湖泊沉积对地震动的响应特征与古地震序列重建

Characteristics of lake sediment response to earthquakes and the reconstruction of paleoseismic sequences

湖泊沉积因其对地震动的敏感而被认为是“天然地震仪”,湖泊沉积古地震研究有机会重建长时间尺度的地震(动)序列,对认识区域发震孕震环境和地震复发规律具有潜在优势,是当前古地震学研究的重要方向之一。本文旨在总结现今湖泊沉积古地震研究的主要进展、存在的问题和未来展望。首先通过与传统古地震研究关注的记录对比,扼要介绍了湖泊沉积地震动记录在形成和保存潜力、空间分布以及感应能力等方面的相对优势。然后从过程角度总结了湖泊沉积对地震动响应的主要机制,着重剖析了液化、流化和沉积物再悬浮等不同机制在控制因素、过程特点、响应阈值等方面的异同。再结合湖泊沉积对地震动响应的过程特点和研究现状,总结了不同类型的湖泊沉积地震动记录,对比分析了变形构造、块体运动堆积、浊流堆积和再悬浮沉积等4种类型记录的沉积学和动力学特征;对不同类型记录的古地震学含义和研究手段进行了梳理。再总结了地球物理勘探、结构构造和理化代理指标等现阶段流行的方法在不同尺度湖泊沉积古地震识别和古地震序列重建中的适用性和局限性,后者主要缘于湖泊沉积系统本身的复杂性和外部扰动过程的多样性。最后指出,当前湖泊沉积古地震研究面临的主要问题是缺乏普适性的响应模式、判别依据和甄别准则;今后工作应致力于对湖泊沉积地震动响应过程的深入理解,积极引进数字或试验模拟等理论工具与方法,从二维观察扩展到三维重建;数据解释力求宏观结合微观,由单一指标转向综合性组合式指标,为最终建立普适性的诊断指标和判别依据服务。

Lake sediment is considered a "natural seismograph" because of its sensitivity to ground motion.Paleoseismic study of lake sediments,an important direction of paleoseismic study,makes it feasible to reconstruct long-term seismic sequences.The purpose of this paper is to summarize the main progress,existing problems,and future prospects of this important research field.The paper first briefly introduces the relative advantages of lacustrine paleoseismic records in terms of formation and preservation potentials,spatial distribution,and sensing ability over the traditional paleoseismic records.It then summarizes the main response mechanism of lake sediments to ground motion from the process viewpoint,with emphasis on the similarities and differences of different mechanisms,such as liquefaction,fluidization,and sediment resuspension,in control factors,process features,and response threshold.Next,combined with process characteristics and research status,the paper summarizes the seismic records of different types of lake sediments,compares and analyzes the sedimentary and dynamic characteristics from four types of records,including deformation structure,mass-transported deposits,turbidity deposits,and resuspension deposits,and discusses the paleoseismic meaning and research methods of different types of records.Lastly,the paper summarizes the applicability of popular research methods,such as geophysical exploration,sedimentary analysis,and physical and chemical proxies,in the identification of paleoearthquakes and reconstruction of paleoseismic sequences of lake sediments on different spatial scales.However,each method has its limitations due to the complexity of the lacustrine system itself and diversity of external disturbances.In the final analysis,it is clear that the main problems in paleoseismic investigation of lake sediments are the lacks of a universal response model,identification bases,and discrimination criteria.In future,we should concentrate our effort toward an in-depth understanding of the response process of lake sediments to ground motion,actively introducing theoretical tools and methods in expanding mathematical or experimental simulation from 2 D observation to 3 D reconstruction;whilst data interpretation should attempt to combine macro and micro aspects,e.g.,changing from single to comprehensive combined indexes,to facilitate the establishment of universal diagnostic indicators and criteria.