华北盆地为我国板内地震多发区域,历史以来相继发生多次破坏性大地震.前人地震勘探与震源定位结果揭示了华北地震的空间分布特征:横向上,华北地震基本发生在地壳的薄弱地带(Moho面上隆),或者地壳厚度的急剧变化带；纵向上,华北地震在地壳一定深度范围内呈现成层分布特征；主震一般在上地壳底部9～15 km深度范围,余震多发生在大约深5～25 km的上地壳与中地壳范围内,在中地壳下层与下地壳中仅有少量或者鲜见有余震发生.为研究解释华北盆地地震空间分布的以上特征,本文建立了华北盆地岩石圈三维黏弹性有限元模型.震源机制和GPS反映华北盆地处于NNE最大主压应力方向挤压,因此对模型边界施以恒定的位移速率边界条件；数值模拟华北岩石圈各层位在数百年以上长期匀速构造挤压作用下的应力积累特征,分析了华北地震空间分布与构造应力积累速率的关系,探讨了地壳结构与地壳分层流变性质对地壳应力积累的影响.计算结果表明,Moho面的隆起与地壳各层位岩石介质的黏滞系数是华北盆地地震孕育的重要因素.华北盆地在构造挤压的持续作用下,Moho面隆起处产生明显应力集中现象.该区域应力在长时期的积累过程中,在脆性的上地壳与中地壳上层,应力表现近于线性增长趋势,上地壳底部较其它深度有最大的应力增长率,其主震可以在应力积累至岩石破裂强度时发生；在脆、韧性转换的中地壳下层,应力增长速率次之,华北地震的大部分余震可能在该层位为主震所触发；而在柔性的下地壳应力增长近于指数形式,稳定状态之后其应力增长速率近于零,而鲜有地震发生.地壳各层位的应力增长率差异与地震成层分布的现象揭示了华北地壳的分层流变性质:脆性(上地壳)-较弱脆性(中地壳上层)-较弱韧性(中地壳下层)-较强韧性(下地壳)-韧性(岩石圈上地幔)的分层流变结构.%Many devastating intraplate earthquakes, such as the 1966 Xingtai earthquake and 1976 Tangshan earthquake, occurred in the North China basin. Previous studies have shown that earthquake occurrence is related to the fluctuating Moho surface. Large earthquakes usually occur in the regions where crustal thickness is relatively thin (Moho surface upheavals) or the gradient of Moho depth is large. It is also noted that main earthquakes usually take place at depths ranging from 9 to 15 km, and aftershocks are located at depths ranging from 5 to 25 km near the bottom of upper crust and in the middle crust. This study aims to explain this earthquake spatial distribution in the North China basin through 3-D numerical modeling based on visco-elasticity with boundary conditions of steady compressional deformation rates constrained by GPS observations. We calculate the long-term stress accumulation in the lithosphere of the North China basin. The results show that the Moho surface upheaval and the viscosity in different layers of the crust determine the stress accumulation in the crust of the North China basin. It is also shown that the highest rate of stress concentrates at zones where either Moho depth is relatively small or the gradient of Moho depth is large. Stress increases almost linearly in brittle upper crust and the upper layer of middle crust. The largest stress rate is located at the bottom of the upper crust where stress may reach the rock failure strength and generate main shocks. Stress in the ductile lower layer of the middle crust and the lower crust increases exponentially to a steady level. Aftershocks in the middle crust could be triggered by the main shock in the upper crust. Our results support that the rheological structure of the lithosphere beneath the North China basin presents a layering style as follows: more brittle (upper crust)-brittle (upper layer of middle crust Mess ductile (lower layer of middle crust )-more ductile (lower crust)-ductile (mantle lithosphere).
展开▼
