西宁市活断层地震构造模型初步研究

A PRELIMINARY STUDY ON SEISMOTECTONIC MODEL FOR THE ACTIVE FAULTS IN XINING URBAN AREA

在西宁市城市活断层探测的基础上,对城市活断层与地层褶皱变形的关系进行了研究,认为湟水河断层和NW向的湟水河北岸断层为背斜顶部张性断层,南川河断层为褶皱不均匀变形形成的横向撕裂断层,北川河东岸断层为向斜核部或翼部发育的挤压断层。并根据褶皱变形平衡剖面和重力资料反演,推测了褶皱变形滑脱面深度,初步建立了西宁市城市活断层的地震构造模型。依据地震构造模型,对城市活断层的发震能力进行了讨论。

The Xining basin is a Cenozoic basin bounded by the Riyueshan Fault and the Lajishan Fault on the south, the Dabanshan Fault on the north. Controlled by these active boundary faults and under the near NE-oriented compressional stress, fold deformation occurred inside the basin. Several active faults of NE or EW trending were detected. Nevertheless, all of them are small and weakly active. By the study on the relationship between fault and folding, we think that the Huangshuihe Fault and the NW-strike north bank of Huangshuihe Fault are tensional faults developed on the apex of an anticline ; the Nanchuanhe Fault is a transverse tear fault resulting from differential folding on the two sides of the fault; and the east bank of Beichuanhe Fault is a compressional fault developed on the core or limb of an syncline. By the balanced cross-section analysis of fold deformation and inversion of gravity anomaly data, we obtained that the depth of detachment plane is about 4 -5km. Thus, based on the above data, the seismotectonic model of the Xining urban area is built. We conclude that the faults of the area are the secondary faults related to folding of overburdens and have no potential of producing earthquakes larger than magnitude 6 for their small scale and shallow depth.