## 6 Focal Mechanism Solutions of the Yangbi Earthquake Sequence  
Using 2389 P-wave first-motion polarities of the above 112 earthquakes, the focal mechanism solutions of 112 earthquakes were inverted by the CHNYTX program [16]. Referring to the evaluation metrics in [16] (Table 3), the inversion results were divided into three classes, A, B, and C. There were 43 in Class A, 9 in Class B, and 60 in Class C. The detailed parameters of the focal mechanism solutions are shown in Supplementary Table S1. The focal mechanism solutions of approximately 30 earthquakes with \(\text{M}_{\text{L}}\geq 3.5\) have been obtained by using the waveform inversion method [52, 53, 54, 55, 57]. In this study, we determined 112 focal mechanism solutions using the P-wave first-motion polarity, which is 3.7 times that of the waveform inversion method (Figure 3). Among the 112 earthquakes, there are 40 earthquakes with \(\text{M}_{\text{L}}\geq 3.5\) and 50 earthquakes with \(\text{M}_{\text{L}}<3.0\), and the minimum magnitude is \(\text{M}_{\text{L}}1.6\). This shows that when the stations are well distributed, the focal mechanism of a large number of small earthquakes can be retrieved using the P-wave first-motion data. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Random Error** & **0** & \(\pm\)**0.1 s** & **+0.1 s** & \(-\)**0.1 s** \\ \hline Accuracy (all) & 98.49\% & 84.26\% & 76.39\% & 92.88\% \\ Accuracy (SNR \(\geq\) 30 dB) & 99.60\% & 87.69\% & 82.85\% & 95.58\% \\ Accuracy (SNR \(<\) 30 dB) & 97.57\% & 81.12\% & 71.96\% & 92.22\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Detection accuracy of P-wave first-motion polarity before and after shifting the arrival time. \begin{table}
\begin{tabular}{c c} \hline \hline
**Classes** & **Evaluation Threshold** \\ \hline A & \(\Psi_{\text{min}}\leq 0.15\), and \(\text{N}_{\text{C}}=1\), and \(\text{RMS}\leq 15^{\circ}\) \\ B & Each parameter is between Class A and Class C \\ C & \(\Psi_{\text{min}}\geq 0.30\), or \(\text{N}_{\text{C}}\geq 2\) \\ \hline \hline \end{tabular} Note: \(\Psi_{\text{min}}\) denotes the minimum weighted contradiction ratio; \(\text{N}_{\text{C}}\) denotes the number of clustering centers; RMS denotes the minimum root mean square of the rotation angle.  
\end{table} 
Table 3: Focal mechanism solution quality evaluation [16]. To verify the reliability of the new inversion results, we selected 10 earthquakes with focal mechanism solutions from published articles that have been commonly studied (Figure 4). We measured the consistency of the focal mechanism solutions [60] using the Kagan angle (\(KA\)). We calculated \(KA\) with the FMcenter program [61]. When \(KA\) is less than 25\({}^{\circ}\), it can be considered that the focal mechanism solutions are consistent [62]. Figure 4 is the comparison result. The standard deviation of the \(KA\) obtained in this paper is less than 24\({}^{\circ}\), which is in good agreement with previous results. It shows that the focal mechanism solution obtained by Fock-Flow inversion is reliable. The leftmost column is the earthquake information; the rightmost column is the standard deviation of the Kagan angle. Previous studies have shown that the Yangbi earthquake sequence occurred on a secondary fault parallel to the Weixi-Qiaohou fault [52; 53; 55; 56]. This right-lateral strike-slip fault can be regarded as the northward extension of the Red River fault zone [63]. We used the FMC program [64] to classify the focal mechanisms (Figure 5). The results showed that there were 31 strike-slip events (SS), 35 strike-slip types with normal fault components (SS-N), 17 normal fault types with strike-slip components (N-SS), 15 strike-slip types with  
Figure 4: Comparison of focal mechanism solutions between different studies. Figure 3: Statistics of the focal mechanism solutions for 1 \(\leq\) M\({}_{\rm L}\)\(\leq\) 6.4. (**a**–**f**) denotes the magnitude distribution of the focal mechanisms obtained in this study and Refs. [52; 53; 54; 55; 57].