Abstract:The characteristics of seismogeology,local tectonics,geophysical field background and seismicity in Tianshui earthquake area located on the northern part of South-North Seismic Belt were demonstrated briefly.The questions answered in DSS were further defined.

Abstract:In this paper it is described that the observation system for deep seismic sounding (DSS) in Tianshui earthquake area.The main profiles include crossing length wises with 9 shots.The length of E-W profile is about 460 km and N-S is 240 kin.In order to study the crustal layering structure and the Moho depth in the both side of E-W line.two shot points had been desigaed as a fan profiles at the correspond area,such a net observation system can obtain more information about crustal deep structure.In sum we get.actual records up to 1300.

Abstract:The shallow crust structure beneath Tianshui earthquake area has been studied by the Pg data of seven inversion groups aquired in Chengxian-Xiji and Lingtai-Amuquhu deep seismic sounding profile.On the premise of careful phase analysis,the shallow structure and velocity distribution of layer above basement interface is obtained by applying classical methods of time section and arithmetic mean.The result show that the average velocity of sedimentary cover above basement interface in this area is 4.17km/s.The speed of basement is 5.84km/s.The sedimentary cover at western of Wushan shot is lower with average velocity 3.88 km/s,interface velocity is 5.59 km/s.The interface velocity at south of Qinan shot is higher with velocity 6.26 km/s and average velocity of sedimenary cover is 4.21 km/s.The average buried depth of basement in Tianshui earthquake area is 1.9 km.The most shallow point is in the vicinity of Chengxian.The deepest point is in the vicinity of Lingtai,the buried depth of basement is about 5.0 km.There are many fractures which dissect basement in this area,especially in western of Lingtai and in the vicinity of Qinan and Tianshui are much many with large faults throw.

Abstract:Based on various kind of geological maps and tables,the stripping stratum method (SSM) had been adopted to bring to light the degree of fluctuation of top of metamorphosed basement in the E-Wand N-S profiles of DSS which are located on the eastern of province Gansu by means of the predecessors data.The sedimantary construction and the features of major faults across both profiles were sumed up and discussed.

Abstract:Using the recording data of the lengthwise profile (from Lingtai to Amquhu) and processing corrected traveltime depend on the station's height and the loessal thickness,the 1-D and 2-D crustal velocity structure in the area was obtained.Along the line in the west the crustal thickness is about 51 km and in the east only 44 km.The eastern of profile is the structure of Erduos Platform style.Guguan town is located in the boundary between geosyncline area and platform area.There is a P-wave lower velocity layer (LVL) which embeds in 15-20 km between Wushan and Zhangjiachuan.Combining the seismic wave's dynamical parameters of observed data,some over-Moho faults,containing the Western Fringe Fault of Erduos Platform-Liupanshan Mountain,the Northern Fringe Fault of western segment of Qinling Mountain and Wushan Fault with N-S direction,have been determined.

Abstract:The Chengxian-Xiji profile of DSS in Tianshui earthquake area is from Chengxian country of province Gansu to Xiji country of Ningxia Hui Autonomous Region,the total length of profile is 242km with 4 shots.The crust can be divided into four layers,sedimentary,upper crust,middle crust and down crust.The deep of Moho increase gradually from south to north,and it rises in the vicinity of Tianshui-Qinan.The low velocity layer (LVL) was found in the top of middle crust.The northern fringe fault of western segment of Qinling Mountain across to the profile is a over-crust fault and it is a boundary fault between Qilian and Qinling Geosyncline.

Abstract:From the data of DSS in Tianshui earthquake area in 1986,a great deal of observational recordes on three fanprofiles with eight shots had been obtained.Designing these fanprofiles aim at distinguishing the Moho feature and fault distribution in this area.According to the interpretated result,we found that the thickness of crust is deeper in north than that in south,in west than that in east.We found not only the Northern Fringe Fault of western segment of Qinling Mountain,whose strike is near EW direction,but also the Tanchang-Tongwei and the Xihe-Jingning NNE deep buried faults.In this area,there are a series parallel deep buried faults on near SN direction and they are segres with the arrangement of M > 7 earthquakes in the area.

Abstract:Several rules to distinguish S-wave phase of vertical records in DSS and a method to inverse simultaneously crustal structure by P-and S-wave have been put forward and sumed up.The crustal structures of Lingtai-Amuquhu profile from Zhangjiachuan shot and Huixian-Lixian profile from Changba shot located on the northern part of South-North Seismic Belt were inversed according to this method,the medium parameters of crust were calculated.The results indicated that the results obtained from simultaneous inversion of P-and S-wave are believable.

Abstract:The experiment results about the excitation and reception,effect measurement window,splitting,relationship between polarized anomaly,anisotropy and principal stress direction of S waves in Tianshui earthquake area have been given in this article.A common hypocenter of the explosion located under the water-bearing bed can excites stronger S waves.The S waves can be received not only from Moho but also from other interface using three component geophone.The optimal window of Sm in this experiment is about 170kin and the obvious S-splitting and polarization anomaly exist there.The predominant polarized direction of faster S-wave is near E-W which is basically agree with the maximum principal compressive stress direction (P-axis),the slower S-wave is near N-S which across modern maximum principal compressive stress direction.

Abstract:Based on the interpretation of DSS in Tianshui earthquakes area,the features of basement,deep velocity structure,fault and tectonics are analysed and sumed up.More profound,systematic and comprehensive understanding about this area from shallow to deep,from geophysics to geology,from crustal structure to tectonic and its formation are obtained.

Abstract:Based on the interpretation of DSS in Tianshui earthquake area,the preliminary understanding on the relationship between deep tectonics and formation of bigger earthquakes has been got.Bigger earthquakes are related to tectonics for M>6,related to across of tectonics for M > 7.Many stronger earthquakes have happened at the location where the deepth of Moho variates obviously or the Moho upheaves.Beneath the location where stronger earthquakes have happened there are low velocity layer (LVL) in crust and low velocity zone (LVZ) on upper mantle.Above preliminary study may be help us to comprehend the cause of earthquakes in this area.