震源机制水平应变花面应变的地震震源机制分类方法及序列震源机制总体特征分析

Focal Mechanism Classification Based on Areal Strain of Horizontal Strain Rosette of Focal Mechanism and Characteristic Analysis of Overall Focal Mechanism of Earthquake Sequence

地震震源机制分类对于地震动力学分析有重要作用.然而目前震源机制分类较为随意,并且存在不确定的类型,增加了后续进一步分析的困难.为解决此问题,引入地震震源机制水平应变花理论的面应变(As)作为震源机制类型划分的标准:正断型:-1≤As<-0.7;正走滑型:-0.7≤As<-0.3;走滑型:-0.3≤As≤0.3;逆走滑型:0.3<As≤0.7;逆断型:0.7<As≤1.由于震源机制水平应变花的面应变以震源引起的垂直变形量和水平变形量的比例来划分震源机制类型,避免了以3个轴倾伏角的多种组合进行划分的麻烦,并且解决了不确定型震源机制的问题.将这种划分方法表示在球面三角形的震源机制分类图中,发现震源机制的类型划分界限是对称的.另外,一条活动断层或地震带上地震的整体滑动行为对于地球动力学研究相当重要.假定活动断层或断裂带上发生的地震只有一个震源机制,即由释放较大矩张量的地震主导,而其他震源机制是由次级破裂或者误差导致的,则得出来的余震整体震源机制解在一定程度上可以反映断层破裂的性质.提出将余震震源机制采用标量地震矩加权平均的方法得到一条活动断层或地震带上的地震的整体震源机制,从而研究主震和余震的滑动行为差别的方法.将上述方法用于2021年玛多地震序列和2022年的门源地震序列中,得到了主震和序列中的其他地震滑动特性的差别,发现2022年的门源地震序列的余震整体震源机制与主震的震源机制几乎相同,而2021年玛多地震序列的余震总体震源机制和主震有一定的差别.该方法为一条断裂带或地震带的地震滑动以及地球动力学分析等提供了工具.

Classification of seismic focal mechanism plays an important role in earthquake dynamic analysis. However, focalmechanism classification is quite arbitrary at present, and the existence of the undefined type increases the difficulty of further analysis. To solve this problem, the authors introduce the areal strain (As) of seismic focal mechanism as the standard for thedivision of focal mechanism types: normal fault type with -1≤As<-0.7, normal strike-slip type with -0.7≤As<-0.3, strikesliptype with -0.3≤As≤0.3, reverse strike-slip type with 0.3<As≤0.7, and reverse fault type with 0.7<As≤1. Because theareal strain of horizontal strain rosette divides the focal mechanism according to the proportion of vertical and horizontaldeformation caused by the source, the trouble of division by multiple combination of three axial plunge angles is avoided, and theproblem of undefined type of focal mechanism is avoided. By presenting this partition strategy in spherical triangle diagram to showthe focal mechanism classification, the type classification boundaries of focal mechanism are found to be symmetric. In addition,the overall sliding behavior of earthquake sequence on an active fault or seismic belt is quite important for geodynamic studies.Supposing that the earthquakes occur on a fault belt ruptured as the same focal mechanism, i. e. the released moment tensordominated by the earthquakes with large moment tensor, and the other focal mechanisms are caused by observational errors, orsecondary/minor fault rupture, then the overall focal mechanism can reflect the rupture property of the fault belt. This studyproposes averaging the seismic moment tensor weighted by scalar seismic moment of the aftershocks to obtain the overall focalmechanism of earthquakes on an active fault or seismic belt, thus the difference of sliding behavior of the main shock andaftershocks can be analyzed. By adopting the above method in 2021 Qinghai Madoi earthquake sequence and 2022 QinghaiMenyuan earthquake sequence, the difference of slip properties between the main shock and other earthquakes is obtained. It isfound that the overall focal mechanism of the 2022 Menyuan aftershocks is almost the same with that of the main shock, while theoverall focal mechanism of the aftershocks in 2021 Madoi earthquake sequence has certain differences with that of the mainshock. The method provides a tool for fault slip characteristics and geodynamic analysis in an active fault or seismic region.