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Method of locating buried active fault by composite drilling section doubling exploration
Abstract
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 composite section exploration in Xietan Village (location shown in Figure 2A,B) was used to detect the Hetan-Guotan buried fault, and the composite section exploration in Lizui Village (location shown in Figure 2A,C) was used to detect the Zhenjing-Zhenbei buried fault. During the construction, the "folding method" is used to approach the positioning fault from the outside to the inside [23], emphasizing dynamic construction and dynamic analysis. The position of the fault is found through continuous trials. ...
By using shallow seismic exploration, composite drilling section exploration and sample dating test, we have obtained precise positions, burial depths of uppermost point and activity characteristics of Hetan-Guotan buried fault and Zhenjing-Zhenbei buried fault in Zhongwei Basin. The results show that the latest active period of Hetan-Guotan buried fault is the middle-late Middle Pleistocene, and the latest active period of Zhenjing-Zhenbei buried fault is the Early and Middle Pleistocene. The two buried faults became inactive at the end of the Middle Pleistocene and have been inactive since the Late Pleistocene.
... According to the nature of the activity and geometric characteristics of fault F 5 , the obvious mutation point of the reflection lineups on the shallow SRS profile was selected, and combined borehole section profiling on both sides of the breakpoint was performed. The borehole arrangement followed a doubling exploration pattern [37,38], with at least three boreholes in the hanging wall and three boreholes in the footwall of the fault, and the spacing between boreholes was no more than 10 m. ...
The Tancheng–Lujiang (Tanlu) fault zone is the most active fault zone in eastern China. In this zone, the Anqiu–Juxian fault represents the most recently active fault and has the clearest surface traces and the highest seismic risk. This study comprehensively analyzes the kinematic characteristics of the Jiangsu segment of the Anqiu–Juxian fault using field geological surveys, trenches, shallow seismic reflection surveys, combined borehole section exploration, and middepth seismic reflection surveys. The results show that the Jiangsu segment of the Anqiu–Juxian fault features a single branch in the bedrock outcrop area, with reverse strike-slip motion near North Maling Mountain and Chonggang Mountain and normal strike-slip motion near South Maling Mountain. The sedimentary zone features two normal strike-slip faults (east and western branches), which represent the synsedimentary boundaries of a half-graben rift basin. The kinematic process is represented by rotational movement along the strike-slip fault with a curved path. The resulting tensile and compressive stresses are accommodated by dip-slip movement at both ends of the strike-slip fault. The activity of the Jiangsu segment of the Anqiu-Juxian fault can be divided into two periods. The first period of activity occurred before the later part of the Late Pleistocene, when movement along this curved segment occurred, forming the western branch of the Xinyi segment and the eastern branch of the Suqian segment. The second period of activity started in the later part of the Late Pleistocene and continues today. It is characterized by activity on the western branch of the Xinyi segment and the western branch of the Suqian segment of the Jiangsu segment, while the eastern branch of the Xinyi segment and the eastern branch of the Suqian segment became inactive and can be considered Late Pleistocene faults. The maximum vertical slip rate of the Jiangsu segment of the Anqiu–Juxian fault since the Pleistocene has been 0.28 mm/a. The Jiangsu segment of the Anqiu–Juxian fault formed via dextral strike-slip faulting, mainly due to the southward movement of the region to the east of the fault.
Studying the faults in Wuhai Basin, which is located in the northwestern corner of the Ordos Block in China, can not only guide earthquake-safe construction in the region, but also provide a greater understanding of the structure of the Wuhai Basin and the dynamic environment of Northwestern Ordos Block. In this study, we effectively detected the Yellow River buried Fault (YRF), which we believe currently presents a risk to human life in the area. First, we laid out a deep seismic reflection profile (DSRP) in the north of the Wuhai Basin and found that the YRF is composed of two subparallel faults, the east branch of the Yellow River buried Fault (EYF) and the west branch of the Yellow River buried Fault (WYF), which formed a Y-type graben in the section. We then used 25 shallow seismic exploration profiles to find out how the faults are distributed. Finally, we used drillings and Quaternary dating methods to certify their existence and obtain their activity parameters. We used the sequence stratigraphy method to compare sediment strata and identified paleo-earthquake events by finding the unequal thickness layers (UTLs) on both sides of the fault. The drillings revealed that four earthquakes occurred on the YRF around 25.6 ± 0.11 ka BP, 39.5 ± 0.45–41.7 ± 0.57 ka BP, 58.25 ± 7.13 ka BP, and 111 ± 1.21 ka BP. Since the last activity on the YRF occurred much longer ago than its estimated recurrence cycle, we believe that the fault YRF currently presents a dangerous risk. Our study confirms that the YRF is a normal fault graben and that the Wuhai Bain is an extensional basin formed by the relative movement of the Tibetan Plateau, the Alashan Block, and the Ordos Block.
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.
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.
Active fault is one of potential geohazards in cities. Locating and dating buried active faults in urban areas have been a difficult issue in active fault exploration. In this paper, we take the detection of the buried active fault performed at Hehuan Road in the north of Suqian city as an example. We preliminarily mapped the fault through field investigation and shallow seismic reflection survey technique. Furthermore, based on the principle of doubling section method, we conducted multiple drilling to constrain the upper faulted point which is located in a range of 5m in horizon and 4.4~6.1 m in depth. Finally, we determined the exact location and latest activity of the fault by trenching. Obviously, good results have been acquired on the accurate location and activity of the Suqian segment of Anqiu-Juxian Fault using multi-level and multi-means detection method. Besides, we observed from the detection at the Hehuan Road site that at least four paleoseismic events occurred during the past 80000 yrs, and the result indicates that the latest faulting event on the fault is younger than (5.9±0.3) ka BP and the buried active fault at the Hehuan Road is a Holocene active fault. The result of buried active fault detection at the Hehuan Road site provides quantitative parameters for evaluation of seismic hazards and planning the width of safety distance in Suqian City.
The Xiadian fault is one of the most important concealed active faults in the northern part of the North China Plain. The Sanhe -Pinggu earthquake (M = 8) is the latest surface rupturing event on this fault. Two stratigraphie drill logs on both sides of the Xiadian fault are chronologized by using thermoluminescene and 14C dating methods. Comparison shows a differential sedimentation on both sides of the fault in the last 26 ka B. P. The stratigraphie marks are established for identifying paleoearthquakes in the drill logs and 11 events are recognized. The main results of this study include: (1)The vertical slip of the Xiadian fault decreased since the Last Glacial Maximum in the past 26ka B. P. The vertical slip rate reached 1. 2mm/a during the period of 21 - 26 ka B. P. , 0.98mm/a during the period of 10 -21 ka and 0.34mm/a in the past 10 ka B.P. , respectively. (2)11 paleoearthquakes are identified by the drill logs which indicate an irregular recurrence behaviour in the past 26ka B.P. The paleoearthquakes clustered with a relatively short interval of 900-1 200 years in the Last Inter - Stadial and Last Glacial Maximum (19.3 -26 ka B.P.), while several paleoearthquakes recurred quasi -periodically with an interval of 3 770 -5 800 years in the Post Glacial. (3)The paleoearthquake recurrence pattern seems to be associated with paleo- climate variation in the Last Glacial. The earthquake clustering occurred in the Last Inter -Stadial and Last Glacial Maximum with a short recurrence interval. The recurrence interval became longer in the Post Glacial.
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 Weihe Fault is an important blind fault in Weihe Basin and controls the formation, evolution and seismicity of Weihe Basin. The deep seismic reflection survey results show that the fault is not a deep crustal fault; it is located right below the C layer at about 15km depth and cuts through the crystalline basement and the C layer, causing a throw of about 4km between the two sides of crystalline basement. The dip angle at the shallow part of the fault (depth < 5 km) is big and flattens with depth, and the fault turns to be a listric fault. Shallow seismic survey results show that the dip angle of the Weihe Fault in the middle and deep parts is about 85°; the attitude is different on the two walls of the fault, the footwall is horizontal and the hanging wall is tilting to the south direction; and its dip angle increases quickly. Drilling survey results show that the fault at the s hallow part is obviously manifested. The lithology, thickness and attitudes of strata are quite different between the two sides of fault. The attitude on the footwall is horizontal and that on the hanging wall tilts a bit to the fault side. The late Pleistocene displacement is about 4-6m. Trenching results show that the Weihe Fault near ground is still active. Since Holocene epoch it has undergone 3 paleoearthquakes and 1 history earthquake, so it is a Holocene active fault.
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.
Shallow seismic surveying is a common method applied to the research of blind fault activity at the Quaternary stratum region, especially in urban area. The bifurcating, uniting, bending, intermitting and ending off of reflection waves on stacked time section are important signs which are often used to judge the existence of blind fault. At the northern area of Songhuajiang River in Harbin City, we finished two seismic reflection profiles. On the shallow seismic reflection profiles of Lugangtun and Jubaotun, the T0 wave group corresponding to the interface between Cretaceous sand-mud stone and lower Pleistocene gravel is clear. T0-1, reflecting wave group corresponding to the contact surface between gravel and clay, or fine sand and gravel, is also clear. Based on the stacking seismic profiles and borehole data, we conclude that the Ashihe fault offset the bottom stratum of lower Pleistocene, and Binzhou fault cut off the bottom stratum of upper Pleistocene. By building high quality composite drilling geological profiles, dating the samples from bores, and contrasting stratum from one bore to the other, we consider that the Ashihe fault does not offset the Quaternary formation, the fault may only exist in Cretaceous sand-mud stone. The Binzhou fault is only an early Pleistocene fault; it doesn't offset the late Pleistocene formation. Finally, we think that the bifurcating, bending, intermitting and ending off of seismic reflection wave group on Luegangtun and Jubaotun seismic profiles are not the result of fault activity, but the phase change of Quaternary formation.
This paper makes a comprehensive analysis of the recent progress of the seismic active fault prospecting in Lanzhou city. Based on the satellite and aerial photos interpretation, geological and geomorphic investigation, geochemistry prospecting, shallow seismic investigation, resistivity imaging, drilling, especially large-scale trenching along the 7 active fault zones in Lanzhou city, we have achieved very important progress and gained new knowledge about the recent activity of main active faults and deformation features in Lanzhou Basin. The main conclusions are summarized bellow: (1) The Jinchengguan Fault is a thru st fault, constituting the northern boundary of the Lanzhou Tertiary Basin. It is revealed by geophysical prospecting and drilling that the newest strata offset by the Jinchengguan Fault are the early-Pleistocene sandstone and conglomerate, and that the overlying second and third terraces of the Yellow River remain intact. So, it's an early and middle Pleistocene active fault. ( 2) The Liujiabu Fault and Shengouqiao Fault constitute the northern and western boundaries of the Qilihe Subsidence, respectively. Revealed by geophysical prospecting, drilling and large trenching, they are not faults but lithologic boundaries of different rocks between Pliocene and early Pleistocene. (3) The Leitanhe Fault is the eastern boundary of Qi lihe Subsidence, a boundary fault separating the Tertiary Lanzhou Basin into the east and west basins. According to the geophysical prospecting and drilling, the Leitanhe Fault is a thrust fault and its newest activity age is early and middle Pleistocene. It is not active since late Quaternary and does not cut the third terrace of the Yellow River. (4) The Siergou Fault is the southwestern boundary of Lanzhou Basin, a thrust fault too. It's an early and middle Pleistocene active fault and does not offset the forth terrace of Yellow River. While the Xijincun Fault is much nearer to the south margin of Lanzhou Basin and forms the southern boundary of the Tertiary Lanzhou Basin. It's an early Pleistocene fault. (5) The northern margin of Maxianshan Mountains faul t is a major seismic fault on the southern margin of Lanzhou Basin, and its movement is characterized by segmentation. The east segment, the Neiguanying sub-fault, is a late Pleistocene fault. The middle segment, the Maxianshan and Qidaoliang faults, are active during late Pleistocene and early Holocene. The west segment, the Wusushan sub fault, is active during late Pleistocene and Holocene, and it's also the seismic fault of the M7 Lanzhou earthquake. On the whole, we correct the previous recogn itions about the activity times of 4 faults, i. e. the Jinchengguan Fault, Leitanhe Fault, Siergou Fault and Xijicun Fault. They are all early and middle Pleistocene instead of late Pleistocene active faults. Especially, we find that the Liujiabu Fault and Sbengouqiao Fault directly across Lanzhou city are not late Pleistocene or Holocene active faults but lithologic boundaries between Pliocene mudstone and early Pleistocene conglomerate. The results are very important for the urban planning and engineering construction, and will produce obvious economical and social benefits.
We have conducted an integrated survey and investigation on the Quaternary activity of the Liaocheng-Lankao buried fault. Methods include geochemical exploration, shallow seismic exploration, drilling geological profile and neo-strata dating. The object is to determine the accurate location of the fault, dislocation amount of each time period since Quaternary and the offset age of the last time of dislocation. The results show that the dislocation of the fault extends upward to the depth 20m or so below the surface. This fault has been active in early Holocene time. The average slip rate of the fault is 0.12mm/a.