Table of Content

04 March 2004, Volume 26 Issue 1

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Brief Report

BASELINE CORRECTION OF NEAR FAULT GROUND MOTION RECORDINGS OF THE 1999 CHI-CHI, TAIWAN EARTHQUAKE

WANG Guo-quan, ZHOU Xi-yuan

2004, 26(1):  1-14. 

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The Chi-Chi earthquake firstly supplied us unprecedented near fault recordings and made it possible to study deeply the characteristics of near fault ground motion. However, there are considerable offsets of baseline of the near fault accelerograms obtained in this event. The reasons of the offset of baseline are more complex than those occurred in other near field recordings with farther fault distance. Therefore, general baseline correction schemes for near field recordings, such as the methods of BAP (Converse, 1992) and Iwan (Iwan et al., 1985), could not be used directly in baseline correction of this event, as pointed out by Boore (1999). As far as our information goes, no robust procedure for the correction of baseline offsets that can be applied to near fault digital recording of this event has been proposed. This has hampered to a considerable degree the further studies on the near fault ground motion of this event. To develop such a procedure, the specific sources of the baseline offsets must be understood. Basing on the result of our systematical analyses, we propose that the tilt of instruments due to differential settlement of the materials in the vicinity of the instrument should primarily be responsible for the large baseline-offset occurred in the near fault recordings of this event. A baseline correction scheme for the offsets caused by the tilt of accelerograph is put forward in this paper, which could eliminate the offsets of baseline effectively. It is noted that a low-cut filter can eliminate the effects of the baseline offsets, but it may cause the loss of information about the permanent displacement. The method given by the authors in this paper does not involve any filter, so that we can get the permanent displacements from the double-integrated accelerograms, which coincide well with the co-seismic displacements measured by GPS instruments. According to this study, the offsets of baseline have fairly small effects on PGA and response spectra within the period of 0.1 to 6 second, which is of great interest to engineering circle. However, PGV, PGD and D-last are strongly affected by the problem of baseline offset. Therefore, the baseline correction of near fault ground motion recordings is of great importance. The method of baseline correction proposed in this paper can be applied to the near fault ground motion recordings of the 1999 Turkey and Chi-chi earthquakes.

TECTONIC BACKGROUND AND SPATIAL CORRELATION FRACTAL DIMENSION OF STRONG EARTHQUAKES IN TAIWAN, CHINA

YAN Yun-peng, XU Jian-dong, DONG Rui-shu

2004, 26(1):  15-23. 

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In recent years, many fractal studies have been made in aspects of time, space, and intensity of earthquakes, but most of them are limited to single fractal (capacity dimension D₁ and information dimension D₂). It is found that single fractal is not enough to characterize the temporal and spatial fractal properties of earthquakes and commonly has a very narrow linear non-scale range. Moreover, single fractal dimensions cannot be obtained in some cases. In this paper, therefore, the correlation fractal dimension is adopted to analyze spatial characteristics of seismicity in Taiwan area. By use of plentiful earthquake records from the last century in Taiwan area, the correlation fractal dimensions for each seismic region, zone, and segment in this area are calculated, and then the fractal characterization for hypocentral distribution of earthquakes can be quantified. Finally, the relationship between the spatial correlation dimensions and the seismogenic environments in this area is construed synthetically. The major results are as follows: 1) the correlation fractal dimensions of the eastern seismic region of Taiwan area differs from that of the western region, owing to the different tectonic settings of the two regions; 2) each seismic region can be subdivided into different seismic zones based on the difference of plate tectonics, crustal structure and the distribution of active faults. Each zone has its distinctive correlation fractal dimension; 3) the different segments of each seismic zone have different characteristics of earthquake clustering, and hence have different correlation fractal dimensions. The results of this study indicate that there exists a good correlation between the spatial images of seismic activity obtained from correlation fractal dimension analysis and the corresponding seismo-tectonic settings.

DEVELOPMENT OF THE QUATERNARY SYSTEM IN THE BOHAI SEA AREA, CHINA

XU Jie, RAN Yong-kang, SHAN Xin-jian, CHEN Guo-guang, LI Xiang-gen

2004, 26(1):  24-32. 

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The Bohai Sea is located in the east of the Bohai Bay Cenozoic rift basin, and has been characterized by intense tectonic activities since Quaternary time. Many major earthquakes of M≥7 had occurred in this area in history. Although a great number of detailed studies on the land part of the basin have been made, little has been done on the research of the Quaternary sediments in the Bohai Sea area. The isopach map of Quaternary system for this area has not been compiled so far. Basing on the previous studies, we used the data of the thickness of Neogene - Quaternary system, and the thickness of the Guantao and Minghuazhen Groups of Neogene to compile the isopach map of the Quaternary system with the help of GIS and image processing. This map shows that the Quaternary system is generally 300～600m thick, and locally 800～1000m thick in the studied area. The isopach in the western and eastern parts of the Bohai Sea exhibits NW- and NE-trending belt-like shape, respectively.

DEGRADATION OF FAULT SCARP AND PALEOEARTHQUAKE RESEARCH

YOU Hui-chuan, DENG Qi-dong, RAN Yong-kang

2004, 26(1):  33-45. 

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This paper deals with the degradation of fault scarp and the identification of paleo-earthquake in the light of theoretical and mathematical analysis of scarp development processes. We propose some models for fault scarp degradation in unstable and diffusion stages, and give out their mathematical solutions. From these solutions, the quantitative relation between geometrical features and/or sedimentary structures of fault scarp is analyzed. Furthermore, the precise expressions of the isochronous form-line of the scarp profile, the maximum gradient of scarp, and the sectional area of scarp deposits are established quantitatively for the use of paleo-earthquake research. Based on the field investigation, as well as experimental and mathematical analysis, the comprehensive indicators for identifying paleo-earthquakes associated with fault scarp, including geometrical, sedimentary, structural and experimental features, are summarized. Case studies on the fault scarp of the eastern Helanshan Fault at Hongguozi and fault scarp of the F 201 Fault at Mengjiawan were carried out. The results of these two studies reveal 2 series of 4 paleo-earthquake events that had occurred on the two faults respectively. The characteristics and recurrence intervals of these events have been studied in details. It is determined that the recurrence interval of paleo-earthquke events on the eastern Helashan Fault is 2.2～2.6ka, and that on the F 201 Fault is 3.3～4.0ka.

GEOCHEMICAL EVIDENCE OF THE ACTIVITY OF THE NW-TRENDING FAULT AT REHAI AREA, TENGCHONG, YUNNAN PROVINCE, CHINA

SHANGGUAN Zhi-guan, GAO Qing-wu, ZHAO Ci-ping

2004, 26(1):  46-51. 

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The Rehai area of Tengchong County, Yunnan Province is a well-known volcanic geothermal field in China. During the last few years, the hydrothermal activity was very strong in this area. Almost all of the boiling springs, spouting boiling springs and more than 20 times of hydrothermal explosion events since 1993 in this area are distributed along the NW-trending fault, indicating that the fault has been recently very active. The results of Th and Rn radioactive gas measurements verify that a NW-trending fault with a strike of 290? does exist in this area, and that the fault is presently still very active. The geological evidence and related research results indicate that the locations of magma chamber in the crust, the ring tectonic zone, the upwarped district of the Maanshan volcano and the Rehai geothermal field are all distributed just along northwest direction, which is roughly the same as the NW-trending fault. This may imply that the formation of this fault might be related to magmatic activity in the crust of the Tengchong volcanic region. As such, the recent strong activity of geothermal fluid in the studied area must be the surficial manifestation of the enhanced activity of deep-seated magma, to which enough attention should be paid.

TEXTUAL RESEARCH ON THE HISTORICAL DATA OF THE 138 AD JINCHENG-LONGXI M6³/4 EARTHQUAKE AND DISCUSSION ON ITS SEISMOGENIC STRUCTURAL BACKGROUND

YUAN Dao-yang, LEI Zhong-sheng, LIU Xiao-feng, LIU Bai-chi, CAI Shu-hua, ZHENG Wen-jun

2004, 26(1):  52-60. 

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On March 1, 138 AD, a great earthquake of magnitude about 6³/4 occurred in Jincheng-Longxi area. This event had caused "surface ruptures, serious damages of towns and houses, death of people, collapse of hills and river banks, and sink of land" in Jingcheng-Longxi area. It is worthy of noting that this earthquake is the only earthquake that had been recorded by seismograph invented by Zhang Heng, the famous Chinese ancient scientist, and it is also the earliest earthquake that was recorded by seismograph in the world. According to the detailed textual research of the historical earthquake records and the ancient town chronicles, it can be concluded that the Qianya town,the capital of the Jincheng state in the Eastern Han Dynasty, which was damaged during this earthquake,is situated in present-day Xiachuankou of Machangyuan village,Minhe Hui and Tu Autonomous County of Qinghai Province. At present, the ancient town site is located on the second level terrace of the south bank of the Huangshui River about 700m to the northwest of the Xiachuankou village. We found the remnants of pottery pots and animal bones from the rammed earth of the ancient town walls. The ages of these remnant pottery pots was dated by themoluminescent method to be about 2.77?0.24ka BP, which is consistent with the epoch of the Eastern Han Dynasty. The ancient Didao town, the capital of ancient Longxi state, is located in the present-day Lintao County, Gansu Province. The epicentral area of the 138 AD earthquake, therefore, can be located in the intersection area of the Yellow River and Huangshui River to the northwest of Yongjin County, Gansu Province. The intersection area of the Yellow River and Huangshui River is also a region where loess landslides are densely distributed, among which the largest ones might be produced by earthquake. Tectonically, this area is located on the frontier of the northern marginal arcuate thrust fault zone of the Lajishan Mountains. This fault zone is a late Pleistocene active fault zone, but some segments of the fault have been reactivated since Holocene. The low-angle reverse faulting toward the Minhe basin along NE direction had caused the intense compressive deformation of Neogene mudstone and early Pleistocene conglomerate on the northern side of the fault, resulting in compressive thrust fold zone. This may indicate that the frontier of the northern marginal arcuate fault zone of the Lajishan Mountains is probably a buried active compressive reverse fault-fold zone. This recognition has been supported by the recent data of petroleum prospecting. It is concluded, therefore, that it was the new activity of the reverse fault-fold zone on the frontier of the Lajishan Mountains that caused the 138 AD Jincheng-Longxi M6³/4 earthquake.

STUDY ON HOLOCENE STRIKE-SLIP RATES OF THE MIDDLE ALTYN TAGH FAULT BY TERRACES OFFSET MEASUREMENT

WANG Feng, XU Xi-wei, ZHENG Rong-zhang

2004, 26(1):  61-70. 

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The strike-slip rate of the Altyn Tagh Fault zone is a topic full of disputation because of its significance to the comprehension of the formation and evolution of the Qinghai-Tibet Plateau. Identifying the strike-slip rate of the Altyn Tagh Fault zone by geological method and comparing the result with that obtained by other method (GPS Observation) is of great importance to the study of the mechanism of intra-continental deformation. With the help of high-resolution digitalized SPOT images, three typical landform sites of strike-slip fault offset have been identified and investigated. In addition, samples for thermoluminescence (TL) dating were collected and dated to get the ages of different terraces at different sites. The studied area of this paper is the E88°30′～93°05′ segment of the Altyn Tagh Fault, where 3 sites including Ananba gully, Qigequanzi and Yuemakeqi were investigated to get the sinistral strike-slip rates on the middle segment of the Altyn Tagh Fault. At Annanba gully, the Altyn Tagh Fault has two main strands, among which the southern one was studied. By reconstructing the offset terraces, the displacement of the T₃ terrace has been identified as 70±10m and the TL ages of the T₂ terrace has been determined to be 9.36±0.73ka BP, so the strike-slip rate of the south strand of the Altyn Tagh Fault since Holocene can be estimated as 7.5±1.7mm/yr. At Qigequanzi, the Altyn Tagh Fault has four strands, among which the second smallest strand was studied. Here the horizontal displacements of the T₂ and T₃ terraces are 12±2m and 32±4m, respectively, while the TL ages of the T₁, T₂ and T₃ terraces are 5.47±0.42ka BP, 13.86±1.07ka BP and 20.18±1.53ka BP, respectively. The slip rate of this small strand since Holocene calculated from these data is abouT₂.3±0.5mm/yr. Regarding the scale and mode of motion of the four fault strands, the deduced slip rate for the whole fault zone here since Holocene varies from 6.9±1.5mm/yr to 9.2±2.0mm/yr. At Yuemakeqi, the horizontal displacements of the T₂ and T₃ terraces are 30m and 50±10m, respectively, while the ages of the T₁, T₂ and T₃ terraces are 3.46±0.26ka BP, 4.73±0.38ka BP and 7.98±0.61ka BP, respectively, so the strike-slip rate of the Altyn Tagh Fault since 4.73±0.38ka BP can be estimated as 10.6±3.0mm/yr. Basing on the data obtained at the three sites described above, we conclude that the horizontal strike-slip rate on the middle segment of the Altyn Tagh Fault since Holocene is about 7～11mm/yr, which is coincident with the latest GPS (Global Positioning System) result about this region. The slip rate of the Altyn Tagh Fault obtained by this study is much less than 44mm/yr or 25mm/yr that were evaluated previously by some researchers. From this point of view, among the models for the formation and evolution of the Qinghai-Tibet Plateau, the "Crustal thickening" model seems to be more reasonable.

LATE QUATERNARY CRUSTAL DEFORMATION FIELD IN CHINA’S CONTINENT AND ITS ADJACENT REGIONS INFERRED FROM ACTIVE FAULT DATA

JING Yan, REN Jin-wei

2004, 26(1):  71-90. 

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Active tectonics is the important manifestation of the present crustal deformation. The data of the major active faults in China and its adjacent regions were collected and sorted according to their scale and significance. Furthermore, the faults were classified roughly on the basis of available quantitative data of active faults. Based on these data, the strain rate field of the crustal deformation and the distribution of horizontal velocity field in China’s continent were modeled by using Haines’ method. The results of modeling were obtained on the assumption that the crustal deformation in China’s continent is continuous deformation. The results of the modeling accord very well with those obtained from GPS observation. In addition, the variation of motion vectors in every block of China’s continent was analyzed on the basis of the modeling result. The effects of the secondary faults on the crustal deformation in this region were estimated quantitatively by comparing with the results of GPS observation. From the afore-mentioned results, some important conclusions can be drawn as follows: (1) crustal deformations are mostly absorbed by the major faults, and only about 10% of the deformation is absorbed by the secondary faults; (2) crustal deformation in west China is much stronger than that in east China. The geodynamics of crustal deformation in east and west China is quite different. The collision between the Indian and Eurasian plates is the dominating cause of crustal deformation in west China; (3) crustal deformation in China is characterized as block movement. The active block boundaries and the major faults are the place where the velocity vector changes significantly; (4) a small-angle clockwise rotation of the deformation vector occurs in South China, which is the result of the combined action of the Philippines and Pacific plates.

PLASTIC-FLOW WAVES ("SLOW WAVES") AND SEISMIC ACTIVITY IN CENTRAL-EASTERN ASIA

WANG Sheng-zu, ZHANG Zong-chun

2004, 26(1):  91-101. 

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The results inferred from the experiments of analogue modeling carried out previously have shown that two types of plastic-flow waves, "fast-wave" and "slow-wave", are induced in the lower lithosphere (including the lower crust and lithospheric mantle) under the driving at plate boundary; and both of them are the viscous gravity waves formed by the superposition of major and subsidiary waves. The major one is similar to solitary wave and the subsidiary one is traveling wave. The plastic-flow waves in the lower lithosphere control the seismic activities in the overlying seismogenic layer and result in the distribution of earthquakes along the wave-crest belts. The "fast-waves" propagated with velocities in the orders of magnitude of 10⁰～10²km/yr have been verified by the wave-controlled earthquake migration, showing the "decade waves" and "century waves" with the average periods of 10.8 and 93.4yr, respectively, which are originated from the Himalayan driving boundary. According to the recognition of the patterns of the belt-like distribution of strong earthquakes with M_S≥7.0, it is indicated further in this paper that the "slow-waves" with velocities in the orders of magnitude of 10⁰～10¹m/yr are also originated under the compression from the Himalayan driving boundary. The strong earthquakes with M S≥7.0 are controlled mainly by the subsidiary waves, because the major waves with the duration up to 10⁶yr for each perturbation are impossible to result in the accumulation of energy enough for generating strong earthquakes owing to the relaxation of the upper crust. The subsidiary waves propagate with an average wave length of 445km and, as estimated in term of the requirement of elastic energy for strong earthquakes in the crust (the constraint of energy for strong earthquakes) and the actual situation of the apparently immobile wave-crest belts (the constraint of the relative immobility of wave-crest belts), with velocities of 0.81～2.80m/yr and periods of 0.16～0.55Ma. The wave-generating time at the Himalayan driving boundary is about 1.34～4.66Ma BP for the "slow-waves", corresponding to the stage from Mid Pliocene to Mid Early-Pleistocene and being identical with one of the major tectonic episodes of the Himalayan tectonic movement. It is shown from the recognition of the wave-controlled belts of strong earthquakes that two optimal patterns of wave-crest belts are originated simultaneously from the eastern and western segment of the Himalayan arc, respectively. The overlap of wave-crest belts of these two systems are responsible for the relative concentration of energy and the formation of the seismic-energy-background zones for strong earthquakes with M_S≥7.0.

MICROSTRUCTURAL ANALYSIS OF MESO-CENOZOIC TECTONITES IN THE NORTHERN SEGMENT OF THE TAIHANG MOUNTAINS

CHEN Gui-hua, WEN Chang-shun, HU Ling, XU Xi-wei

2004, 26(1):  102-110. 

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The northern segment of the Taihang Mountains is a typical tectonic region located in the middle segment of the Daxing’anling-Taihang-Wuling tectonic belt. Mesozoic and Cenozoic tectonites of cataclasite series developed along the 2 main fault zones in the region: the Zijingguan and Wulonggou Fault zones. Microstructures of these tectonites were studied in detail according to the different lithologies of their host rocks, which may influence the deformation mechanism. Accordingly, the observed geological bodies can be divided into carbonate and granitoid rocks. According to their occurence, microstructures and microscopic deformation mechanism, the tectonites can be divided into cataclased rocks or fault breccias, protocataclasites and cataclasites. Furthermore, three stages of deformation can be distinguished from the superimposition of cataclasis of different stages exposed in these tectonites. The tectonites produced in different stages are different. The product of the first stage cataclasis can be assigned to cataclasite, which is the product of relatively intense deformation. In the second stage, the product was mainly protocataclasites or cataclased rocks, and the deformation shown in the matrices superposed upon the previous one. In the third stage, the pre-existing tectonites was cataclased in a relatively low intensity, and the veins cut into the matrices formed in the earlier stages. By combining the afore-mentioned information with the related geological data, it can be inferred that 3 significant faulting events had occurred on the Zijingguan Fault Zone in Mesozoic and Cenozoic, and each stage of faulting had its own feature, by which a trend line from high to low intensity of deformation can be traced. Three stages of tectonites corresponded with the three stages of significant activity of the fault zone after the emplacement of granitoid intrusions along the Taihang tectonic belt.

CHARACTERISTICS OF LATE QUATERNARY ACTIVITY OF THE HAIHE BURIED FAULT IN TIANJIN MUNICIPALITY

CHEN Yu-kun, REN Feng, ZHANG Cui-fen, SHAO Yong-xin

2004, 26(1):  111-121. 

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On the basis of precise locating of the Haihe buried fault, the characteristics of late Quaternary activity of the fault was identified through drilling, identification of microfossils, stratigraphic correlation and chronological dating, by taking 4 marine strata of late Quaternary ages in Tianjin area as key horizon. The result indicates that the upper breaking point of the Haihe Fault in Tianjin downtown area is buried 21m below ground surface, and the corresponding latest movement of the fault had continued through the late stage of late Pleistocene to the early stage of Holocene. On the Tanggu segment of the fault, however, the upper breaking point is buried 16.4m deep, and its latest active period was in the early and middle stage of Holocene. Therefore, the average vertical movement rate on Tianjin downtown segment since the late stage of late Pleistocene (36,290a BP) can be estimated as 0.248mm/yr, and that on the Tanggu segment since the middle stage of Holocene (7200a BP) may reach 0.333mm/yr. This may indicate that the activity of the Haihe Fault on Tanggu segment (eastern segment) is obviously stronger than that on the Tianjin downtown segment (western segment).

THE FEATURE OF SEISMICITY IN NORTHEAST CHINA AND ITS RELATION TO THE SUBDUCTION OF THE JAPAN SEA PLATE

SUN Wen-bin, HE Yue-shi

2004, 26(1):  122-132. 

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Based on the analyses of grouped activities of deep-focus (m_b≥6.0) and shallow-focus (M_s≥5.0) earthquakes in Northeast China, the spatial and temporal correlation between the deep-focus "strong earthquake group" and shallow-focus "strong earthquake group" was studied. The study was focused mainly on the characteristics of earthquake distribution along the collision zone between the west Pacific plate and Eurasian plate, as well as its relation to the form of the west Pacific subduction zone. Moreover, emphasis was also laid on the analyses of the effect of the west Pacific plate on the seismicity in Eurasian plate. The results show that in the region where the west Pacific plate subducts at low angle, the seismicity along the plate collision zone is very strong, and the effect of plate subduction on the seismicity in Eurasian continent is most significant. In this condition, the subduction zone is subjected to relatively strong compressive stress. However, in the region where the west Pacific plate subducts at high angle, the seismicity along the plate collision zone becomes weaker, and the effect of plate subduction to the seismic activity in Eurasian continent becomes less significant. In this condition, the tensile stress produced by the subduction zone at depth is enhanced. We propose, therefore, that the seismic activity of Northeast China will enter the active period of shallow "strong earthquake group" in forthcoming 10 years. In this period, 6 earthquakes of M_s≥5.0 may occur in this region. It is emphasized that the monitoring and prediction of seismic activity in Northeast China should be enhanced.

RESEARCH ON THE PROBABILISTIC ASSESSMENT OF POTENTIAL GROUND OFFSET ALONG ACTIVE FAULT

RAN Hong-liu, ZHOU Ben-gang

2004, 26(1):  133-140. 

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The deterministic method is currently adopted in offset-resisting design for fault-crossing linear engineering project. However, it considers merely the maximum displacement of the fault, and thus cannot meet the actual demand of anti-disaster designing. In this paper, we develop a probabilistic method for evaluation of the potential abrupt displacement on various positions of the fault. This method combines the distribution of the maximum displacement positions and the distribution of displacements on both sides of the positions with the recurrence model of strong earthquake. As a case study, the risk of potential offset on the Shengzhuang-Changtuan segment of the normal fault on the northern boundary of Huailai-Zhuolu Basin in future 100yr was estimated quantitatively. This estimation provides the potential offset along the active fault under different probability levels. The result of this study may provide basic data for offset-resisting design of fault-crossing linear engineering project.

SIMULATION OF MODERN TECTONIC STRESS FIELD IN YANQING-HUAILAI BASIN AND SEISMIC SAFETY ASSESSMENT OF THE GUANTING RESERVOIR DAM

WEN Yan-jun, ZHENG Wen-tao

2004, 26(1):  141-152. 

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The research of regional seismic risk is fairly mature at present. Recently, a lot of papers concerning the simulation of the tectonic stress field of certain earthquake areas have been published. And by combining the obtained results with the other data, the seismic risk of the areas has been discussed and the risk areas of strong earthquake have been determined. In Yanhuai Basin where the Guanting reservoir is located, exist many active faults. The neotectonic movement in the basin has been very intense since Cenozoic era. The characteristics of historical earthquake activity and the crust-mantle structure of the basin indicate that strong earthquake is possible to occur in the basin in the future. Basing on the models and methods used in previous work, this paper studies the seismic hazard in Yanhuai Basin. Furthermore, by considering the possible impact of the presumed earthquake on the Guanting reservoir dam, the authors have assessed preliminarily the seismic safety of the dam. This paper focuses mainly on the possible impact of the expected earthquake on the dam. On the basis of filed work, together with the displacement data of active faults in Yanhuai basin, the modern tectonic stress field of the region has been simulated in this paper by using ANSYS finite element compute program. The result of simulation reveals that the orientation of the maximum principal compressive stress is N80°～85°E. Combining this result with the characteristics of some risk areas of strong earthquake in North China, we propose that the potential seismogenic fault in this basin should be the Fangjiachong-Yingmenkuang Fault, and according to previous studies the maximum magnitude of the potential earthquake is considered to be 7.25. Moreover, the intensity attenuation equation for the Yanhuai Basin and its vicinity is regressed, through which the intensity of presumed earthquake at the Guanting reservoir dam is determined to be 7 degree. The corresponding seismic vibration parameters of the presumed earthquake on the dam are estimated. The results show that the peak velocity is 165cm/s, the peak acceleration is 19 cm/s² and the minimum duration time of the shock is 20s. Taking the intensity factor singly into consideration, it seems that the presumed shock will have small effect on the dam, because the dam had been fortified to resist 9 degree earthquake intensity. However, the situation may change when some other factors such as the engineering conditions of the dam, the spatial relationship between the seismogenic fault and the dam, and the impact of historical strong earthquake on the dam are taken into account. The seismic risk of the Guanting reservoir dam may increase due to all these factors.

PRECISE LOCATING OF JIASHI, XINJIANG EARTHQUAKE BY USING INVERSION METHOD

PAN Su-zhen, ZHANG Xian-kang, YANG Zhuo-xin, ZHANG Cheng-ke, DUAN Yong-hong, ZHAO Jin-ren, JIA Shi-xu

2004, 26(1):  153-160. 

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Jiashi region, Xinjiang is a strong earthquake area in western China. In recent years, several great earthquake swarms have occurred in this region, causing tremendous hazards. In order to get an insight into the relation between the deep structures and the generation of great earthquake swarm in this region, a generalized inversion technique for determining probability distributions of spatial locations of earthquake events through the travel times of P and S waves in random and vertically inhomogeneous medium, is used in this paper. Seismic data recorded by temporal digital seismic network deposed in this region were located using this method. The located earthquakes show linear distribution in north-northwest and north-northeast directions, among which the former is more distinct than the later. Based on the obtained results, the deep structural background of seismic activities in Jiashi region is discussed. The results indicate also the close relation between the earthquake and fault structures in Jiashi region. It is clear that the generation of earthquake swarm might be related to the buried fault in the vicinity of seismic source area, and might be the result of violent crustal deformation on the northern margin of Tarim basin and the present tectonic movement. These complicated structural framework and peculiar environmental conditions might be responsible for the development and generation of the strong earthquake swarm in Jiashi region.

APPLICATION OF IKONOS IMAGERY TO EARTHQUAKE DISASTER REDUCTION AND DAMAGE ASSESSEMENT IN URBAN AREA

NING Bao-kun, QU Guo-sheng, ZHANG Ning, LI Yi-gang

2004, 26(1):  161-168. 

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IKONOS satellite launched in 1999 is one of the commercial satellites with highest spatial resolution, which has five spectral bands including red, green, blue, near-infrared and pan. The maximum spatial resolution of IKONOS is 1 meter. From the imagery, researchers can easily identify the basic elements needed for earthquake hazard reduction business in urban areas, such as buildings, structure, roads and bridges. With the aid of computer, a great amount of information including the shape, position and attribute of ground objects can be automatically or semi-automatically extracted from IKONOS digital imagery by using the remote sensing imagery processing software. In this way, lots of manpower and finance can be saved. Because the renewal of IKONOS imagery is faster than aerial photos or large-scale maps that are used for urban earthquake hazard assessment, the latest information can be ensured. It is very useful for solving the aging of data for the Ninth Five Year Project on the Research of Seismic Hazard Assessment and Disaster Prevention and Mitigation. The repetition interval of IKONOS imagery is about 3 days, so the imagery can be applied to the rapid assessment of earthquake disasters. Under ideal weather condition, users can get IKONOS imagery of earthquake-stricken area within 3 days (even several hours) when an earthquake occurs. Owing to the high resolution of IKONOS, researchers can use it for earthquake site investigation instead of aerial photos. Remote sensing imagery including IKONOS is in digital format, and thus it can be easily processed and analyzed by computer, so that it is convenient for the computer-aided assessment and emergence decision of earthquake disaster. The methodological feasibility of IKONOS application to earthquake disaster is discussed in this paper. It is concluded that the high-resolution satellite imagery including IKONOS has the broad prospects in obtaining and renewing the basic data of urban areas, as well as in the rapid assessment and emergence decision of earthquake disaster.