Focal Mechanism Classification Based on Areal Strain of Horizontal Strain Rosette of Focal Mechanism and Characteristic Analysis of Overall Focal Mechanism of Earthquake Sequence
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 Ⅰ ncreases 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.
classification of focal mechanismhorizontal strain rosette of focal mechanismareal strainspherical triangle diagramseismology
维普
万方数据
