# Coseismic Stress Change and Viscoelastic Relaxation after the 2008 Great Sichuan Earthquake
## 3 Results
### Long-Term Evolution of Stress Changes  
After a large earthquake, the changes in coseismic stress are further affected by the relaxation of the upper mantle and the viscous lower crust, which transfers the stress from these regions up to the middle crust and outward to a wider area. To quantify the potential stress changes on active faults in the study area, the cumulative changes in Coulomb stress are simulated after the six events (Figure 8). Figure 8 shows the stress changes caused by viscoelastic relaxation over the past 10+ years. The evolution of postseismic stress changes after earthquakes can be tracked systematically. The cumulative stress changes caused by these events show complex distributions. The simulated stress changes produce both positive and negative stresses on the source faults near these major events. Figure 8: Long-term evolution of stress changes induced by all the previous earthquakes with viscoelastic relaxation effects and resolved at the depths of the mainshocks with \(\mu^{\prime}\) = 0.6: (**a**) immediately before the 2010 Ms 7.1 Yushu earthquake, (**b**) immediately after the 2010 Ms 7.1 Yushu earthquake, (**c**) immediately after the 2013 Ms 7.0 Lushan earthquake, (**d**) immediately after the 2014 Ms 5.8 Kangding earthquakes, and (**e**) stress changes calculated on 1 January 2020. The black star indicates the epicenter of the 2008 Ms 8.0 Wenchuan earthquake. The green star denotes the 2010 Ms 7.1 Yushu earthquake. The orange star represents the 2013 Ms 7.0 Lushan earthquake. The gray and pink stars indicate the 2014 Ms 6.3 and Ms 5.8 Kangding earthquakes, respectively. The blue star represents the 2017 Ms 7.0 Jiuzhaigou earthquake. Immediately before the 2010 Yushu earthquake (Figure 8a), the positive and negative lobes of stress changes were similar to those previously observed (Figure 3a), but the positive and negative lobes were enlarged or reduced slightly around the 2008 Wenchuan ruptures. The slight change was that the average stress level increased due to nonuniform viscoelastic relaxation over two years. Immediately after the Yushu earthquake in 2010 (Figure 8b), the negative stress shadow of the Yushu earthquake enlarged the previous negative stress change in the middle of the study area. However, in the northwest, the Yushu earthquake caused a series of changes in the positive and negative stress lobes, although the changes in stress were not large enough to disturb the stress distribution in the eastern area controlled by the Wenchuan earthquake. The 2013 Lushan earthquake (Figure 8c) had just occurred, and the stress field in the southern part of the study area (\(\sim\)30\({}^{\circ}\) N) increased, forming a wider positive stress change area. The positive stress change zone also changed the stress in the southern section of the Xianshuihe fault. To the south of the Xianshuihe fault (\(\sim\)30\({}^{\circ}\) N), due to the Lushan event in 2013 and nonuniform viscoelastic relaxation, the change in positive stress was further expanded. After the two Kangding earthquakes (Figure 4d), the stress distribution in the southern segment of the Xianshuihe fault system (\(\sim\)30\({}^{\circ}\) N) changed slightly. The stress reduction areas on both sides of the Longmenshan fault were further strengthened. Those in the northern part of the study area (\(\sim\)34\({}^{\circ}\) N) also increased due to viscoelastic relaxation, forming a wide positive area. The stress pattern in Figure 8 evolved as a result of postseismic relaxation of the lower crust and upper mantle. The postseismic relaxation effect leads to the increased \(\Delta\)CFS near the northern and southern tips of the Longmenshan fault. At a distance of less than 300-400 km from the fault, the relaxation effect dominates because the shadow obviously extends outward. However, the relaxation effect tends to be reloaded near the fault zone. Comparing the coseismic and postseismic stress changes reveals that the Wenchuan earthquake caused more coseismic stress changes than other earthquakes.