全球海啸地震震源机制解及海啸成因探讨

Focal mechanism solutions for global tsunami earthquakes and future tsunami threat to China

海啸作为五大海洋自然灾害之一,严重威胁着人类生命财产安全。近些年来,国内外学者对地震海啸进行了大量研究,主要针对海啸的生成、传播、爬高和淹没的数值模拟,以及古海啸沉积物进行研究,但是对于海啸地震震源机制的研究还比较欠缺,尤其是缺乏对震级小于6.5的海啸地震的研究。针对我国的地震海啸研究现状,强调震级小于6.5地震引发海啸的问题不容忽视。本文归纳整理了全球766次地震海啸,利用三角图分类基本法则对海啸地震震源机制解进行分类,并对其中341个发生在1976年后的海啸地震进行震源机制解分析,对其中633次海啸浪高进行统计学方法分析研究。本文认为逆冲型、正断型、走滑型和奇异型机制地震均能引发海啸,逆冲型地震引发的海啸占比最大,震级小于6.5级地震引发的海啸的浪高也有高达10 m的情况,也能产生巨大破坏性。逆冲型、正断型、奇异型地震可直接引起海底地形垂向变化,进而引发海啸,而走滑型地震引发海啸则可能有两种原因,一种是走滑型地震并非纯走滑型而是带有正断或逆冲分量从而引发海啸,另外一种是走滑型地震引发海底滑坡导致海底地形变化进而产生海啸。从海啸地震震源深度分析,能产生海啸的地震震源深度97%以上都是浅源地震,主要集中在30 km深度以内,但是也有中深源地震海啸。本文综合海啸地震的震源特点、我国地理位置以及以往海啸发生的情况,认为未来我国沿海地区威胁性的地震海啸主要集中在马尼拉海沟和台湾海峡区域,在今后海啸预警方面需要格外重视这些区域,通过建立完善海啸预警系统来减少损失。

Tsunamis are one of the five major marine natural disasters that can seriously threaten human life and property.In recent years international researchers have conducted a large number of studies on earthquake-induced tsunamis.These studies mainly focused on numerical simulation of tsunami generation,propagation,climb and inundation,as well as on paleo-tsunami sediments,but lacked attention to the focal mechanism of tsunami earthquakes,especially of those below magnitude 6.5.As revealed by the most recent studies in China on earthquake-induced tsunamis,tsunami hazards caused by earthquakes of magnitude less than 6.5 cannot be ignored.In this paper,766 global earthquake-generated tsunamis are summarized;focal mechanism solutions for tsunami earthquakes are classified using triangulation techniques;and focal mechanism solutions for 341 tsunami earthquakes(since 1976)are analyzed in detail,with 633 wave heights analyzed by statistical methods.As results show,all types of earthquakes—strike-slip,normal and thrust earthquakes as well as unusual earthquakes—can cause tsunamis;whilst most tsunamis are caused by thrust fault earthquakes,and tsunami earthquakes below magnitude 6.5 can produce tsunami wave height up to 10 m to cause major damages.Mechanistically,thrust and normal earthquakes as well as unusual earthquakes can directly cause vertical changes in submarine topography to generate tsunamis;whereas strike-slip earthquakes can cause tsunamis for one of two reasons:they are not pure strike-slip type but have normal/thrust component,or they trigger submarine landslide to cause changes in seabed topography in generating tsunamis.According to focal depth analysis,more than 97%of tsunami earthquakes are shallow earthquakes less than 30 km deep,but intermediate and deep earthquakes can also cause tsunamis.Based on seismic source characteristics,geographical location of China mainland,as well as historical occurrences of tsunamis,we propose that earthquake-induced tsunamis that may threaten China’s coastal areas in the future are concentrated in the Manila trench and Taiwan strait.These areas should be monitored closely and early warning systems should be established to reduce disaster damage.