## 4 Results and Analysis
### Time-Space Analysis of \(b\) Values  
Earthquake frequency will increase immediately within a short time after a large event and may exceed the recording capacity of the seismic network. Before establishing the time-space series of \(b\) values with aftershocks, we should eliminate the events documented in the early catalog, which is somewhat heterogeneous and incomplete in small events [41]. In this work, the exclusion period depends on the magnitude of completeness over time. Therefore, we first removed the events documented in the initial catalog within two months after the Wenchuan \(M_{\rm S}\) 8.0 and Lushan \(M_{\rm S}\) 7.0 earthquakes, a period for which the data are highly incomplete. Then, we calculated the spatiotemporal distributions of \(b\) values before and after two large events that occurred from 2000-2019 along the Longmenshan fault by selecting events with \(M\geq M_{\rm C}\) = 1.5 and using a time window and spatial grid to calculate the \(b\) values. In this computation, the window lengths were set to at least 500 events in the  
Figure 4: Frequency-magnitude distribution and NLIndex. Figure 3: Frequency-magnitude distribution of the seismicity of the Wenchuan-Lushan sequence from 1 January 2000 to 1 January 2019. Wenchuan aftershock zone and 200 events in the Lushan aftershock zone. Each window was moved forward by one event at a time. Figure 5a,b display time series of the \(b\) value in source regions, and the overall change trend conforms to our postulation that \(b\) values will undergo relatively significant changes in a period of time before and after a large earthquake. Specifically, \(b\) values show a decreasing trend before the occurrence of both large earthquakes in both zones. To ensure that this trend is statistically significant, we quantitatively assessed the temporal variation in \(b\) values using the \(P\) parameter test and selected three windows before the \(M_{\rm S}\) 8.0 event (\(W_{1}\), \(W_{2}\), and \(W_{3}\)) and the \(M_{\rm S}\) 7.0 event (\(L_{1}\), \(L_{2}\), and \(L_{3}\)). Window selection was based on the significance of changes in \(b\) values (Figure 5a,b). The results are shown in Table 1. The \(b\) value in the sample window has significantly changed when \(\Delta\)AIC \(\geq\) 2 (\(P_{b}\)\(\approx\) 0.05). Table 1 shows that the \(b\) values decreased before both large earthquakes with statistically significant variations. After the Wenchuan \(M_{\rm S}\) 8.0 event, the \(b\) values in the Wenchuan aftershock zone experienced a period of dramatic fluctuation (indicated by the pink shading lasting not more than one year) before gradually stabilizing within a small fluctuation range (Figure 5a), which was similar to the range of \(b\) values in the third period (Figure 5c, \(b\) = 0.84). After the Lushan \(M_{\rm S}\) 7.0 event, the \(b\) values in the Lushan aftershock zones increased rapidly and then slowly dropped to a stable state (Figure 5b), which was similar to the FMD in the first period (Figure 5d, \(b\) = 0.76). As shown in Figure 5b (red shading), the \(b\) value required less than ten months to return to a stable state. The reference \(b\) values can be estimated for the background levels (for the period, \(b\) = 1.21 in the Wenchuan aftershock zone and \(b\) = 0.76 in the Lushan aftershock zone). When the perturbation effect of the mainshocks gradually decreases, the \(b\) values in the Lushan aftershock zone eventually return to the background level (\(b\) = 0.76), whereas those in the Wenchuan aftershock zone drop below the background level (from 1.21 to 0.84). To date, there have been no earthquakes that have significantly changed the stability of the Longmenshan fault zone since the \(M_{\rm S}\) 7.0 Lushan earthquake. The temporal distribution of earthquakes also indicates the change in stress state of faults and their surroundings.