Study On The Regional Stability Dynamical Background In The Forming Of Xi'an Ground Fissures

As one of the main geological disasters in Xi'an city, ground fissures are meancing the engineering field of Xi'an city. At present, researches on the geodynamical background of the ground fissures in Xi'an city are few at present, and the relationship between the geophysical background and the geological structures in the deep is also indefinite. In this paper, under the significant project "the formational mechanism of the ground fissures in Fenwei Basin and its continental geodynamics" supportted by National Science Foundation, by using travel time data of P wave and Pn wave of earthquakes recorded by the seismic stations in Shaanxi Province and its adjacent areas, the velocities of crust in depth of 10 and 20 kilometers and the velocity image of Moho discontinuity in Weihe fault depression and its adjacent areas are determined by means of seismic tomography, the tectonic stress field in Weihe fault depression and its adjacent areas are determined by analyzing the focal mechanisms of earthquakes and by inversing the main strain rate and its direction of Fenwei Basin through analyzing the earthquake moment tensors, by using the broad-band digital seismic data recorded by Shaanxi digital seismic network, the distribution of Q values and the ambient shear stress field in Weihe fault depression and its adjacent areas are determined through coda wave analyzing and spectra analyzing, the earthquake activity is also studied in Weihe fault depression and its adjacent areas through wavelet analysis and other methods. The characteristics of ground fissures' distribution in Xi'an city and their relationship with the deep faults are also studied through the systemic analysis of the recent reconnaissance of ground fissures in Xi'an city. Under the direction of the dynamics of the regional stability theory sparkplugged by Professor Peng Jian-bing, based on the previous researches about ground fissures, combining with analysis of the results of the reconnaissance of the active faults in Xi'an city and other geophysical data, the dynamical background of the forming of Xi'an ground fissures is studied systematically, and the regional unstabilized dynamical model of the forming of Xi'an ground fissures is put forward finally.The results show that the occurrence of Xi'an ground fissures in large number is rooted in special geological environments, those are:the crust depth of the Weihe fault depression is about 10km thinner than its adjacent tectonic units such as Ordos massif and Qinling Mountains uplift, the forces from upper mantle's thermal up welling and its expanding are the dynamical sources of the tensile deformation in shallow part of the crust.②The basement in Xi'an region is separated into many block structures by numerous extensional faults, so the geological structures are broken into many parts. The developing space and stress conditions for the forming of ground fissures are provided by the movement of the tilted blocks and the extension of these faults.Among the faults, Kouzhen-GuanShan fault, Liquan-Heyang fault, Jinghe-Chanhe fault, Weihe fault, Lintong-Chang'an fault, Yuxia-Tieluzi fault and the northern border fault of Qingling, are the main faults controlling and influencing the formation and development of ground fissures in Xi'an city. Kouzhen-GuanShan fault,Liquan-Heyang fault, Weihe fault and Lintong-Chang'an fault, are the main controlling tectonics in this region, and the ground fissures are concomitant or derived with the creeping slip of these four faults.③In Quaternary period, multilevel and many-group fractures are formed in Quaternary system owing to the successive extension of Weihe basin and the coexistent extensional dislocations of these faults, and the tectonic prototypes of the tectonic fractured surface are formed then.④At present, under the durative jostling northward of the Indian plate, Gansu-Qinghai land block is crushing into the FenWei region, so a successive tensile field in the direction of NNW-SSE in Xi'an region has formed. This field is just the driving force for the movement of ground fissures.⑤The broad covering loess in this region, with collapsibility, rheology, disintegration and fracture, provides suitable solum for the forming fo ground fissures.⑥The morphosturcture separated altermatively by ridges and swales provides favorable boundary conditions for the forming of ground fissures.⑦When earthquakes occurred in Xi'an in the history, ground fissures often emerged concomitantly, these two phenomena have contemporary at a certain extent, but the ground fissures are usually coseismic or postseismic ruptures, so they are not earthquake precursor.On the whole, in the formation of ground fissures in Xi'an city, tectonic factor is dominant, so ground fissures in Xi'an city are essentially tectonic ground fissures. The mechanisms of the formation of ground fissures in Xi'an city are listed below. The ground fissures in xi'an city are pregnant from deep tectonic, they germinated with the extension of the Weihe fault depression, they are formed by the creeping of the faults, they are enlarged with the activity of regional tectonic stress, they are induced by earthquakes and they can be reopend and increased by over-extraction of groundwater and intense infiltration of surface water.