Current Crustal Deformation Variation Characteristics Of The Fenwei Basin And Its Surrounding Areas Revealed By Gps Data

The Fenwei basin is of great importance in research of tectonic activity and geological hazards (such as the ground fissures and earthquakes) in China. This basin is actually a deep rift that divides North China into eastern and western parts. Bounded by the Qinghai-Tibet, Ordos, North China and South China blocks, the Fenwei basin is a accommodation zone of differential motions of these blocks, and the boundaries and decoupled strips of tectonics between western and eastern North China. We focused on the current crustal deformation characteristics of the Fenwei basin and its surrounding areas, especially the influence of the 2008 Wenchuan earthquake. Meanwhile, we further analyzed the intrinsic relationship between the tectonic deformation features of the overall Fenwei basin and the multiple ground fissures within the basin.Using high-precision GPS data for the periods of 2001-2007 and 2009-2011 from the projects "Crustal Movement Observation Network of China" and "Chinese Continental Tectonic Environment Monitoring Network", we have constructed reasonable crustal deformation models of the Fenwei basin and its surrounding areas based on the strain theory of the block movement combined with mathematical statistical hypothesis testing method. We then established the crustal deformation models of the Datong, Xinzhou-Taiyuan, Linfen-Yuncheng and Weihe basins, as well as the eastern Qinghai-Tibet and the northern edge of the South China blocks. Based on the strain results solved by these deformation models, the current crustal strain fields and their distribution characteristics were obtained, and the intrinsic relationship between the principal strain vectors of the overall Fenwei basin and the multiple ground fissures within the basin were discussed in detail.The results are as follows: (1) The deformation models after selection by hypothesis statistical testing can better describe the strain distribution characteristics, especially the inherent differences in the interiors of the basins or blocks. (2) During the periods of 2001-2007 and 2009-2011, the principal strain vectors of the eastern Qinghai-Tibet block are all mainly of compressional strains in the nearly E-W direction, with a value higher than -3.8 x 10(-8)/a and -3.2 x 10(-8)/a, respectively. And the principal strain vectors of the west part of the Weihe basin are all also of compressional strains in the nearly NNW SSE direction, but with a value higher than -2.8 x 10(-8)/a and -1.6 x 10(-8)/a, respectively. (3) The whole strain distribution characteristics of the Fenwei basin and its surrounding areas have no significant change before and after the Wenchuan earthquake. The eastern Qinghai-Tibet and west part of the Weihe basin remain compressional strains. The Yinchuan basin and the North China block are still of tensional strains. And the overall Fenwei basin including Datong, Xinzhou-Taiyuan, Linfen-Yuncheng basins and most parts of the middle east of the Weihe basin are all of tensional strains in the nearly NW SE direction. (4) The strain vector fields of the whole Fenwei basin mainly show extension strains in NW SE direction, which are nearly vertical to the direction of the ground fissures (NE SW direction). It indicates that the regional tensional stress fields with NW SE direction is the power source mechanism of numerous ground fissures developed within the Fenwei basin.We obtained following conclusions: (1) Judging by the statistical test, the choice of reasonable crustal movement and strain models plays an important role in obtaining the strain parameters which can truly reflect the real regional tectonic deformation characteristics. (2) The 2008 Wenchuan earthquake has affected the strain fields in the eastern edge of the Qinghai-Tibet block and west part of the Weihe basin. However, after this earthquake these above-mentioned regions did not experience significant strain accumulation but showing lower magnitude of strain. The reasons may be that these areas were not the zones with significant increase of Coulomb stress caused by the earthquake, but in a tectonic stress relaxation adjustment period during 2009 to 2011. Furthermore, the Wenchuan earthquake did not significantly alter the overall current. tectonic deformation background characteristics of the Fenwei basin and its surrounding areas, and regional tectonic activity characteristics still have good inheritance. (3) We preliminary infer that the multiple ground fissures within the Fenwei basin were the response of surface rupture under the role of tensile stress fields with NW SE direction, based on that the tectonic blocks (or basins) have the deformation nature of continuous isotropic elastic characteristics.