Study On Dynamics Characteristics Of GPS Crustal Deformation In He Northern Section Of North-south Seismic Belt

Earth’s interior strength could make the rock to produce inconspicuousnessdeformation gradually, however, the change could be observed by a number ofhigh-precision observational techniques. The crustal movement observationtechnology developed from the global satellite positioning system (GPS) hassuccessfully been used to detect several seismic regions in China, by which we canobtain relative movement velocities of continental blocks and a wide range of crustaldeformation motion pictures. It also provides the direct evidence and quantitativecharacteristics of the crustal movement. These techniques possess the importantsignificance to the seismic activity background research and prediction of the largeand moderate earthquakes. By using the GPS observation data of the North-Southseismic belt during the period of1999~2013, the multi-stage velocity fieldcharacteristics are analyzed in the study area. The homogeneous strain rate parametersof each block are obtained by REHSM model, and the characteristics of multi-stagestrain rate field in study area are analyzed. The strain accumulation states of the majoractive faults are studied through fault profile analysis. The baseline expansion ratebetween GPS station and the baseline dynamic change of main faults are analyzed.Eventually, the characteristics and dynamic change of the crustal movement in studyregion are studied combined with multiphase flow observation data. Finally,combined with several moderate earthquakes in recent years, the present seismicity instudy area is synthetically analyzed. The research mainly got the followingpreliminary conclusions:(1) By using the GPS observation data from1999to2013, the multi-stagevelocity field characteristics of study area are analyzed. The results show that thevelocity becomes larger from the east segment of Gansu-Sichuan border to the eastpart of North Margin of West Qinling fault after the Wenchuan Ms8.0earthquake. Itreflects that the big rupture of the Longmenshan fault caused by Wenchuanearthquake had a larger scale "unlocked" on the pushed movement from NW to SE. Itcaused the enhancement of the SE motion which associated with the NW side ofLongmenshan fault. The push effect by the Qilian block and the Qaidam block to thenorth-east reduced significantly. The push effects by the Bayan Har block to eastweaken distinctly. Recently, the movement velocities of the north border of Qilianblock and the North Margin of Western Qinling fault exist differences, hence, there may be a strong earthquake risk.(2) The homogeneous strain rate parameters of each block obtained by REHSMmodel shown that the responses of each block to the Wenchuan earthquake aredifferent in the study area. The Alashan block and the South China block graduallyrestored to the original state after years of adjustment. The principal compressivestrain rate and the maximum shear strain rate of the Bayan Har block and the Ordosblock decreased year by year, and their strain accumulation rate slowed down. Themovement and deformation status of the Qaidam and the Qilian block have obviouslychanged, but the response of the Longzhong basin tectonic zone to the Wenchuanearthquake is not obvious, which indicate that this area has a strong earthquake risk.(3) The multi-stage profile analysis results to main fault in the study zone showthat the impacts on the movement characteristics of each fault zone resulted from theWenchuan earthquake are different. Recently, the extrusion and dextral sheardeformation rate on eastern side of Zhuanglanghe fault is very small, hence it mayexist a higher strain accumulation background. The squeeze shifted to tension onnorth side of the Haiyuan fault, whereas the sinistral strike-slip and compressionaldeformation enhanced on its south side. The LiuPanShan fault zone generally showsthe compressional deformation, meantime its dextral shear deformation is very smalland extrusion rate is reduced recently, there has a background of the higher strainaccumulation and the strong earthquake preparation. The North Margin of WesternQinling fault accumulated the higher strain before the Wenchuan earthquake, but theoccurrence of the Wenchuan earthquake promoted the strain accumulation level of theNorth Margin of Western Qinling fault and its adjacent faults, and the north side ofthe fault is in a strong locked state recently. The tensional tensor of the East Kunlunfault also increased obviously, although it was recovered after the quake to someextent, its sinistral deformation is strengthened and the extensional deformation rateof south side is very small, hence, it may be in a certain state of block. The EasternFoot of the Helan Mountain is in a state of compression at present, and theright-lateral strike-slip of the east side and sinistral strike-slip of the west side amountare very small, which indicates that a certain number of strain energy may have beenaccumulated in this area.(4) The statistical results of the GPS baseline expansion rate shows that GPSbaseline had long trend movement characteristics that NE shorten, NW extension, EWshorten and SN extension, and the GPS baseline shorten or elongation rate of the western region is greater than that in the eastern region significantly. It shows that theoverall rigidity movement which the Qinghai-Tibet block is relative to the SouthChina block has obviously enhanced in recent years. This kind of large scaleenhancement of the NE crust shortening movement is advantage to the rupture ofWenchuan earthquake, especially the occurrence of the dextral shear rupture.(5) The cross-fault baseline results suggest that the north border of the Qilianblock may have accumulated a certain degree of the strain background currently. Theelongation rate of the baseline which crosses the Zhuanglanghe fault is greater, andthis region has a higher degree of strain accumulation. The sinistral slip rate of theHaiyuan fault has slowed down, which shows a degree of strain accumulationbackground may exist. Currently, the strain accumulation rate of the Liupanshan faultzone is decreasing. The North margin of Western Qinling fault was locked before the2013Minxian-Zhangxian Ms6.6earthquake. The sinistral slip rate of the East Kunlunfault has slowed down and a degree of strain accumulation background may exist aswell.(6) The analysis results to the relationship between the GPS crustal deformationcharacteristics and the strong earthquake activity shows that the occurrence ofmoderate and strong earthquake would cause the short-term relaxation and thedeformational adjustment in some regions near the epicenter, and it unload theaccumulation of strain energy overall. In addition, it may enhance the extrusionbetween blocks, and result in the enhancement of the internal strain accumulation ofthe pushed block. Finally, it becomes the conditions of the pregnant earthquake. Theadjustments after a big earthquake affect the movement rate of the large fault near theblock boundary and make the movements between both sides of the fault to bedifferences, which may cause the strong earthquake in the area nearby the fault.