Table of Content

02 December 2004, Volume 26 Issue 4

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Brief Report

RECOGNITION OF “THE WORKING GUIDELINES FOR EXPLORATION AND SEISMIC RISK ASSESSMENT OF ACTIVE FAULTS IN URBAN AREAS (ON TRIAL)”

WANG Yi-peng

2004, 26(4):  559-565. 

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The exploration and seismic risk assessment of active faults in 20 capital cities of China will be carried out during the period of the 10^th“Five-Year Plan”. This paper elaborates the essential contents and key points of the technical guiding document for this project,“the Working Guideline for Exploration and Seismic Risk Assessment of Active Faults in Urban Areas (on Trial)”,demonstrating the scientific,commonly applicable,inclusive,operable,and restrictive attributes of the regulation,defining the main objects of the exploration engineering: the active fault and the seismic active fault,explaining the connotation of the working areas and the target areas,as well as the difference of the working method in these two areas,analyzing the three level scientific problems which the project would face and the five steps that should be taken in executing process,and discussing the points of attention in each step and countermeasures that should be taken. This paper emphasizes the necessity of the middle-phase checking during the exploration,and points out that this project is the continuing,developing and deepening of all the earthquake hazard prevention work that has been done before.

RELATION BETWEEN CRUSTAL MOVEMENT AND MANTLE DEFORMATION IN CHINA CONTINENT

REN Jin-wei

2004, 26(4):  566-575. 

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Plate motion is one of the major dynamic sources for crustal and mantle deformation. As crustal deformation can be measured by GPS and geologic observation,the comparison between crustal deformation and mantle deformation is an effective approach to understanding the coupling relationship between the crustal movement and the mantle deformation. In this study,regional crustal strain rate tensors for China inferred from GPS measurements,slip rates of Quaternary active fault and earthquake deformation data within an area of approximately 200×200km²,are incorporated with smooth,continuous functions (sp line) to determine the self consistent model of present day velocity gradient tensor field for Chinas continent. In the incorporation process,GPS velocity vectors are also matched,within a defined frame of reference,with the model velocity field. The results are then compared with the anisotropy of the Pn wave and with the fast directions of shear-wave splitting inferred from the SKS phases,which may represent the shear deformation in the mantle and the deep crust,respectively. The comparison shows that there is relatively large difference between the average directions of crustal shearing and that of mantle shearing in the regions of active tectonics. This may indicate the decoupling of crustal movement and mantle deformation in these active regions.

SEISMOGENIC FAULT OF THE 1999 CHI CHI EARTHQUAKE IN TAIWAN PROVINCE AND THE FEATURES OF EARTHQUAKE DAMAGES

PENG Fu-nan, YE Yin-can

2004, 26(4):  576-585. 

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A large earthquake of M7.3 took place on September 21,1999 at Chi-Chi of Western Foothill,central Taiwan region. It caused severe damage and a great number of casualties that had gone beyond the destruction and casualty caused by any one of the historical earthquakes in the past 20^th century in Taiwan region. The magnitude of the earthquake is also the largest one of the shocks occurred in the inland of Taiwan Island. The main shock was located at 23.85°N,120.82°E,with a depth of 8 km,and the focal mechanism was of a thrust type with a strike of 5°,dipping SE with a rake of 65°,which is consistent with the surface rupture along the Chelungpu earthquake fault (80km in length). The P axis is striking 293°with a plunge angle of 13°consistent with the tectonic setting in which the Philippine Sea plate collides with the Eurasian plate. Especially,the horizontal distribution of the aftershocks appeared as a semicircular form around the Peikang basement high,rather than a single line along the earthquake fault. Vertically,the aftershocks were approximately distributed on the hanging wall of the thrust fault. The other distinct features of the earthquake are the differential displacement between the hanging wall and footwall,the horizontal ground displacement of up to 9.8m,and the vertical ground elevation of up to 10m at the northern terminal of the earthquake fault,where both the surface and the subsurface data (horizontal PGA=502gal and the vertical PGA=519gal) showed accumulation of stresses in this area which caused the destruction of Shihgang Dam and other damage. Correspondingly,the deformation of the ground and the destruction of the constructions on the hanging wall of earthquake thrust were more severe than those on the footwall. All of the above characteristics show clearly the close relationship between the deep geologic structure,the surface rupture,and the seismic phenomena. Therefore,we support the“Thin-skinned-Thrust”model proposed by Taiwan seismologists for explaining the mechanism and kinematics of the Chi-Chi earthquake. The generation of the Chi-Chi earthquake can be attributed to the low-angle northwestward pushing of the Philippine Sea plate upon the imbricated structure of Jura-type on the Eurasian plate. In this paper,we describe also the features of the earthquake damages,especially the severe damage on the hanging wall and on the north terminal of the earthquake fault.

DISCUSSION ON CHARACTERISTICS OF CRUSTAL DEFORMATION ALONG THE ZHANGJIAKOU-BOHAI SEISMOTECTONIC ZONE

WANG Ruo-bai, GU Guo-hua, XU Jie, ZHOU Wei

2004, 26(4):  586-596. 

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The Zhangjiakou-Bohai fault zone in the northern part of North China area is a seismotectonic zone controlling the present-day strong earthquake activity. Under the effect of NEE-directing regional principal compressive stress,a series of NE trending active tectonic zones were engendered in this area. Together with the Zhangjiakou-Bohai fault zone,they make up a group of conjugate shear fracturing systems,controlling the occurrence of the present-day strong earthquakes. This paper focuses mainly on the feature of crustal deformation around this fault zone. The secular GPS crustal deformation image has reflected the relatively complete left-lateral strike-slip movement of this active fault zone. However,the crustal deformation images of different stages indicate that a series of NE-trending large-scale anomalous gradient zones appeared along the Zhangjiakou-Bohai zone before the occurrence of moderate strong earthquakes. These anomalous zones have indicated separately the activities of the Tangshan-Hejian,Sanhe-Laishui and Yan-huai-Shanxi seismotectonic zones,and represented the medium term precursors of the moderate strong earthquake along the Zhangjiakou-Bohai fault zone. Furthermore,the results of this study suggest that the crustal deformation that appears before the occurrence of strong earthquake reflects the strain activity in the deep seismogenic zone,while the disorderly and unsystematic crustal deformation images that occur after the occurrence of strong earthquake reflect the adjusting movement of the covering strata.

LARGE-SCALE DEXTRAL STRIKE-SLIP MOVEMENT AND ASOCIATED TECTONIC DEFORMATION ALONG THE RED RIVER FAULT ZONE

XIANG Hong-fa, HAN Zhu-jun, GUO Shun-min, ZHANG Wan-xia, CHEN Li-chun

2004, 26(4):  597-610. 

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Field investigation has revealed that the large-scale dextral strike-slip movement and the associated tectonic deformation along the Honghe (Red River) Fault zone have the following features. Geometrically,the whole Red River fault system can be divided into three deformation regions:the north,central and south deformation regions. On the eastern side of the north region lies the northwest Yunnan extensional taphrogenic belt,which is characterized by three sets of rift-type faulted basins striking NNW,NNE and nearly N-S since the Miocene time. From Miocene to Quaternary epoch,the faulted basins spread or migrated southwestwardly,but the large-scale and intensive rift-depression mainly occurred in the late Pliocene to Quaternary. The extension amount of the basin since Quaternary is about 5.6 km. On the western side of the north segment lies the Lanping-Yunlong Tertiary compressive deformation region. The deformation in the central segment is characterized by dextral strike slip or shearing,resulting in a dextral displacement of about 7.4km since Quaternary. The east Yunnan Miocene compressive deformation region lies on the eastern side of the fault in the south,and the extensive fault-depressed region is located on its western side. In tectonic-geomorphology,the afore-mentioned deformation features appear as basin range tectonics in the north,linear fault valley depressed basins in the central part and compressive (or extensive) basins in the south. Among them,the great variance of the planation surfaces on both sides of the fault in Cangshan to Erhai area is the prominent expression of the normal faulting along the Red River fault zone since Pliocene time. From the view-point of spatial-temporal evolution,the main active portion of the Red River fault zone from Eogene to Pliocene was the southern segment,which was characterized by tearing from south to north. The main active portion of the fault has migrated to the north segment since Pliocene,especially in late Quaternary,and was characterized by extensional slipping from north to southeast. The range of deformation region and the magnitude of deformation show that the eastern plate of the Red River Fault zone is always the active plate for the relative movement of the fault blocks.

GEOLOGIC-GEOMORPHIC EVIDENCE AND RATE OF LATE PLEISTOCENE DISPLACEMENT OF THE YOUJIANG FAULT ZONE

SONG Fang-min, LI Chuan-you, CHEN Xian-cheng, ZHANG Lan-feng

2004, 26(4):  611-621. 

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The Youjiang Fault zone is located in the Guixi (Western Guangxi) fault block region. Since the beginning of seismic records,22 earthquakes of magnitude 4.0～6 occurred in this region,among which the largest one is the magnitude 6/2 earthquake occurred in the area between Leye,Guangxi Autonomous Region and Luodian,Guizhou Province in 1875. Of these events,15 earthquakes of magnitude 4.0～5.0 occurred on the Youjiang Fault zone. The Tianlin Bagui M5.0 earthquake of 1962 and the Pingguo M5.0 earthquake of 1977 had caused certain damages of basic installations in the regions. Obviously,the Guixi region is an active region of moderate earthquake,and the Youjiang Fault zone is an active belt of moderate earthquake,which plays an important role in the seismicity in Guixi fault block region and in the territory of the Guangxi Autonomous Region. Based on the interpretation and analysis of satellite images,aerophotos,and large-scale topographic maps,as well as field investigation,a line of geologic geomorphic evidence of late Pleistocene activity of the Youjiang Fault zone have been obtained,and the left-lateral displacements on the fault zone have been measured. This paper presents all these results and provides the horizontal and vertical slip rates of the fault zone since mid-late Pleistocene. The Youjiang Fault zone can be divided into 3 segments. They are the west of Bose,Bose-Silin and Silin-Tanluo segments,each of which can be subdivided into several sub-segments. The offset of late Pleistocene terrace deposit and talus can be observed along each segment of the fault zone. The ages of the deposits have been dated to be (3.28±0.25)×10⁴a BP～(10.16±0.79)×10⁴a BP. Geomorphically,the fault zone controlled the development of the Bose-Tiandong late Quaternary basin. A series of fault valleys,troughs,and scarps were developed along the fault strand,while the drainage system crossing the fault zone was left-laterally offset. According to the comparison of the amplitudes of vertical and horizontal displacements on the fault zone,it is inferred that the activity since late Pleistocene of the fault zone has been dominated by left-lateral strike-slipping accompanied by extensional differential motion. The horizontal displacement rate since late Pleistocene on the fault zone has been determined to be 1.47～1.98 mm/a,the vertical displacement rate since middle Pleistocene is 0.74～0.76 mm/a,and the vertical displacement rate since late Pleistoce is 0.1～0.35mm/a. All these values are significantly lower than those on the fault zones surrounding or within the Chuandian fault block. The recent results of GPS observation support also this conclusion.

QUATERNARY ACTIVITY OF THE BAISHAN SAPU SEGMENT OF THE TANCHENG-LUJIANG FAULT ZONE

YAO Da-quan, LIU Jia-can, ZHAI Hong-tao, TONG Yuan-lin

2004, 26(4):  622-629. 

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Remote sensing analysis of tectonic landforms revealed that the Baishan (Lujiang County)-Sapu (Tancheng County) segment of the Tancheng-Lujiang Fault zone appears as a series of parallel faults. Field investigation has confirmed that these parallel fault series are active fault. Moreover,in-situ observation of fault profiles,as well as microstructural analysis of samples collected from the faults reveal that stick-slip and creep-slip deformation have occurred alternately along the Baishan-Sapu segment of the Tancheng-Lujiang fault zone since late Quaternary. In Ketan-Sapu area,the youngest drainage system was dextrally offset by the NE-trending faults. Microscopic observation and dating of the samples collected from the fault show that during Quaternary time the deformation along the fault was characterized by brittle-ductile transition,and the strongest faulting event occurred in Early-Middle Pleistocene. The dating results of fault gouges collected from Baishan profile indicate that this fault segment had been strongly active during Middle-Upper Pleistocene. SEM and optical microscopic observations also show that stick-slip and creep-slip deformation had alternately occurred along this fault segment,and the stick-slip deformation occurred earlier than the creep-slip deformation. In consideration of the distortion of drainage system and the fact that minor earthquake occurred frequently along this fault segment in recent years,it can be postulated that the recent movement along the fault has been dominated mainly by creep-slip,that is to say that the accumulated stress on this segment has been released slowly by creep slip or micro-earthquake. Therefore,it is unlikely that a strong earthquake will occur on this fault segment in the near future.

HUJIAMENGYAN SURFACE RUPTURE IN JUXIAN COUNTY,SHANDONG PROVINCE—A NEW DISCOVERY ON THE TANLU FAULT ZONE

HE Hong-lin, SONG Fang-min, LI Chuan-you, YANG Xiao-ping

2004, 26(4):  630-637. 

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Tanlu Fault zone is one of the most important active faults in east China. Yishu Fault,the middle section of the Tanlu Fault zone,presents a graben system consisting of two grabens and four main boundary faults. The eastern graben between Weifang and Jiashan is the most active segment of the Tanlu fault zone,along which developed a 360km long Holocene active fault zone (F₅). The F₅ fault zone has been defined as what consists of all Holocene faults in the eastern graben. The Anqiu earthquake of AD 70 and the Tancheng earthquake of 1668 occurred along the northern and middle segments of the Holocene active fault,respectively. We found a 7km long surface rupture between the main boundary faults of the eastern graben in Juxian County,when we made an investigation on the Yishu Fault in 2003. As an active fault,it should belong to the F₅ fault zone. The carbon date of the un-faulted deposit covering the newly-found surface rupture shows that no earthquake has occurred along this rupture since 2 140?190 yr BP. Therefore,we infer that this newly-found surface rupture is independent of the surface rupture of 1668 Tancheng earthquake,although it is necessary to make a further research to verify this inference.

A NEWLY DISCOVERED E-W-TRENDING ACTIVE FAULT IN THE NORTHEAST PART OF SHANDONG PENINSULA

MA Bao-qi, SHU Sai-bing, LIU Guang-xun

2004, 26(4):  638-644. 

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Late Quaternary tectonic movement in the northeast part of Shandong Peninsula was dominated mainly by integral uplift,while fault activity inside the peninsula was relatively weak. Late Pleistocene faults are limited to some local areas,and one of them is the newly discovered Dongdianhou Fault. The fault is located to the north of Taiboding Mountains,associated with a broad fracture zone of 100m wide. Outcrops of fault profile can be observed in many places along the fault strand. The fault has a general strike of nearly East-West and a total length of about 20km. Geomorphically,the fault appears as a straight fault valley consisting of 3 upper reaches of stream. The mountains on the southern side of the fault are obviously higher than those on the northern side. The cross section of the fault to the west of Xinshangzhuang Village shows that the fault displaces the unconsolidated rubble bed dated to be 84～75ka BP in age,resulting in a 1 8m high colluvial wedge,which is overlain by talus bed dated to be 64ka BP in age. The vertical slip rate of the fault is estimated roughly to be about 0.16mm/a. Soft fault gouges were observed on the fault planes exposed to the south of Diaoyushi Village and to the east of Taoyuan Village,and the age of the gouges is dated by thermoluminescent method to be 82ka BP. The characteristics of the fault and the age of overlying layers indicate that the latest faulting event occurred in early and middle stage of Late Pleistocene,while since the late stage of Late Pleistocene the fault has not ruptured the surface. According to the relation between surface rupture parameters and earthquake magnitude,it is estimated that the magnitude of the maximum potential earthquake on this fault is 6/2.

CHARACTERISTICS OF PALAEO-EARTHQUAKE ACTIVITY ALONG THE ACTIVE TIANQIAOGOU HUANGYANGCHUAN FAULT ON THE EASTERN SECTION OF THE QILIANSHAN MOUNTAINS

ZHENG Wen-jun, YUAN Dao-yang, HE Wen-gui

2004, 26(4):  645-657. 

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The Tianqiaogou Huangyangchuan Fault (called also Gulang Fault) lies to the east of Lenglongling,the highest peak on the eastern section of the Qilianshan Mountains. It is one of the important active faults in eastern Qilianshan. From west to east,the fault is about 86km long,initiating from Hongyaoxian,extending eastward along Tainqiaogou,Qianjin,Guanjiatai,and Huangyangchuan villages,and ending at Jiapigou. The western segment of the fault is NWW-trending,the middle is nearly E-W-trending and the eastern is NEE-trending. Among them,the middle segment is convex slightly to the south. The fault is basically continuous,except that a small step-over exists near Guanjiatai village,dividing the fault into two sub-segments:the Tianqiaogou sub-segment in the west and the Huangyangchuan sub-segment in the east. Field investigation revealed that offset landforms are well developed along the fault,such as offset gully,fault scarp,sag pond,offset terrace and alluvial fans. All the offset landforms indicate that the fault was dominated by left lateral strike-slip with a component of normal faulting. Tracing along the fault and trenching across 6 typical offset landforms revealed the evidence of Holocene activities and several palaeo earthquakes along the fault. By the use of progressive constraining method,7 Holocene palaeo-earthquakes and one historic earthquake are identified. The occurrence time of these events are determined to be 10,743±342a BP (Event Ⅰ),9038±39a BP (Event Ⅱ),6910±438a BP (Event Ⅲ),4847±185a BP (Event Ⅳ),3562±190a BP (Event Ⅴ),2476±194a BP (Event Ⅵ),1505±253a BP (Event Ⅶ) and 1927AD (Event Ⅷ,possibly the Gulang earthquake). The distribution and recurrence behavior of strong earthquakes along the fault show a distinct linearity and sub-periodical recurrence,for which the linear regression equation of event-time sequence is T_i=1516i-11734. Moreover,during field investigation we found new evidence indicating that surface rupture occurred along the Tianqiaogou-Huangyangchuan fault during the Gulang M_S8.0 in 1927.

LATE CENEZOIC NORMAL FAULTING ON THE WESTERN SIDE OF WENQUAN GRABEN,CENTRAL QINGHAI-TIBET PLATEAU

WU Zhong-hai, YE Pei-sheng, LIU Qi-sheng, WU Zhen-han, HU Dao-gong, ZHAO Xi-tao, ZHOU Chun-jing

2004, 26(4):  658-675. 

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The north-south-trending Wenquan graben is located to the north of Tanggula Mountains in central Qinghai-Tibet Plateau,having a length of about 40km and a width of 8～12km. The graben is filled with Quaternary moraine,fluvioglacial deposits and alluvia. An east-dipping boundary normal fault of about 45km length is developed along the western margin of the graben,where the relief is dramatically varied. The fault offsets vertically the bedrock,alluvial fan,river terraces and travertine platform. On the upthrown side of the fault,there are 4 prominent fault facets,which are ～600m,～400m,～200m,and ～80m above the surface of piedmont plain,respectively. At the base of fault facets,sustained faulting has given rise to the formation of east-facing fault scarps and surface ruptures that cut the terraces,alluvial fans and travertine platform. Leveling survey and in-situ measurement with tape measure show that the 6 sets of fault scarps are about 0 3～0.6m,4～5m,8±1m,15±3m,24±4m and 45±5m in height,respectively,and that the higher the scarp,the older the strata cut by the scarp. Among them,the 0.3～0.6m high fault scarp might be associated with M_S6～7 earthquake of middle-late Holocene,the 4～5m and 8±1m fault scarps offset separately the terraces Ⅰ and Ⅱ,while the 15±3m,24±4m and 45±5m high fault scarps offset separately the terrace Ⅲ,Ⅳ,Ⅴ and travertine platform. U-series dating results indicate that the fault scarp that offsets the terracesⅠand Ⅱ was formed 11～7ka BP,the scarp that offsets the terraces Ⅲ was formed 21～25ka BP,and the scarps that offset the terrace Ⅳ and Ⅴ and the correlated travertine platform were formed 45 2～53 6ka BP and 103～127ka BP,respectively. The 80～200m high scarps might be formed since 324～521ka BP. Moreover,the dating results suggest also that the average vertical slip rate along the normal fault on the western side of the Wenquan graben is 0.45mm/a with the maximum value of less than 1.2mm/a. The relief across the normal fault indicates that the minimum cumulative displacement on the fault is about 2.1km. The maximum cumulative throws on the fault is about 8.2km. The trenching across the fault revealed that at least 3 paleoseismic events have occurred along the fault since late Pleistocene. The magnitude of those events can be estimated to be M_S6～7 and the approximate recurrence interval to be from about 400 to 3000yr. The slip rate on normal fault from Wenquan graben implies that the east-west extension rate absorbed by a single graben in central Tibet is significantly lower than that of the grabens in southern Tibet,but this does not imply that the east-west extension rate in central Tibet must be significantly lower than that in southern Tibet. This is because that the east-west extension in central Tibet might be accommodated by a larger number of normal and strike-slip faults distributed in this area. On the basis of the total cumulative throws and the average vertical slip rate on the fault,it is inferred that the initiation time of the normal faulting to be at about 18 2～1 8Ma BP. The broad similarities in the magnitude of slip,the direction of extension and the initiation time of normal faulting in both central and southern Qinghai-Tibet Pateau imply that the east-west extension within the Qinghai-Tibet Plateau has occurred synchronously,and that the crustal thickness in central and southern Tibet Plateau has reached its maximum value since Miocene,while the whole Plateau has reached its present elevation.

HOLOCENE SLIP RATE ALONG THE EASTERN SEGMENT OF THE KUNLUN FAULT

LI Chun-feng, HE Qun-lu, ZHAO Guo-guang

2004, 26(4):  676-687. 

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The Eastern Kunlun Fault zone in northern Tibetan Plateau is one of the largest active fault zones in the world,demonstrated by a composite surface rupture zone of more than 700km long. However,the quantitative studies of the fault zone have remained scarce until recently mostly due to the difficulties of carrying out field investigation in this area. In this paper,we analyze offset values along the fault and their relevant TL and 14 C dating samples collected during our field investigation,from which we estimate the Holocene horizontal slip rates and earthquake recurrence intervals along the fault segment between the Toson Hu Lake and Kendingna (east Maqen). It is found that roughly along the Maji peak of the A'nyemaqen Mountains,the eastern Kunlun Fault section can be subdivided geometrically into two discontinuous segments:the Huashixia segment (from the Toson Hu Lake to the Maji Peak) and the Maqen segments (from the east of the Maji Peak to east of Kendingna). These two segments have significantly different Holocene slip rates and paleo-earthquake activities,with increased activity on the Huashixia segment. The Holocene horizontal sinistral slip rate on the Huashixia segment is estimated to be 11.5?1.1mm/a from five offset locations,while that of Maqen segment to be 7.0?0.6mm/a from study of four offset measurements. Furthermore,the vertical differential rising rates caused by the fault are estimated to be 2 1?0 3mm/a for the Huashixia segment since 4ka BP,and about 0.55mm/a for the Maqen segment since 10ka BP. These significant differences in the slip rates between the two segments indicate strongly some behavioral changes for the eastern Kunlun Fault striding over the Maji peak of the A'nyemaqen Mountains; meanwhile,the relatively large vertical differential rising rates may support a rapid Holocene rising of this region.

STUDY ON THE SEISMOGENIC STRUCTURE OF THE YUMEN, GANSU PROVINCE M_S5.9 EARTHQUAKE OF DECEMBER 14, 2002

HE Wen-gui, ZHENG Wen-jun, ZHAO Guang-kun, MA Er-man, DONG Zhi-ping

2004, 26(4):  688-697. 

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On December 14,2002,an earthquake of M_S5.9 occurred in Yumen,Gansu Province. Field investigation revealed that both the macroscopic and measured epicenters of this event,the coordinate of which are 39.8°N,97 3°E and 39.7°N,97 3°E,respectively,are located on the northern marginal fault of the Qilian Mountains. The epicentral intensity was determined to be Ⅶ. The epicentral area appears as an elongated ellipse with N65°W-trending long axis of 15km long and N25 E-trending short axis of 12km long. The earthquake took place in the Yumen area,which is located on the northern margin of the Qinghai Tibet Plateau and the western section of the Qilian Mountains. There are several Holocene active faults,of which the most famous ones are the Altun and the Changma faults. Historically,several large earthquakes occurred on these two faults,such as the Changma (M_S=7.6) earthquake of 1932 and the earthquake (M_S=5.5) occurred on the eastern section of the Altun fault in 1933. The Yumen earthquake is a moderate-strong one,which didn't produce distinct surface rupture zone. The event was the result of the most recent movement on the secondary fault of the northern marginal fault zone of the Qilian Mountains,namely the Hanxia-Dahuanggou Fault. The evidence is as follows: Field investigation shows that the macroscopic epicenter of this earthquake is located in the area involving the Hanxia Coal Mine of Yumen,Yuerhong Village of Subei County and Yaomoshan. The trend of the epicentral area is just consistent with the Hanxia-Dahuanggou Fault. Focal mechanism solution shows that the nodal plane A appears as thrust fault trending to 147°with a relatively small dip angle of about 26°,in accord with the general strike of the Hanxia-Dahuanggou Fault. The P axis is oriented in 277° direction having an elevation angle of 27°. This may indicate that this earthquake was the result of stress concentration due to the action of nearly SN-trending horizontal compressive stress. Aftershocks of this event were very frequent,and 113 aftershock of M_S≥0.5 had been recorded before Feb. 20,2003. Most aftershocks were distributed near the Hanxia Dahuanggou Fault and located to the south of the main shock,making a NE-trending zone. This may indicate that the faulting propagated from south to north along the fault,and that the event was the result of thrusting along the reverse fault. Several lines of evidence shows that a low angle major detachment plane exists at a weak zone 6～9 km beneath the surface of Yumen area. All folding and thrusting deformation occurred in the strata above the detachment plane (Fig.6). The northern marginal fault of the Qilian Mountains consists of several secondary reverse faults dipping southwest at an angle of 25°～70°. They all converge on the low angle detachment plane. The Hanxia Dahuanggou Fault is a part of the fault zone,and has a nappe structure. The focal depth of 31 recorded aftershocks coincides well with the depth of the detachment plane. Therefore,it is unlikely that a greater earthquake will occur on this fault in the near future,and this recognition may provide credible evidence for earthquake trend assessment after the Yumen earthquake.

SHALLOW SEISMIC EXLORATION FOR HUANGZHUANG-GAOLIYING BURIED FAULT IN THE VICINITY OF LISHUIQIAO,BEIJING

MA Wen-tao, XU Xi-wei, HAO Shu-jian, YIN Gong-ming, YU Guang-ming, YU Gui-hua, CHU Bao-gui, ZHENG Bao-gui, ZHANG Lan-feng

2004, 26(4):  698-705. 

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This paper introduces the results of shallow seismic exploration on five traverse lines across the Huangzhuang Gaoliying buried fault in the vicinity of Lishuiqiao,Beijing area. The section of the Huangzhuang Gaoliying fault in the vicinity of Lishuiqiao can be distinguished distinctly on the spot map,but the other sections of the fault along river valley are undistinguishable. The shallow seismic exploration and geological information reveal that the velocity model of the shallow part around the Lishuiqiao area is characterized by 4-layer structure. The layers 1 and 2 are located at a depth range of 0～150m beneath the surface,and they can be assigned to Tertiary or Quaternary deposits,having a velocity of 800m/s to 2000m/s. The layers 3 and 4 are identified at a depth range of 130m to 300m,which are bedrock consisting of mudstone and sandstone,having a velocity of larger than 2000m/s or 2500m/s. At shallow depth,the Huangzhuang-Gaoliying Fault in Lishuiqiao area is composed of two sub-parallel faults about 1300m apart from each other. The two faults are N23°E-striking,dipping southeast at an angle of 22° for the western fault and 87° for the eastern one. At a depth of 634m the two faults converge into one fault,appearing as a branching fault. The buried depth of the highest point of the hanging wall of the fault is 101m,while that of the footwall is 109 m. The throw of the fault is about 8m. It is concluded,therefore,that the fault is a normal fault with strike-slip component,dissecting the T₂ and T₃ stratigraphic interfaces.

SEISMIC REFLECTION SURVEYING OF THE PUJIANG-XINJIN DEYANG BURIED FAULT IN FENGHUANGSHAN AREA AND THE STUDY OF FAULT ACTIVITY

HE Qiang, LI Lu-ming, LAI Min, LI Da-hu, HUANG Wei

2004, 26(4):  706-715. 

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The spatial distribution,scale and property of the Pujiang-Xinjin-Deyang buried fault in Fenghuangshan area were investigated by the use of seismic reflection survey technique. The result shows that the fault cut off the late-pleistocene sediment in the process of eastward underthrusting,and has been significantly active since Quaternary. It has controlled to some extent the formation of the Fenghuangshan platform. Surface geological investigation and historical earthquake research have revealed that the Pujiang-Xinjin-Deyang buried fault were active during late Pleistocene. The activity of the south segment of the fault is obviously stronger than that of the north segment,indicating that the north segment of the fault is a zone of weak seismicity in the Chengdu Plain. At the same time,the layout of observational systems and the selection of seismic parameters for seismic reflection surveying of buried fault in densely populated suburban areas have been studied as well. It is found that the use of small group intervals,small offset,multiple channels,short array length,multiple coverage and high-frequency receivers,as well as the appropriate selection of seismic parameters are of great significance to the effective separation of reflected wave from disturbing wave,and also to the improvement of the resolving power of seismic prospecting technique.

OPTICAL STIMULATED LUMINESCENCE (OSL) DATING OF LATE PLEISTOCENE TERRACE DEPOSITS IN THE EASTERN SEGMENT OF THE ALTYN TAGH FAULT AND ITS TECTONIC IMPLICATION

WANG Ping, LU Yan-chou, CHEN Jie

2004, 26(4):  716-726. 

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The ages of terrace deposits along the Shulehe River system are determined by the use of Optical Stimulated Luminescence (OSL) dating technique. The possible tectonic implications of the obtained age data are discussed as follows: (1) The Duanjiagou,Shulehe,and Tashihe Rivers are three rivers of different characters from the Shulehe River system. Several levels of terraces of different altitudes and corrosion rates were developed in the intermont basin and mountain front on the eastern segment of the Altyn Tagh fault. These terraces are the result of differential uplift of the fault blocks. (2) On the southern margin of the Changma basin,7 levels of terraces were developed along the Shulehe River. OSL dating results show that those over 100m high terraces were formed mainly about 40000 years ago,and this may indicate the intensive uplift of the Daxueshan-Xiangmaoshan Mountains and the new activity of the Changma seismic belt. The rate of tilted uplift of the southern part of the Changma basin is inferred to be 2.5mm/a. (3) At Zhaobishan gorge,five levels of terraces of the Shulehe River are still well preserved. Among them,the T₃ terrace of 40～60m elevation above the stream level might be related to the new tectonic activity of the region. The T₅ terrace of over 150m elevation above the stream level observed at the boca has an ESR age of approximately 200ka and an uplift rate of 0.7mm/a. (4) On the Hongliuxia fault block uplift,4 levels of terraces were developed along the Duanjiasha River. Dating results of the T₁,T₂ and T₃ terraces show that a linear relation exists between the heights and the ages of the terraces,from which an uplift rate of 0.6mm/a can be deduced. The Hongliuxia and the Zhaobishan fault block uplifts are the fault blocks controlled by the strike-slip movement of the Altyn Tagh Fault. They have approximately the same rate of uplift that is quite different from that of the range-front uplift of the Daxueshan-Xiangmaoshan Mountains,indicating that the uplift deformation of the Tibetan Plateau in the late stage of late Pleistocene occurred mainly along the Changma active fault zone. (5) Four levels of terraces were developed at the fan apex of the Tashihe River,and the age of the T₃ terrace is dated to be 78.8±5.5ka. It is postulated,therefore,that the terraces at the apex of the Tashihe River alluvial fan were formed in the middle to late stages of late Pleistocene. On the basis of OSL dating results of terrace deposits and the investigation of tectonic landforms,it is concluded that periodic uplift has occurred in the Nanjieshan fault block at 300 ka BP or during the middle to late stage of late Pleistocene.

DISCUSSION ON DIFFUSE SEISMICITY ASSESSMENT RELATED TO NUCLEAR POWER PLANT SITING

ZHANG Yu-ming, SHI Zhen-liang, CHANG Xiang-dong, ZHOU Ben-gang, HUAN Wen-lin, DONG Rui-shu, RAN Hong-liu

2004, 26(4):  727-732. 

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Diffuse earthquake refers to the maximum earthquake potential not associated with the identified seismogenic structures. The assessment of diffuse earthquake is a key point in nuclear power plant siting. As one part of the regional seismotectonic model,the diffuse earthquake plays an important role in SL-2 determination,because nuclear power plant is usually sited in the region of lower seismicity. Diffuse earthquake usually consists of moderate or small earthquakes of magnitudes less than 6,and the evaluation of diffuse earthquake is carried out in the seismotectonic province. The concept of“seismotectonic province”has been used to represent diffuse seismicity for the purpose of seismic hazard assessment,with each seismotectonic province assumed to encompass an area of equal seismic potential,that is to say the“diffuse earthquake”. The principles and methods of delineating seismotectonic province are discussed in detail in this paper. According to the related nuclear safety statutes,it is necessary to make reasonable assessment of diffuse earthquake for each seismotectonic province,especially for area where the nuclear power plant is sited. After discussing the principles for diffuse earthquake assessment,the paper puts forward the methods for evaluating the magnitude of diffuse earthquake based on the concept of diffuse earthquake by using seismic and geological data. The paper has also discussed how to determine the specific distance of the diffuse earthquake to the site in the seismotectonic province where the nuclear power plant is located.

ACTIVE TECTONIC RESEARCH FOR SEISMIC SAFETY EVALUATION OF LONG-LINE ENGINEERING SITES IN CHINA

RAN Yong-kang, CHEN Li-chun

2004, 26(4):  733-741. 

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Long-line engineering sites usually have to pass through some active tectonics,so that the research of active tectonics is of great importance to seismic safety evaluation of this sort of site. In this paper,basing on the summarization and analysis of the requirements for seismic safety evaluation of long-line engineering sites and the present situation of active tectonics research,we propose the focal points of active tectonics research for seismic safety evaluation of long-line engineering sites,including research contents,technical target and routes,and the submission of the achievements,etc. Furthermore,we make a preliminary analysis and discussion about some problems existing in the present-day active tectonics research for seismic safety evaluation of long-line engineering sites.

APPLICATION OF GIS TO THE COMPILATION OF SEISMIC ZONING MAP OF CHINA

YE Hong, ZHOU Qing, CHEN Guo-guang, SHAN Xin-jian, QU Chun-yan, CHEN Xiao-li

2004, 26(4):  742-749. 

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Geographic Information System (GIS) technique provides a powerful tool for managing,manipulating,analyzing and displaying space related data set. The compilation of seismic zoning map requires a vast amount of spatially referenced data. Therefore,the application of GIS to the seismic zo￣ning work allows the users to take advantage of various functions of GIS into the seismic zoning work. This paper introduces briefly the procedure and results of GIS application in the compilation of the New Seismic Zoning Map of China (2001). The GIS Database of the New Seismic Zoning Map includes three sub-databases: 1. Sub-database for seismic environment and potential seismic sources,including: (1)Earthquake catalog; (2)Quaternary active faults; (3)Cenozoic basins; (4)Earthquake mechanism; (5)Geophysical anomalies and deep structure; (6)Seismotectonic provinces and zones; (7)Potential seismic sources. 2. Sub-database for earthquake ground motion and seismic attenuation,including: (1)Seismic intensity and isoseism; (2)Observed seismic intensity; (3)Strong ground motion. 3. Sub-database for the results of probability calculation of seismic parameters.

DISCUSION ON THE APPLICATION OF THE PROPERTIES OF SEISMOGENIC STRUCTURES TO THE DETERMINATION OF POTENTIAL EARTHQAUKE SOURCE PARAMETERS

ZHOU Ben-gang

2004, 26(4):  750-760. 

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Studies of the properties of seismogenic structures play an important role in delineation of potential earthquake source and in determination of parameters of seismic activity. The location,direction,size and the maximum potential earthquake magnitude of a seismogenic structure can be used separately to determine the location,direction,extent and upper limit magnitude of the related potential earthquake sources. The recurrence behavior of large earthquakes on seismogenic structures is closely related to the estimation of the annual average occurrence of the high magnitude grade earthquakes. In consideration of the spatial heterogeneity of seismicity,two-grade delineation of potential earthquake source has been adopted in seismic hazard assessment in China since the late 1980s. Moreover,a relatively large region is usually taken as the seismictiy statistical unit (seismic region or seismic zone) in order to gain sufficient seismic data,so that it might contain some seismotectonic blocks with different background earthquake magnitudes. For the sake of more reasonable seismic hazard assessment,it is necessary to consider these seismotectonic blocks as the second grade potential earthquake sources,which are then subdivided into the third grade potential earthquake sources. On the basis of research on the recurrence behavior of large earthquakes on seismogenic structures,this paper proposes a method for calculating the annual average occurrence of the high magnitude grade earthquakes by applying the equivalent probability transferring in the predicted time scale. This method may greatly enhance the reasonability of seismic risk assessment. The frequency insufficiency of the second grade magnitude earthquakes of potential earthquake sources and the spatial inhomogeneity of seismogenic structures,as well as their applications to determining the parameters of potential earthquake sources are also discussed in this paper.

PRINCIPLE AND METHOD OF DELINEATION OF POTENTIAL SEISMIC SOURCES IN NORTHEASTERN YUNNAN PROVINCE

ZHOU Qing, GUO Shun-min, XIANG Hong-fa

2004, 26(4):  761-771. 

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Lying on the eastern margin of the Tibetan Plateau,the northwestern Yunnan Province has attracted many tourists all over the world for its beautiful scenery and maltinational cultures. At the same time,however,it is also famous for frequent destructive earthquakes that have caused serious loss of life and properties. With rapid economic development,deficiency of electric power in China has become very serious. Therefore,it is inevitable to build a lot of hydropower stations in southwestern China,where hydropower resources are very abundant. A series of dams are planed to construct along the Jinshajiang River. The seismic safety evaluation should be done before the construction of hydropower station,while the appropriate delineation of potential seismic source (PSS) is a key point of the evaluation. In the probabilistic seismic hazard analysis,the distribution and upper bound magnitude of the PSS affect the determination of the design parameters in different engineering sites. For this reason,we have collected the former research results and the data obtained from recent geologic investigation on engineering sites of a series of hydropower stations and on regional seismicity. In order to get an insight into the seismotectonic indicator for the northwestern Yunnan Province,we have analyzed the seismogenic structures of several typical strong earthquakes,including the 1515 Yongsheng M 7^3/4,1925 Dali M 7.0 and 1996 Lijiang M 7.0,and delineate again the PSS for the northwestern Yunnan Province. The following principles and methods have been applied to the delineation of PSS: In the region where strong earthquake has occurred,upper bound magnitude of PSS is determined in the light of “Earthquake Recurrence” principle; in the region where seismogenic structures are clear but no strong earthquake was recorded,PSS are delineated according to“Tectonic Analog”principle; in the region where the segmentation of active fault has been studied thoroughly,the PSS can be demarcated precisely; early-middle Pleistocene faults can be taken as tectonic indicator for moderate-strong earthquake; in the region of buried active fault,images of seismic activity and geophysical anomaly can be used to determine the extension direction of the PSS. In the studied region,21 PSS are delineated with different upper bound magnitude,among which 1 is of upper bound magnitude 8.0,3 are of magnitude 7.5,9 are of magnitude 7.0,and 8 are of magnitude 6.5.

DESIGN RESPONSE SPECTRA OF LONG PERIOD-GROUND MOTION FOR THE BEDROCK OF THE TAOYAOMEN BRIDGE SITE

LIANG Xiao-hua, JIANG Pu

2004, 26(4):  772-783. 

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With the construction of more and more super-high buildings and large scale flexible structures,great attention has been paid to information about long-period response spectrum exceeding 4～6s,which is the basis of anti seismic design of large scale flexible structure. However,most of current anti-seismic codes,such as the anti-seismic design codes for highway construction used for the basis of large highway bridge design,are inadaptable just due to shortage of available information of the spectrum noted above. In consideration of the need of long period spectrum in the design of Taoyaomen large bridge in Zhoushan City,this paper focuses on the investigation and analysis of the basic characteristics of seismo-geologic environments of the bridge site and presents the state of art of the research of long period ground motion spectrum. On the basis of the analysis of the basic characteristics of long period component of 72 ground motion records chosen from 11 representative strong earthquake records,as well as in the light of the determining method of ground spectrum for specific sites of the Nuclear Safety Code [HAF0101(1)],the long period response spectrum based on middle response spectrum is established in this paper through statistic analysis method. According to the basic forms and characteristics of the average spectrum of strong motion records,this paper utilizes zigzag spectral form to represent the average spectrum of strong motion records under various ground condition. In consideration of the reliability of strong motion records,especially the long period part,the average value of seismic response spectrum of the bedrock within period field spectrum is processed separately: those of less than 4.0s period build on data from all samples,and those of greater than 4.0s period build on more reliable ground motion records,involving mainly the samples with PGA of above 100gal (0.1 g ). Moreover,comparison has been made with some standard seismic response spectrum related to the Taoyaomen site,which includes: 1) horizontal and vertical average probabilistic spectrum obtained from ground probabilistic earthquake risk analysis; 2) Probabilistic spectrum (50yrs,above 10% exceeding probability) for Fuchi Island near the Taoyaomen large bridge presented in the report of earthquake safety assessment of the bank area of Yeyashan-Cezi Island,Zhoushan City submitted by Institute of Crustal Dynamics,China Seismological Bureau in 1988. The comparison shows that the average spectrums of bedrock records of the period of 6～10s,whether the mean spectrum or the mean plus 1σ one is much higher than the probabilistic spectrum of the site. The following two factors are considered: 1) the site where Taoyaomen large bridge is situated belongs to low seismicity and relatively stable area,and the basic earthquake intensity for the area is assigned to be Ⅵ Degree; 2) As a whole,the standard of seismic design of highway architecture is lower than that of nuclear facilities and hydro constructions,so taken the 3% exceeding probability per century as the basic estimation of large bridge deformation is approximately equal to the seismic activity level of one event per 3000～4000years. Therefore,6～10s long period response spectrum can be used for the design spectrum of Taoyaomen bridge,similar to the determination method used in most of the engineering seismic design regulations. The comparison of the results shows that it is feasible to use long period response spectrum based on middle response spectrum as the basis of design spectrum of Taoyaomen bridge in Zhoushan City. From the point of view of the present research situation of long period ground motion response spectrum and the rapid growth of the need of long period design spectrum in engineering design,this research topic is not only of universal significance,but also provides operable reference for seismic design of engineering that needs the long period design spectrum. In consideration of the certain limitation of frequency characteristics of simulated strong-motion records.

INDUCED EARTHQUAKE RISK ASSESSMENT OF JIN'ANQIAO RESERVOIR ON THE JINSHAJIANG RIVER,YUNNAN PROVINCE

CHEN Xian-cheng, YANG Qing-yuan

2004, 26(4):  784-794. 

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Basing on geological field investigation and other relevant data,this paper analyzes the geologic-tectonic features,seismic activity background,regional stress field and hydrogeologic conditions of the Jinanqiao reservoir area. Moreover,the comparison with the other reservoirs of the similar conditions has been carried out. The results indicate that induced earthquake is possible to occur in the Jinanqiao Reservoir of the Jinshajiang River. The bedrock in the dam site to Guoshi segment of the reservoir consists of basalts produced by multicycle eruptions in Permian period. Joints and cracks in the basalt are mainly south-north-trending,consistent with the orientation of the principal regional compressive stress. Under the action of regional stress field,these fissure systems will propagate and enhance the percolation of water. As a result the stress produced by the mountain and rock mass would significantly increase when the water is stored in the reservoir. In consideration of the activity of the Shudi fault,it is calculated by probability method that the probability of the occurrence of type-Ⅰ induced earthquake in this segment is 2‰,that of type-Ⅱ induced earthquake is 5%,and that of type-Ⅲ induced earthquake is 20%. Therefore,an induced earthquake of magnitude 5.0 is possible to occur in the vicinity of Shudi. The bedrock in the Guoshi to Paimai segment of the reservoir consists mainly of limestone and dolomitic limestone,and karsts are well developed in the canyon area of this segment. At the intersection of faults in the vicinity of Guoshi,Huangyangguo and Wajinping,an induced earthquake with maximum magnitude of 5.0 is possible to occur. In the other places of this segment induced earthquake of magnitude 4.0 is possible to occur owing to the development of karsts,joints and fractures. Probability calculation shows that the probability of type-Ⅰ induced earthquake in this segment is 3‰,that of type-Ⅱ induced earthquake is 9%,and that of type-Ⅲ induced earthquake is 20%. The bedrocks in the segment from Paimaidang to the tail of the reservoir are relatively complex. The karstification in this segment is relatively weak owing to shallow water level and the existence of impure limestone and insoluble interbeds of sandstone and shale,so that the percolation of water in this reservoir segment is relatively weak. The probability of the occurrence of type-Ⅰ and type-Ⅱ induced earthquake is very low,and that of type-Ⅲ induced earthquake is lower than that in the other segments of the reservoir. However,the probability of type-Ⅳ induced earthquake is rather high,i e. there is high possibility of the occurrence of induced earthquake of magnitude 3.0. Comprehensive analysis of the probability of induced earthquake in Jin'anqiao Reservoir indicates that induced earthquake that may damage the hydraulic structures is impossible to occur in the basaltic segment and the head of the reservoir. However,induced earthquake of magnitude 5.0 is possible to occur at the intersection of faults in Shudi,Wajinping,Huangyangguo and Guoshi area,and the epicentral intensity may reach Ⅶ. The intensity affected to the dam buildings will be Ⅵ. Induced earthquake with magnitude 3.0 is possible to occur along the carbonate segment from Paimaidang to the tail of the Reservoir,but the damage effect of this earthquake can be neglected.