The Langshan range-front fault (LRF)is a Holocene active normal fault that bounds the Langshan Mountain and Hetao Basin at the northwest corner of the Ordos Plateau. Paleoseismic trenching research at three sites, Dongshen Village trench (TC1), Qingshan trench (TC2)and Wulanhashao trench (TC3)from north to south was performed in this study to reveal the seismic hazard risk in Hetao Basin. The paleoevents ED1, ED2, ED3 from TC1 can be constrained to have occurred (6±1.3)ka, (9.6±2)ka and (19.7±4.2)ka respectively, while the paleoevent EQ1 from TC2 occurred about (6.7±0.1)ka and the paleoevents EW1, EW2, EW3 at TC3 took place about (2.3±0.4)ka, (6±1)ka and before 7ka respectively. In combination with paleoseismic results of previous researchers, the Holocene earthquake sequence of the LRF could be established as 2.3~2.43ka BP (E1), 4.41~3.06ka BP (E2), 6.71~6.8ka BP (E3), 7.6~9.81ka BP (E4), and (19.7±4.2)ka BP (E5). Although the possibility of missing events cannot totally be ruled out, based on the analysis on faulted geomorphology at Wulanhashao site, we argue the paleoearthquake history of the LRF during Holocene may be complete with an average recurrent interval about 2500 yrs. The apparent displacements associated with events E1, E3 and E4 are significantly larger than that of event, E2, that suggests that they might be great events with magnitudes 7.5 to even over 8 that ruptured the entire LRF, while the event E2 may be a smaller event that only ruptured a segment of the fault. The magnitude of event E2 might be about M7. This poses a significant seismic hazard to the area of the Linhe depression in the western Hetao graben region. With the further limitation of previous radiocarbon dating result near our trench site at Wulanhashao, the slip rate at Wulanhashao should be not smaller than, but close to 0.66mm/a since 15ka BP. And the slip rate at Qingshan site is supposed to be about 1.4~1.6mm/a since 6.8ka BP. Both our combined most recent paleoseismic cognition and current tectonic geomorphologic research results supports to reveal that the Langshan range-front fault now is an unsegmented fault, preferring to rupture the whole fault in a surface-rupture event. Considering the most recent event E1 and fault slip rate obtained above, the accumulated strain on the LRF could be estimated as about 1.52~3.94m. Given the ~2500a recurrent interval, we argue that the elapsed time since last major quake, E1, is approaching or even over the recurrence, and the seismic risk for another major quake is imminent, at least cannot be ignored.

Traditional method to generate Digital Elevation Model (DEM)through topographic map and topographic measurement has weak points such as low efficiency, long operating time and small range. The emergence of DEM-generation technology from high resolution satellite image provides a new method for rapid acquisition of large terrain and geomorphic data, which greatly improves the efficiency of data acquisition. This method costs lower compared with LiDAR (Light Detection and Ranging), has large coverage compared with SfM (Structure from Motion). However, there is still lack of report on whether the accuracy of DEM generated from stereo-imagery satisfies the quantitative research of active tectonics. This research is based on LPS (Leica Photogrammetry Suit)software platform, using Worldview-2 panchromatic stereo-imagery as data source, selecting Kumishi Basin in eastern Tianshan Mountains with little vegetation as study area. We generated 0.5m resolution DEM of 5-km swath along the newly discovered rupture zone at the south of Kumishi Basin, measured the height of fault scarps on different levels of alluvial fans based on the DEM, then compared with the scarp height measured by differential GPS survey in the field to analyze the accuracy of the extracted DEM. The results show that the elevation difference between the topographic profiles derived from the extracted DEM and surveyed by differential GPS ranges from -2.82 to 4.87m. The shape of the fault scarp can be finely depicted and the deviation is 0.30m after elevation correction. The accuracy of measuring the height of fault scarps can reach 0.22m, which meets the need of high-precision quantitative research of active tectonics. It provides great convenience for rapidly obtaining fine geometry, profiles morphology, vertical dislocations of fault and important reference for sites selection for trench excavation, slip rate, and samples. This method has broad prospects in the study of active tectonics.

The horizontal movement of the Helan Shan west-piedmont fault is important to determination of the present-day boundary between the Alashan and North China blocks as well as to the exploration of the extent of the northeastward expansion of the Tibetan plateau. Field geological surveys found that this fault cuts the west wing of the Neogene anticline, which right-laterally offset the geological boundary between Ganhegou and Qingshuiying Formations with displacement over 800m. The secondary tensional joints (fissures)intersected with the main faults developed on the Quaternary flood high platform near the fault, of which the acute angles indicate its dextral strike slip. The normal faults developed at the southern end of the Helan Shan west-piedmont fault show that the west wall of this fault moves northward, and the tensional adjustment zone formed at the end of the strike slip fault, which reflects that the horizontal movement of the main fault is dextral strike slip. The dextral dislocation occurred in the gully across the fault during different periods. Therefore, the Helan Shan west-piedmont fault is a dextral strike slip fault rather than a sinistral strike slip fault as previous work suggested. The relationship between the faulting and deformation of Cenozoic strata demonstrates that there were two stages of tectonic deformation near the Helan Shan west-piedmont fault since the late Cenozoic, namely early folding and late faulting. These two tectonic deformations are the result of the northeastward thrust on the Alashan block by the Tibet Plateau. The influence range of Tibetan plateau expansion has arrived in the Helan Shan west-piedmont area in the late Pliocene leading to the dextral strike slip of this fault as well as formation of the current boundary between the Alashan and North China blocks, which is also the youngest front of the Tibetan plateau.

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.

LiDAR, as a newly developed surveying technology in recent decades, has been widely used in engineering survey, protection of cultural relics and topographic measurement, and it has also been gradually introduced to studies of tectonic activities. Although the digital photography technology has been used in the study of palaeoearthquake, the information would be still acquired by traditional geological sketch from trenches. Due to the limitation of photography itself, it is difficult to overcome the distortion of information. With its high information content, accuracy, convenience, safety and easy operation, LiDAR, as a new technology, broadens the access to data and information for palaeoearthquake study.

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.

The Yabrai range-front fault is a normal fault,which is about 120km long,trends N60°E and distributes along the southeast margin of the Alashan block. In this paper,we focus on the geomorphology and kinematics of the Yabrai range-front fault,and discuss the implications of the fault for the regional tectonics.
This fault consists of three segments and the most active one is located in the southwest,which has a length of about 35km. The about 1～2m-high scarp,stretching almost the full segment,might be the result of the latest earthquake event. Fresh free surface indicates that the elapsed time of the last event should not be long.
The middle segment is about 31km in length. The results suggest that just a single fault is developed along the piedmont of the Yabrai Shan,and there is no evidence of recent activity on this fault. In contrast to the simple geometric structure of the middle segment,the northeast segment consists of several faults. The scarps of the most recent earthquake event,which are clear but discontinuous,are about 0.5～1.5m high and some are up to 2m. Although the scarps along the southwest and northeast segments of the fault are similar,it is difficult to suggest they are caused by the same earthquake without precise dating.
The seismic reflection profile suggests that the Yabrai range-front fault came into being as a normal fault in Cretaceous,when the Tibetan plateau did not emerge at that time. Therefore,we conclude that the Yabrai range-front fault is not the consequence of the Indo-Asian collision. But this region plays a great role in constraining the tectonic evolution of the Alashan block and therefore,the Tibetan plateau.

As the outermost fault zone in the northeastern margin of the Tibetan plateau,the deep structures,distribution,movement feature and deformational mechanism of the Niushoushan-Luoshan Fault zone are crucial to understand the formation and evolution of the arcuate fault zones in the northeast corner of the Tibetan plateau. In this paper,we analyze four seismic reflection sections across the Niushoushan-Luoshan Fault zone and map in detail the area within the fault zone. These data indicate that the Niushoushan-Luoshan Fault zone is a discrete fault zone. The fault zone can be subdivided into three parts: the south part,i.e.the Luoshan Fault,is characterized by positive flower structure,shown as remarkable right lateral strike-slip; in the middle segment,that is,the Niushoushan Fault,no active fault exists on the east flank of the Niushoushan,and this region is dominated by intensive folding; the north part,the Sanguankou Fault,is a left-lateral strike-slip fault. The discontinuity and segmentation feature of the Niushoushan-Luoshan Fault zone suggest different deformational styles in different locations of the fault zone associated with the process of northeastward propagation of the Tibetan plateau.

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.

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.