Abstract: Coulomb stress change on active faults is critical for seismic hazard analysis and has been widely used at home and abroad. The Sichuan-Yunnan region is one of the most tectonically and seismically active regions in Mainland China, considering some highly-populated cities and the historical earthquake records in this region, stress evolution and seismic hazard on these active faults capture much attention.
    From the physical principal, the occurrence of earthquakes will not only cause stress drop and strain energy release on the seismogenic faults, but also transfer stress to the surrounding faults, hence alter the shear and normal stress on the surrounding faults that may delay, hasten or even trigger subsequent earthquakes. Previously, most studies focus on the coseismic Coulomb stress change according to the elastic dislocation model. However, the gradually plentiful observation data attest to the importance of postseismic viscoelastic relaxation effect during the analysis of seismic interactions, stress evolution along faults and the cumulative effect on the longer time scale of the surrounding fault zone. In this paper, in order to assess the seismic hazard in Sichuan-Yunnan region, based on the elastic dislocation theory and the stratified viscoelastic model, we employ the PSGRN/PSCMP program to calculate the cumulative Coulomb stress change on the main boundary faults and in inner blocks in this region, by combining the influence of coseismic dislocations of the M≥7.0 historical strong earthquakes since the Yongsheng M7.8 earthquake in 1515 in Sichuan-Yunnan region and M≥8.0 events in the neighboring area, and the postseismic viscoelastic relaxation effect of the lower crust and upper mantle.
    The results show that the Coulomb stress change increases significantly in the south section of the Xianshuihe Fault, the Anninghe Fault, the northern section of the Xiaojiang Fault, the southern section of the Longmen Shan Fault, the intersection of the Chuxiong-Jianshui Fault and the Xiaojiang Fault, and the Shawan section of the Litang Fault, in which the cumulative Coulomb stress change exceeds 0.1MPa. The assuming different friction coefficient has little effect on the stress change, as for the strike-slip dominated faults, the shear stress change is much larger than the normal stress change, and the shear stress change is the main factor controlling the Coulomb stress change on the fault plane. Meanwhile, we compare the Coulomb stress change in the 10km and 15km depths, and find that for most faults, the results are slightly different. Additionally, based on the existing focal mechanism solutions, we add the focal mechanism solutions of the 5 675 small-medium earthquakes(2.5≤M≤4.9)in Sichuan-Yunnan region from January 2009 to July 2019, and invert the directions of the three principal stresses and the stress shape factor in 0.1°×0.1° grid points; by combining the grid search method, we compare the inverted stress tensors with that from the actual seismic data, and further obtain the optimal stress tensors. Then, we project the stress tensors on the two inverted nodal planes separately, and select the maximum Coulomb stress change to represent the stress change at the node. The results show that the cumulative Coulomb stress change increase in the triple-junction of Sichuan-Yunnan-Tibet region is also significant, and the stress change exceeds 0.1MPa.
    Comprehensive analysis of the Coulomb stress change, seismic gaps and seismicity parameters suggest that more attention should be paid to the Anninghe Fault, the northern section of the Xiaojiang Fault, the south section of the Xianshuihe Fault, the southern section of the Longmen Shan Fault and the triple-junction of the Sichuan-Yunnan-Tibet region. These results provide a basis for future seismic hazard analysis in the Sichuan-Yunnan region.

Key words: Coulomb stress change, stratified viscoelastic model, seismic hazard, Sichuan-Yunnan region

摘要： 文中基于弹性位错理论与黏弹性分层介质模型, 考虑川滇地区及邻区历史强震的同震位错与震后黏弹性松弛效应的影响, 计算给出了1515年永胜M7.8地震以来川滇地区主要块体边界断裂带与构造块体内部的库仑应力变化。 计算结果显示, 鲜水河断裂带南段、 安宁河断裂带、 小江断裂带北段、 龙门山断裂带南段、 楚雄-建水断裂带与小江断裂带的交会处、 理塘断裂带沙湾段等断裂带上的库仑应力增加显著(≥0.1MPa)。 同时, 块体内部川滇藏交界区的库仑应力增加同样显著。 综合地震空区、 地震活动性参数及文中所给出的应力场变化结果分析认为, 安宁河断裂带、 小江断裂带北段、 鲜水河断裂带南段、 龙门山断裂带南段和川滇藏交界区未来的强震危险性值得密切关注。 文中研究结果可为川滇地区未来的地震危险性分析提供力学参考。

关键词: 库仑应力变化, 黏弹性分层模型, 地震危险性, 川滇地区