Table of Content

03 December 2002, Volume 24 Issue 4

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Brief Report

EVIDENCE AND METHODS FOR DETERMINING THE SAFETY DISTANCE FROM THE POTENTIAL EARTHQUAKE SURFACE RUPTURE ON ACTIVE FAULT

XU Xi-wei, YU Gui-hua, MA Wen-tao, RAN Yong-kang, CHEN Gui-hua, HAN Zhu-jun, Zhang, Lan-feng, YOU Hui-chuan

2002, 24(4):  470-483. 

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A large number of observed data about the widths of earthquake surface ruptures produced on diverse types of active faults, as well as the widths of intense deformation zones in trench logs across active faults are presented in this paper. Combining these data with the close relation between the damage zone of surface construction and the spatial position of active fault, the authors propose that a minimum distance(safety distance)of 30m away from active fault is essentially required for constructions to prevent the effect of faulting. The more accurate safety distance required for various types of active faults can be tested and modified through the analysis of the deformation features of strata in trench logs across the fault. The required safety distance in some specific sites, such as the step-overs of active faults, as well as the area defined by sub-parallel secondary faults on both sides, should be the sum of the width of the area and 15m from area boundaries. It is suggested that this "safety distance" should be taken as a legal regulation for constructions and buildings. In addition, further attention should be paid to the identification of active fault and precise determination of the geometric structures of the surface fault strand, so that earthquake hazards can be positively and effectively reduced.

PRIMARY STUDY ON POSSIBLE WIDTH AND DISPLACEMENT OF FUTURE SURFACE RUPTURE ZONE PRODUCED BY BURIED ACTIVE FAULT

HAN Zhu-jun, RAN Yong-kang, XU Xi-wei

2002, 24(4):  484-494. 

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Okada's(1992)elastic half-space theory is applied to calculate the surface displacement and the amount of differential displacement produced by buried active faults. The theory has been verified to some extent by the 1992 Landers earthquake, United State, the 1994 Northridge earthquake, United State, the 1995 Kobe earthquake, Japan and the 1999 Izmit earthquake, Turkey. According to the criteria given in 《The Regulations of Seismic Design of Building》(GB50011-2001),the threshold value of differential displacement of buried seismogenic fault for producing surface rupture is estimated to be 0.1m for two adjacent points of 5m distance. This means that if the surface displacement produced by any buried fault is larger than 0.1m, then surface rupture zone will occur. If it is smaller than 0.1m, then the hazard that the buried fault may cause can be neglected. How the width and displacement of surface rupture zones vary with the buried depth, dip angle, motion mode and offset of the active fault is analyzed and discussed by applying the same theory and regulations. The results show that: 1)for buried normal fault, with increasing of the buried depth of the fault the width of surface rupture zones increases gradually to a peak value, and then decreases again. The point of peak value migrates toward the footwall of the fault(downfaulted block); 2)when the dip angle of the fault(normal fault)becomes small and the other parameters remain unchanged, the surface rupture zone will distribute mainly on the downfaulted block and the width becomes narrower; 3)as compared to the case of buried normal fault, the differential displacement produced by buried strike-slip fault attenuates more quickly when the buried depth of the fault becomes greater. It may imply that the normal fault is more risky; 4)the width and displacement of the surface rupture zone increase significantly with increasing displacement on the buried active fault. These primary results provide scientific basis for city planning and the seismic design of lifeline engineering and constructions across buried active faults. However, how to decide the threshold value of surface rupture and how to build up a more reasonable model for the middle and shallow crust need far more detailed study.

RISK EVALUATION OF SURFACE DISLOCATION CAUSED BY EARTHQUAKE

LIANG Xiao-hua, JIANG Pu, DONG Jin-cheng, LIU Shou-hua

2002, 24(4):  495-502. 

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In this paper we introduce the concept of the risk of surface dislocation. The possibility of surface dislocation caused by earthquake is expressed here by probability. The risk of surface dislocation involves earthquake risk as well as the occurrence and distribution feature of surface dislocation. The risk of surface dislocation caused by earthquake can be expressed as(Jiang Pu et al., 1998): P=P₁?P₂?P₃ where P is the risk of surface dislocation caused by earthquake, P₁ represents earthquake risk; P₂ is the probability of the occurrence of surface dislocation at different earthquake magnitudes; P₃ is the distribution probability of the amount of surface dislocation. The earthquake risk can be determined through conventional analysis methods applied in engineering-seismology or through seismo-geological investigation and the analysis of seismic activity. Based on the worldwide data of surface dislocations produced by earthquakes, two parameters for evaluating the risk of surface dislocation are derived. They are the probability index of earthquake magnitude-surface dislocation(P₂)and the probability index of dislocation amplitude and its distribution(P₃), which involves the amplitude, distribution and width of dislocation, as well as the effect of overburden thickness. According to the principle, method and the related probability indexes given in this paper, the risk of surface dislocation in a certain urban area or an engineering site can be quantitatively evaluated.

DIVISION OF LATE QUATERNARY STRATA AND ANALYSIS OF PALAEOENVIRONMENT IN FUZHOU BASIN

ZHENG Rong-zhang, XU Xi-wei, ZHU Jin-fang, JI Feng-ju, HUANG Zong-lin, LI Jian-ping

2002, 24(4):  503-513. 

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Fuzhou basin is tectonically located in the eastern part of South China fold system, within the Fuding-Yunxiao Fault depression zone of East Fujian volcanic fault depression zone. It is a fault depression basin developed in the middle and late Quaternary, filled with marine, terrestrial and alternating marine-terrestrial facies sediments with complicated structures. Two test holes(SZK-1 and SZK-2)were drilled in the Gutian Street, Fuzhou City, with the purpose of setting up a standard section of Quaternary strata in Fuzhou basin. The SZK-1 test hole is 56.3m deep, in which the thickness of the Quaternary sediments is 51.1m. From the cores of this borehole, 25 samples were collected for thermoluminescent(TL)dating and 73 samples were collected for sporopollen analysis. The SZK-2 test hole is 53.2m deep, in which the thickness of the Quaternary sediments is 50.9m. From the cores of this test hole 18 samples were collected for TL dating. The distance of these two test holes is 113.5m. Basically, these two boreholes reveal the same features of deposition. The TL dating results of samples from these two test holes show good vertical time sequence and lateral correlation. The relative ages of sediments dated by sporopollen analysis in SZK-1 test hole show a good consistency with the ages dated by TL method. This study has led to some new insights about the sedimentary facies and succession, as well as the paleo-environment of the Fuzhou basin:(1)The sedimentation in Fuzhou basin initiated at middle Epipleistocene, at least at 54.3ka B.P., and terminated after 1ka B.P.(2)The ages of sediments from the two test holes can be assigned to Epipleistocene and Holocene. The Epipleistocene sediments can be subdivided into the sediments of middle and late Epipleistocene, i.e. 54.3ka B.P.～24 ka B.P. and 24 ka B.P.～ 11.3 ka B.P. Moreover, the Holocene sediments include the sediments of early, middle and late Holocene, i.e. 9ka B.P. to later than 1ka B.P.(3)The strata in the two test holes can be divided into three sections. The lower section corresponds to middle Epipleistocene, the middle section to late Epipleistocene, and the upper section to Holocene.(4)The sediments of middle Epipleistocene are composed of celadon gravel, coarse sand and clay, indicating a temperate and dry climate, while the sedimentary facies of the sediments can be assigned to lacustrine and lakeshore facies. The sediments of late Epipleistocene are composed of gray and yellow, coarse and medium sand, indicating a temperate and dry climate, while the sedimentary facies of the sediments can be assigned to fluvial facies. The early Holocene sediments consist of gray medium and coarse sand with a small amount of sludge, indicating a temperate and relatively wet climate, and littoral deposition environment. The middle Holocene sediments are composed of gray interbedded sludge and fine silt, indicating a warm and wet climate, and shallow-sea deposition environment. The late Holocene sediments consist of gray medium-coarse sand, sludge and clay, indicating a transition from littoral, neritic to lacustrine environment, and a warm and wet climate.

STUDY ON ACTIVE FAULTS IN FUZHOU BASIN THROUGH TRENCHING

MIN Wei, ZHU Jin-fang, RAN Yong-kang, SONG Fang-min, YANG Xiao-ping, HUANG Zong-lin

2002, 24(4):  514-523. 

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On the basis of 1/10,000 active fault mapping in Fuzhou Basin, interpretation of aerial photos and detailed field investigation, 11 sites on six faults in the basin were selected for exploratory trenching. The main results of trenching are as follows:(1)Two trenches were dug across the Hutou-Miaopu Fault. One of the trenches reveals a fault plane, which is developed in lava bed and covered by Quaternary strata with an age of 75?6ka B.P. The other trench reveals that a series of joints are developed in tuff bed, but they do not offset the overlying strata of 119?10ka in age. These facts may indicate that the fault had ceased moving since late Pleistocene.(2)Two trenches were excavated across the Gushan Fault. No fault plane can be identified in one of the trenches, but the second trench reveals a minor fault plane, which is developed in granite and does not offset the overlying strata of 100?9ka in age. It can be concluded, therefore,that the fault had also ceased moving since late Pleistocene.(3)Two trenches were excavated across the Wuhushan Fault. In both trenches, the relationships between the volcanic rocks and Quaternary deposits, as well as the fault planes in volcanic rocks and overlying Quaternary deposits are well exposed. A series of minor fault planes developed in granitic rocks are well exposed in one trench, and these minor faults do not offset the overlying strata of 98?8.3ka in age. It is postulated, therefore,that the fault might have ceased moving since the early stage of late Pleistocene.(4)The micro-gemorphological expression of the Minhou-Nanyu Fault can be distinctly traced along the fault strand. A fault trough is observed at Heshangsi Temple, where two trenches were excavated to identify the two faults on the eastern and western sides of the trough. The trench logs show that the fault planes that are developed in granitic rocks do not offset the overlying Quaternary deposits of 29?2ka in age. Obviously, the Minhou-Nanyu Fault had ceased moving since the late stage of late Pleistocene.(5)One trench was excavated across the Tongkou-Hongshanqiao Fault. The trench log reveals two fault planes, which dislocate the same bed of Quaternary strata and are covered by the younger bed. The age of offset bed is dated to be 193?16ka B.P, and that of the overlying bed is 100?9ka. It is suggested, therefore, that the fault had also ceased moving since late Pleistocene.(6)A 4-5 meters deep trench was excavated across the Bayishuiku-Shanggan buried fault. Geophysical prospecting data suggested that the fault probably passes the trenching site. However, no fault is found in the trench, and the Quaternary deposits here are nearly horizontal. The age of bottom deposits is dated to be 39?3ka B.P. It seems that the Bayishuiku-Shanggan Fault had ceased its activity since the late stage of late Pleistocene. The afore-mentioned conclusions are merely drawn from the results of trenching, and more detailed study is needed in the future.

ACQUISITION TECHNIQUE OF HIGH-RESOLUTION SHALLOW SEISMIC DATA FOR SURVEYING OF URBAN ACTIVE FAULTS

LIU Bao-jin, ZHANG Xian-kang, FANG Sheng-ming, ZHAO Cheng-bin, DUAN Yong-hong, ZHU Jin-fang, HUANG Zhao, HUANG Zong-lin, WANG Shan-xiong, ZHENG De-gang

2002, 24(4):  524-532. 

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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.

COMPARATIVE EXPERIMENT ON SEISMIC SOURCES IN HIGH-RESOLUTION SEISMIC EXPLORATION FOR URBAN ACTIVE FAULTS

PAN Ji-shun, LIU Bao-jin, ZHU Jin-fang, ZHANG Xian-kang, FANG Sheng-ming, WANG Fu-yun, DUAN Yong-hong, XU Zhao-fan

2002, 24(4):  533-541. 

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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.

REFRACTOR IMAGING IN COMPLEX STRUCTURES BY USING HAGEDOORN WAVEFRONT RECONSTRUCTION PRINCIPLE

XU Zhao-fan, ZHANG Xian-kang, ZHU Jin-fang, DUAN Yong-hong, TIAN Xiao-feng, PAN Ji-shun

2002, 24(4):  542-548. 

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In urban active fault prospecting, the shallow structures usually display strong lateral inhomogeneity, appearing as the heavy fluctuation of interfaces and considerable variation of layer velocities. In this case, the traditional refraction data processing and interpreting methods based on homogeneous layered structures with level interfaces can't be directly applied to the prospecting. It is very important, therefore, to study the seismic behaviors in these complex structures and to deve~lop a new technique that can be used to process and interpret seismic refraction data obtained from urban areas. In this paper, forward computing of wave field is carried out by using wavefront expanding method in terms of Huygens' principle. Furthermore, in the light of Hagedoorn wavefront refractor imaging principle a new processing method of seismic refraction data and the corresponding interpretation software are developed, in which Hole's original finite-difference codes were modified with Lecomte's five operators for computing seismic travel times. Applying this technique, we successfully process the data from two refraction profiles recently completed in Yixu, Fuzhou City during urban buried fault prospecting. The results show that the shallow structures in the investigation area display three layers, which are sedimentary cover, strongly weathered layer and bedrock, respectively. The buried depth of the upper surface of bedrock ranges from 52m to 58m or so. The variation of P wave velocity in sedimentary cover is considerable.

EFFECTS OF URBAN NOISE ON ELECTROMAGNETIC METHODS

BAI Deng-hai, WANG Li-feng, SUN Jie, ZHU Jin-fang, HUANG Zong-lin, HUANG Dan-qing, HE Zhao-hai, ZU Jin-hua, LIAN Yu-fang, Quentin Yarie, Volker Schaepe

2002, 24(4):  549-556. 

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Of the geoelectromagnetic techniques, multi-electrode Direct Current(DC)and Transient Electromagnetic(TEM)methods are the most powerful tools for shallow soundings. For city active fault detection, urban noise is a key problem in the use of electrical or electromagnetic methods. Effects of some major noises on DC and TEM are discussed on the basis of the experiments carried out in Fuzhou City in 2001. The experiments show that underground noises(pipes, cables, etc.)are most harmful to DC soundings, while for TEM, in addition to the underground noises, the aerial noises(power lines, metal sheds, etc.)will also lead to serious effects. Even so, effective soundings can be obtained providing that the noises are not too strong and some proper countermeasures are taken. In the experiments, specific measurement environments, including aerial and underground power lines and cables, water supply pipelines, roads, metal sheds, waste disposal sites, etc., were chosen as the urban noise sources. A set of RESECS instruments from DMT, Germany, were used for DC test, and EM-47/EM-67 by Geonics, Canada, for TEM test. The results of the experiments show that for DC soundings if the underground noise is not too strong, an effective record generally can be obtained, and especially the results would be much improved if the sampling time window of the instrument could be adjusted according to the noise distribution. We strongly recommend, therefore, that an instrument with real-time display of injection current and measurement potential, having adjustable time window be used for the active fault detection in urban areas. The configuration of electrodes is also important in some cases. For TEM soundings, it is better to set the measurement traverse at least 50m away from power lines, roads, cables, and big pipes. If the pipes are not big and not densely distributed within the transmitter loop, good data can be obtained at the sites several meters away from the pipes. In an area with electric current channeling, different configurations of the transmitter loops of different dimensions should be tested before the measurement.

DC AND TEM TEST SOUNDING FOR THE BAYISHUIKU- SHANGGAN FAULT IN FUZHOU CITY, FUJIAN PROVICE, CHINA

BAI Deng-hai, WANG Li-feng, SUN Jie, ZHU Jin-fang, HUANG Zong-lin, HUANG Dan-qing, HE Zhao-hai, ZU Jin-hua, LIAN Yu-fang, Quentin Yarie, Volker Schaepe

2002, 24(4):  557-564. 

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To test the effectiveness of DC and TEM soundings for urban active faults, experiments were carried out in Fuzhou City, Fujian Province in 2001. This paper is focusing on the test results of the Bayishuiku-Shanggan Fault, which is the major fault crossing the downtown of Fuzhou City. Two DC(DC-Ⅰ-1 and DC-Ⅰ-23)and three TEM(TEM-Ⅰ-1, TEM-Ⅰ-2 and TEM-Ⅰ-3)traverses were deployed in Bayishuiku area. DC experiment used a RESECS equipment. An array of 96 electrodes(192m long)was employed for the measurement, while a 16 electrode string(32m long)is prepared for rolling ahead at the end of the array for a long traverse. Electrode spacing is 2m. TEM used EM-47 and EM-67 systems. For EM-47 the measurements were made at the center of the transmitter loop with a dimension of 40m×40m. A large fixed loop with side length of 100～200m was employed for EM-67. Two adjacent loops overlap half of the side length. Site spacing is 10m for both EM-47 and EM-67. DC models show the electric structures at a depth range of 1 to 30m, while TEM models indicate those of 10 to 300m depths. DC model coincides excellently with the TEM model in the same traverse. All the DC and TEM models represent a layered structure with a strong vertical low resistivity anomaly in the middle part of each profile, which is inferred to be the main strand of the Bayishuiku-Shanggan Fault in the area. The fault appears to be several ten meters wide dipping NNE into the basement. The experiments suggest that the results will be much improved by using the combined DC and TEM method.

SHALLOW SEISMIC EXLORATION OF THE ACTIVE LIUWU-DAFOSI FAULT IN THE VICINITY OF LHASA CITY

MA Wen-tao, LING Hong, CAO Zhong-quan, CHU Bao-gui, ZHANG Lan-feng

2002, 24(4):  565-570. 

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This paper demonstrates the results of shallow seismic exploration of the Liuwu-Dafosi Fault in the vicinity of Lhasa City. On spot map, the segment of the fault in mountainous area can be identified distinctly, but the segment in river valley is unidentified. Shallow seismic exploration reveals that the fault dissects Tertiary or Quaternary strata. Geological interpretation of shallow seismic reflection profile reveals a velocity structure of five layers. Layer 1 to layer 4 have a velocity ranging from 0.8km/s to 2.0km/s or 2.8km/s, and can be assigned to Tertiary or Quaternary strata. Layer 5 has a velocity ranging from 3.0km/s to 4.0km/s, which is supposed to be granite bedrock according to its scattered distribution in CDP stacked section and geological evidence. The Liuwu-Dafosi Fault can be identified at 320m distance of the shallow seismic reflection profile located to the southwest of the Lhasa City. The fault is dipping northeast at an angle of 80?, the highest point of which is at 100m depth. It is determined that the fault is an active reverse fault.

STUDY ON THE RELATIONSHIP BETWEEN SHALLOW AND DEEP STRUCTURES IN THE 1976 TANGSHAN EARTHQUAKE AREA

YOU Hui-chuan, XU Xi-wei, WU Jian-ping, HE Zheng-qin

2002, 24(4):  571-582. 

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In this paper, field observation on earthquake surface rupture zone and high-resolution seismic reflection profiling of faults in combination with related geological data have led to some new insights about shallow Quaternary geology and underground fault structure in the 1976 Tangshan earthquake area. On this basis, we analyze prospecting results of deep structure and then study the relationship between deep and shallow structures in the 1976 Tangshan earthquake area, as well discuss tectonic setting and seismogenic model for this earthquake. Tanshan fault zone is a complex structure zone associated with faults and folds in this area. It is mainly developed on southeastern flank of an anticline and northwest flank of the Kaiping syncline, and consists of a series of NNE-NE-trending parallel faults. This fault zone can be divided into southern and northern segments and/or bifurcated into eastern and western branches, controlling seismic activity and geological motion in this area. The eastern branch of southern segment of this fault zone is Tangshan Fault, which strikes to N30°E and consists of two parallel faults. The eastern fault dips to WNW at an angle of 70°～80° and represents the main fault with reverse-strike-slipping. The 1976 Tangshan M 7.8 earthquake produced four surface rupture zones. The main surface rupture zone is ～8km long and generally strikes to N30°E. It consists of many secondary right-lateral en echelon ground fissure zones on both southern and northern segments, distributed along Tangshan Fault. This surface rupture zone underwent horizontal and vertical dislocations, with horizontal separation of 1.53m in maximum and vertical offset of 0.2～0.7m. The western side of this zone has been uplifted and the eastern side subsided. The high-resolution seismic reflection profiling revealed the following relations between Tangshan Fault and earthquake surface rupture zone:(1)their positions are consistent;(2)their geometric attitudes are consistent, dips to west at angle of 70°～80°;(3)their movements are of reverse-strike-slip faulting, while strong-reflection interfaces show a compressive bending and strike-slip motion characters. These facts indicate that there exists a high-angle reverse strike-slip fault, i.e. the NNE-trending and WNW-dipping Tangshan Fault, the sudden motion of which produced the 1976 Tangshan M 7.8 earthquake. The high-resolution seismic reflection profiling also revealed a SE-dipping reverse fault with a dip angle of about 70°. It occurs on the western steep limb of the Kaiping syncline east of the Tangshan Fault, and may be a shallow bending-slip or bending-moment fault, on which a ground fissure zone produced by the 1976 Tangshan earthquake. Analysis on deep seismic sounding result indicates that the relation between shallow and deep structures is close in spatial location, geometric structure and movement nature. There exist W-dipping slope of the Moho discontinuity, crustal "anticline", horizontal detachment and propagation fault in the middle crust, high-angle reverse-strike-slip fault and fold structure in the upper crust. They form a structural pattern of multi-order and multi-step composite fault-propagation fold, and controll the tectonic deformation and seismic activity in this area. The 1976 Tangshan earthquake is a result of stick-slipping along Tangshan Fault zone caused by the detachment and fault-propagation in the middle crust and by the stress concentration on the base of upper crust. The cumulative vertical offset has reached 15m along the Tangshan Fault since the late Pleistocene. The vertical offset of ground is 0.3m caused by the Tangshan earthquake and 0.5m obtained by inversion of seismic wave data. If creepslip is not taken into account, the recurrence interval of earthquakes on the Tangshan Fault is 2.4～4.0ka, which is comparable with the result of regional paleo-seismological research.

HIGH-RESOLUTION SEISMIC REFLECTION PROFILING OF THE FENHE FAULT, TAIYUAN CITY

YOU Hui-chuan, HE Zheng-qin, DING Zhi-feng, WU Jian-ping, WU Qiang-ju

2002, 24(4):  583-592. 

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This study focuses on a NNE-striking, normal-right-lateral-strike-slip fault, the Fenhe Fault in Taiyuan City. It is one of the faults from the NNE-trending fault system developed within the Shanxi faulted basin zone, buried beneath the ground surface. In this paper, we demonstrate the high-resolution seismic reflection data for a depth range of several hundred meters across the Fenhe Fault. In combination with the relevant borehole logs, these data provide useful constraints on the accurate position, geometry and deformation rate of the fault, as well as the kinematics of recent fault motion. Two high-resolution seismic reflection profiles across the Fenhe Fault in Taiyuan City clearly demonstrate two continuous, strong-reflection interfaces(at two-way time of 85～110ms for the upper one and of 180～200ms for the lower one), and three reflection layers separated by these two interfaces. In addition, they reveal the near-surface location, geometry and activities of this oblique-normal right-lateral fault. Although the westward dipping fault at shallow level cannot be ruled out, we consider the eastward dipping high angle fault, which is located between the canal and the western embankment of the Fenhe River, to be the west branch of the Fenhe Fault. Based on the geological and drilling data, three seismic reflection layers are inferred to represent the Holocene-upper Pleistocene, middle-lower Pleistocene and Neogene strata, respectively. A total vertical separation of ～10m has occurred on the fault since the beginning of Quaternary, terminating at ～70m below the ground surface. In the upper ～190m section of the borehole log near the two seismic profiles, two closely spaced, subvertical strike-slip faults can be recognized. The eastern branch fault is more active than the western one. It lies beneath the eastern embankment of the Fenhe River, dipping to the west and cutting into the upper layer of Holocene-late Pleistocene strata with a maximum vertical offset of ～8m. Another borehole log across the northern segment of the Fenhe Fault reveals an eastward dipping subvertical, right-lateral strike-slip oblique normal fault, which has cut into the upper layer of Holocene-late Pleistocene strata with a maximum vertical offset of ～6m. The afore-mentioned data provide a minimum average Pleistocene-Holocene vertical slip rate of ～0.06～0.08mm/a and a maximum average recurrence interval of 5.0～6.7ka for the Fenhe Fault, providing that the vertical offset caused by large earthquake is 0.4m per event(similar to the offset caused by the Tangshan earthquake). This is a long-recurrence interval and a low vertical-slip rate probably similar to or more than those of the Tangshan No.5 Fault. If the 0.08mm/a vertical-slip rate characterized the history of faulting at seismic profiling site, then the currently active strand of the east branch of the Fenhe Fault had been active since middle Pleistocene(～150ka), and before that time the west branch of the Fenhe Fault was active but probably had stopped moving at that time. Epicentral distribution of earthquakes clearly demonstrates the seismogenic structures of the Shanxi faulted basin zone. Within these NNE-NS-trending basins, some large historical earthquakes have occurred on the NNE-NS-trending faults. Although within the Taiyuan basin and on the Fenhe Fault occurred only some moderate earthquakes, their structures and activities are similar to those of the large earthquakes. It can be concluded, therefore, that the seismic risk of the Fenhe Fault is an objective reality. As the fault just lies beneath the urban population center of the Taiyuan City, a large earthquake(M_S 7.0～7.5)on it may cause enormous damages. An insight into the activity of the fault is needed for the earthquake prediction and hazard reduction of the Taiyuan City.

A TRIAL GEOCHEMICAL PROSPECTING FOR BURIED ACTIVE FAULTS IN FUZHOU CITY

WANG Guang-cai, WANG Ji-hua, LIU Cheng-long, LIU Wu-zhou, ZHANG Pei-ren, LIN Yuan-wu, ZHU Jin-fang, HUANG Zong-lin, ZHAO Zhi-wei

2002, 24(4):  593-600. 

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A trial geochemical prospecting for active faults in urban area through the survey of soil gases has been conducted in Fuzhou City, China. This paper demonstrates the principal methodology and results of the trial prospecting. As compared with field investigation of active fault, the detection of buried active faults in urban area is more complicated since there are additional unfavorable environment and conditions such as road, constructions, refilled soils, rubbish dumps and soil/water pollution etc. The trial is aimed at the assessment of the effectiveness of soil gases survey for buried fault prospecting in urban area, and the identification of the surface locations of two buried faults in Fuzhou City. The trial includes the following comparative surveys: 1)different types of sites(soils); 2)different radon detectors and 3)different detecting items(adsorbed mercury, free mercury and radon in soil). Totally, 18 traverses of free mercury gas surveying have been conducted, along with 8 traverses of free radon gas and 1 traverse of adsorbed mercury surveying. The anomalies of different items are basically accordant, but it seems that radon gas is more sensitive to the influence of environmental factors such as groundwater level. The locations of two buried faults determined by soil gases are in good accordance with those determined by seismo-geologic investigation. The results of soils gases survey have also been compared with those of shallow seismic exploration. The comparison shows that the anomalies detected by the two different prospecting techniques are fairly coincident(the "fitting ratio" is about 70%); the false soil gas anomalies seem to occur in such a site as bridge side, abandoned construction bases and rubbish dumps.

EXPLORATION AND SEISMIC HAZARD ASSESSMENT OF ACTIVE FAULTS IN URBAN AREAS

DENG Qi-dong

2002, 24(4):  601-605. 

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Displacements along active faults buried directly beneath major cities create devastating earthquakes that seriously threaten the safety of human lives and properties. Exploration and seismic hazard assessment of active faults in urban areas are thus an important systematic engineering for disaster mitigation in major cities. It is also a new field for active tectonic studies. The kernel of this work includes determining of the exact location of active faults, dating the ages of last tectonic activity, relating the shallow level faults to structures in the crustal depth, assessing seismic hazard and potential for surface offsets, and formulating countermeasures for disaster mitigations. We use the following simple phrases to express these key scientific problems: Where are the faults? Are they active? How deep are they? Will they create earthquake? Will they form surface offsets? What are the countermeasures? This paper explains these key scientific problems in detail.

GEOPHYSICAL METHODS FOR THE EXPORATION OF URBAN ACTIVE FAULTS

FANG Sheng-ming, ZHANG Xian-kang, LIU Bao-jin, XU Xi-wei, BAI Deng-hai, JI Ji-fa

2002, 24(4):  606-613. 

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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.