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.

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.