前震研究进展综述

简要介绍了当前国内外前震研究领域的一些进展。对比了不同前震定义条件下前震序列的共性特征，对几种主要的前震机理及主要的前震识别方法进行了简要综述，对其特点及存在的问题进行了评述和讨论。前震指主震之前在主震断层面上、紧邻主震破裂起始点发生的小地震。主震发生之前的一系列前震活动构成前震序列。在不同的前震定义条件下，具有“直接前震”的震例比例从10%至40%不等。理论上前震可用级联应力触发或预滑动模型进行解释。前震空间上主要集中分布在距离主震10~75km范围内，但其时间分布形式非常复杂，大多发生在主震前1~2天，部分前震序列的地震活动率显示明显的加速特征，但许多前震序列单独来看却往往显示主-余型序列的衰减特征。序列地震震源机制一致、序列b值偏低是前震序列最突出的特征。前震的发生与主震破裂形式和构造环境似有一定关系，在有限的前震震例中，逆冲型主震似乎具有相对更多的前震。部分震例的研究结果显示，随主震的临近，前震震源深度有逐渐下迁的特点。到目前为止，震前很难判定一次地震或一个地震序列是否前震或前震序列，所使用的前震识别主要有基于统计类比的方法、基于震源机制一致及衍生的相关方法以及基于对地震成核过程精细检测的方法。从现有不多的震例研究结果来看，尽管地震时空丛集和震源机制高度一致是前震序列的最显著特征，但却不是判定前震序列的充分条件。由于成核的破裂扩展速度和滑动位移有随时间较快增大的趋势，因而基于对地震成核过程精细检测的方法有望在前震识别中发挥更为重要的作用，但需更多震例进行验证。

A Review on Research of Foreshocks

This paper briefly introduces the current research progress in the field of foreshocks, both in China and abroad, and contrasts the common characteristics and the mechanisms of foreshocks of the sequence under the definition of different conditions of foreshocks. The main recognition methods of foreshocks are briefly reviewed, and their characteristics and existing problems are reviewed and discussed. Foreshocks are small earthquakes that occur before the mainshock and adjacent to the main source location. A foreshock sequence is constituted of a series of foreshock activities that occur before the mainshock. The proportion of earthquake cases includes direct foreshocks ranging from 10%-40% at different defined conditions of foreshocks. Theoretically, cascade model or pre-slid model can explain foreshocks. Foreshocks are mainly concentrated in the range of 10km-75km of the mainshock, but their time distribution form is very complicated, mostly prior to the mainshock from 1 or 2 days, the seismicity rate of part of foreshock sequences shows significant acceleration features, but many foreshock sequences often show the attenuation characteristics of mainshock-aftershock sequences. The most prominent feature of foreshocks is a focal consistent mechanism and low b-value of earthquakes of the earthquake sequence. Foreshocks seems to have a certain relationship with tectonic environment and the rupture form of the mainshock, in limited foreshocks earthquake cases, the dip-thrusting earthquakes seem to have relatively more foreshocks. The results of some of the earthquake cases show that the focal depth of foreshocks gradually moved downward with the mainshock approaching. So far, it is difficult to determine whether an earthquake or an earthquake sequence is a foreshock or foreshock sequence before the mainshock. The identification methods of foreshocks mainly include a statistical method of analogy, focal consistent mechanism and related derivative method, the fine detection of the earthquake nucleation process. From a few existing studies of earthquake cases, in spite of earthquake temporal clusters and focal consistent mechanism being the most significant features of foreshock sequence, there were not sufficient conditions for judging a foreshock sequence. Because the rupture rate of expansion and sliding displacement have a tendency to increase faster with time, the method of fine detection of the earthquake nucleation process is expected to play a more important role in the identification of foreshocks, but it needs more earthquake examples to be verified.