## 6 Focal Mechanism Solutions of the Yangbi Earthquake Sequence
reverse components (SS-R), 9 normal fault types (N), 2 reverse fault types (R), and 3 reverse fault types with strike-slip components (R-SS). We divided the magnitude-time (M-T) diagram of the Yangbi earthquake sequence from 18 to 28 May into two phases: foreshock and aftershock sequences. Figure 6 shows the focal mechanism solutions of this study for some large earthquakes (M\({}_{\rm L}\geq 3.5\)). We found that the focal mechanisms of the foreshocks were consistent, and they were all strike-slip-type events. The earthquakes were distributed along the NW-SE direction, and the largest M\({}_{\rm 1}\)5.4 foreshock occurred at the southeast end of the foreshock sequence. The focal mechanisms of the aftershocks were complex. The focal mechanism of the mainshock was the strike-slip type, and the focal mechanism of the largest aftershock M\({}_{\rm L}\)5.1 was the strike-slip type with a normal fault component. Earthquakes at the periphery of the source region had prominent normal fault components, and very few of them had normal fault events. The focal mechanisms with the strike-slip and normal-fault components accounted for more than 92% of the total inversion results in this study, which was consistent with the regional tectonic background (Figures 5 and 6). In particular, the focal mechanism solutions for the foreshock sequence were not only highly consistent, but their nodal planes had an NW-SE orientation (Figure 6b), which is consistent with the seismogenic tectonics of this earthquake, as suggested by previous studies. Although the focal mechanisms of the aftershocks were relatively complex, the orientation of the nodal planes of the source mechanism solution for most aftershocks was also consistent with the seismogenic tectonics. This indicates that the focal mechanisms obtained in this study are highly credible. It was found that there were differences in the consistency of the focal mechanisms between foreshocks and aftershocks [65; 66; 67], which was proposed to describe the seismicity with parameter \(K\) (\(K\)\(\in\)\([-1,\)\(1]\)) using the consistency of the focal mechanisms. High \(K\) means high consistency of the focal mechanism. In addition, \(KA\) can also reflect the consistency of the focal mechanisms. Except for the largest foreshock (M\({}_{\rm L}\)5.4) whose \(KA\) was more than 30\({}^{\circ}\), the \(KA\) values of other foreshocks were all less than 25\({}^{\circ}\), indicating that the focal mechanisms were consistent (Figure 7c). On the contrary, the \(KA\) of the aftershocks has a wide distribution range and low consistency. The seismicity parameter \(K\) and the \(KA\) of the foreshock and aftershock changed before and after the mainshock. The focal mechanisms of the foreshock sequence were consistent, all of which were of the strike-slip type with high \(K\) values, which reflects that the stress in the source area accumulated continuously before the mainshock, resulting in the stress field tending to be uniform. This is consistent with the previous research that considered that the foreshocks were controlled by a unified stress field, and the focal mechanism solutions had high similarity [8; 65; 68; 69]. After the mainshock, the number of aftershocks with different types of focal mechanisms increased,  
Figure 5: Types of focal mechanism solutions of the Yangbi earthquake sequence. the \(K\) value decreased, and the \(KA\) increased, all of which reflected the adjustment process of the stress field in the source area [8,70, 71, 72].