Table of Content

09 June 2007, Volume 29 Issue 2

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Brief Report

REMARKS ON URBAN ACTIVE FAULTS EXPLORATION AND ASSOCIATED ACTIVITY ASSESSMENT

DENG Qi-dong, LU Zao-xun, YANG Zhu-en

2007, 29(2):  189-200. 

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According to the practice of urban active fault exploration and associated activity assessment conducted in recent years,this paper summarizes the problems encountered in geological,geomorphological,geochemical and geophysical explorations,and proposes the following means and suggestions to solve these problems.To determine the most recent faults or fault zones,emphases should be placed on iden～tifing the youngest active faults and offset geomorphology.To understand the history of faulting and to discover the last offset event,it is suggested to combine geophysical prospecting,drilling and trenching into one profile.Because of significant uncertainties in late Quaternary dating,it is advised to sample systematically and to use multiple dating methods.Shallow seismic reflection has been proven to be the most useful method in urban active fault exploration.However,it is a pressing need to increase the quality of data acquiring and data processing to obtain high resolution images,so that the abi～lity of identifying active faults could be enhanced.Combination of seismic P-wave reflection and S-wave reflection methods is proved to be a powerful means to investigate tectonic environments of deep crust.

NEAR-SURFACE CHARACTER OF PERMANENT GEOLOGIC DEFORMATION ACROSS THE MEGA-STRIKE-SLIP FAULTS IN THE NORTHERN TIBETAN PLATEAU

XU Xi-wei, YU Gui-hua, CHEN Gui-hua, LI Chen-xia, ZHANG Lan-feng, Yann Klinger, Paul Tapponnier, LIU Jing

2007, 29(2):  201-217. 

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The Altyn Tagh,Kunlun and Haiyuan Faults are three major left-lateral strike-slip faults with high geologic and GPS-derived horizontal slip rates as well as frequent surface-rupturing earthquakes in the northern Tibetan Plateau.There exist local structures,such as pull-apart basins in stepovers and sag ponds,where fine-grained and/or organic interfaulting sediments have been continuously filled and co-seismic faulting traces have been well preserved in those sediments.Trenching across the strike-slip faults and those local structures,stratum-logging of the trench walls and structural-stratigraphic examination can uncover basic features of the permanent and cumulative geologic deformation zone of a strike-slip fault that has experienced several surface-rupturing earthquake cycles.The geologic section of the Banguoba trench 9km east of Old A'kesai Town across the recent traces of the Altyn Tagh Fault records 7 paleoearthquake events with a co-seismic left-lateral slip of 7±1m for the latest event and its cumulative geologic deformation zone is only 8m in width.The geologic section of the western Old A'kesai trench across a pull-apart basin of the Altyn Tagh Fault records at least 4 paleoearthquake events and its permanent and cumulative geologic deformation zone is only 13m in width.The geologic section of the Xidatan trench across the Kunlun Fault reveals 5 paleoearthquake events and their cumulative geologic deformation zones are 12～13m in width.The Maqin trench across a pull-apart of the Kunlun Fault also reveals 5 paleo-earthquake events and most of the structural deformation,about 15m wide,is concentrated in the pull-apart,while the widest structural deformation,including the associated distortion nearby the boundary fault of the pull-apart,is less than 35m.Two trenches excavated across the southern and northern boundary faults of the Songshan pull-apart basin along the Maomaoshan-Laohuashan segment of the Haiyuan Fault show up 6 paleoevents and their permanent geologic deformation zones are less than 10m wide for single boundary fault.Of course,as a extensional jog,the pull-apart basin over a hundred meters wide will experience severe tensional and transtensional surface ruptures during an earthquake,and the pull-apart basin itself may be taken as one part of the permanent geologic deformation zone.Thus,the repeatedly faulting of the Altyn Tagh,Kunlun,and Haiyuan Faults during the past several surface-rupturing earthquake cycles is localized along their strike and the width of their permanent geologic deformation zone for a single strike-slip fault is over 10 meters,but less than 30 meters in general.

XISHAN FAULT GROUP NEAR URUMQI CITY AND PALEOEARTHQUAKE IDENTIFICATION ON REVERSE FAULT

RAN Yong-kang, CHEN Li-chun, SHEN Jun, LI Jun, GONG Hui-ling

2007, 29(2):  218-235. 

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Xishan Fault group is distributed in the transition zone between the fold-reverse fault system along the front of the north Tianshan Mountains to the west and the thrust tectonics of Bogeda to the east.It is a tectonics that thrusts from the basin in the north to the mountains in the south,consisting of 4～5 faults that are more than 10 to about 30km long,showing low angles near surface and converging on the detachment surface at about 11km deep.We discovered that the activity of Xishan Fault group is distinct during late Quaternary by doing field investigation of geology and geomorphology,excavating trenches along faults and analyzing deep structure of the fault group.The faults offset the second and above terraces of Wanjiagou creek and created fault scarps of 0.5～5.4m high on the terraces.And traces of paleoearthquakes can be found easily.The younger two events on F₁,F₂ and F₃ are confined in(22.7±5.2)ka and 40ka BP by OSL samples dating,respectively and the traces of the youngest event on F₄ and the front fault of Xishan are covered by deposits whose ages of OSL samples are about(31.1±3.2)ka and(37.9±3.8)ka BP,respectively.It means that there was grouped faulting in late Quaternary in the Xishan Fault group.F₁,F₂ and F₃ or F₄ and the front fault of Xishan might rupture in a same event on near surface.Event traces on the Xishan Fault group and other reverse faults of low angle show that the deposits along the front of fault scarp,the offset relation between fault and deposit bed,and the abrupt increase and diminution of displacement on difference markers or unconformable surfaces on both sides of fault are important identification marks of paleoearthquakes along surface rupture-type reverse fault.The deposits along the front of fault scarp on reverse faults of low angle are much more different from those on normal fault.For ideal mode,the deposit in front of fault scarp of reverse fault of low angle is characterized with that the original structure of the collapsing thrust sheet front is not broken entirely on the lower part and the sloping deposit on the upper part may exist covering on both sides of the fault.We think that it is very important for reducing uncertainty of paleoearthquake identification to seek for evidences as many as possible and analyze the different influencing factors,such as tectonics,climate,environment and anthropic activities.

FREQUENCY-MAGNITUDE RELATIONSHIP MODELS FOR ASSESSMENT OF MAXIMUM MAGNITUDES OF POTENTIAL EARTHQUAKES ON MODERATELY AND WEAKLY ACTIVE FAULTS IN EASTERN CHINA MAINLAND

WEN Xue-ze, XU Xi-wei, LONG Feng, XIA Cai-fang

2007, 29(2):  236-253. 

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In this paper,we make an effort to study the feasibility to assess magnitudes of maximum potential earthquakes in sub-areas of moderately and weakly active faults in eastern China mainland by using parameters of frequency-magnitude relationship,and develop the corresponding methodology.Data suggest that frequency-magnitude relationships of fault sub-areas in eastern China mainland accord with the characteristic earthquake model,meaning that for a fault sub-area,the ratio a/b of the constants,which is also called as the maximum intercept magnitude of the exponential portion(i.e.the G-R relationship)of the frequency-magnitude relationship,is obviously less than that of the characteristic magnitudes M_C.In order to make the ratio a/b be usable in indirectly assessing the magnitudes of maximum potential earthquakes in fault sub-areas,we develop a method to establish long-duration frequency-magnitude relationships for fault sub-areas by combining data of both historical and modern seismicity.In this method,event's numbers from the two sources of data are all normalized to a duration of t=500years.We then calculate a_t/b values of the normalized G-R relationships for 130 fault sub-areas.Our analyses reveal that maximum magnitudes,M_max,of earthquakes occurring and recorded in the studied fault sub-areas are positively correlative with sizes of a_t/b values,and with the increase of a_t/b values the upper-limits of the maximum magnitudes,M_max,show the feature of monotonously rising and relatively smooth variation.Based on such feature we develop three empirical formulae of the relations between the upper-limits M_mu of the maximum magnitudes,and a_t/b values,for the three regions,i.e.North China,Central and East China,and South China and the southeastern coastal area,respectively,and take them as empirical models to estimate magnitudes of maximum potential earthquakes in fault sub-areas.By using the newly developed method and empirical models we estimate magnitudes of maximum potential earthquakes in several fault sub-areas.Our research also suggests that several types of the abnormal seismicity,such as the swarms of moderate and small size earthquakes,aftershocks and triggered earthquake sequences,and artificially induced seismicity,as well as the determination of minimum complete magnitudes,have influences to the calculation results of a_t/b values,and that severely influenced a_t/b values are overestimated and show deflecting to the right on M_max-a_t/b diagrams.The empirical models and method developed in this study can be applied to the assessment of magnitudes of maximum potential earthquakes for sub-areas of moderately and weakly active faults in eastern China mainland.

THE APPLICATION AND LIMITATION OF MULTIPLE SEISMICITY PARAMETERS TO ASSESSING CURRENT FAULTING BEHAVIOR AND SEISMIC POTENTIAL OF ACTIVE FAULT ZONES

YI Gui-xi, WEN Xue-ze

2007, 29(2):  254-271. 

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In order to analyze the feasibility using seismicity parameters to identify current faulting behavior and hence to assess long-term seismic potential of active fault zones,we introduced the technique of spatially scanning and mapping b values along fault zones,as well as the procedure of data processing,calculating and result analyzing of multiple-seismicity-parameter-value combinations(i.e.b,√E,n and a/b)for various fault-segments in this paper.We then brought forward several reference criterions for fault segmentation,that is,the calculation of multiple seismicity parameters.Moreover,we generalized the approach and thinking of synthetically identifying spatial differences of current faulting behavior from the calculated parameters by combining histories of strong and major earthquakes and spatial distributions of modern seismicity.As an application,we took 5 active fault zones with various current seismicity levels and seismic detectabilities as the experimental targets to analyze synthetically the spatial differences of current faulting behaviors and identify fault segments with potential risk of strong earthquakes for each of the 5 fault zones basing on the generalized approach or methods.Meanwhile,the issues,such as the relation between the postseismic fault healing and relatively low b-value,the proper use of high-accurate re-located seismic data in the parameter calculating,and etc.have also been discussed.The results mainly suggest that the spatial distribution of multiple seismicity parameter values,among which the b-value is the most important one,can be effectively applied to analyzing current faulting behaviors and hence to identifying fault segments with potential risk of strong or major earthquakes.The two methods,the scanning and mapping of b-values along fault zones and the analysis of multi-parameter-value combinations for various fault-segments,can be applied to active fault zones in areas of high or moderate seismic detectability,respectively.And if the two methods are employed to areas with higher seismic detectability,more reliable results will be obtained.Fault-segments with activity of late-stage aftershocks or with postseismic fault healing may have relatively low b or b-values too.We believe that once a catalog of earthquakes accurately relocated is used in calculation of seismicity parameters,it must contain the complete data of earthquakes occurring during the period and in the area studied,rather than only a part of data selected.

ACTIVITIES AND SLIP RATE OF THE FRONTAL FAULTS OF THE KALPINTAG NAPPE,TIANSHAN MOUNTAINS,CHINA

SONG Fang-min, MIN Wei, HAN Zhu-jun, XU Xi-wei

2007, 29(2):  272-281. 

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The Kalpintag nappe is located at the southwestern foot of the Tianshan Mountains,consisting of several rows of NE-to EW-trending fold-reverse fault zones.This paper demonstrates the activities and slip rate of the frontal faults of the first to third row fold-reverse fault zones located to the west of the Piqiang-Bachu phosphorite mine.The newly-found evidence shows that the front of each fold-reverse fault zone is composed of several faults of typical reverse fault type.The faults with newest activity are located at the forefront of the fold-reverse fault zones,and the active period of the faults is late Pleistocene-Holocene.They dissect the T₀,T₁,T₂ and T₃ terraces,resulting in fault scarps of different heights.According to the in-situ measurement of the fault scarps and the dating data of the relevant samples,it is estimated that the amount and rate of vertical displacement along the faults since the formation of the T₀ terrace are 0.9～1.1m and 0.53～0.65mm/a,respectively,while the amount and rate of the corresponding crustal shortening are 1.93～2.56m and 1.14～1.52mm/a,respectively.Similarly,it is estimated that the amount and rate of vertical displacement since the formation of the T₁ terrace are 1.4～1.8m,and 0.36～0.46mm/a,with the corresponding crustal shortening of 3.00～3.86m,and shortening rate of 0.77～0.99mm/a.The vertical displacement amount and rate since the formation of the T₂ terrace are about 2.1～3.0m and 0.31～0.45mm/a,respectively,and the amount and rate of the corresponding crustal shortening are 4.50～6.98m and 0.67～1.04mm/a.The amount and rate of vertical displacement since the formation of the T₃ terrace are 3.4～4.2m and 0.28～0.35mm/a,and the amount and rate of the corresponding crustal shortening are 7.29～9.22m and 0.61～0.77mm/a,respectively.Based on the obtained amount and rate of crustal shortening since the formation of T₀ terrace,the total amount and rate of crustal shortening of the Kalpintag nappe since 1.7ka can be estimated to be 9.65～12.80m and 5.68～7.53mm,respectively.

SOME PROBLEMS WORTH CONSIDERING IN THE GEOLOGICAL EXPLANATION OF SHALLOW SEISMIC PROSPECTING DATA

YANG Xiao-ping, ZHENG Rong-zhang, ZHANG Lan-feng, CHEN Xian-cheng, MA Wen-tao, XU Xi-wei, WANG Rui-guang, LIANG Hui, WANG Yan

2007, 29(2):  282-293. 

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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.T_0-1 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.

MICROSTRUCTURAL AND MICRO-SEDIMENTOLOGIC ANALYSES OF THE TRENCH ACROSS THE JIAOCHENG FAULT,XIZHANGCUN VILLAGE,TAIYUAN,SHANXI PROVINCE

LIN Chuan-yong, CHEN Xiao-de, SHI Lan-bin

2007, 29(2):  294-310. 

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Faulting process in soft sediments usually did not leave any macroscopically observable trace,so the occurrence of the fault as well as its activity and mode of motion cannot be identified by naked eyes in the field.Available data have shown that microstructural and micro-sedimentologic analyses are effective method for identifying obscured fault and paleoearthquake event in soft sediment.Recently,a large-scale trench has been excavated across the Jiaocheng Fault by the Institute of Crustal Dynamics and Earthquake Administration of Shanxi Province.They believe that the trench reveals 3 paleoearthquake events of M>7.0.In this study,we attempt to seek for the microstructural and micro-sedimentologic indicators of obscured fault and paleo-seismic event in soft sediment,through the microscopic analysis of this trench.Microstructural observation reveals that the fault in this trench microscopically appears as micro-fracture zone,slip surface,deformation bands and the bands of clay fillings,while deformation microstructures can be observed in the samples collected from the vicinity of the fault.Micro-sedimentologic analysis shows that the angularity,grain-size and particle size distribution of the samples collected from the fault differ to some extent from those of the samples collected outside the fault.The fractal dimension of the particle size distribution of the sample collected from the fault is D=2.0,which indicates the existence of cataclasis by controlled particulate flow during faulting process.Microscopic indicators comparable to the macroscopic indicators of seismites,such as ball and pillow structure,load structure,fluid flow,mass flow and the floatation of organics,are observed widely in the samples studied.All the results show that microstructural and micro-sedimentologic analyses are effective approaches to the identifying of obscured fault and paleo-seismic event in soft sediments.

A PRELIMINARY STUDY ON SEISMOTECTONIC MODEL FOR THE ACTIVE FAULTS IN XINING URBAN AREA

TIAN Qin-jian, LI Zhi-min, ZHANG Jun-long, REN Zhi-kun

2007, 29(2):  311-319. 

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The Xining basin is a Cenozoic basin bounded by the Riyueshan Fault and the Lajishan Fault on the south,the Dabanshan Fault on the north.Controlled by these active boundary faults and under the near NE-oriented compressional stress,fold deformation occurred inside the basin.Several active faults of NE or EW trending were detected.Nevertheless,all of them are small and weakly active.By the study on the relationship between fault and folding,we think that the Huangshuihe Fault and the NW-strike north bank of Huangshuihe Fault are tensional faults developed on the apex of an anticline;the Nanchuanhe Fault is a transverse tear fault resulting from differential folding on the two sides of the fault;and the east bank of Beichuanhe Fault is a compressional fault developed on the core or limb of an syncline.By the balanced cross-section analysis of fold deformation and inversion of gravity anomaly data,we obtained that the depth of detachment plane is about 4～5km.Thus,based on the above data,the seismotectonic model of the Xining urban area is built.We conclude that the faults of the area are the secondary faults related to folding of overburdens and have no potential of producing earthquakes larger than magnitude 6 for their small scale and shallow depth.

CONSTRUCTING 3D SUBSURFACE STRUCTURE MODEL FROM ACTIVE FAULT SURVEY DATA OF THE YINCHUAN AREA

YU Gui-hua, XU Xi-wei, CHAI Chi-zhang, WANG Yin, LIU Bao-jin

2007, 29(2):  320-329. 

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The project of "Yinchuan active fault exploration and earthquake risk assessment" is aimed at strengthening the fundamental work of engineering construction for earthquake resistance and prevention in Yinchuan City.In order to achieve this general goal,a joint multi-disciplinary exploration of the Yinchuan buried fault and the Luhuatai fault has been carried out on the basis of collection and analysis of predecessors'data.This paper discusses the methods of constructing three-dimensional subsurface structure model of the project area of Yinchuan active fault exploration by using the achievements of the Yinchuan active fault exploration,and puts forward main work steps and suggestions for the model construction.The steps for 3D modeling of subsurface structure using active fault survey data are as follows:To collect and pre-process the data of seismic exploration of active fault;Using seismic interpretation software to interpret the fault and the bed position,draw bed plane structure map,and output the fault and bed position data,meanwhile,to further complete the 3D visual modeling with this software;To load fundamental data,bed plane and fault data into the 3D modeling software GOCAD,carry on the fault plane and the bed plane revision and reconstruct 3D model,construct three-dimensional cutting section,and demonstrate the three-dimensional model achievements.Since the seismic survey data are not standard,it increases the difficulty in earthquake data explanation and affects the explanation precision.Therefore,we propose to standardize the archiving of primary seismic survey data,add a cross line in the survey area of each active fault,and if possible,use the regional time-depth conversion used by petroleum sector to enhance the explanation precision of seismic section.

ACTIVITY OF THE LINTONG-CHANG'AN FAULT ZONE SINCE THE LATE QUATERNARY

DAI Wang-qiang, HAN Heng-yue, FENG Xi-jie, SHI Ya-qin, LI Xiao-ni

2007, 29(2):  330-335. 

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The Lintong-Chang'an Fault zone locates in the middle part of Cenozoic Weihe depression.It is the boundary fault controlling the Lishan diamond block and Xi'an sag.The landforms are obviously different between the sides of the fault,and the geomorphic forms are stepped fault scraps and loess scraps.In the paper,by field geological survey to the Zhongdi Village,Wangjiabian Village and Qiaogou profiles on the Lintong-Chang'an Fault,and in combination with the dating data of regional loess and paleosol profile(An Zhi-sheng and Sun Jian-zhong),the fault is studied in order to explore the times of its latest activity and the characteristic of its late Quaternary movement.The fault strikes NE as a whole and is characterized with tensile vertical movement.The fault obviously offset the first paleosol layer S₁ in loess stratum,indicating that it is still active since late Pleistocene epoch.But most fault displacements are less than 2m,the slip rate is low,and the activity level is higher in the northern and central segments than that in the southern segment of the fault.Regarding that the Lintong-Chang'an Fault consists of several secondary faults,its whole activity should be much higher than the local slip rate of the fault we have derived.The fault displacements show an increasing trend with depth and the slip rates calculated using the dating data of different strata are almost the same.So perhaps,the fault is mainly dominated by vertical creep-slip since the late of middle Pleistocene epoch.

A TEST STUDY OF URBAN FAULTS USING ANALYTICAL METHOD OF MAGNETIC ANOMALY

ZHANG Xian, ZHAO Li

2007, 29(2):  336-353. 

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Beijing area is chosen as the testing field to study the urban fault.Aeromagnetic data in the field are processed using analytical methods of magnetic anomaly such as analytical downward continuation,spectrum analysis,regularization filtering,directional derivative,wavelet analysis,fractal dimension,inversion of apparent magnetization and inversion of magnetic surface.The effect,function and adaptability of various interpreting methods of magnetic prospecting are evaluated on the basis of analysis and contrast of a lot of data in the urban fault study.Instructional opinions are proposed about magnetic prospecting techniques in the future urban fault exploration after the test results are summarized.For example,the upward continuation and the regularization filtering or multi-scale separation of wavelets are effective for revealing deep faults.The height of upward continuation and the wavelength of regularization filtering should not be too big,otherwise,it is possible to enhance regional structures and restrain the information of local faults.Downward continuation and vertical derivative are effective for revealing shallow faults.It is necessary to control the largest downward continuation depth to the top of field source,otherwise,the data can not be used because of vibratory results.Horizontal derivative or wavelet analysis of high frequency is effective for revealing faults in the pre-judged direction.Inversion of apparent magnetization is effective for revealing faults in the area where there is a distinct difference of rock magnetism between two sides of the fault.Inversion of base magnetic surface is effective for revealing the vertical basement fault with a big shift of base magnetic surfaces between the two blocks along the fault.In addition,the testing results also show that spectrum analysis has a good effect on computing the field source depth for local anomaly,but not for regional anomaly.We hope that the study results can play an active role in the future exploration of urban faults.

A PRELIMINARY STUDY ON TANGXI FAULT USING FAULT ZONE TRAPPED WAVES

LI Song-lin, LAI Xiao-ling, HE Jia-yong, TIAN Jian-hua

2007, 29(2):  354-362. 

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As the first application of fault zone trapped waves to the active fault exploration in urban areas,an experiment aimed at Tangxi Fault of Taihang Mtns.Fault Belt was performed with explosion sources.By using digital filtering,spectral analysis and other techniques,the fault zone trapped waves were separated from seismograms.Based on the observational data of two transverse profiles,the position of the fault was detected,and its width was assumed to be 200～300m.The great width of the fractured zone may be due to the tensional action.The normal dispersion nature of the fault zone trapped waves was revealed from multiple band-pass filtered seismograms.The waves with lower frequencies travel faster than those with higher frequencies.This indicates that the fault zone trapped waves are surface waves and their dispersions can be used to study physical properties of the medium in the fault zone.The results also show that the fault zone trapped waves can be formed and propagate not only in new fractured zones,but also in old faults.Indeed,they can be generated if the physical properties of the mediums inside and outside the fault zone are quite different.The method using fault zone trapped waves is of great advantage for active fault exploration,especially in urban areas.

APPLICATION OF SHALLOW SEISMIC EXPLORATION IN DETECTION OF BURIED FAULT IN COASTAL AREAS

HE Zheng-qin, CHEN Yu-kun, YE Tai-lan, WANG Xiang-dong, WANG Hui, JIA Hui

2007, 29(2):  363-372. 

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The depth of Quaternary in China coastal zone varies greatly.In most areas,silt,clay and sand are always multiply interbedded.This kind of formation can attenuate greatly the high frequency signal,and is unfavorable to propagation of seismic wave.At the same time,the density of population in the area is much higher and the industrial noise is strong,so the trigger and receiver conditions are not ideal.The paper presents some application examples of detecting buried faults with near-surface seismic methods in east Guangdong,Tianjin and Tangshan where the thickness of overburden varies greatly,and systemically introduces the methods of fieldworks,data processing and explanation approaches.It also illustrates the effective methods of how to improve SNR and resolution in environment with strong noise.In the application,the signal of refection is strong and the buried faults can be reflected evidently.Proofs of drilling at both sides of the faults approve that the depth of interface,location of fault point and throw of basement determined by seismic exploration are all reliable.The techniques and methods mentioned in this paper are of important reference value in urban active fault detection and engineering geophysical prospecting in similar areas.

EXPERIMENT ON JOINT DETECTION BY SEISMIC REFLECTION AND ELECTRICAL IMAGING:WITH AN EXAMPLE OF EXPLORATION OF ACTIVE FAULT IN ZIBO CITY

ZHU Tao, HE Zheng-qin, FENG Rui, HAO Jin-qi, ZHOU Jian-guo, WANG Hua-lin, WANG Shuo-qing

2007, 29(2):  373-380. 

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In exploration of a shallow active fault in urban area,seismic reflection survey and electrical imaging are the two main geophysical tools.Both of them can be implemented independently.However,the complexity of subsurface geologic tectonics,especially the lateral irregularities close to fault and geologic tectonic zones could result in great deviation from the detected results.In the meantime,the geophysical inversion has not only one result.These would lead to a large difference of stacked velocity model established only by its own information from real one in seismic data processing.In consequence,the geologic interpretation would be harmed.On the other hand,electrical imaging is very sensitive to the lateral irregularities of rock properties but has weak resolution in depth.Therefore,the two techniques can just compensate each other and a joint detection can be carried out to interpret a complex geologic tectonics more reasonably.In this paper,experiment on the joint detection using seismic reflection survey and electrical imaging is made basing on the simple relationship between resistivity and seismic P-wave velocity:ρ_α∝|Z|²∝V_p².It is shown that a reasonable quasi 2-D seismic stacked velocity model can be obtained referring to the resistivity image from electrical imaging.In the end,the reasonable seismic reflection and time-depth conversion profiles are gotten,which is helpful for further understanding and more reasonably interpreting the geological structures.

STATISTICS ANALYSIS ON THE ¹⁴C AGE OF HUMIC ACID AND HUMIN IN PALEOSOL

YIN Jin-hui, ZHENG Yong-gang, LIU Yue-xia, LU Yan-chou

2007, 29(2):  381-389. 

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Based on 44 ¹⁴C dates of humic acids and humins under different weather conditions,a statistics analysis was made to estimate their performance in paleosols.The following suggestions are reached:(1)A normal distribution analysis for HA/HM values deduced from 44 ¹⁴C dates results in average value of 0.98 with standard deviation of 0.20.This seems that the humin may be the oldest fraction of soil and its ¹⁴C age can represent the age of the paleosol.(2)The relationship between HA/HM value and the age of paleosol samples shows that the ages of humic acid and humin are indistinguishable for samples deposited in late Quaternary,but are obviously different from paleosol samples in Holocene.Their different behaviors in late Quaternary and Holocene may be related to the pollutant due to Holocene soil exposed to the surface and easily influenced by pollutant.Therefore it is suggested to use the different fractions for Holocene soil to obtain reliable radiocarbon ages for soil samples.(3)Assuming that the modern carbon pollutant entered into soil one-off,the amount of modern carbon into original carbon is estimated to be about 1.49% for late Quaternary soil sample,but this value increases to 21.37% in Holocene soil.Therefore,the amount of pollutant for Holocene soil is over 15 times than that for late Quaternary soil.If such pollutant continues to enter into soil with similar quantity during deposition,the impact percentages are 5.71% and 30.46% for late Quaternary and Holocene soil samples,respectively.The amount of modern carbon by continuously entering into soil is 2.6 times that of the one-off entering into soil for one sample in the same condition.It is further needed to state that which fractions could obtain more reliable ages for soil samples still depends on other factors such as geomorphic location,climatic zone,and soil type.

OSL DATING OF TWO BURNT REWORKED LOESS SAMPLES FROM CIERGOU PROFILE,HAIYUAN FAULT ZONE:RESULTS AND GEOLOGICAL IMPLICATIONS

ZHANG Ke-qi, CHEN Jie, LIU Jin-feng, YIN Jin-hui, WANG Chang-sheng, WANG Tong-li

2007, 29(2):  390-401. 

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The fine-grain quartz extracted from two burnt reworked loess samples LEDL0588H and LEDL0365 was measured by modified SAR and SMAR protocols for OSL dating.The fast component is dominant in OSL signals.The identical De in the thermal treatment from 180～260℃was showed in preheat plateau test using both protocols.As was shown in measuring the values T_X/T_N and L_N/L_N as a function of preheat temperatures,in dose recovery test in SAR and the value of L_N/L_N in SAR and SMAR protocols,OSL signal sensitivity changes were well corrected.Both methods can be used to estimate De of the two samples properly for OSL dating.Although located at different strata levels,the similar OSL ages of two samples suggest the latest time they were burnt may be at a same period of human activity.

STANDARDIZED GROWTH CURVES (SGC) FOR OPTICAL DATING OF FINE-GRAINED QUARTZ FROM WATER-LAIN SEDIMENTS

YANG Chuan-cheng, CHEN Jie, ZHANG Ke-qi, LIU Jin-feng, WANG Chang-sheng, LEI Sheng-xue

2007, 29(2):  402-411. 

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Some water-lain sediments samples with different ages were selected from several regions in China,and their Standardized Growth Curves(SGC)were constructed for fine-grained quartz OSL signals that were measured by sensitivity-corrected multiple aliquot regenerative-dose protocol(SMAR).The equivalent dose of each sample can be determined by plotting their standard OSL intensity((L_N/T_N)*T_D)into the SGC curve.To estimate the reliability of this method in water lain sediments,their SGC E_D values were compared to that of SMAR E_D using their own individual growth curve.It is found that the uncertainty is up to 19% for natural samples with E_D values of 12～65Gy.For the E_D values greater than 65Gy,their uncertainty is much larger.While for samples with E_D values of less than 12Gy,the maximum uncertainty is up to 25%.Using the information of E_D values estimated by SGC protocol,we can easily and accurately obtain regeneration doses for SMAR protocol and further obtain the E_D value for each sample.This will save much time in dating application.

COMPILING THE 1:250,000 REGIONAL SEISMOTECTONIC MAP USING THE 1:200,000 GEOLOGIC MAPS WITH ArcGIS METHOD

ZHANG Lan-feng, YU Gui-hua, LI Chen-xia, WANG Yin, LI Ce

2007, 29(2):  412-417. 

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1:250,000 regional seismotectonic map is one of graphic documents achieved in the urban active fault survey project.At present,the 1:250,000 regional seismotectonic maps of various cities are drawn mostly basing on the existing 1:200,000 geological maps.The available paper-made 1:200,000 geological maps are the graphic documents under Beijing 54 Coordinates Systems.However,according to Stipulation on Technical System for China Earthquake Active Fault Surveying issued by China Earthquake Administration,the 1:250,000 regional seismotectonic maps are the graphic documents under Xi'an 80 Coordinate Systems.This article introduces the basic concepts of coordinates systems and the map projection and proposes explicitly that when scanning and digitizing paper geological map,the coordinate's projection parameters indicated in the paper geological map shall be taken for setting up projection information.The conversion of screen coordinates to map coordinates is realized through space coordinate registration,and by projection conversion,the Beijing 54 Coordinates System is converted to Xi'an 80 Coordinates System.Finally,with the aid of ArcGIS software,the 1:250,000 regional seismotectonic maps are compiled based on 1:200,000 geologic maps.

ACTIVE TECTONIC RATES CONSTRAINED BY TERRESTRIAL IN SITU COSMOGENIC NUCLIDES DATING

ZHANG Hui-ping, ZHANG Pei-zhen, ZHENG Wen-jun, ZHENG De-wen, CHEN Zheng-wei

2007, 29(2):  418-430. 

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Active tectonics studies focus on the active faults,active folds,active basins and the crustal and lithosphere blocks confined by the above-mentioned active structures.Tectonic deformation style,magnitude,and corresponding rate determined by active tectonic studies are fundamental for testing and constructing some kinematic and kinetic models.And these rates are necessary boundary conditions for testing and proposing the kinematic and kinetic models.Therefore,active tectonic rates must be quantified for better understanding and interpreting the above key models.Terrestrial in situ cosmogenic nuclides(TCNs)dating techniques have been discovered and developed for decades following the AMS emergence.And then they were widely utilized for geological issues,especially for the geomorphic processes and active tectonics.Production rates of cosmogenic nuclides can change spatially and temporally,therefore,they must be corrected before being applied and interpreted.During the field investigation,the influences such as sample depth,height,latitude and shielding features,intermittent covering and sample location shifting,should be carefully and impersonally considered because of their necessity for applying the results.Based on the samples and detailed field investigation,slip rates of the active faults,river incision rates within active domain,paleoearthquake events and active volcanic eruption events can be constrained by TCNs dating.Active fault slip rate is widely utilized for paleoearthquake research and earthquake prediction,and many classic models for plateau uplift and evolution were conceived based on distribution features and slip rates of active faults.While the river incision processes within active orogen are becoming key issues for studying tectonic and fluvial geomorphology.The incision process and its rate are one of the new insights for understanding the tectonic uplift and climatic change processes.Paleoearthquake and historical earthquake studies can provide the parameters for earthquake recurrence interval,and for consequent earthquake prediction and mitigating earthquake disasters.Besides the above active rates,the volcanic eruption events during the Quaternary can help us know the relative tectonic activities,crustal and mantle geochemistry process and landscape evolution history.After briefly introducing the basic theory of TCNs,the present paper will generalize recent results and data by TCNs dating in active tectonic studies,and then put its emphasis on the procedure and consequent interpretation for these active tectonic rates.

SOME ANALYSES ON THE RELATION BETWEEN THE UPPER OFFSET POINT AND THE LATEST ACTIVITY TIMES OF BURIED FAULTS IN CITIES OF EASTERN CHINA:TAKING THE XINGTAI AND TANGSHAN EARTHQUAKE REGIONS AS AN EXAMPLE

LI Chuan-you, WANG Yi-peng, WANG Zhi-cai

2007, 29(2):  431-445. 

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The urban area in the eastern China region is mostly covered by the relatively thick loose Quaternary deposits,below which,in many cities,there exist many considerably large buried faults.In these thick-Quaternary-covering areas,does the date of the upper layers dislocated by the buried faults represent the latest faulting? In this paper,based on the integrated analysis on the data of geology,geophysics and earthquakes of cities in the east of China,including Xingtai and Tangshan,we discussed the relation between the upper offset point and the latest activity times of the buried faults in these areas covered by thick Quaternary.Our study shows,in the area with very thick recent deposits in East China,one should not determine the latest faulting of one fault fully according to the younger layers displaced by the fault.To a fault running through the area covered with thick young deposits,its latest active period should be determined comprehensively by the tectonic settings,the controlling of the fault to the young strata,the youngest layer displaced by the fault,the thickness of the young deposits,seismicity and modern tectonic stress field,etc.