华北地区地下水开采对地壳应力的影响

近50年来华北地区遭受持续大面积过量开采地下水,已形成区域地下水漏斗、地面沉降、地陷地裂等地质灾害.然而,地下水的抽取减小了地壳的载荷,造成地壳应力场变化,这一点至今尚未被充分认识.为探索华北地区地下水超采对地壳应力场的影响,本文建立了二维有限元模型,定量计算地下水超采引起地壳变形和应力场变化.结果表明:华北地区地下水开采会引起地表抬升达+12.4 cm;漏斗区上、中地壳的水平拉应力增量分别达到70 kPa和35 kPa;而在地下水开采区外围,水平压应力增量达20 kPa;而华北地区构造主压应力积累速率约为0.5 kPa·a-1.通过对比华北地区1980年前后5级以上地震的分布状况,本文认为地下水开采对区域构造应力场的扰动不可忽略, 其卸载过程可能对华北地区大地震孕震过程存在减缓作用.

Changes of crustal stress induced by groundwater over-pumping in North China Plain

It is well known that excessive mining of groundwater has induced surface subsidence in North China, especially in the past 50 years. While how groundwater mining unloads the lithosphere and causes stress changes is not fully understand. In this study, we set up a 2-D finite element model across a groundwater depression zone to quantitatively analyze deformation and crustal stress changes of North China Plain. The numerical results show that the maximum surface vertical uplift is 12.4 cm, extensional stress changes are up to 70 kPa and 35 kPa in upper and lower crust in the groundwater exploration area, respectively; while compressional deformation occurs at the edge of exploration zone with values up to 20 kPa. The annual tectonic stress change is 0.5 kPa. By comparing historical earthquakes (＞M5.0) in North China Plain between two periods 1945—1979 and after 1980—2015, it is preliminary concluded the influence of groundwater unloading is significant, which may to some extent reduce the risk of great earthquakes in North China Plain.