# Coseismic Stress Change and Viscoelastic Relaxation after the 2008 Great Sichuan Earthquake
## 1 Introduction
Multidisciplinary studies have explored the characteristics of these large regional earthquakes to improve seismic rupture forecasting and thus reduce earthquake disaster risk (e.g., [29; 30; 31]); these studies have focused on stress accumulation  
Figure 1: Topographic map including the distribution of the 2008 Ms 8.0 great Sichuan (Wenchuan) earthquake and five other large earthquakes. The epicenter loci of the 2008 Wenchuan earthquake and 2013 Ms 7.0 Lushan earthquake are from the USGS, whereas the other epicenter loci (2010 Ms 7.1 Yushu earthquake, 2014 Ms 6.3 and Ms 5.8 Kangding earthquakes, and 2017 Ms 7.0 Jiuzhaigou earthquake) are from the China Earthquake Networks Center (CENC). The focal mechanisms of the Wenchuan and Lushan events are from the Global Centroid Moment Tensor (CMT), that of the Yushu event is from [PERSON] et al. [18], those of the Ms 6.3 and Ms 5.8 Kangding double earthquakes are from [PERSON] et al. [19], and that of the Ms 7.0 Jiuzhaigou event is from the USGS. along fault systems as one of the main determining mechanisms and properties. The major factors causing the stress concentration in Sichuan are suggested to be the regional tectonic load of the India-Asia collision and the southward movement of the southeastern Tibetan Plateau relative to the South China block, as supported by the variation in tectonic stresses (e.g., [32; 33]). In addition, the authors of recent studies on stress transfer and fault interactions have proposed that previous large earthquakes may favor or advance subsequent earthquakes [34; 35; 36; 37; 38]. Considering that the stress redistribution induced by previous large earthquakes may change the properties of adjacent faults and promote fault rupture, such stress redistribution will ultimately lead to changes in seismicity [34; 39; 40; 41]. Hence, the stress variations caused by large earthquakes might be another significant factor in adjusting the stress state of fault systems. The authors of relevant studies have used the Coulomb stress change in a pure elastic crust to quantify whether a fault rupture was advanced by a previous earthquake with a positive Coulomb stress change or delayed by one with a negative Coulomb stress change [37; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52]. The Earth's lower crust and mantle might deform as inelastic bodies, and the external stress relaxes with time (e.g., [53]). Therefore, the upper crust is elastic at the initial stage of stress loading and continues to deform due to stress relaxation. Furthermore, because inelastic stress relaxation can also lead to a change in the stress distribution, it is necessary to consider inelastic behavior in the analysis of stress transfer and earthquake interaction. In addition, the postseismic stress changes caused by the viscous relaxation of the lower crust or upper mantle are also used to analyze the aftershock distribution, especially delayed triggered seismicity [36; 49; 54; 55; 56]. For example, [PERSON] and [PERSON] [36] pointed out that the 1999 M 7.1 Hector Mine earthquake in southern California might have been caused by the 1992 Landers earthquake (M = 7.3) through postearthquake stress changes within seven years after the Landers earthquake. [PERSON] et al. [56] analyzed a sequence of 10 magnitude Ms > 6.5 earthquakes from 1939 to 1999 and studied the evolution of the Coulomb stress field, including viscoelastic relaxation along the North Anatolian Fault; their results show that the Marmara Sea area is currently loading at the rate of Coulomb normal stress rather than due only to the stable tectonic load on the North Anatolian fault. Accordingly, both elastic coseismic deformation and postearthquake viscoelastic relaxation have important impacts on the stress state around a fault system [54; 57]. The areas of stress increase and stress decrease produced by coseismic dislocations occur alternately [39].