现间断
沉积构造	没有或很少有层理	可能有平行层理	西沙群岛和广东的海啸沉积内部未见层理构造,均为单层沉积,与理论模型相符
粒序	多为正粒序或无粒序构造	正粒序或逆粒序	西沙群岛和广东南澳的海啸沉积未找到明显的粒序构造,随深度变化较小,事件类型根据其粒径分布进行判断
粒径分布	多是单峰分布,以砂、粉砂粒径为主	单峰或双峰分布	粒径分布受沉积物源、海岸地形等因素影响很大,可能会出现多组分、多峰混杂的情况
物质组成	砂、粉砂、卵石、贝壳碎片	砂、粉砂、卵石、贝壳碎片	二者区别不大,海啸的粗颗粒的成分比例可能多一些
地球化学指标	应用较少,西沙的海啸沉积中的有机质(TOC)含量下降明显,显著区别于上下层位	Cl、Sr及Cl/Br对古风暴有较好的指示	目前的研究中指示意义相对较小,用得不多,其意义和应用仍有待进一步发展
有孔虫等微型生物壳体	受保存环境影响很大,热带地区湿热的海岸平原可能使古海啸层中的有孔虫发生溶解	同样存在溶解的可能	南澳的海啸层中未观察到有孔虫,可能是事件发生时间太长溶解了,西沙湖泊中保存着有孔虫,可能是湖水和上层沉积物对雨水的隔离作用令溶解过程不易发生;海底沉积中未见相关报道
砾石沉积	可将砾石搬运至距海岸线近1 km处	最远约搬运至二三百米处	在南澳岛距海岸线500多米的山坡处找到了磨圆较好、岩性不同于周围基岩的砾石,西沙东岛的砗磲、珊瑚也类似
人工产物或遗迹	南澳海啸层中发现了宋代瓷陶器残片,以色列凯塞利亚港海啸沉积中存在古代港口的建筑遗迹,日本2011年海啸层中存在福岛核事故造成的人工放射性核素含量异常	未见报道	较少见,目前的研究中均指示海啸事件,可能是海啸灾害的有力证据,但特大风暴的破坏作用可能也会形成人工产物遗迹,未见报道并不表示没有这种可能
洞穴沉积	可在海岸十余米高的洞穴深处形成过去几千年连续的沉积,保存好	未见报道	风暴沉积难以到达并深入洞穴内部,通过沉积环境避开了风暴沉积的干扰
深海沉积	日本海沟7 500 m水深处发现2011年海啸和过去一千多年的两次特大海啸形成的浊流沉积,沉积构造层理清晰、保存完好	未见报道,理论上不会形成深海沉积	深海沉积指示性更好,且很少受环境变化的扰动,可能会提供完整的长时间尺度记录,包括末次冰期的古事件,但采样困难、不易获取,故相关研究很少,未来可能是重建古海啸的方向之一