【摘要】 川滇地区构造活动强烈，活动断裂众多，强震活动频繁.了解主要活动断裂的应力应变积累速率对于评估该区的长期地震危险性具有重要参考作用.本文新建立川滇地区及其主要活动断裂的三维黏弹性有限元模型，其中考虑了该区的岩石圈结构分层以及介质在水平和深度方向的不均匀性；采用1996—2018年的平均GPS水平速度场约束模型的侧边界，同时考虑川滇地块深部介质的拖曳力过程，模拟计算了构造加载作用下模型的动力学响应——区域变形特征和断裂的应力积累速率.模拟的位移场能很好拟合GPS观测的速度场，模拟的水平主应力方向在川滇块体和华南块体西缘与地震震源主应力方向相符，反映所建模型与模拟结果有较好的可信度.尝试将模拟的应力积累速率与基于GPS/GNSS观测的应变速率相结合，计算能反映区域地壳应变积累程度的弹性应变能密度分布；讨论了主要断裂带的背景应力、弹性应变能密度和库仑应力积累速率与M≥6.0地震发生的关系.结果显示除了模型边界附近的应力奇异地带外，川滇其余地区1996年以来M≥6.0地震大多数发生在模拟应力积累速率高或次高的断裂带(段),以及弹性应变能密度积累速率中偏高的断裂带(段).在模拟的背景应力速率高而且库仑应力增长快的龙门山断裂带南段和鲜水河断裂带最南段，分别在本文写作和定稿阶段发生2022年6月1日四川芦山M 6.1地震和2022年9月5日四川泸定M 6.8地震.认为研究区具有类似高或者次高的模拟应力和弹性应变能密度积累速率、但至今无强震发生和记录的断裂带(段),潜在的地震危险性需要关注.更多还原

【Abstract】 In the Sichuan-Yunnan region, China, many active faults have complex tectonic structures and relatively high seismicity. To know the stress and strain accumulation rates on these faults is significant work to assess the long-term seismic potential risk in the region. In this study, for major active faults in the region we construct a three-dimensional(3D) viscoelastic finite-element model(FEM) in which both the viscosity stratification of regional lithosphere and the heterogeneity of medium in horizontal and depth directions are considered. Using the average GPS horizontal velocity field from 1996 to 2018 to constrain lateral boundaries of the finite-element model, considering basal shear force beneath the Sichuan-Yunnan Block, the geodynamic response of the model to tectonic loading, i.e. the regional crust deformation features and stress accumulation rates associated with the these faults, was numerically simulated. For the area of the Sichuan-Yunnan Block and the western South China Block, the modeled horizontal velocity field fits the GPS observed velocity field very well, and the modeled principal stress orientations are mostly consistent with those inverted from earthquake focal mechanism solutions suggesting that both our FEM model and the simulated results have good reliability. We try to combine modeled stress accumulation rates with GPS/GNSS-observed strain rates to compute the distribution of elastic strain energy density that may reflect different degrees of strain accumulation in the crust of the region. We then discuss the connections among background stress rate, elastic strain energy density, Coulomb stress rate, and the occurrence of M≥6.0 earthquakes. The comparison shows that most M≥6.0 events since 1996 in Sichuan-Yunnan occurred on the major fault zones(or segments), where the modeled stress rates are either high or secondary high and the modeled strain energy density rates are moderate to medium-high, except for a few faults near the model′s lateral boundaries where the stress rates show “singular values”. While we were drafting and preparing this paper, two events, the M 6.1 Lushan earthquake on 1st June 2022 and M 6.8 Luding earthquake on 5th September 2022, occurred in Sichuan on the southern segment of the Longmenshan fault and the southernmost segment of the Xianshuihe fault, respectively, both of which have high accumulation rates of the modeled background stress and Coulomb stress. Our study suggests the potential seismic risk of those faults(zones or segments) with similar(high and secondary high) stress and strain accumulating rates, and so far without occurrence or record of strong earthquakes needs to be concerned.更多还原