Table of Content

19 March 2008, Volume 30 Issue 1

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Brief Report

A REVIEW ON THE RESEARCH OF ACTIVE TECTONICS——HISTORY, PROGRESS AND SUGGESTIONS

DENG Qi-dong, WEN Xue-ze

2008, 30(1):  1-30. 

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This paper reviews the history and progresses of the research on active tectonics in China and overseas. By giving a brief introduction on histories of active tectonic researches in China and some other countries,the paper sums up the process and development on the quantitative investigation of active tectonics since the 1980's,and puts the focus on the main efforts and progresses that have been made in China on some aspects of the research,such as the basic survey and research and the applied investigation of active tectonics,the study on theories about regional active tectonics and their kinematics and geodynamics,the survey on the coupling relations between deep and shallow structures,the project on active fault survey and prospecting and seismic hazard assessment in urban areas,as well as the efforts on using the Quaternary geochronology. Further,the paper looks back to the Chinese efforts on the quantitative investigation of active tectonics,sums up those cognitions from studies on the determination of several basic and measurable parameters of active tectonics,such as the length of fault and fault-segment,coseismic slip and cumulative slip,fault slip rate,sequence of paleo-earthquake events,and the time elapsed from the latest event. At the same time,efforts and progresses in China on assessing long-term seismic potential for active fault and evaluating potential risk from potential active fault movement have been reviewed by summarizing researches on developing theories,models,methods and on application to the probabilistic assessment of time-dependent seismic potential,magnitude estimation for potential earthquake on active fault,and forecasting the potential risk caused by potential active fault movement. Finally,considering the realities and problems in the research of active tectonics in China,the authors put forward several suggestions for issues worthy of paying more attention to for further investigation in the future.

CURRENT ADVANCE OF OVERSEAS RESEARCH ON NEOTECTONICS:A REVIEW AND COMMENTS

CHENG Shao-ping, YANG Gui-zhi

2008, 30(1):  31-43. 

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During the past more than ten years, the research front on neotectonics concentrated on the field observations and theoretical models of the interaction between surface and crustal processes. This paper reviews for and comments on the current advance of overseas research on neotectonics in terms of four aspects: interaction between tectonics, surface erosion processes, and climate in orogens, interaction between rift segments in continental rift zones, transverse drainages, and planation surfaces. The including of surface processes in geodynamic models is a remarkable progress in understanding the development of orogens and rifted margins. The classical geomorphogical concepts on the geneses of transverse drainages and planation surfaces still have their vitality.

Active structures and neotectonics

STRATH TERRACE FORMATION AND STRIKE-SLIP FAULTING

ZHANG Pei-zhen, LI Chuan-you, MAO Feng-ying

2008, 30(1):  44-57. 

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Slip rates along major active faults are important components of quantitative studies of active tectonics. Slip rates can be directly used to seismic potential evaluation of active faults and seismic safety assessments of major engineering. In principle,dividing total displacement by its initial time yields slip rate along the fault. But,accurate determination of slip rate along a particular fault is not a simple task in practices for which the rates may deviate as much as 3 times among different researches and different methods. We argue that offset terrace risers that are protected by topography upstream of them are more closely dated by the age of the upper terrace than by that of the lower terrace. In some cases,valleys upstream of the fault have been incised into bedrock,and few if any terrace risers can be seen within the valleys. Such streams debouch onto alluviated floodplains or fans that become incised,presumably during climate changes,to create terrace risers. The terrace risers are then displaced so that they lie downslope from bedrock ridges on the upstream side of the fault,and thus the risers become protected from further incision. In such cases,dates of upper terraces should more closely approximate the ages of the risers than those of lower terraces. As noted above,whether the age of the upper or of the lower terrace more closely approximates the age of the riser will depend upon how the stream flowing over the flood plain that becomes the lower terrace alters the riser,and therefore at least in part on whether the offset riser moves into the path of the active stream or becomes shielded from it. Of cause,the age of the riser should be neither greater than the age of the upper terrace nor smaller than the age of the lower terrace. In an ideal situation,the ages of both would be sufficiently similar that they would place nearly equal upper and lower bounds on the slip rate. In many regions,however,the ages of the two terraces are so different that the bounds that they place on the slip rate are too large to be useful. We propose three methods to determine slip rate based on offsets of terrace risers. The first is to use both upper and lower terraces to constrain the maximum and minimum age of the offset of the riser. The second is to use the abandonment age of upper terrace as the initial age of the offset on the side of stream moving away from the river course. The third is to use the inception of sedimentary deposition on the lower strath terrace as the initial age of terrace riser offset. We use these methods to study slip rates along the Haiyuan Fault and the Altun Fault. The results show consistency of slip rates among different time scales,and are also consistent with other independent studies.

KINEMATICAL TRANSFORMATION AND SLIP PARTITIONING OF NORTHERN TO EASTERN ACTIVE BOUNDARY BELT OF SICHUAN-YUNNAN BLOCK

CHEN Gui-hua, XU Xi-wei, WEN Xue-ze, WANG Ya-li

2008, 30(1):  58-85. 

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It is a fundamental study for the research of evolution of Qinghai-Tibetan Plateau to resolve the kinematics of active boundary belt around the plateau quantitatively. With the measuring technology of photogrammetry and chronological method of analysis among regional climate,geomorphology and tectonics,we get a set of quantitative kinematical data for the main faults along the northern-eastern active boundary belt of Sichuan-Yunnan block. There were centre-symmetrical dip-slips locally in many fault segments while they were mainly strike-slip. With vector computation within a regional system including the north-east active boundary belt of Sichuan-Yunnan block and related blocks,we analyzed the longitudinal kinematical transformation and transversal slip partitioning quantitatively based on the parameters of main faults kinematics. From these data,there were a vertical uplift rate of 6.2mm/a in the Gonggashan area in distributed deformation form,and a mainly dip-slip fault at least with a rate of 1.45mm/a along the inner side of eastern terrace of Anninghe valley. We set up quantitative slip partitioning models for the northeast corner and eastern belt of Sichuan-Yunnan block. A quantitative kinematical model is set up for the regional system which consists of the northern-eastern active boundary belt of Sichuan-Yunnan block and related blocks.

LATE QUATERNARY GEOMORPHIC DEFORMATION AND DISPLACEMENT RATES OF THE ANNINGHE FAULT AROUND ZIMAKUA

RAN Yong-kang, CHENG Jian-wu, GONG Hui-ling, CHEN Li-chun

2008, 30(1):  86-98. 

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The Anninghe Fault is an important active fault along the eastern boundary of Sichuan-Yunnan active tectonic block,and the study of its distribution feature of displacement rates in the late Quaternary is of fundamental importance for understanding the dynamic theory on Chinese continent,boundary dynamic process of the active block and the recurrence interval of large earthquakes. The active Anninghe Fault in late Quaternary has the length of 160km and consists of two segments. Zimakua is an area where the latest neotectonic rupture trajectories are relatively simple and the offset geomorphologic sequences are well developed. Using the methods of detailed geomorphic and geological survey,digital image analysis,total station instrument survey,excavation of trench and dating,the paper makes an analysis on the geomorphologic sequences of the strata and obtains some new results as follows: The terrace(step)T₃ consisting of moraine deposits was formed at about 20ka BP and offset by the Anninghe Fault with a left-lateral displacement of (84±3)m and a vertical displacement of 18m. On the base of the moraine deposits overlies terrace(step)T₂ of mainly alluvial deposits formed at 10～7ka BP with the maximum left-lateral displacement of about 40m,an average of (36±4)m,and a vertical displacement of about 11m. Terrace T1 is composed mainly of the alluvium and partial dammed-pond deposits which were formed during 3～2.2ka BP. The earliest event that offset the T1 happened at 1.7kaBP.Three events have been discovered since 1.7kaBP and the cumulative left-lateral displacement is 10.5m and the vertical displacement 2.3m. So,the left-lateral displacement rates are 6.2mm/a,3.6～4mm/a,3.8～4.2mm/a and the vertical rates are 1.4mm/a,1.1mm/a,0.9mm/a(at least)on average in about 2.6ka,10ka and 20ka,respectively. The proportion of horizontal to vertical displacements is about 4:1. That means the vertical rate on Anninghe Fault is about 25% of horizontal slip rate. The left-lateral slip rate in late Holocene is well consistent with GPS measurement. The change of the left-lateral rates in different stage is also consistent with recurrence interval of paleoearthquakes. It means that there is alternation between strong and weak activities of the Anninghe Fault.

NEOTECTONICS AND FAULT ACTIVITY IN THE ANQING-MA'ANSHAN SECTION OF THE CHANGJIANG RIVER VALLEY

SONG Fang-min, DENG Zhi-hui, MA Xiao-jing, ZU Jin-hua, CHU Quan-zhi, YIN Gong-ming, ZHOU Qing

2008, 30(1):  99-110. 

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The neotectonic movement and characteristics of fault activity in the Anqing-Ma'anshan section of the Changjiang River valley are analyzed on the basis of data obtained from field investigation,shallow seismic prospecting and drilling. The results show that during neotectonic time this river valley section and its both sides as a whole was dominated by weak and intermittent uplift movement. As a consequence,owing to the effect of the activity of NE-and NNE-trending faults,relatively strong vertical differential movement occurred in Wuwei-Anqing area during Paleogene-Neogene,and had continued to early and middle Pleistocene. The NE-NNE-trending and NW-trending faults were developed in the bed rocks of the valley and its both sides. The former was formed during Indo-Chinese epoch,while the later was formed during Yanshan epoch. The most recent active period of the larger faults controlling the development of Cenozoic Basins is middle Pleistocene,while the newest activity of the relatively small faults developed within pre-Cenozoic group is pre-Quaternary. The Quaternary system in the valley is \{10～\}50m thick,consisting mainly of Pleistocene-Holocene deposits. The isopach of these deposits is smoothly distributed,indicating normal valley deposition. Seismic activity along the valley and its both sides is relatively weak,and historically only 4 destructive earthquakes have been recorded. Among these events,the largest one is the M5(3/4) earthquake occurring at Chaohu in 1585,and the other events including one with M5(1/4) and two with M4(3/4). Since the beginning of instrumental records in 1970,the largest magnitude that has been recorded so far is ML 3.7. All these results may provide better constraints on the assessment of the crustal stability for this river valley section.

CRUSTAL SHORTENING OF MAJOR NAPPE STRUCTURES ON THE FRONT MARGINS OF THE TIANSHAN

YANG Xiao-ping, DENG Qi-dong, ZHANG Pei-zhen, XU Xi-wei

2008, 30(1):  111-131. 

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We present the results of selected balanced cross-sections across four active reverse fault-fold belts at the front margins of the Chinese Tianshan,an intracontinental mountain belt formed in response to the India-Eurasia continental collision. It consists of Paleozoic,Mesozoic and Cenozoic strata in active fold-and-thrust belts at two sides of Tianshan, and the stratum in active fold belt changes younger from west fold-and-thrust belt to east. The middle part's structure is complicated in Kalpin active fold-and-fault belt,the width gradually diminishes towards east. At the west of Akesu city,the Kalpin active fold-and-thrust belt vanishes, but at the east part of Akesu city,the Kuche active fold-and-thrust belt emerges on the Tarim desert. In the middle section of the Kuche active fold-and-thrust belt near Baicheng County,the structure is most complicated,and the width is also maximal. The width of Kuche fold-and-thrust belt gradually decreases towards east,and disappears near 85° longitude line. But at the northern piedmont of Tianshan,the Manas active fold-and-thrust belt presents itself approximately at the same longitude. The longitude of the west end of Turpan centre uplift active fold-and-thrust zone is approximately the longitude of the east end of the Manas active fold-and-thrust belt. The four active fold-and-thrust belts which we studied are located at the north and south sides of Tianshan Mountains. Two balanced cross-sections traverse the Kalpin active fold-and-fault belt and one crosses the Kuche fold-and-fault belt. Two balanced cross-sections cross the Manas active fold-and-thrust belt,and one balanced cross-section crosses the Turfan centre uplift zone. The crustal shortening of Kalpin fold-and-thrust belt is 40～45km,that of Kuche fold-and-thrust belt about 27～37km,and of Manas,Turpan centre uplift active fold-and-thrust belt are 8.5～10.5km and 6～7km,respectively. The four active fold-and-thrust belts at south and north Tianshan are not superposed in longitude. So their crustal shortenings approximately represent the minimal crustal shortening of Tianshan at different segments,and also reflect the reducing trend of crustal shortening from west to east. It is very difficult to calculate the crustal shortening across entire Tianshan,owing to deficiency of data about active reverse fault and strike-slip fault within Tianshan Mountains. If the time of initial deformation is the starting time of Xiyu conglomerate deposition(since 2.5Ma),and considering the shortening component on south-north direction of Bo-A NW strike-slip fault,the minimum crustal shortening rates at four segments of Tianshan would be 15.4～17.3mm/a,12.7～16.5mm/a,3.8～4.5mm/a and 2.3～2.7mm/a,respectively.

CRYSTALLINE BASEMENT FAULTS IN JUNGGAR BASIN DETERMINED BY GRAVITY AND GEOMAGNETISM

ZHAO Jun-meng, LU Zao-xun, YAO Chang-li, LI Yi-shi, LIU Zhan-po

2008, 30(1):  132-143. 

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Based on the 1:200000 gravitational and geomagnetic data in Junggar Basin and its adjacent regions,47 faults in Junggar Basin were determined by means of frequency domain transform in gravity and geomagnetism fields,regular filter,upward continuation of Bouguer anomaly,derivative,polarization of geomagnetic anomaly and tendency analysis. These faults can be divided into three groups: 6 of the first class,6 of the second class and 35 of the third class. The faults F1 in SN direction and F6 in EW direction were determined especially by detailed data processing,combined with sediment features of the cover layers. The two main faults divide the basement of the Junggar Basin into four quadrants,which provide an important evidence for developing the new basement structure frame.

THE QUATERNARY NORMAL FAULTING OF THE CONA-OIGA RIFT

WU Zhong-hai, ZHANG Yong-shuang, HU Dao-gong, ZHAO Xi-tao, YE Pei-sheng

2008, 30(1):  144-160. 

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The Cona-Oiga rift zone is N10～12°E trending,about 220km long,and is the only rift located at the east of Yadong-Gulu rift in southern Tibet. It is located around 92°E and between 27°40'N and 29°40'N and cuts the south Tibet Detachment into High Himalaya block to the south and strides across Yarlung Tsangpo Fault(or Great Counter Thrust fault)into eastern Gangdese batholith zone to the north. There are 3 independent grabens contained in the rift. They are Oiga graben,Qungdo'gyang graben and Cona graben from north to south. The earthquake activity is very prominent along the rift. There are two large earthquakes of M7.5 and M7.0 occurring at northern Cona in 1806 and southern Oiga in 1915, respectively. In the rift,late Quaternary tills and fluvioglacial deposits may be divided into 4 sets corresponding to the so-called Nyanyxungla Ice Age,Jilongshi Ice Age,Ronbushi Ice Age and Holocene glaciations respectively from oldest to youngest. Their TL and U-series ages show that the first(or the oldest)tills formed before marine isotope stage(MIS)6,the second tills formed during about 200～140ka BP,the third and fourth tills formed during the last Glacial maximum(about 28～15ka BP)and Holocene glaciations respectively.The Oiga graben is located at the north of Yarlung Tsangpo Fault. It is about 50km long and widens from 3～5km in the south to 15～18km in the north. The graben is limited to the east and west by two N 18±1°E-trending boundary normal faults of opposite dips, indicating that the extension direction is 108±1° in the region. Field survey shows that the master boundary fault is the eastern margin fault of Oiga Basin which has been active from Quaternary or Pliocene to Holocene. The boundary fault shows obvious activity during late Pleistocene. Based on measurement of fault scarp,the vertical displacements are 50～90m, (24.0±1.5)m, (16.0±1.0)m or (13.7±0.5)m and (3.7±0.4)m since MIS6,24～18ka BP and 15～11ka BP since middle-late Holocene, respectively. Given such displacements and ages of fault scarps,the average throw rates are limited between 0.4～0.9mm/a since MIS6,and about (1.2±0.3)mm/a since MIS2. The Qungdo'gyang graben is a N18°E-trending and 11～20km-wide basin. It cuts across the eastern segment of Yala Xiangbo gneiss dome and is a half-graben limited by the west boundary fault which is a N 18±1°E-trending,east dipping and about 40km long normal fault. It has been active since Pliocene. Based on measurement of late Quaternary fault scarp,the vertical displacements are (29.8±1.0)m and (12.0±0.5)m since about 28～15ka BP, (4.6±0.4)m and (7.0±0.7)m since middle-late Holocene, respectively,and the most probably values of throw rates of main boundary fault aren't less than about 0.5mm/a,and the average throw rate is about 1.0～1.5mm/a since MIS2. The Cona graben is a north-trending basin,about 80km long and 1～10km wide. Its main boundary fault is located on the western margin of basin. It is a N-trending and east dipping normal fault, about 110km long. Based on measurement of fault scarp offset tills and fluviaoglacial terraces,the displacements are 44～80m, (27±1)m and (15.5±0.5)m since MIS6,24～18kaBP and 15～11kaBP, respectively. The average vertical throw rates are between 0.3～0.8mm/a and about (1.3±0.3)mm/a since MIS6 and MIS2 respectively. The late Quaternary throw rates show consistency and constrain the uniform long-term slip rate along Cona-Oiga rift. The obvious increase of throw rate during Holocene most probably results from earthquake cluster in Holocene along the rift.This new observation on Cona-Oiga rift shows that the extension direction of rifts is strictly limited to 100° in southern Tibet,and is parallel with Yadong-Gulu rift. The long-term and short-term slip rates of main bounding normal faults of Cona-Oiga rift are also distinctly similar to the throw rates of Yadong-Gulu graben system. The strict geometry pattern,pronounced similar and consistent to the active magnitude and trend of bounding normal faults in southern Tibet suggests that the N-trending rifts most probably result from the uniform extension deformation controlled by middle-lower crust lateral flow or extension parallel to orogen caused by India lithosphere insert under south Tibet.

QUANTITATIVE DATA OF ACTIVE FAULTS WITHIN THE ACTIVE TECTONIC BLOCK IN NORTH QINGHAI-XIZANG PLATEAU

LIU Bai-chi, CAO Juan-juan, YUAN Dao-yang, HE Wen-gui

2008, 30(1):  161-175. 

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This paper defines the boundaries of north Qinghai-Xizang plateau active tectonic block and offers quantitative information of active faults within the north Qinghai-Xizang plateau active tectonic block in the past dozen years. The information mainly include: the serial number,name,attitude of active fault,main geological geomorphic signs,activity age,fault segmentation,fault slip rate,paleo-earthquake and its date,and the major features of earthquake rupture zones and so on. These data indicate that great earthquakes with M≥8 concentrate on the boundary active faults of the north Qinghai-Xizang plateau active tectonic block,and the slip rate of the faults all reaches about 5～12mm/yr. Earthquakes of about M7 occurred on the smaller-scale active faults inside the north Qinghai-Xizang plateau active tectonic block. Generally,the slip rates of these active faults are 1～3mm/yr.The north Qinghai-Xizang plateau active tectonic block can be divided into several sub-blocks,and these active sub-blocks are dominated with deformation,and no rotation occurred. Our result supports the hypothesis of continuous deformation of Qinghai-Xizang plateau active tectonic block.

PRELIMINARY SURVEY ON THE QUATERNARY ACTIVITIES OF THE QIANLIYAN FAULT IN THE NORTHERN PART OF THE SOUTH YELLOW SEA

WANG Zhi-cai, CHAO Hong-tai, DU Xian-song, JIA Rong-guang, ZHOU Bin, LU Zi-lin

2008, 30(1):  176-186. 

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It is the first time that several profiles of acoustic survey have been conducted across the Qianliyan Fault in the northern part of the South Yellow Sea. According to the difference of the late Quaternary fault activities revealed by the acoustic survey,the Qianliyan Fault may be divided by the Chaoliandao Fault into two segments. The southern segment is not active since the late Pleistocene,no offset in late Pleistocene strata has been observed on the acoustic profiles to the southeast of Rizhao. The northern segment is active in the late Pleistocene,normal faults are observed on the acoustic profiles west of Qianliyan island and offset the middle and upper part of the late Pleistocene layers,but no offset has been found in the latest Pleistocene and Holocene strata. In addition,it is found that the Qianliyan Fault offsets the late Pleistocene strata in the sea region 15km east of Shidao,Rongcheng County. In summary,the northern segment of the Qianliyan Fault is active in the latest Pleistocene in the sea region from the west of Qianliyan Island to the east of Shidao. Besides the Qianliyan Fault,other faults develop in the region to the southeast of the Qianliyan Island,which show obvious late Pleistocene active evidence. Therefore,the relative uplift status of the block where the Qianliyan Island lies on may be related to the late Pleistocene tectonic activities including that of the Qianliyan Fault. Both modern and history seismicity are relatively week in the region along the Qianliyan Fault. Since the establishment of the seismograph network in this region,no earthquake with magnitude equal or larger than 5and no small earthquake clusters have occurred along the Qianliyan Fault and in the surrounding area. The only historic earthquake is the 1932 M6(1/2) southern Yellow Sea earthquake which occurred in the sea region 28km southeast of the Qianliyan Fault. It is obviously that the seismicity is not concordant with the late Pleistocene active features of the Qianliyan Fault. Above all,the Qianliyan Fault is over 100km long from the Qianliyan Island to the offshore near Shidao and it is obviously active in the latest Pleistocene,the probability of the occurrence of a some M6.5 earthquake along the Qianliyan Fault should be considered in the practice of earthquake prediction and seismic hazard analysis.

Brief Report

STEPWISE LANDFORMS AND QUATERNARY EPISODIC UPLIFTS OF MOUNTAINS AROUND XINDING BASIN

ZHANG Shi-min, REN Jun-jie, LUO Ming-hui, DING Rui, L? Zhi-qiang

2008, 30(1):  187-201. 

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Xinding Basin is a typical Cenozoic graben-type basin in the northern Fenwei graben system,which is surrounded by mountains of 2000 to 3000m high above sea level. Downward from the peak to the piedmont on the basin side are respectively peneplanes of Beitai stage and Dianziliang stage in Paleogene,peneplane of Tangxian stage in Neogene,and seven-level river terraces and six-level pediments in Quaternary. Loess and paleosol sequences on the landforms are correlative to typical loess section,which is an advantage for landform dating.A study has been made of the paragenetic relations of the pediments and river terraces on the northern piedmonts of Wutai Mountains and Xizhou Mountains via geomorphologic mapping on 1:10000 scale,and an analysis on the role of tectonic,climatic,and drainage factors in the parageneses. The Quaternary pediments and river terraces on the north piedmont of Wutai Mountains are united to constitute six steps of geomorphic surfaces. The episodic uplifting of fault blocks is deemed to be the dominant factor in the formation of the unified surfaces.Episodic uplifts result in longitudinal profiles of riverbeds alternated by cascade and gentle reaches one in mountainous areas. Terraces disappear upstream near the vertex of cascade reach. Cascade reaches represent quickly uplifting stages,and gentle ones represent stable stages. On condition that waterfalls between adjacent terraces recede at same speed,durations of active or stable stages can be estimated by ages of terraces and relative length of cascade reach and gentle one,and then ages of uplifting stages since 1.20Ma are estimated. The four quickly uplifting stages are from 1200 to 1059ka BP,from 600 to 501ka BP,from 130 to 103ka BP,and from 20ka BP to present. Other intervals are stable stages.

A PRELIMINARY STUDY ON SEISMOGEOLOGIC EVIDENCE FOR MODERATE EARTHQUAKES IN EAST CHINA

XIANG Hong-fa, HAN Zhu-jun, ZHANG Wan-xia, ZENG Jian-hua, XIAO He-ping

2008, 30(1):  202-208. 

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Few large earthquakes(M_S≥7)have ever occurred in the historical records in eastern China,especially in southeast continental region of 105°～120°E,20°～35°N. However,many moderate-strong earthquakes with magnitude between 5 and 6 occurred there,such as the 1962 Heyuan M_S 6.4 earthquake and 1969 Yangjiang M_S 6.7 earthquake in Guangdong province,the 1974 Liyang M_S 6.0 earthquake in Jiangsu province,1979 Guzhen M_S 5.0 earthquake in Anhui province,and the 2005 Jiujiang M_S 5.7 earthquake in Jiangxi province and so on. These earthquakes have no significant earthquake ruptures,and few late Pleistocene active faults were discovered. So,the seismogeological background of the above moderate-strong earthquakes is still unclear up to the present,that is to say, it is very necessary to study the seismogeologic evidence for moderate earthquakes. In the paper,by analysis of the seismogeologic evidence of the 1979 Liyang M_S 6.0 earthquake in Jiangsu province,the 2005 Jiujiang-Ruichang M_S 5.7 earthquake in Jiangxi province,the 1710 Xinhua M_S 5(1/2) earthquake in Hunan province,the 1917 Huoshan M_S 6(1/4) earthquake in Anhui province and the moderate-strong earthquakes around Dongting Lake,some conclusions are obtained as follows: 1)most moderate-strong earthquakes in east China occurred at regions near the early Quaternary active faults,and regions with early-middle Pleistocene active faults; 2)most moderate-strong earthquakes in East China are related to the development and distribution of the Quaternary down-faulted basins. 3)moderate-strong earthquakes in East China may occur at regions with evident active tectonic geomorphological characteristics in Quaternary,such as the areas with linear fault geomorphology or geomorphologic surface; 4)moderate-strong earthquakes may occur in the seismic gaps along the seismic zones where earthquakes with magnitudes 4～5 occurred in the history.

DISCUSSION ON ENGINEERING APPLICATION BASED ON THE EXPLORATION OF YUWANGSHAN FAULT ZONE

ZHOU Ben-gang, SONG Xin-chu, YANG Xiao-ping, HAN Zhu-jun, DU Long, DONG Shao-peng

2008, 30(1):  209-217. 

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Combined with the consideration of relative codes,this paper mainly presents a preliminary analysis on the engineering application of Yuwangshan Fault zone based on the results achieved in the project of active fault exploration and seismic hazard assessment in the city of Ningbo. The conclusions we get from it are as follows. We don't have to take safety distance into account as to the west section of Yuwangshan Fault in engineering and construction because it has not been active since mid-Pleistocene,whereas,as to the east section of this fault,there's no need to consider the safety distance about Ⅲ-type and Ⅳ-type buildings,nevertheless,the case is different for Ⅰ-type and Ⅱ-type buildings because the potential surface deformation to some extent will occur with the scenario earthquake of 6.0,even if no direct surface offset. Due to the existence of about 6km² area where near-fault peak ground accelerations lightly exceeds the present-day standard,the authors put forward a package of aseismatic solutions which consist of the identification of aseismatic capability to buildings in the area and consideration of this factor in new buildings' construction.There's a potential area of seismic settlement of 30km² which may sink 5 cm or above according to the characteristics of distribution of soft soil as well as the assessment of near-fault strong ground motion in the east of the west section of Yuwangshan Fault. We should not ignore this important result.Given the importance of the results to the policy-decision for the mitigation of seismic disasters,we should seek for the trade-off between the safety and economic rationality in engineering application analysis.

APPLICATION OF MICROSCOPIC ANALYSIS TO THE PROSPECTING OF ACTIVE FAULTS IN URUMQI CITY, XINJIANG UYGUR AUTONOMOUS REGION, CHINA

LIN Chuan-yong, SHEN Jun, CHEN Xiao-de, SHI Lan-bin, HU Jun, XIANG Zhi-yong

2008, 30(1):  218-235. 

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In recent years,the project of active fault prospecting in major cities has been widely carried out in China. However,active faults are usually developed in soft sediments,and sometimes do not leave any macroscopically observable trace. Therefore,even though fault-crossing trench is excavated,the key problems concerning the upward or downward propagation and the property of these obscured faults,as well as the mode of faulting and the time of the last faulting event can not be solved by macroscopic observation alone. In this paper,a microscopic observation method for resolving these problems is introduced through a case study of the active fault prospecting in Urumqi,Xinjiang Uygur Autonomous Region. Macroscopic observations have been carried out on 13 trenches and natural outcrops,and 53 samples have been systematically collected. Detailed micro-structural investigations have been carried out on 106 thin sections cut from the samples,while the grain-size analysis,particle size distribution(PSD)analysis and the statistic measurement of the angularity of the clastic grains from the samples have also been carried out. The problems concerning the mode of faulting,the occurrence of obscured fault,and whether the fault has cut through the overlying strata are resolved through the discovery of microscopic appearances of the faults,the finding of microstructures indicative of seismites,as well as the variation of angularity,grain-size and PSD of the clastic grains of the samples collected on and outside the fault. The results show that microscopic observation is an effective approach to the identification of obscured active faults in soft sediments,and it is widely applicable to the project of active fault prospecting in major cities currently carried out in China.

NEW PROGRESS OF SEISMIC ACTIVE FAULT PROSPECTING IN LANZHOU CITY

YUAN Dao-yang, WANG Lan-min, HE Wen-gui, LIU Bai-chi, GE Wei-peng, LIU Xing-wang, LIANG Ming-jian, ZHENG Wen-jun

2008, 30(1):  236-249. 

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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 thrust 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 Qilihe 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 fault 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 recognitions 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 Shengouqiao 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.

COMPOSITE DRILLING SECTION EXPLORATION OF YINCHUAN BURIED FAULT

LEI Qi-yun, CHAI Chi-zhang, MENG Guang-kui, DU Peng, WANG Yin, XIE Xiao-feng, ZHANG Xue-hui

2008, 30(1):  250-263. 

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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.

MANIFESTATIONS OF WEIHE FAULT AT DEEP, MIDDLE, SHALLOW AND NEAR-SURFACE DEPTH

FENG Xi-jie, LI Xiao-ni, REN Jun, SHI Ya-qin, DAI Wang-qiang, WANG Fu-yun, MIAO Kang-yun, HAN Heng-yue

2008, 30(1):  264-272. 

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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<5km)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 shallow 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.

BRIEF INTRODUCTION ON THE PREDOMINANT RESULTS OF THE ACTIVE FAULT DETECTING AND SEISMIC RISK ASSESSMENT IN URUMQI CITY

SHEN Jun, SONG He-ping

2008, 30(1):  273-288. 

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The predominant results of the project "Active fault detecting and seismic risk assessment in Urumqi City" are introduced in this paper. The active faults detecting target region covers the Urumqi City and its urban planning area. Deep seismogenic tectonics of the faults was detected systematically during this project. The seismic risk and damage of the active faults were estimated preliminarily. Two groups of Holocene active faults,namely,Wangjiagou Fault group and Jiujiawan Fault group,were identified in the target region. The Wangjiagou Fault group is composed of four north-dipping thrust faults,while the Jiujiawan Fault group is composed of four north-dipping normal faults. Four late Pleistocene active faults,namely,Bagang-Shihua buried fault,Xishan Fault,Wanyaogou Fault and Bayangnangou Fault,were detected in the target region. A clear image of the thin-skinned thrust system under the city was made by deep seismic profile carried out by this project. Combined with the observation of mobile seismic station array and precise location of micro quakes in the target region,as well as other deep structure detecting results,such as the petroleum seismic profile,two seismic models of the target region were established. The seismic risk of the Holocene active faults and late Pleistocene active fault were estimated by paleo-earthquake trenching and seismicity analysis along the Holocene active faults,as well as the microstructure study along the late Pleistocene active faults. Meanwhile,the strong ground motion generated by potential strong earthquakes directly under the target region was predicted by numerical modeling. The surface displacement and deformation zones of the future strong earthquake along the Holocene and late Pleistocene active faults were estimated.

TESTING GEO-SLICER ON THE RUPTURE OF THE M8 SANHE-PINGGU EARTHQUAKE OF 1679

HE Hong-lin, MIN Wei, Tsuyoshi HARAGUCHI

2008, 30(1):  289-297. 

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Geo-slicer method,a newly developed technique for detecting active fault,is designed to take a slice of unconsolidated Quaternary strata whose texture and structure are not destroyed. The Sanhe-Pinggu M8 earthquake of 1679 is the biggest one recorded in Beijing area. The macroscopical epicenter is located around Pangezhuang Village,Xiadian Town,Hebei Province. We successfully applied the geo-slicer method to detecting the rupture of this earthquake near its epicenter. Through our test research,we gained the following knowledge: 1)the detailed deposit structure of the unconsolidated strata can be kept in the geo-slicer; 2)the power unit should be selected to fit to different site conditions,and for Beijing area,a power unit with "grab+vibrant hammer" is better than that with "crane+hammer" ; 3)there is some shortening due to the vibration of the power,but the shortening is generally less than 5%. Moreover,synthesizing the information from the geo-slicers and trench,we found two earthquake events. One is the 1679 earthquake and the other is the one before the 1679. The vertical seismic displacements are 1.4m and 1.2m,respectively.

BUILDING TO THE ACTIVE TECTONIC DATABASE OF CHINA

QU Chun-yan

2008, 30(1):  298-304. 

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Based on ArcGIS and MapInfo software,we digitized the active tectonic map(1:4000000)of China,which was compiled and revised by Academician Deng Qidong,and built the spatial database of active tectonics of China. The database integrates rich active tectonic data,such as earthquake catalogue with magnitude above 6,active faults,the Quaternary basins,active folds and their associated attribute parameters,and implements the scientific and effective management to these active tectonic data. At the same time,the spatial database joins the spatial map data and its associated attribute data together,which implements the data query between spatial feature and its attribute parameters and also makes it possible to do spatial analysis with different data layers. These provide much convenience for earthquake study and engineering construction institutions to use these data in practical application.

DISCUSSION ON SEISMOLOGICAL PRINCIPLE OF GEOLOGIST

JIANG Wa-li

2008, 30(1):  305-323. 

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Geologist thinks that fault is the surface expression of focus. The paper discusses how to predict strong earthquake by using information of quantitative research on active faults on the basis of the principle. Prediction of places where strong earthquake occurs depends on the investigation of fault activity. Especially,interpretation of air photo and field investigation of faulted landform is emphasized to get the latest active time of fault. The fault which was active during Late Quaternary is recognized to have larger seismic potential in the future. The intensity of strong earthquake has close relations with the length of active fault and the rupture accompanied with earthquake. It is in direct proportion,which was confirmed by a lot of historical earthquakes and recent earthquakes occurring in the whole world. Prediction of time of strong earthquake needs to get the recurrence interval and the elapse time of latest activity on different fault zones. Average interval of strong earthquake may get from the ratio between active rate and the offset accompanied with earthquake on the fault. Besides,trench investigation to get times of paleoearthquakes on different faults is very important for distinguishing individual character of seismic-activity on different fault zones. A lot of information on research of active fault and recognition from Japan and Professor T. Matsuda is quoted in the paper. At last the paper briefly reviews the development of research on active fault in China.

NEW FINDING OF EARTHQUAKE RUPTURE IN THE EASTERN MARGIN OF MONGOLIAN ALTAI

TIAN Qin-jian, ZHANG Jun-long

2008, 30(1):  324-332. 

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The active faults in Altai Mountains are mainly in NW—NNW direction and can be divided into three belts:the western margin of Altai Mountains active fault zone, the central Altai Mountains active fault zone and the eastern margin of Altai Mountains active fault zone. The eastern marginal active fault zone consists of two right-lateral strike-slip faults,the Hovd active fault and the Har-Us active fault,and the compressional basins between them. Several segments of earthquake ruptures have been reported along the strike-slip faults in former studies. We also found two new segments of earthquake rupture on the middle to south parts of the fault zone in recent field investigation. One segment is on the Jargalant Fault in the middle part of the Har-Us Fault zone,the other segment is on the Tugen Gol Fault in the south part of the Hovd Fault zone. The earthquake surface rupture on the Jargalant Fault has been studied in more detail in this paper. The rupture zone is 50km long with about 4～5m dextral displacement,implying a large and recent earthquake. Earthquake ruptures have been observed on each segment of the eastern margin of Altai Mountains active fault zone. So,the eastern margin of Altai Mountains is a strongly active tectonic zone with large earthquake.

DISCOVERY OF A NW-TRENDING QUATERNARY FAULT AT NANLING, ANHUI PROVINCE AND ITS IMPLICATION

YIN Gong-ming, CHEN Xian-cheng, SONG Fang-min, DENG Zhi-hui, ZHOU Ben-gang, LIU Jing-wei

2008, 30(1):  333-338. 

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In the eastern mainland of China there are few cross sections of faults where dislocation of Quaternary strata can be observed. However,recently we found such a profile about 2km away from the Nanling county,Anhui Province(30°55'456″N,118°177'74″E),west to the highway from Nanling to Fanchang. This fault has been identified on the satellite image,but its trace is confined to the southern side of the Nanling Basin. Our field investigation indicates that the northwestern end of this fault lies at the Xiaodanyang-Fangshan Fault. It is only 20km long,striking in NW,dipping to southwest. From observations on the profile,it consists of two small fractures and has two periods of activity at least. The first active period is before middle Pleistocene time,or probably in early Pleistocene. And the second active period is in or after middle Pleistocene. Its latest motion is of thrust with an amount of dislocation of 40cm. This fault cross section shows that the NW-trending faults in the eastern Mainland of China have new activities,though on small scales in general.

THE METHOD OF TERRACE ANALYSIS BASED ON DEM——A CASE STUDY IN ZIMAKUA OF ANNINGHE FAULT

GONG Hui-ling, RAN Yong-kang, CHEN Li-chun

2008, 30(1):  339-348. 

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Quantitative study of active tectonics needs to get a series of deformation parameters,and fault displacement is one of the most basic active parameters. Alluvial fan and terrace around active fault zones can record the information of time and strength of fault movement. River terrace as a most common landform contains structural information. It is meaningful for quantitative tectonic movement study to ascertain river terrace. In the article we use photogrammetric software virtuo NT to extract high resolution DEM. Based on DEM,the method and program for terrace analysis are built on the platform of ARCGIS. In the light of the distribution characteristics of slope and height,we generate the slope classification map and height classification map. And by multiplication of the two maps,the terrace landform map is generated. In the Zimakua region of Sichuan province,we applied the method to extract terrace information and got its distribution. We compared the terrace extracted based on DEM with the terrace interpreted in the filed,and the result shows that they have good consistency. The terrace boundaries record the information of fault dislocation. By measuring the dislocation of terrace boundary,we get the average dislocation of secondary fault to be 85.4m,which is in accord with the filed surveys. The results indicate that the method of terrace extraction based on DEM has the properties of high precision, high efficiency,visualization and so on.