## 4 Results and Analysis
### Time-Space Analysis of \(b\) Values
As shown in Figure 2a, before the \(M_{\rm S}\) 8.0 Wenchuan earthquake, the frequency of events in the Wenchuan aftershock zone had been decreasing for a year since 2006, and only two events greater than \(M_{\rm S}\) 4 occurred during the period when \(b\) values were significantly decreasing (Figure 5a). However, earthquakes that were greater than \(M_{\rm S}\) 4 struck the entire Wenchuan aftershock zone after the \(M_{\rm S}\) 8.0 event (Figure 1). In addition, there were no strong aftershocks above magnitude 6.5 along the faults, and only six events greater than \(M_{\rm S}\) 4 occurred within two months after the mainshock. These phenomena indicate that without the continuous perturbation of strong aftershock, the \(b\) value gradually stabilized to the state mainly determined by the background earthquakes, which tended to shift to larger events following the \(M_{\rm S}\) 8.0 event in the Wenchuan aftershock zone. After the \(M_{\rm S}\) 8.0 Wenchuan earthquake, the Lushan aftershock zone also experienced a \"seismic quiescence\" of approximately two years, and only one event greater than \(M_{\rm S}\) 4 occurred before the \(M_{\rm S}\) 7.0 Lushan earthquake during the period when \(b\) values were significantly decreasing (Figure 2b). Moreover, only a few events greater than \(M_{\rm S}\) 4 occurred within two months after the mainshock; the subsequent events basically returned to the magnitude of the background earthquakes before the mainshock, which shows that the \(b\) values in the Lushan aftershock zone eventually returned to the background level (Figure 5b). To analyze the spatial footprints of the changes in the \(b\) values, we divided the two study regions into 0.1\({}^{\circ}\)\(\times\) 0.1\({}^{\circ}\) grids, sampled the 300 events nearest to each grid node within radius of 30 km, and re-estimated the \(M_{\rm C}\) in each node. For this purpose, we used a bootstrap approach to sample the events 1000 times randomly. The spatial footprints of the changes in the \(b\) values are consistent with the FMDs (Figure 5c,d). Figure 5e-g demonstrate the spatial variation in the \(b\) values throughout the Wenchuan aftershock zone. Figure 5h-j show the spatial variation in the \(b\) values in the Lushan aftershock zone. Figure 5: Temporal and spatial analysis of the \(b\) values for the Wenchuan-Lushan sequence. (**a,b**) Temporal variation of \(b\) values for the Wenchuan and Lushan source regions. The dashed black lines represent the times of the \(M_{S}\) 8.0 (Wenchuan) and \(M_{S}\) 7.0 (Lushan) events, and the dashed blue lines show the background \(b\) values for the Wenchuan and Lushan source regions. The dashed red line represents the \(b\) value after the time of the \(M_{S}\) 7.0 event (Lushan) for the Wenchuan source region. The shaded regions represent the uncertainties in the \(b\) values. (**c,d**) FMDs for the two aftershock zones in three different periods. (**e–g**) Map showing the spatial footprint of \(b\) values for the Wenchuan aftershock zone in three different periods. (**h–j**) Map showing the spatial footprint of \(b\) values for the Lushan aftershock zone in three different periods. The \(b\) value in the southern part of the Wenchuan source region and Lushan source region was lower than that in the northern part of the Wenchuan aftershock before the \(M_{S}\) 8.0 Wenchuan event (Figure 5e,h). This pattern is consistent with the characteristics of the Longmenshan fault, which is a strike-slip fault in the north and a thrust fault in the southern section. It is generally considered that the \(b\) value is inversely proportional to stress, and the \(b\) values of different types of faults are as follows: \(b\) (normal) \(>\) 1, \(b\) (strike-slip)-1, and \(b\) (thrust) \(<\) 1 [24,25]. The conditions changed markedly after the \(M_{S}\) 8.0 Wenchuan event; the \(b\) values decreased in the Wenchuan source region and Lushan source region (Figure 5f,i). This finding illustrates the effect of the Wenchuan earthquakes on stress change along the Longmenshan fault.