陕西渭河盆地地裂缝的构造机理

The tectonic mechanism of ground fissures in Shaanxi Weihe basin

分析研究了渭河盆地及邻区介质品质因子分布、震源机制解与应力场和形变场特征。研究结果表明,渭河盆地地裂缝,特别是渭河盆地东部的西安地裂缝,存在明显的深部构造背景:1在渭河盆地的东部地区存在结构较为松散、介质强度较弱、近北东向展布的巨厚沉积层,渭河盆地东部的西安地区位于地震波低Q值区的梯度陡变带边缘和第四系沉降梯度陡变带上,两梯度陡变带的延伸方向均接近北东向,与临潼—长安断裂的展布方向一致,也与西安地裂缝的延伸方向相同,表明此区域深部的地质结构为地裂缝的形成提供了构造基础。2震源机制解的研究结果表明,渭河盆地处于NNW向水平拉张应力场的作用下,容易产生NE—NEE向的正断层活动,地表则表现为以垂直沉降和水平张开活动为主,呈NEE向展布的地裂缝。3形变测量结果表明,盆地中部与南北两侧相比,张应变较大,为地裂缝的形成提供了更为宽松的应力环境。

The distribution of Q values,the focal mechanisms of the earthquakes after 1965 with magnitudes greater than 3. 0 and the characteristics of the stress and deformation field in Weihe basin and its adjacent areas were studied in this paper. The results show that in Weihe basin,especially in the eastern part of the basin,such as the area of Xi＇an city,the ground fissures are rooted in the deep geological structures. The results show that in the eastern part of Weihe basin,there lies loose and weaker sedimentary layers with very large thickness which extended nearly in the direction of NE. Xi＇an area lies in the eastern part of the basin with low Q value,and also lies in the edge of the abrupt gradient belt of Q value and the sedimentation gradient belt of Quaternary system. These two gradient belts all lie in the direction of fairly NE,this direction is consistent with the strike of Lintong-Chang＇an fault and the extended direction of the ground fissures in Xi＇an city. These facts show that the ground fissures in this area are rooted in the deep geological structures. The mechanisms of the earthquakes in Weihe basin and its adjacent areas show that the main stress is horizontal tensile stress with NNW direction in this region. Under this stress field,in Weihe basin,the normal faultsgenerate easily and behave actively with the direction of NE-NEE,and in the area near the surface of the earth,the ground fissures emerge with vertical subsidence and horizontal extension,so the direction of these ground fissures are also NEE. The results of deformation survey in this region shows,compared with the northern and southern edges of Weihe basin,the tensile strain in the central part of Weihe basin is higher,it is easier for the forming of ground fissures there under this stress field. Thus,the ground fissures in Weihe basin mainly emerge in the central parts of the basin,such as Xi＇an,Xianyang,Lintong and Sanyuan.