Focal Mechanism Classification Based on Areal Strain of Horizontal Strain Rosette of Focal Mechanism and Characteristic Analysis of Overall Focal Mechanism of Earthquake Sequence
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摘要: 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.Abstract: Classification of seismic focal mechanism plays an important role in earthquake dynamic analysis. However, focal mechanism 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 the division of focal mechanism types: normal fault type with -1≤As < -0.7, normal strike-slip type with -0.7≤As < -0.3, strike-slip type 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 the areal strain of horizontal strain rosette divides the focal mechanism according to the proportion of vertical and horizontal deformation caused by the source, the trouble of division by multiple combination of three axial plunge angles is avoided, and the problem of undefined type of focal mechanism is avoided. By presenting this partition strategy in spherical triangle diagram to show the 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 tensor dominated by the earthquakes with large moment tensor, and the other focal mechanisms are caused by observational errors, or secondary/minor fault rupture, then the overall focal mechanism can reflect the rupture property of the fault belt. This study proposes averaging the seismic moment tensor weighted by scalar seismic moment of the aftershocks to obtain the overall focal mechanism of earthquakes on an active fault or seismic belt, thus the difference of sliding behavior of the main shock and aftershocks can be analyzed. By adopting the above method in 2021 Qinghai Madoi earthquake sequence and 2022 Qinghai Menyuan earthquake sequence, the difference of slip properties between the main shock and other earthquakes is obtained. It is found that the overall focal mechanism of the 2022 Menyuan aftershocks is almost the same with that of the main shock, while the overall focal mechanism of the aftershocks in 2021 Madoi earthquake sequence has certain differences with that of the main shock. The method provides a tool for fault slip characteristics and geodynamic analysis in an active fault or seismic region.
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图 2 三种典型类型震源机制的水平应变花表示
a.正断型（走向30°，倾角45°，滑动角‒90°）；b.走滑型（走向90°，倾角90°，滑动角0°）；c.逆断型（走向135°，倾角45°，滑动角90°）
Fig. 2. Horizontal strain rosettes of the 3 typical focal mechanisms
图 3 本研究的震源机制分类边界在球面三角形中的表示
三角形的3个边分别为震源机制P、T和B轴倾伏角的刻度，三角形中的点划线表示震源机制类型分类的界限.相对面应变（As）以图下方的色棒为标准采用背景颜色绘制.白线为网格线；SS.走滑型；NS.正走滑型；N.正断型；RS.逆走滑型；R.逆断型
Fig. 3. Focal mechanism classification boundaries of this study represented in spherical triangle diagram
图 4 2021年玛多地震序列的前震和余震震源机制分类及总体特征分析
a.前震和余震震源机制在球面三角形分类中的表示；余震用海滩球表示，其膨胀区颜色按照正断‒走滑‒逆冲逐渐由红色变为蓝色，主震和总体震源机制分别用’☆’和‘Δ’表示，其他表示与图 3相同.图b和图c主震震源机制和其他地震事件总体震源机制的比较.红色到蓝色表示压缩到膨胀的过渡；P、T、B分别表示压轴、张轴和中间轴
Fig. 4. Focal mechanism classification in 2021 Madoi earthquake sequence and its overall feature analysis
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