Table of Content

04 December 2001, Volume 23 Issue 4

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Brief Report

TEMPORAL SCANNING OF b-VALUE AND SPECTRUM OF AE ACTIVITY FOR SAMPLES WITH DIFFERENT TEXTURES AND THEIR PHYSICAL IMPLICATIONS

Liu Liqiang, Ma Shengli, Ma Jin, Hu Xiaoyan, Liu Tianchang, Wu Xiuquan

2001, 23(4):  481-492. 

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A comparative study was performed on the temporal characteristics of b-value and spectrum of acoustic emission during the deformation of samples with different textures, including sample of intact granite, sample of granite with naturally healed joints, and samples with simple, extensional or compressional en-echelon simulated faults. The results indicate that the temporal characteristics of b-value and spectrum of AE events may not only be affected by the sample texture but also differ in the different deformation phases for the same sample. This is closely related to whether the deformation is dominated by fracturing or by frictional sliding. When the deformation is dominated by fracturing, b-value and spectrum of acoustic emission decrease obviously before instability events although fluctuations may occur sometimes. When the deformation is dominated by frictional sliding, b-value and spectrum of acoustic emission show relatively small changes before instability events, and both decreasing and increasing are possible. If the deformation is a process including both fracturing and frictional sliding, b-value and spectrum of acoustic emission may show complex changes. The experimental results imply that b-value and spectrum of acoustic emission as precursors of earthquakes may show different characteristics in different tectonic setting. Using such precursors to predict strong earthquakes may be useful for fracturing type earthquakes, and a little difficult for stick-slip type earthquakes, but very difficult for mixed type earthquakes. Moreover, the preparation of a strong earthquake usually involves quite a big area, in which faults with different geometry may exist. This can cause complexity of b-value and spectrum in statistics and affect their reliability as prediction parameters.It is concluded that fault geometry may be an important factor affecting the heterogeneity in temporal and spatial distribution of short-term precursors.

PRELIMINARY STUDY ON THE DEFORMATION FEATURES AND SEISMOGENIC MODEL OF THE 1902 ARTUX, XINJIANG EARTHQUAKE OF M_S 8(1/4)

Zhao Ruibin, Shen Jun, Li Jun

2001, 23(4):  493-500. 

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The Tuotegongbaizi-Aerpaleike fault is located in the epicentral area of the 1902 Artux,Xinjiang earthquake of M S 81/4. It is a gently dipping reverse fault, merging downward into the decollement of South Tianshan Foreland Thrust belt at about 3km depth below the Earth surface. To the north of the epicentral area, lies the Muziduke thin-skinned arcuate nappe of the South Tianshan Foreland Thrust belt, while to the south lies the Mushi-Kashi-Artux arcuate recoil fold-thrust belt of northwest Tarim. The epicentral area is just located at the triangle zone of frontal structure of the juncture between these two major tectonic units. In the epicentral area a deep fault exists under the decollement of the thin-skinned nappe, extending downward to the Moho. It is located at the sharp gradient zone of the crust-mantle boundary between the South Tianshan and the Tarim basin. The projection of epicenter falls at a zone of abrupt change of crustal thickness. It seems that this great earthquake was the result of rapid faulting at the gradient zone of the crust-mantle boundary due to the action of N-S trending regional tectonic stress field. Because the focal depth is great, no earthquake faulting is observed on the surface. The deformation of the magistoseismic area is characterized mainly by physicogeologic phenomena such as collapse, landslides, and ground-fissures, accompanied by coseismic ruptures and folding of the active thin-skinned structures above the decollement.

SHORTENING AND UPLIFT OF THE ACTIVE YAKEN ANTICLINE, SOUTHERN TIANSHAN, CHINA SINCE LATE PLEISTOCENE

Yang Xiaoping, Zhou Bengang, Li Jun, Sun Jianbao

2001, 23(4):  501-509. 

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The Yaken anticline is located on the southernmost side of Cenozioc Kuqa rejuvenated foreland basin in southern Tianshan. The anticline can be assigned to fault-bend-fold, which involved the Jurassic, Cretaceous, Tertiary and Quaternary strata. The length of the Yaken fold belt is about 80km in east-west direction, and the width in north-south direction is about 6～7km. The oldest deposits exposed in the anticline core is Pliocene in age, while the deposits exposed on both limbs and periclinal part are early Pleistocene Xiyu conglomerate, as well as late Pleistocene and Holocene alluvial-pluvial gravel beds. The anticline appears as a broad fold, both the southern and northern limbs of which are basically symmetric. Dip angles of the deposits on both limbs of the anticline are only 5°～17°. The reverse fault controlling the development of Yaken anticline is not exposed on the surface and occurs in the form of a blind reverse fault. Seismic reflection data indicate that the deposits above the Jurassic strata are folded, and that the reverse fault merges downward into the horizontal detanchment surface at about 7km depth. Three levels of terraces were developed in Yaken anticline zone. Among them, the third level terrace has the highest elevation, and was strongly dissected to form some gullies. The second level terrace consists of a large paleo alluvial pluvial fan, and connects with the modern desert plane on both sides of the Yaken anticline. The surface of the terrace is broad and smooth. The age of deposit collected at 0.3～0.5m depth below the surface is determined to be 39.5～43.6ka by thermoluminescent dating, while the time of the abandon of this terrace is about 4～1.2ka. The first level terrace exists only in large south-north-trending gully or on the plunging crown of the anticline. The amounts of uplift of the first, second, and third level terraces to the northwest of Yaha village are 6.5m, 40m and 50m, respectively. To the northwest of Shiyou-Wuqi farm, the amounts of uplift of the second and third level terraces are about 60m and 100m, respectively, while the amount of shortening of the second level terrace is about 47m. The rates of uplift and shortening for the second level terrace in the middle segment of the anticline in the vicinity of the Shiyou-Wuqi farm, are 1.5～5mm/a and 1.2～3.9mm/a, respectively. On the western plunging crown of the anticline, the time of the abandon of the first level terrace is less than 7.8ka. The amounts of uplift and shortening here are 20m and 17.9m, respectively, while the rates of uplift and shortening are 2.56mm/a and 2.29mm/a, respectively.

BASEMENT FORMATION AND STRUCTURAL FEATURES OF THE QIANGTANG BASIN AND THEIR IMPLICATIONS

Lu Bing, Liu Chiyang, Liu Zhong, Li Yongtie

2001, 23(4):  510-520. 

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The Pre-Devonian metamorphic rocks exposed in the middle part of Qiangtang Basin, Qinghai-Xizang(Tibet) Plateau were formerly termed Armugang Group or Gemuri Group, which is composed of schists, gneisses, amphibolites and blue-schists. Basing on the metamorphic ranks and deformation degree of these rocks, the Qinghai-Xizang(Tibet) Petroleum Exploration Project Management Department of CNPC has recently divided the group into Guoganjianianri, Mayer Kangri, Gomori and Armugang Formations from top to bottom. The Gomori and Armugang Formations made up the crystalline basement of the Qiangtang Basin. Recently, it has been found that this series of rocks contain also radiolarian fossils of middle to late Permian and Triassic in ages, as well as early Permian and late Triassic mafic rocks. This may indicate that tensional taphrogeny had occurred during that time in the region. The results of aeromagnetic survey show that the strong magnetic anomalies spread as stripes along Rongma-Shuanghu, Geladaindong and Purog Kangri, as well as along both the southern and northern boundaries of the Qiangtang Block. The anomaly intensity varies between 100～300nT. Based on the aeromagnetic anomalies, the basement of the Qiangtang Basin is divided into two depressions and one uplift. The northern depression is 11km deep, and the southern one is 9km. Each depression can be subdivided into smaller units. After 20km upward continuation, the spreading direction of the aeromagnetic anomaly changes from east-west-trending to south-north-trending. Along Rongma, Shuanghu area and the E90° line there exist south-north-trending transverse tectonic zones, which separate Qiangtang Basin into western and eastern secondary blocks. The results of Magnetotelluric (MT) Sounding also support the existence of this situation. Moreover, the MT profile shows that the geo-electric structures of the basement in the basin vary significantly from north to south. For example, in south Qiangtang basin many crustal high-resistance bodies are developed, and the high-conductivity layer is buried at a greater depth and deformed intensely. In northern Qiangtang, however, many low-resistance bodies are developed, and crustal high- conductivity layer is buried at a shallow depth and deformed weakly. All of these situations are the result of later tectonic transformation. The crystalline basement of the basin was intensely transformed during the later uplifting of the Plateau. The basement was filled with high- or low-resistance compositions, which changed the continuity and homogeneity of the basement.

NEW INSIGHT INTO THE QUATERNARY ACTIVITY OF WUGUISHAN SOUTHERN PIEDMONT FAULT IN ZHUJIANG DELTA

Song Fangmin, Wang Yipeng, Li Chuanyou, Chen Weiguang, Zhao Hongmei

2001, 23(4):  521-526. 

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A number of NE-, NW- and E-W-trending faults are developed within and around the Zhujiang Delta, controlling the formation, evolution and even the seismic activity of the Delta. The previous studies on the Zhujiang Delta focused mainly on the geometry and the nature of recent activity of the Delta. It is a pity, however, that no good surface exposure was found to reveal the cross-cutting relationship between the fault and the Quaternary deposits, so that the recent activities of the faults was deduced indirectly from the interior structures, development features and seismic activity of the Delta. In 1998 and 1999, the authors carried out field investigation in the Zhujiang Delta. During the investigation, we found at the southern foot of Wuguishan mountain 2 good surface exposures showing clearly the cross cutting relationship between the fault and the Quaternary deposits. This is the direct evidence of Quaternary faulting firstly discovered in the Zhujiang Delta area. The Wuguishan Southern Piedmont fault lies in the Zhujiang Delta, extending northeastward from Hutiaomen in the southwest, passing through Dachi, Dutou, and terminating at Yakou, having a total length of 55km. It is striking 60°, dipping southeast or northwest at an angle of over 65°. Geomorphic landform related to faulting, such as terminal facet, fault cliff, and fault valley or nek are well developed along the fault. The newly discovered 2 exposures are located at the southern foot of the Wuguishan mountain, to the west of the newly built Zhuyuan high way to the northeast of Sanxiang. The first exposure is located beside a gully in the vicinity of a stone quarry. At this exposure 3 faults can be recognized with general occurrence of 35°～45°/NW∠65°～85°, the two of which dissect the Quaternary sandy gravel and gravel beds. The second exposure is located to the south of a bedrock hill. At a cross section exposed by quarrying, a 20m wide crushed zone can be recognized between the granite and the Quaternary coarse gravel bed. It seems that the two units are in faulted contact with each other. The fault is composed of a series of compressional planes, striking 65°, and dipping northwest at an angle of 75°～85°. The lower part of the Quaternary deposits that are dissected by the 2 faults consists of yellow gravel bearing coarse grained sands, while the upper part consists of slightly bedded gravel, granule and coarse grained sands. The former is comparable with the regional mid Pleistocene alluvium (sandy gravel and pebble beds), while the latter is comparable with the regional late Pleistocene alluvium (medium fine sands and gravel beds). They are all of terrestrial origin, but span mid-late Pleistocene in ages. According to these 2 exposures, therefore, it is confirmed that the two faults were active during mid-late Pleistocene.

THE RATE OF LATE QUATERNARY VERTICAL MOTION OF THE ZHUJIANG DELTA

Chen Weiguang, Zhao Hongmei, Chang Yu, Lu Banghua

2001, 23(4):  527-536. 

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Based on 166 age data of Quaternary deposits in fossil delta, the rate of the vertical motion of the Zhujiang Delta since late Quaternary has been estimated by eliminating the non-tectonic effects, such as the change of ancient sea level, the compaction of sedimentary layer, and the isostatic crustal subsidence. The estimations have been conducted separately for 11 fault block tectonic regions, which are bounded by NE-trending, E-W-trending and NW-trending faults. The results have indicated that the rate of vertical motion of the Doumen fault block in the south and the Guangzhou-Panyu fault block in the middle parts of the Zhujiang Delta is relatively high. This conclusion has been supported by the results of the studies on the changes of ancient geographic environment, the timing of faulting, and the late Quaternary paleo-seismicity in the Zhujiang Delta area. In contrast to the previous studies, the present study emphasizes the changes of vertical motion rate in different fault blocks and in different time. The result of this study indicates that the secondary fault blocks of the Zhujiang Delta were dominated by elevation motion at a rate of 1～2mm/a during the early Holocene (10～4ka), and were dominated by subsidence at a rate of -3～-4mm/a during the late Holocene (～4ka). The elevation rates of Guangzhou-Panyu, Guanghua Plain, Shunde and Doumen fault blocks were greater than 1mm/a, while the subsidence rates in Guangzhou-Panyu, Shenzhen-Hong Kong, Zhongshan and Doumen fault blocks exceeded -2mm/a.

MICROSTRUCTURAL FEATURES AND SEGMENTATION OF PREFERRED FAULTS AT ZHENJIANG-YANGZHOU YANGTZE RIVER HIGHWAY BRIDGE FOUNDATION SITE

Wang Mingwu, Zhang Yangsong, Li Li, Luo Guoyu

2001, 23(4):  537-544. 

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In this paper we present the results of the study on the compositions, structures, and seismic reflection of the preferred faults at Jiajiang and Shiyezhou sites of Zhenjiang-Yangzhou highway bridge foundation, as well as the results of the quantitative observation of the microstructural features of fault gouges from these faults by means of scanning electron microscopy (SEM). The segmentation of these faults has been further studied in the light of the preferred plane theory. The compositions and types of cementation of the fault gouges from the eastern segment of the Jiajiang fault are different from those of the fault gouges from the western segment. The main clay mineral of the fault gouges is illite, and chlorite comes to second. The eastern segment of the fault displays a distinct throw in seismic reflection, and here the surface textures of quartz grains from fault gouges can be assigned to group Ⅱ and subgroup Ⅰ_c, which are characterized by subangular shape of grains, subconchoidal texture and medium degree of chemical effect. The western segment of the fault displays a discontinuos and weakened reflection. The fault gouges from this segment contain more quartz grains than those in the eastern segment, while the surface textures of quartz grains belong to subgroups Ⅰ_b and Ⅰ_c, which are characterized by step-like texture and weak chemical effect. The main clay minerals of fault gouge from Shiyezhou fault are illite and montmorillorite, and the granulometric compositions of the gouges can be assigned to group Ⅱ or group Ⅲ. The value of fractal dimension and maturity of the gouges from the northern segment is lower than those of the gouges from the southern segment. The present fault activity in the intersection segment is higher than that in the southern segment. The surface textures of quartz grains in fault gouges from the northern segment belong to subgroup Ⅰ_b , and those from the intersection segment and the southern segment belong to subgroup Ⅰ_c . However, the percentage of perfectly round grain shape and step-like texture is higher in the southern segment than that in the intersection segment. The percentage of indentation and impact pits in the southern segment is higher than those in the intersection segment. It is clear that the Jiajiang and Shiyezhou faults display a significant segmentation in their geometry, activity and structure, while the intersection of the two faults is a combination preferred segment. The results of statistic analysis of surface textures of quartz grains show that both the Jiajiang and Shiyezhou faults have been active for several times, and were characterized by both stick slip and creep slip. The age of latest activity of the Shiyezhou fault is younger than that of the Jiajiang fault. In consideration of the geological condition, therefore, the middle bridge site is not an ideal bridge site. This study has great importance and practical significance to the assessment of fault activity and stability. It provides a geological basis for site selection and design, as well as the safety assessment of oversize bridge project that should be built on the fault zone.

RESEARCH ON TWO BURIED QUATERNARY FAULTS IN SHANGHAI

Zhang Zhenquan, Liu Changsen

2001, 23(4):  545-555. 

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In this paper, the Quarternary activity of two main faults in Shanghai has been studied by means of shallow seismic prospecting. The nine seismic reflection profiles crossing the Fengjing-Chuansha Fault have shown that the fault can be divided into three segments. The southwestern segment is a middle Pleistocene active fault. The middle segment is composed of two normal faults which make up a horst. The northeastern segment displays only the northern branch fault. The latter two segments were active during the late stage of early Pleistocene. The Taicang-Fengxian Fault is under the control of five seismic reflection profiles. The northern segment of the fault is a normal fault that was active during middle Pleistocene. The southern segment consists of two normal faults bounding a horst. The age of the latest activity of the fault is about 100ka B.P. The offsets of the upper reflector by the two faults are 2～5m in general. The average rate of vertical displacement is about 0.02mm/a for the Taicang-Fengxian Fault, and n ×10^-3mm/a for the Fengjing-Chuansha Fault. The characteristics of the activity of these two buried faults in Shanghai can be summarized as follows: (1) The NNW-, NE-, and E-W-trending faults are predominant over the fault structures of Shanghai. (2) The latest activity of the faults occurred generally in early-middle Pleistocene. (3) The age of the latest activity of the NNW-trending fault is younger than that of the other faults. The latest activity occurred generally in middle Pleistocene, but in some segments it might continue to late Pleistocene. (4) Normal fault is the only fault style in this area. The dislocation produced by the faults is 5～ 15m, and the maximum dislocation may reach up to 40～50m. It seems that the younger the age of activity, the smaller the dislocation is. (5) The Taicang-Fengxian Fault has the greatest average displacement rate, it is about 0.01～ 0.028mm/a.

THE LATEST TECTONIC MOTION AND SEISMIC RISK IN ZHANGZHOU BASIN, FUJIAN PROVINCE

Chen Yuantian, Xie Zhizhao, Wang Zhipeng, Ye Jianhui, Chen Xiangxiong

2001, 23(4):  556-563. 

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Zhangzhou basin is the largest Quaternary fault basin in Fujian Province. Five earthquakes of magnitudes 5 1/2, 6 1/2, 6 1/4, 5.0 and 5 1/4 had successively occurred in this basin in 1067, 1185, 1445, 1549 and 1601. It is an area with highest intensity and frequency of historical earthquakes in Fujian Province. In this paper we discuss the latest tectonic motion and the seismic risk of this basin on the basis of the analysis of the rejuvenation of the basin, fault activity, neotectonic differential movement within the basin and the periodicity of strong earthquakes. We have also compared this basin with the other basins or plains in the coastal areas of Fujian Province. It is found that since the late Pleistocene the basin has got rid of the constraint of the NE-trending faults to continue its propagation toward the southeast. At that time, the NE-trending faults within the basin were dominated by strong dextral strike slip with dip-slip component, while the NW-trending faults were dominated by sinistral strike slip with dip-slip component. Elevation and subsidence have frequently occurred in this basin since late Quaternary. As a result, the early Holocene alluvial deposits overlapped the late Pleistocene alluvial beds, forming the first level terrace. The lower part of the late Pleistocene beds is still below the present river level. The elevation of some early Holocene deposits is higher than that of the middle Pleistocene deposits, indicating that during the late Pleistocene time the amplitude of subsidence was greater than the amplitude of elevation in this basin. The differential movement within the basin was very intensive. In elevated area the level of early Holocene alluvial deposits is 20m higher than the river level, while in subsided area the top of these alluvial deposits is 27 10m below the earth's surface, resulting in a height difference of 47 10m between the two. The latest historical earthquake in this basin has occurred for 400 years, which exceeds greatly the 52～260 years interval of strong earthquakes in this area. This area, therefore, is a high seismic risk area. As compared with the other basins and plains in the coastal areas of Fujian Province, this basin has some peculiarities, and that is why 5 strong earthquakes had occurred here. It is expected that a destructive earthquake may occur in this area in the future. Moreover, the historical strong earthquakes in this basin all occurred directly beneath the city, causing more severe damage than the earthquakes of the other types.

A RESEARCH ON QUATERNARY VOLCANISM IN NORTH CHINA

Chen Xiaode, Shi Lanbin, Lin Chuanyong

2001, 23(4):  564-573. 

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The Quaternary volcanoes in North China are distributed mainly in Shanxi and Shandong Provinces. In Shanxi Province, the famous Datong volcanic cluster consists of ten odd well preserved volcanic craters, volcanic cones, and volcanic necks. In Shandong Province, isolated volcanoes are distributed in Penglai and Wudi areas. Stratigraphic correlation has revealed that the volcanic products are middle to late Pleistocene in ages, while dating data have indicated that the ages of these volcanic products are in the range of 0.8～0.1Ma, and that the climax of eruption occurred in middle Pleistocene. The Quaternary volcanics in North China can be assigned to alkaline and tholeiitic basalts. The chemical compositions of these basalts indicate that they are the products of partial melting to different extents in mantle source area, having the basic features of primitive mantle-derived magma that has not been affected by crystallization differentiation. The basalts from three localities all contain mantle xenoliths comprising mainly spinel lherzolites of proto-granular and porphyroclastic textures. The rheological parameters of the upper mantle beneath these three regions have been inferred from the detailed study of these mantle xenoliths. The differential stresses are determined to be 21～31MPa, 23～26MPa and 13～24MPa for Datong, Wudi and Penglai, respectively, while the strain rates are 10^-17～10^-15s^-1, 10^-18～10^-17s^-1 and 10^-20～10^-14s^-1, respectively. The equilibrium temperatures are 900～1 050℃ for Datong xenoliths, 840～900℃ for Wudi xenoliths and 800～1 000℃ for Penglai xenoliths. The constructed differential stress and strain rate profiles of the upper mantle beneath these three areas coincide well with those of the upper mantle in continental extension zone or rift proposed by Ave Lallemant et al (1980). Volcanism is the product of the combination action of deep-seated tectonic and shallow tectonic processes. The Datong region is a part of Shanxi depression belt, Wudi volcano lies within the North China depression, and Penglai volcano is located on the outer margin of the Bohai depression. Tectonically, volcanism bears a close relation with fault depression and down-warping tectonics. Seismic sounding data and the study of mantle xenoliths have confirmed the existence of upper mantle uplift or diapir beneath these depressions. In consideration of the scale, range and distribution of Quaternary volcanism, as well as the results of the study of mantle xenoliths, it is suggested that the features of the upper mantle of North China in Quaternary, inherit the properties of the upper mantle in Tertiary time. The volcanism in Quaternary time, however, was much weaker than that in Tertiary time.

EFFECT OF EARTH RESISTIVITY ON ELF/SLF TRANSMISSION TECHNIQUE

Wang Xiangshu

2001, 23(4):  574-580. 

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Since the last decade, the applicability of source-controllable powerful Super Low Frequency/Extremely Low Frequency (SLF/ELF) electromagnetic waves to the reconnaissance of resources and monitoring of earthquake has attracted great attention of the geophysical communities for its high precision and vast area of service region. It has become a new and prospecting field of research. By using this technique, the signal of high precision can be transmitted and received at each frequency under control. The observed data can then be analyzed by using correlation analysis technique. This will greatly enhance the signal-to-noise ratio and the precision of the observed data even in strong noise area. The signal can penetrate down to several dozens kilometers depth, so that it is widely applied to the reconnaissance of resources, detection of subsurface nuclear waste, environmental research, as well as the monitoring and prediction of earthquake. The construction of a high power (not less than 100～200kW) SLF/ELF transmitter is one of the most important bases of the system used for geophysical investigation and application. The analysis has shown that the height of earth resistivity of the antenna site plays a key role in increasing the efficiency of the transmitter. The earth resistivity bears a close relation with lithology and tectonic structure of the area. It is suggested, therefore, that the high resistivity of the antenna site is the necessary condition for the transmission of effective SLF/ELF electromagnetic waves. The measurement method and strategy for ELF/SLF antenna site selection are proposed in this paper as well.

QUANTIFICATION OF SEISMOGENIC ENVIRONMENT IN SOUTH CHINA COASTAL AREAS AND ITS APPLICATION TO SEISMIC ZONATION

Zhou Qing, Ye Hong

2001, 23(4):  581-587. 

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It is known that strong earthquakes are usually associated with regional active tectonics and certain geophysical anomalies reflecting the features of regional deep structures. In this paper, the Bayes formula in statistic is used to quantify the relationship between the earthquake and the factors of seismogenic environment. These factors include active fault, Quaternary basin, Bouguer anomaly and geomagnetic anomaly.