# An Analysis of the 2008 \(M_{\rm s}\) 8.0 Wenchuan Earthquake's Aftershock Activity
## 4 Results
### Spatial Distribution of b- and p-Values  
To investigate the pattern of the spatial variation in the Wenchuan aftershock sequence, we divided the aftershock area into 0.01\({}^{\circ}\)\(\times\) 0.01\({}^{\circ}\) grids and calculated the \(b\)- and \(p\)-values at each grid point (Figures 6 and 7). The \(b\)- and \(p\)-values were calculated based on the earthquakes within 5 km around each grid point, and the minimum number of earthquakes required was 100, meaning the \(b\)- and \(p\)-values were calculated only when more than 100 earthquakes were within 5 km around the grid point. The calculations were performed using the ZMAP6.0 software [18].  
Figure 6: Spatial distribution of \(b\)-value for Wenchuan aftershock sequence. Red star marks mainshock hypocenter location; red lines mark faults.  
Figure 7: Spatial distribution of Wenchuan aftershock sequence. Red star marks mainshock hypocenter location; red lines mark faults.  
The \(b\)-value spatially varied in the aftershock area within the range of 0.6-1.2 (Figure 6), and the \(b\)-value was generally small. The regions with relatively large \(b\)-values (\(b>0.9\)) were mainly located around the mainshock epicenter and in the central and northeastern parts of the aftershock zone, and the area with the largest \(b\)-value was located around Qingchuan. The regions with small \(b\)-values (\(b\leq 0.9\)) were mainly located in the southwestern part of the aftershock zone, and the area with the smallest b-value appeared at the northeast end of the aftershock zone. In addition, we found that the \(b\)-value was typically smaller along the Longmenshan central fault (i.e., near the Sichuan Basin) than along the Longmenshan back fault.  
The \(b\)-value in the northeastern part of the aftershock zone was relatively large, potentially because the fault ruptures in this area were newly generated during the earthquake. The rupture mechanism was relatively simple, and the fault planes were relatively smooth, which is favorable for rapid stress adjustments. As a result, although the aftershocks were dense in this area, their intensity was low, ultimately leading to the large \(b\)-value. In the southwestern part of the aftershock zone, the aftershock intensity was relatively high because of the complex rupture mechanism, and the \(b\)-value was generally small. In addition, several strong aftershocks occurred in the southwest area of Wenchuan and at the northeast end of the aftershock zone, where the \(b\)-value was the smallest. The subsurface anomalies in these two areas may have impeded the rupture and slip of faults, leading to the occurrence of strong aftershocks and the small \(b\)-value.  
The \(p\)-value spatially varied, mainly between 0.6 and 1.8 (Figure 7), which is a large range, indicating the spatial heterogeneity of the aftershock decay. A large \(p\)-value indicates the rapid decay of aftershock activity. Areas with high \(p\)-values (\(p\geq 1.2\)) were mainly distributed in the aftershock area along the Longmenshan central fault, spreading northeast from the mainshock epicenter. The areas with the largest \(p\)-values were mainly located around Yingxiu-Hanwang, Beichuan, and Qingchuan. In contrast to the distribution pattern of large \(p\)-values, the areas with small \(p\)-values were mainly concentrated in the aftershock area along the Longmenshan back fault.  
The map in Figure 7 shows that the distribution of areas with large \(p\)-values is consistent with the projection of the shallow parts of the faults on the ground. The reason for this may be that in the shallow part of the fault plane, the fault rupture mechanism was simpler than that in the deep part, the fault plane was relatively smooth, and the stress adjustments were rapid. As a result, the aftershock activity quickly decayed, and the \(p\)-value was relatively large.