The Anqiu-Juxian Fault is the latest active fault in Tanlu fault zone, which is also the seismogenic fault of Tancheng M8.5 earthquake in 1668. In order to probe the shallow structure and the characteristics of faults in the eastern graben of Tanlu fault zone, we applied the high-resolution shallow seismic reflection method with multifold overlaying and stacking. In addition, we laid out two shallow seismic reflection lines across the Anqiu-Juxian Fault and the eastern graben of Tanlu fault zone. The shallow seismic profiles clearly reveal the stratigraphic interface morphology and shallow fault characteristics. The results show that the eastern graben of Tanlu fault zone is a graben basin consisting of multiple faults, and the thickness of Quaternary strata and graben structure characteristics are obviously affected and controlled by Changyi-Dadian Fault F₁ and Baifenzi-Fulaishan Fault F₂. Also, the eastern and western sides of the graben are the basement uplift areas, and the sediment thickness of the Quaternary strata in uplift areas is less than 30m. There are thick Cenozoic strata deposited in the barben, the stratigraphic morphology changes greatly laterally, showing an inclined form which is shallow in the west and deep in the east, and the Cenozoic strata are in angular unconformity contact with the overlying strata. The deepest part of Quaternary strata in the graben is located near the horizontal distance of 7400m, and its depth is about 190m. The Anqiu-Juxian Fault revealed by the shallow seismic reflection profile is composed of two branch faults dipping in opposite direction, which merge into one fault in the deep section. According to the discernible buried depth of the upper breakpoints of these faults and the characteristics of the Quaternary activity, the activity of Baifenzi-Fulaishan Fault on the western boundary of the eastern graben of Tanlu fault zone is relatively weak and the discernible depth of the upper breakpoint is 53m, we infer that the Baifenzi-Fulaishan Fault is a pre-Quaternary fault. The Changyi-Dadian Fault on the eastern boundary of the eastern graben of Tanlu fault zone not only cut the bedrock’s top interface, but also revealed signs of dislocation since Quaternary. The discernible depth of the upper breakpoint of Changyi-Dadian Fault is about 26~33m. The Anqiu-Juxian Fault is the latest active fault in the study area, which possess the characteristics of large scale and large penetration depth. The fault controls the deposition of the Cenozoic strata in the graben and plays an important role in the formation of the the eastern graben of Tanlu fault zone. The discernible depth of the upper breakpoint of Anqiu-Juxian Fault is about 17~22m. Therefore, we infer that the active ages of Changyi-Dadian Fault and Anqiu-Juxian Fault are the late Pleistocene and Holocene, respectively. The research results can provide seismological evidence for further understanding of activity mode and activity age of the seismogenic fault of the 1668 Tancheng M$8\frac{1}{2}$ earthquake, as well as the near-surface characteristics and activity of the Banquan segment of the Tanlu fault zone.

The eastern marginal fault of Daxing Uplift is located in the southeast of the Beijing Plain, which is a boundary fault that controls the Daxing Uplift and the Langgu Sag. It intersects obliquely with the NNE-trending Xiadian Fault in the north where a magnitude 8 earthquake occurred in 1679. The overall strike of the fault is northeast, dipping southeast. Previous studies have suggested that the youngest stratum of the fault is the Mid Pleistocene of the Quaternary and it is not an active fault since the Late Quaternary. Based on high-precision shallow seismic exploration data, this study carried out high-density composite drilling geological section surveys and obtained evidence of obvious activity of the fault since the Late Quaternary. The fault is shown as an active normal fault in the composite drilling geological section. The top of the footwall of the fault is the 7m-thick silty clay marker layer buried at the depth of 74m and the top of the hanging wall is 102m deep, the amount of dislocation is about 28.0m. Fault slip surfaces were found in the cores of two of the boreholes, with depths of 54.2m and 39.4m, respectively. The buried depths of the top surface of the marker layer in the two boreholes with a horizontal distance of 2m are 8m and 10m, respectively, the dislocation amount is 2m. Combined with the observation of core deformation characteristics of the two boreholes, it is believed that the buried depth of the upper breakpoint of the fault may be shallower. This research has changed the understanding that the fault zone on the eastern margin of the Daxing Uplift is not active. This new discovery not only has great application value for understanding the risk of large earthquakes of this fault zone and the risk of earthquake disasters in Beijing, but also has scientific significance for the study of fault development and evolution and the deep-shallow coupling process in North China since the late Cenozoic. The main knowledge gained is as follows: 1)Through high-precision shallow seismic exploration, it is found that the Neogene and above strata in the study area generally show an inclined morphology which is deep in the south and shallow in the north. The strata below the Neogene are in angular unconformity contact with the bottom interface of the Neogene, and the depth of the shallowest upper breakpoint is about 38~43m. 2)The combined drilling geological section exploration reveals rich dislocation information of stratigraphic markers and further confirms the existence of active faults by borehole stratigraphic correlation. In the drill cores, fault slip surfaces were observed in the late Pleistocene strata at the depth of 39.4m, 51.5m and 54.2m, respectively. The stratigraphic comparison of the boreholes 5^# and 9^# with a hole spacing of 2m further reveals a fault throw of about 2m in the stratum at the buried depth of 8~10m, thus, it is inferred that the depth of the upper breakpoint on the fault may be 8m or shallower. According to the stratigraphic age data of adjacent boreholes in this area, it is considered that the fault is a Holocene active fault. The specific age of the latest activity and its activity parameters will be further studied through the subsequent borehole chronological tests and large-scale trench excavation.

The study area is located at the junction of the northern margin of the Qinling orogenic belt and the southern margin of the North China Block. In order to study the fine crustal structure and the deep-shallow structural features of faults in this area, we conducted deep seismic reflection profiling with the seismic profile of 100km long, directing NE-SW in Zhumadian City, Henan Province, and got clear lithospheric structure images along the profile. As regards the data acquisition, we applied the geometry of 25m group interval, 1000 recording channels and more than 60 folds. Seismic wave exploding applies the 30kg shots of dynamite source with the borehole depth of 25m. The shot interval is 200m. In data processing, we focused on improving the signal-to-noise ratio. Data processing methods mainly include first break removal, tomographic static correction, abnormal amplitude elimination, amplitude compensation, pre-stack denoising, surface consistent deconvolution, velocity analysis, several iterations of the residual static correction, dip moveout, post-stack time migration and post-stack denoising, etc. The profile with high signal-to-noise ratio was obtained. The reflection wave group characteristics is obvious in the crust, which reflects abundant information about geological structure. Along the profile, the crust is characterized by double-layer reflection structure, and the Moho surface is composed of a series of laminated arc-shaped strong reflections. The thickness of the upper crust is about 14.8~20.7km, and the total thickness of the crust is about 32.0~35.1km. The upper crust is dominated by the inclined, densely stratified or arc-shaped reflections. The lower crust is dominated by arc-shaped and inclined reflection, and there is a reflective transparent zone under the Moho surface. The reflection sequences with different directions and shapes in the upper crust constitute the nappe structure in southwest segment and the structural model of two concaves with one uplift in NE segment, which correspond to the north Qinling nappe, Zhumadian-Huaibin depression, Pingyu-Xiping uplift and a secondary depression, respectively. There are abundant arc-shaped reflection sequences in the lower crust, which may represent multi-stage magmatic activities. The deep seismic reflection profile shows that faults in the upper crust are well developed. According to the characteristics of reflected wave field in the profile, four groups of fault structure which contain ten faults with different scales are interpreted. Among them, faults F_P1, F_P2 and F_P3 constitute the thrust fault system in the northern margin of Qinling Mountains, and F_P5 and F_P7 are boundary faults of Zhumadian-Huaibin depression. These faults are all developed within the upper crust. In addition, the Fault F_PM is a large fault that cuts through the lower crust and Moho surface. The deep seismic reflection profile reveals the crustal structure and deep-shallow structural features of faults at the junction of the northern margin of the Qinling orogenic belt and the southern margin of the North China block, which provides seismological evidence for the analysis of structural differences, the deep earth's interior processes and deep-shallow structural relationships between the Qinling-Dabie orogenic belt and the southern margin of the North China block. The lower crust of the study area is divided into two parts by deep faults that dislocate the Moho surface. These two parts have distinct reflective structures, suggesting that the area has experienced intense complex tectonic movements. The faults in the upper crust control the formation of basin-mountain structure and stratigraphic deposition of this area. And deep faults in the crust that disrupt Moho surface create conditions for the upwelling and energy exchange of deep materials. All of these have regulated the composition of material and the distribution of energy in the crust. The deep faults cutting through the lower crust and Moho surface and the south-dipping arc-shaped and inclined strong reflection sequences developed in the lower crust should indicate the large-scale subduction of the southern margin of the North China block towards the south-trending Qinling orogenic belt.

The Zhuyangguan-Xiaguan fault is a major fault in the Nanyang Basin. Together with the the Shangxian-Danfeng fault in the south and the Tieluzi fault in the north, it serves as the north boundary of the East Qingling Mountains, as well as the dividing line between North China and South China blocks. This work studied the spatial extension, activity and shallow structure of Zhuyangguan-Xiaguan Fault by combination of shallow seismic exploration of three profiles across the fault and a composite drilling cross-section data.
The anti-interference and high resolution shallow seismic reflection exploration method based on Vibseis techniques was used in the seismic survey. The results show the existence of the main fault and its southern branch. It can be determined that the the Zhuyangguan-Xiaguan fault is a NWW-trending normal fracture. The composite drilling cross-section reveals that the buried depth of the fault's up-breakpoint is about 17.6 to 20.5 meters and the latest active time is the late Middle Pleistocene.
As one of the major buried faults in the Nanyang Basin, the Zhuyangguan-Xiaguan fault has restricted the development of Nanyang City for a long time due to its unclear location and activity characteristics. The results of this study can provide geological and geophysical evidence for seismic risk assessment and site selection for the major lifeline projects in Nanyang City.

To test the effect of three-dimensional seismic reflection methods used in active fault survey, we have done a three-dimensional shallow seismic reflection exploration experiment around the Luhuatai Fault in the west of Yinchuan Basin. The experiment uses swath geometry of 8 lines and 10 shots. Every two adjacent swaths overlap 3 survey lines, thus 5 swaths and 28 survey lines are laid in total. The ground sampling grid is 5m×20m and the CMP grid is 2.5m×5m. The data volume that reflects the three-dimensional spatial structure of the Luhuatai Fault is obtained.In data processing, we select the suitable three-dimensional seismic data process modules. The main processes are composed of raw data input, three-dimensional geometry defining and checking, anomalous trace edit and first arrival mute, spherical divergence compensation, surgical filtering to eliminate surface waves, surface-consistent amplitude compensation, surface-consistent deconvolution, velocity analysis and residual static correction(twice iteration), DMO and the third time velocity analysis, final stacking, three-dimensional post-stack de-noising and horizontally interpolating, one-pass 3-D migration.
3-D seismic data interpretation uses the way of human-computer interaction. Through a variety of methods such as multi-line profiles contrasting, time slicing, three-dimensional visualization, and 3-D coherence cube technology, the reflection horizons are discerned and tracked, and the three-dimensional data volume reflecting the spatial variation of strata and faults is obtained. The results after fine processing and synthetical interpretation show that the Luhuatai Fault consists of two normal faults that incline to each other. The major fault inclines to SE, and the minor fault inclines to NW. The distance between them gradually increases from north to south. In addition, the minor fault merges into the major fault at the depth of approximately 780~800m. The up-breakpoint of the major fault has a tendency of deepening from north to south. The up-breakpoint depth is about 25~30m in the northern part of experimental area, and about 35~40m in the southern part of experimental area.
The experimental results show that the three-dimensional seismic data has the advantages of large volume of data, information-rich, high accuracy of migration, and high precision of tomography. It can reflect the three-dimensional spatial distribution of strata and faults in different aspects, and it is beneficial for the imaging of complex structures and faults.

On July 28,1976,the great Tangshan earthquake(M7.8)occurred in the Tangshan area of Hebei Province,which shocked the whole world.Before this earthquake,there was no earthquake with magnitude over M7.0 in this area.After this earthquake,the crustal structures and tectonics around Tangshan earthquake area remain unclear.In order to investigate the fine crustal structures,the main fault geometries and the relations between the deep-shallow tectonics in this area,a deep seismic reflection profiling with 40m receiver spacing and 200m shot spacing as well as 60-fold across the Tangshan Fault zone was carried out in the Fengnan region of Tangshan in 2009.Because our results have much higher spatial resolution than that of previous results of deep geophysical prospecting,some new features of the crustal structures and fault tectonics were revealed by this study.The results show that the thickness of the crust is about 32～34km along the profile,the Moho gradually deepens from east to west.Between Fengnan county and Xuanzhuang town,the reflections in the middle-lower crust and crust-mantle transitional zone are staggered by the deep Tangshan Fault,and dislocation occurs on the Moho on both sides of the deep fault,indicating the strike-slip effect of the deep Tangshan Fault.Tangshan Fault belt revealed by deep seismic reflection profile is a huge intra-continental strike-slip fault,and its shallow part appears as a typical flower-shaped structure,incising and disturbing the lower crust and crust-mantle transitional zone in the deep part.The complex faults and structures coexisting in both deep and shallow parts of the crust are the tectonic background for the Tangshan Earthquake,and also an important factor controlling the earthquake activity in the area.

The M_S 8.0 great Wenchuan earthquake of May 12th,2008 was generated by abrupt faulting of the Yingxiu-Beichuan Fault along the Longmenshan Fault zone.The earthquake produced not only the surface ruptures along the Yingxiu-Beichuan Fault and Guanxian-Jiangyou Fault,but also surface rupture,highway's pavement arching,sand-boils and waterspouts in various degrees in the areas such as Shifang and Mianzhu of the Chengdu plain.To know the shallow geological structures under the surface rupture zone,a high-resolution shallow seismic reflection profile of 6350m long in near-EW direction was completed.This profile is located at the Shigu town,Shifang city,where a suspected earthquake surface rupture zone was discovered.In this study,a trace interval of 3m,shot interval of 18m,and a 300-channel 25-fold observation system was used.In order to give consideration of both near-surface reflections and dipping interface imaging,we adopted the split-spread geometry and asymmetrical zero-offset receiving technique.For suppressing random-noise and raising signal to noise ratio of seismic data,30 times vertical stacking of vibrator signals was made for each common-shot gather after correlation of individual records.By using the above work method and spread geometry,we obtained high-resolution images of structures in the depth range of 15～800m after data processing.The result shows that there are the buried thrust fault thrusting to the plain and the back-thrust fault under surface rupture zone,and that the activity of the buried thrust fault may be the main causes for folding and deformation in near-surface strata and for the coseismic surface rupturing.

The investigation and study of fault activities are a basic work for urban earthquake prevention and disasters reduction.In order to find out the location,characteristics and activities of the Laoyachen Fault in Zhengzhou,the high-resolution shallow seismic P and S wave survey profiling across the Laoyachen Fault was carried out at the end of 2006,and different seismic sources along with combinations of diverse observation geometries with different parameters were used.The fine structures in different depths beneath the profile were obtained and the patterns as well as characteristics of the Laoyachen Fault were determined.The results show that the Laoyachen Fault,running in NW and dipping in NE,is a normal fault and its dip angle is about 60°～70°,which incises strata of Eocene,Permian,Carboniferous or Ordovician epoch and goes up to the top boundary of Eocene stratum at the 800～850m depth.There is no any reflector of offset stratum found in Q+N strata.The borehole geological sections across steep slopes of earth surface present that the layers inferred from reflected seismic wave groups of shallow seismic profile are well correspondent with boring geological layers.The borehole results reveal that the three reference laminas,i.e.the boundary between Malan loess and silt with clay soil at about 21m in depth,the calcareous gravel clay layer of 53.9m deep,and the calcareous silt layer of 61.9m deep,all have not depth variations at the two sides of surface steep slopes and are situated almost at the same ground surface elevations,which suggests that the steep slopes at the earth's surface should not result from the activities of Laoyachen Fault.In this study,through shallow seismic P wave and S wave exploration as well as combination of joint borehole geological sections,not only the location and characteristics of Laoyachen Fault was determined,but the geological and seismological evidences for the fault activity estimations were provided.

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.

This paper introduced the basic principles of various seismic prospecting methods and working methods briefly according to the nationwide practices of seismic prospecting for active faults beneath big cities in recent years.Furthermore,it mainly analyzed the available range of different seismic prospecting methods,main achievements and solutions,and discussed the best combination of seismic exploration methods for detecting crustal structure and locating the faults used in the present stage,that is,to trace the faults which are at the depths of hundred meters underground using shallow seismic investigation,to detect the basement faults which are above basement(at depth of kilo～meters)using high resolution refraction sounding,and the deep crustal faults using combined seismic prospecting method of reflection seismic sounding and wide-angle reflection/refraction sounding,and furthermore,to adopt 3-D deep seismic sounding method to get 3-D velocity structure beneath city area.Thus,we can get information about fault attitude and distribution at different depths and a complete image of fault from shallow part to deep part using the combined seismic exploration method.Some application examples are presented in the article.

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.

The method and principle of common offset seismic survey as well as the field data gathering and processing technique were introduced briefly. Through two urban active fault survey examples in Fuzhou and Shenyang, the efficiency and limitation about using common offset seismic reflection technique to carry out urban active fault survey were probed. The results show that this technique has the properties of high resolving power, better reconstruction of subsurface structures, and real-time analyzing and interpreting of the investigating results on site. This method can be used to quickly locate the investigating objects accurately in the areas with thinner Q overburdens and strong bedrock interface fluctuations.

This paper gives a brief account of the significance of urban active fault exploration and presents a general review of active fault exploration around the world. Earthquakes provoked by active faults directly beneath the metropolises can cause serious disasters to the cities. If urban active faults are precisely determined and effective precautions are taken, losses at the time of earthquake occurrence can be greatly reduced. We elaborate on various kinds of possible geophysical methods for seismic active fault exploration and their main features. We also discuss the scope of application of related geophysical methods and the major problems that they can solve in the different periods of active fault exploration, such as regional survey or preliminary investigation, detailed exploration or precise location, and the identification of seismogenic structures.

Research on a large number of seismic events at home and abroad has indicated that tremendous earthquake hazards in urban areas are mostly attributed to earthquakes caused by active faults buried beneath the cities. The identification of urban buried active faults, therefore, is an important and urgent task. High-resolution seismic exploration is an effective geophysical technique that can be used to identify urban buried active fault at present. High-resolution seismic exploration for urban buried active faults is a sophisticated and systematic project, which involves excitation and receiving techniques, observational system, as well as seismic data processing and interpretation. The seismic source is of the first importance among the other problems that should be solved during the exploration. High-resolution seismic exploration for urban active fault calls for specific performance of the seismic source, because of peculiar environment in urban areas and particular characteristics of urban buried faults. For examples, relatively small offset of the fault requires a wider source spectrum, while strong disturbances in urban areas need a higher anti-jamming capability of the source. A comparative experiment on various types of sources, including vibroseis, vacuum accelerating weight drop, hammer-blow, air gun and explosive is carried out along the traverse across the Bayishuiku Fault. The features of various source spectrums are obtained by using spectrum analysis technique. The comparison of time-stacked sections obtained by using vibroseis, vacuum accelerating weight-drop and hammer blow from the traverse across the Bayishuiku Fault in Fuzhou City is presented in this paper. The effectiveness of various seismic sources in the exploration of urban buried active faults is discussed in detail.

In addition to a brief account of characteristics of disturbing waves in urban shallow seismic exploration, an exposition of technical facilities and seismic data acquisition techniques for anti-jamming and high-resolution shallow seismic exploration is given in this paper on the basis of shallow seismic experimental data of active fault surveying in Fuzhou City. The technical measures taken for anti-jamming, improving signal to noise ratio and resolution of seismic data are expounded as well. The experiment shows that the effective approach to accomplishing anti-jamming, high-resolution shallow seismic data acquisition is receiving with mini trace intervals, mini offsets, multi-channel and high-frequency Geophones by using mini-vibrator and the matched seismograph.