摘要
大地震的发生不仅与断层几何形态、应力积累状态有关,更和周围介质结构存在密切关系.近年来,随着观测手段的不断丰富和技术的不断发展,越来越多的研究工作聚焦于大地震震源区的精细结构,并将观测结果与发震机制联系起来.目前,地震学家普遍用凹凸体(asperity)模式解释大地震的发震机制,即断层从闭锁到突然滑断的过程.在这个过程中,断层深部几何形态和应力状态较难准确获取,而介质结构却可以通过地震层析成像和大地电磁探测等手段获得.本文将从大地震震源区介质结构的角度,系统总结已有的观测结果,探讨结构特征与凹凸体的关系.为此,我们对全球震级大于6级且震源区开展过凹凸体研究的地震事件进行了统计分析.本文共统计了123个不同震源机制的地震,包括54个板内地震和69个板间地震.这些地震研究主要分为两类:一类是结构研究;另一类是震源破裂过程研究.在结构研究中,共有17个板内地震和14个板间地震.这些地震的结构研究表明,震源区发震断层附近均存在高强度(高地震波速或高电阻率)的异常体,这些异常体被认为是地震发生的凹凸体,对地震的发生起到控制作用.在这31个大地震中,有4个地震发生在俯冲的海山处,海山即为明显的结构异常体,且有效正应力较高,是孕育大地震的高强度凹凸体.研究结果可以作为凹凸体对大地震发生起到重要作用的直接证据.在震源破裂过程研究中,共有92个大地震,研究表明断层附近存在较大的同震滑移带,推测存在高强度的凹凸体.并且,大多数地震具有显著的单侧破裂特征,而这与结构不均匀展布密切相关.破裂过程研究结果可以作为凹凸体对地震发生起到关键作用的间接证据.结构和破裂过程研究均表明,结构控制的凹凸体在大地震孕育过程中发挥着十分重要的作用.凹凸体具有高速、高阻及高力学强度的特征,更容易在长期的断层闭锁期中积聚构造应力,进而导致大地震的发生.因此,我们可以通过结构研究,利用高速、高阻异常体识别潜在的大地震凹凸体,评估断层的发震能力,从而为防震减灾工作提供重要依据.同时,我们发现凹凸体与震源机制无关,在逆冲断层、正断层和走滑断层中发挥着同样重要的作用.
The occurrence of large earthquakes is not only related to the fault geometry and state of stress accumulation, but also closely related to the structure of surrounding rocks. With the continuous enrichment of observation methods and development of techniques in recent decades, various studies have been conducted to explore the fine structure of large earthquake source regions and link the observations to the mechanism of large earthquakes. Seismologists generally use the asperity model to explain the generation mechanism of large earthquakes,that is, the process from locking to sudden slip of the seismogenic faults. The detailed geometrical shape of the fault planes and accurate state of stress accumulation are usually tough to depict during the process, while the structure features of the surrounding rocks are available through seismic tomography and magnetotelluric studies. In this paper, we aim to systematically summarize the results of structure studies in the focal areas of large earthquakes, and discuss the relationship between structure features and asperities. We therefore made statistics on the studies of asperity in the source regions of worldwide large earthquakes with magnitude larger than 6.0. There are totally 123 events with different focal mechanism, among which 54 events are intraplate earthquakes, while the other 69 events are interplate earthquakes. These large earthquakes are well studied by structure and/or source rupture process researches. Structure studies carried on 17 intraplate and 14 interplate earthquakes suggest the presence of high strength(tomographic high-velocity and/or magnetotelluric high-resistance) bodies surrounding the seismogenic faults. These anomalous bodies are considered to be the asperities, which play an important role in the generation of large earthquakes. Among the 31 large earthquakes, there are 4 events that took place on subducted seamounts,which are characterized as patches of anomalous structure features as well as elevated effective normal stress and are asperities with high strength in the evolution of these large earthquakes. The structure studies provide direct evidence for the important role of asperity in the generation of large earthquakes. The indirect evidences come from the source rupture studies of 92 large earthquakes, which show large coseismic slip zones near the seismogenic faults, manifesting the existence of asperities with high strength. The predominance of unilateral rupture in earthquakes has also been revealed, which has a close relationship with the structural heterogeneity. Through statistical analysis of asperity mechanism studies on large earthquakes, we find the asperity characterized by high velocity and resistivity plays an important role in the generation of large earthquakes. The asperities are strong in mechanical strength and could accumulate tectonic stress more easily during the long frictional locking periods, large earthquakes are therefore prone to generate in these areas. If the close relationship between asperity and high-velocity and high-resistivity bodies is valid for most of the large earthquakes, it can be used to predict potential large earthquakes and estimate the seismogenic capability of faults in light of structure studies, which will provide important clues for earthquake prevention and disaster reduction. Furthermore, the asperity model is irrelevant to focal mechanism and plays a same important role in thrusting, normal and strike-slip faults.
作者
孙权
裴顺平
郭震
陈永顺
Sun Quan;Pei Shunping;Guo Zhen;Chen Yongshun(Department of Ocean Science and Engineering,Southern University of Science and Technology,Shenzhen 518055,China;State Key Laboratory of Tibetan Plateau Earth System and Resources Environment(TPESRE),Institute of Tibetan Plateau Research,Chinese Academy of Sciences,Beijing 100101,China)
出处
《地球与行星物理论评》
2022年第2期138-147,共10页
Reviews of Geophysics and Planetary Physics
基金
国家自然科学基金资助项目(U2039203,42130306)
国家自然科学基金青年基金资助项目(42104050)
中国博士后科学基金资助项目(2021M701559)。
关键词
结构
凹凸体
大地震
发震机制
structure
asperity
large earthquake
seismogenic system