Tectonic activity is intense and destructive earthquakes occur frequently in the northern section of the North-South Seismic Belt(NSSB). After the May 12, 2008 Wenchuan earthquake, the North-South Seismic Belt enters a new period of high seismicity. On July 22, 2013, the Minxian-Zhangxian earthquake occurred, which broke the 10-years seismic quiescence of magnitude 6 of the area, indicating an increasing trend of strong earthquakes in the region. Earthquake is the product of crustal movement. Understanding the dynamics of the process of crustal movement may provide basis for earthquake prediction. GPS measurement can provide high-precision, large-scale, quasi-real-time quantitative crustal movement data, that allows us to explore the evolution of crustal movement and its relationship with earthquake, thus providing the basis for determining the seismic situation. Since 2009, the density of mobile GPS measurement stations has significantly improved in the Chinese mainland, and moreover, the Wenchuan earthquake has brought about adjustment of the regional crustal deformation regime. So the introduction of the latest repeat GPS data is important for understanding the features of crustal movement in the northern section of the North-South Seismic Belt. In this paper, we obtained the GPS velocity field, fault profile and baseline time series and analyzed the dynamics of recent crustal movement in the northern section of the North-South Seismic Belt using the 1999a-2014a GPS data of mobile and continuous GPS measurement stations. The results show that: the Qilianshan Fault has a high strain accumulation background. There are locked portions on the Liupanshan Fault, especially in the region of Jingning, Pingling, Dingxi, Longxi. In 2004-2009a, the degree of locking of the Liupanshan Fault got higher. In 2009—2013a, crustal movement on the northern section of the North-South Seismic Belt weakened compared with 1999-2004, 2004-2009, and showed some features as follows: ① The velocity field weakened more markedly near the Qilian-Haiyuan-Liupanshan faults; ②The velocity decreased more significantly in the region north of Qilianshan-Haiyuan Fault than that of the south, resulting in enhanced thrust deformation on the Qilianshan Fault in 2009-2013a and the decreased sinistral shear deformation on the Qilianshan Fault and Haiyuan Fault; ③the velocity field decreased more remarkably at 50km west of Liupanshan Fault, compared to the east region, which led to the locked range on the Liupanshan Fault extending to the range of 100km near the fault zone during 2009-2013 from the previous locked range of 50km near the fault. The GPS baseline time series analysis also reveals a number of structural features in the region: Yinchuan Graben is continuing extending, and the extension in the west is stronger than that in the east. On the southern end of Yinchuan Graben, the deformation is very small.

The 1739 M8.0 Pingluo earthquake is the largest destructive earthquake occurring on the Yinchuan plain in history. However, there are different understandings about the seismogenic structure of this earthquake. In this paper, we re-evaluate the seismogenic structure of the 1739 M8.0 Pingluo earthquake after our investigation and detailed measurement of the seismic dislocations on the Great Wall and the surrounding tableland, and also the latest results of trenching, drilling, and shallow seismic exploration are considered as well. The results show that the latest rupture event of the Helanshan piedmont fault occurred after 600~700a BP, the Great Wall built in Ming Dynasty about 500 years ago was faulted by Helanshan piedmont fault. Although the distribution of Yinchuan buried fault coincides much with the distribution of the meizoseismal area, the fault's northward extending stopped at Yaofu town, and its Holocene active segment is less than 36km in length. The latest surface rupture occurred shortly before 3400a BP. The 1739 Pingluo earthquake did not rupture the ground surface along the Yinchuan buried fault. The presence of growth strata and the non-synchronous deformation of strata near the fault demonstrate that Yinchuan buried fault did not rupture at all or there was rupture but absorbed by the loose layers in the 1739 Pingluo earthquake. Therefore, the Helanshan piedmont fault is the seismogenic structure of the 1739 M8 Pingluo earthquake, rather than the Yinchuan buried fault, and there is no synchronous rupture between two faults. The difference of location between the seismogenic structures and the meizoseismal area of the Pingluo M8 earthquake may be caused by the factors, such as fault dip, groundwater depth, basin structure, loose formations, the degree of residents gathering, so on. The phenomenon that the meizoseismal area shifts to the center of the basin of earthquake generated by faulting of a listric fault on the boundary of the basin should be paid more attention to in seismic fortification in similar areas.

Based on the discussions on the basic ideas, methods and procedures for detecting buried faults and taking the example of Luhuatai buried faults in Yinchuan Basin, the paper introduces in detail the multi-means, multi-level detection methods for gradually determining the accurate location of faults. Multi-means refer to the technical methods such as shallow seismic exploration, composite drilling section, trenching, dating of sedimentary strata samples and calculation of upward continuation of fault's upper breakpoints, etc. Multi-levels refer to gradually determining accurate location of fault at different levels with the above means.
Results of shallow seismic exploration reveal that the Luhuatai buried fault has a strike of NNE in general, dip SEE, with the dip angle between 73° to 78°. Geometrically, the fault consists of a main fault and a small north-segment fault in plane. The main fault runs along the NNE direction from Xixia District of Yinchuan City, passing through Jinshan Township to Chonggang Township, and there is a 4km or so intermittent zone between the main fault and the small north-segment fault. The small north-segment fault is 9km long, distributed between the north of Chonggang Township to the south of Shizuishan City. According to dating of sediments sampled from drill holes, the main fault can be further divided into the southern segment and the northern segment. The southern segment of Luhuatai buried fault is active in Pleistocene, while the northern segment is active in Holocene.
Shallow seismic exploration can detect the upper breakpoint of fault deeper than drilling or trenching does. If simply connecting the vertical projections of these breakpoints on the surface, there is a certain bias of fault strike. To this end, we did accurate location for the Holocene active northern segment of Luhuatai buried fault, in which upward continuation calculation is done based on the fault dip to match the upper breakpoint of fault obtained from shallow seismic exploration with the depth of the upper breakpoints obtained from drilling. Through the accurate location of the fault, we get the geometric distribution, occurrence and segmentation of activity of Luhuatai buried fault at the near-surface. Our results provide reliable basis for the safety distance from active faults for engineering construction projects in the Luhuatai buried fault area of Shizuishan City. The methods discussed in this paper for accurate location of buried active faults are of reference value for buried fault exploration in other similar cities or regions.

Yellow River Fault is the longest, deepest fault in the Yinchuan Basin, also is the eastern boundary of the basin. Because its north section is buried, its activity and slip rate remains unknown, which made a negative impact on understanding the evolution and seismic hazard of the Yinchuan Basin. In this study, a composite drilling section with a row of drillholes were laid out along the northern section of the Yellow River Fault based on the results of shallow seismic exploration near the Taole Town, where oil seismic exploration data are available. Fault activity and slip rate are obtained by measuring the age of samples of holes. The results show that the northern section of the Yellow River Fault is a late Pleistocene or Holocene Fault, its accumulative displacement is 0.96m since (28.16±0.12)ka BP, with an average slip rate of 0.04mm/a, which is significantly lower than the southern section. The activity intensity of the northern section of the Yellow River Fault is significantly lower than the southern section since Late Quaternary. In the Yinchuan Basin, the Helanshan eastern piedmont fault is the most active fault since late Quaternary, next is the Yellow River Fault, then, the Yinchuan buried fault and Luhuatai buried fault. Although the Yellow River Fault is the deepest and the longest fault, its maximum potential earthquake is magnitude 7, this seismogenic capability is weaker than the relatively shallower Helanshan eastern piedmont fault, on which occurred the Pingluo M8 earthquake in 1739 AD. Yinchuan Basin is the result of long-term activities of the four major faults, which shaped the special structure of the different parts of Yinchuan Basin. The Yellow River Fault controlled the evolution of the south part of Yinchuan Basin. The two-layer crustal stretching model can help us understand the structural deformation between the upper crust and the lower crust beneath Yinchuan Basin. Deformation of the upper crust is controlled by several brittle normal faults, while the deformation of the lower crust is controlled by two ductile shear zones. The shear sliding on Conrad discontinuity coordinates the extensional deformation of different mechanical properties between the upper and the lower crust. Yellow River Fault might have cut deeply into the Moho in Mesozoic, the tectonic activity in Yinchuan Basin began to migrate and was partitioned into several faults since the beginning of the Cenozoic, mainly in the Helanshan eastern piedmont fault. This may be the reason why the Yellow River Fault has lower seismogenic capability than the shallower Helanshan eastern piedmont fault.

Luhuatai Fault is one of the important buried tectonics in Yinchuan Basin.Based on the results of shallow seismic exploration,we conducted composite drilling section exploration and dating of the samples of borehole.Some useful data of the fault were obtained,such as the depth of upper breaking point,the latest activity age,displacement in late Quaternary,and slip rates,etc.This study shows that the activity is different between the north and south segment along Luhuatai Fault.The north segment is a Holocene fault,while the south segment is a late mid-Pleistocene fault. From north to south along the north segment of Luhuatai Fault,the activity has enhanced,and the faulting is stronger in late Pleistocene than Holocene.

The concentration of soil gas He, H₂, N₂, O₂, CH₄, C₂H₆, Rn, Hg and flux of soil gas He, H₂, CH₄, Rn, Hg were surveyed at four sites(Xiaokou,Bazhiyao,Caixiangpu and Xiaonanchuan)in the southeastern part of Haiyuan Fault.Soil-gas concentrations of more than 200 samples were obtained.The results show that the background values of N₂/O₂,Hg,Rn were 4.2,50.4ng/m³and 5.8k Bq/m³,respectively. The maximum concentrations of He and CH₄ were 65.3 and 537.7ppm,respectively,at the end of the southeastern part of Haiyuan Fault.Furthermore,soil gas He and CH₄ were intensively degassed.The maximum flux of He and CH₄ in soil gas was 6.9and 390mg m^-2d^-1,respectively.These may be caused by stress concentration at the end of the southeastern part of Haiyuan Fault.H₂ and Rn in soil gas were powerful components as indicators of location of the southeastern part of Haiyuan Fault.The maximum concentrations of H₂ and Rn in soil gas were 369.7ppm and 38.3k Bq/m³ near the middle of the southeastern part of Haiyuan Fault.The maximum fluxes of H₂ and Rn in soil gas were 5.5mg m^-2d^-1 and 828.6m Bqm^-2s^-1,respectively.These may be related with the intensive rupture of the middle of the southeastern part of Haiyuan Fault.The anomalies of Hg in soil gas at the fault were good reference indicators.The maximum flux of Hg in soil gas was 211.2ng m^-2h^-1.

In this paper,an optimized drilling exploration method,the doubling section method,was summarized after many composite drilling section explorations of buried active fault in urban areas.Operation steps of this method are as follows: Firstly,drill a borehole at each of the two ends of the drilling section to make sure that fault is between the two boreholes,then,drill the third borehole at the middle of the two holes; and secondly,confirm again the segment where the fault is and drill the next borehole in the middle of it.By repeating the similar practice,the accurate location of fault can be constrained progressively.Meanwhile,this paper also uses a quantitative indicator,the key horizon gradient between two boreholes,instead of stratigraphic throw,to determine the location of buried fault and puts forward two criterions: 1)the fault is located between two boreholes if the key horizon gradients between these two boreholes are positive and increase with depth; and 2)the fault is located where the key horizon gradients between two boreholes increase obviously relative to the previous values and that of adjacent segments,besides the increase with depth.While in contrast,the key horizon gradient in a normal fault segment decreases obviously.Application cases show that the method can determine precisely the location of buried active fault.

The eastern piedmont fault of Helanshan Mountains is an important tectonic controlling the west boundary of Yinchuan graben.The south segment of the fault locates right in the west of Yinchuan city,which has a length of about 13.2km,strikes NNE-NE and dips south-east at an angle of 50°～80°.The main part of the fault lies between the Ordovician and Quaternary systems,forming the borderline between the hills and diluvium.Parts of the segment of the fault appear in alluvial fans and are displayed as geomorphic scarps.The paper selects the region on the two banks of Dashitou channel to excavate two trenches along the fault based on 1:10,000 geological mapping of the fault area.The result reveals three events since 14ka BP with the ages of 13.8,7.9 and 3.0ka BP and the recurrence intervals of 6 and 5ka,respectively.

This paper introduces the result of exploration of the Yinchuan buried fault using the composite drilling section method. As one of the main buried faults in Yinchuan plain,the Yinchuan buried fault has restricted seriously the development of Yinchuan City for a long time due to its indistinct location and unclear activity property. So the Yinchuan buried fault was taken as one of main tasks of active fault exploration in Yinchuan City. Most of shallow seismic explorations had been done before the drilling. However,due to the limited precision of shallow seismic exploration,the actual location of the Yinchuan buried fault can't be explored. For obtaining the information about the location and the depth of the upper break point, the active time and slip rate of the Yinchuan buried fault,three composite drilling sections,Xinqushao, Manchun and Banqiao,were laid out along the Yinchuan Fault based on the result of shallow seismic exploration. After comparing with the marker horizons disclosed by drilling,the position,scale and the depth of the upper break point of Yinchuan buried fault were found,and the buried active fault was located precisely. From the exploration result we get the apparent dip of the Yinchuan buried fault as 71 degrees at Xinqushao,71 dgrees at Manchun and 66 degrees at Banqiao,and the depth of the upper break points as 5.18～8.30m,5.01～6.50m and from 10.0～13.59m,respectively. Therefore,the latest active date of the Yinchuan buried fault is determined and the question whether the fault is active or not is answered by dating. The Yinchuan buried fault at Xinqushao and Manchun sections is manifested as a Holocene active fault, and at Banqiao,it is shown as a late Pleistocene active fault. The slip rate of the Yinchuan buried fault since late Pleistocene is 0.14mm/a at Xinqushao,0.05mm/a at Manchun and 0mm/a at Banqiao. Based on the result obtained from seismic exploration and the spatial positions of the three composite drilling sections,we draw the following conclusions:the Yinchuan buried fault can be divided into two segments with Yingu Road as the boundary; the northern segment was active in Holocene and the southern one was active in late Pleistocene; the activity of the northern segment is more recent than that of the southern one.

The newest active time and segmentation of the fault are of special significance in the safety evaluation of major engineering projects.This paper discusses the active times and segmentation characteristics of the Guanguanling Fault zone through interpreting aerial photos,field investigation,topographic and geomorphic surveys and analysis of trench logs on paleoearthquake in connection with the study of the geological and seismologic problems in Heishanxia project of the Huanghe River.The Guanguanling active fault zone lies on the northeastern margin of the Qinghai-Tibet block.It is part of the Zhongwei-Tongxin arc active fault zone,striking near EW generally,with a total length of about 60km.It consists of 5 discontinuous secondary faults arranged in left step en echelon,namely,Jingtaixiaohongshan(F_1-1),Guanguanling(F_1-2),Shajing(F_1-3),Zhongweixiaohongshan(F_1-5)and Qingshan-Gushanzi(F_1-4),respectively.Since Late Quaternary,the fault is characterized with intense sinistral strike-slip and compressional thrust and has offset a series of ridges and small gullies and terraces.At the same time,fault scarps were developed along the fault zone.The study reveals that the latest earthquake occurred 700～1200a BP,the largest displacement took place in Guanguanling,and the maximum horizontal sinistral displacement reaches 6m since Holocene.

The project of "Yinchuan active fault exploration and earthquake risk assessment" is aimed at strengthening the fundamental work of engineering construction for earthquake resistance and prevention in Yinchuan City.In order to achieve this general goal,a joint multi-disciplinary exploration of the Yinchuan buried fault and the Luhuatai fault has been carried out on the basis of collection and analysis of predecessors'data.This paper discusses the methods of constructing three-dimensional subsurface structure model of the project area of Yinchuan active fault exploration by using the achievements of the Yinchuan active fault exploration,and puts forward main work steps and suggestions for the model construction.The steps for 3D modeling of subsurface structure using active fault survey data are as follows:To collect and pre-process the data of seismic exploration of active fault;Using seismic interpretation software to interpret the fault and the bed position,draw bed plane structure map,and output the fault and bed position data,meanwhile,to further complete the 3D visual modeling with this software;To load fundamental data,bed plane and fault data into the 3D modeling software GOCAD,carry on the fault plane and the bed plane revision and reconstruct 3D model,construct three-dimensional cutting section,and demonstrate the three-dimensional model achievements.Since the seismic survey data are not standard,it increases the difficulty in earthquake data explanation and affects the explanation precision.Therefore,we propose to standardize the archiving of primary seismic survey data,add a cross line in the survey area of each active fault,and if possible,use the regional time-depth conversion used by petroleum sector to enhance the explanation precision of seismic section.

Yinchuan Basin is a graben-like downfaulted Cenozoic era basin located on the west edge of Ordos Massif.Its activity is violent and deposition is very thick.Yinchuan City is located in the middle of Yinchuan Basin.The seismic petroleum exploration shows that a buried active fault lies in the east of Yinchuan City,named as the Yinchuan buried fault,which strikes NNE and dips west,with a total length of more than 80km.Because the seismic petroleum exploration did not gain any explained signals at the depth ranging from 0 to 400m,so whether the Yinchuan buried fault is active or not in the late Quaternary and its exact surface projective location hasn't been known yet.It has been a “worry” in the urban planning and development of Yinchuan for a long time.Under the financial support of the national and local governments,we launched the project entitled “The prospecting of active fault and earthquake risk assessment in Yinchuan City”.In order to facilitate the exploration,we selected Xinqushao village in the southeast suburb of Yinchuan City to be the site for the integrated test exploration of the Yinchuan buried fault before the exploration,based on the information obtained from the seismic petroleum exploration.Considering that the thick Quaternary sediment in Yinchuan reaches to 1609m,and that the depositional environment is the Yellow River flood plain and the lateral change of lithology is complex,we adopted in the test exploration the train of thoughts of “inferring an unknown fact from a known fact,and from deep to shallow and directly to the top”.The experimentation has been developed step by step according the working order of multilevel seismic exploration→composite geological profile drilling→trenching.Along the same measuring line at Xinqushao,first,we adopted the seismic reflection exploration of primary wave in three levels with the group interval of 10m→5m→1m to catch the master fault of the Yinchuan buried fault,and by tracing upward layer by layer in the order of the three exploration ranges,i.e.1400～400m→600～80m→150～20m,the position of the master fault at ±20m depth under the ground and its offset trace were primarily identified.And then,along the master fault and within the range of 100m at its both sides,9 boreholes of 20.5～100m were arranged for the composite geological profile drilling.The resulting information about the throws of the master fault was obtained,they are 20.34m,9.66m and 2.25m respectively at the depth of 43.75m,20.33m and 13.04m from the ground,and the buried depth of the upper offset point ≤8.34m.At the same time,using the intact core specimen from the fault plane of the borehole No.7,we calculated the dip angle of the fault as 71°at the depth of 55.27m and figured out the exact position of its extension to the earth's surface.Finally,a large-scale trial trench,which is 40 meters long,8～12 meters wide and 6 meters deep,was arranged across the master fault.The trenching revealed that the actual buried depth of the upper offset point of the master fault is 1.5m and there are seismic remains,such as offsets of 5 stages,sand liquefaction and surface rupture,etc.Among the 5 stages offsets,4 events occurred prior to 3170±80 a BP,belonging to the mid to late Holocene paleo-earthquakes.The age of the last event cannot be determined and it is inferred to be the result of the M8.0 Yinchuan-Pingluo earthquake in 1737.In a word,through the comprehensive test exploration,we find that the Yinchuan buried fault is a Holocene active fault,which lays solid base for the next exploration.