Table of Content

04 June 2003, Volume 25 Issue 2

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Brief Report

NEOTECTONIC ACTIVITY OF THE SOUTHERN MARGINAL FAULT ZONE OF THE TAIHANGSHAN MOUNTAINS AND ITS REGIONAL KINEMATIC IMPLICATIONS

ZHANG Yue-qiao, MA Yun-sheng, YANG Nong

2003, 25(2):  169-182. 

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Basing on TM imagery interpretation and field analysis of striated fault planes, we present in this paper morphological and structural evidence for Quaternary left-lateral strike-slip motion along the southern marginal fault zone of the Taihangshan Mountains and discuss their implication for regional kinematics. It is shown that the southern marginal fault zone of the Taihangshan Mountains behaves as a transtensional left-lateral strike-slip fault during Quaternary. Morphological evidence for left lateral strike-slip motion includes offsets of stream channels across the fault trace and offsets of late Quaternary alluvial fans developed on the hanging wall of the range front fault zone. Field analysis of striated fault planes shows two families of striations. The older one resulted from NE-SW to NNE-SSW extension, probably Miocene in age, and the younger one from NW-SE to NNW-SSE extension, possibly Pliocene Quaternary in age. These observations are consistent with those established in the Weihe graben, where these two extensional stresses have been well dated. Structural data from the basins indicate that the southern marginal fault zone of the Taihangshan Mountains extends eastward along the Miyang Kaifeng Shangqiu fault depression zone, making up a tectonic boundary that separates the northern Bohaiwan rhombic basin from the southern North China triangular basin. Neotectonic deformation is dominated by right slip faulting on NNE-striking faults in the northern domain and left slip faulting on E-W-striking faults. It is postulated that the southern marginal fault zone of the Taihangshan Mountains and its eastward continuation into the North China basin forms the northern boundary of a broad distributed deformation zone bounded to the south by the Qinling fault system. Left-lateral strike-slip faulting along this distributed deformation zone during the Pliocene-Quaternary accommodated the SEE-ward extrusion of the South China block with respect to North China.

LATE QUATERNARY RIGHT-LATERAL STRIKE-SLIP FAULTING ALONG THE BOLOKENU-AQIKEKUDUKE FAULT IN CHINESE TIAN SHAN

SHEN Jun, WANG Yi-peng, LI Ying-zhen, JIANG Hui, XIANG Zhi-yong

2003, 25(2):  183-194. 

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The Bolokenu-Aqikekuduke Fault extends over 700km and obliquely cuts the North Tian Shan. The fault is quantitatively studied in this paper based on remote sensing data, field observation, and the analyses of the relationship between the morphology and the climate change. The fault is composed of two portions, the NW-striking western portion and the NWW-striking eastern portion. The western portion is right lateral strike-slip with a rate of 5mm/a, having a length of about 250km and extending northwestward into the Kazakhstan. Along the fault 3～4 rupture zones produced by Paleoearthquakes or historic earthquakes were found. It has the potential for generating strong earthquakes of Magnitude around 7.5. The NWW-striking fault of east portions is dominated by right lateral slip with a rate of 1～1.4mm/a. Relatively small rupture zones, no more than 10km long, were found along this fault. It has the potential for generating strong earthquake of magnitude around 7. The western section of the Bolokenu Aqikekuduke Fault is one of the major NW-striking right lateral strike-slip faults in central Asia, and it is the result of the regional clockwise shearing. It accommodates the different deformations between western and eastern Tianshan and transforms the tectonic deformation among different compressive area. The NWW-striking fault of eastern section was developed from the NWW-striking suture zone in Northern Tianshan. It is accompanied by the E-W-striking thrust system in the front of the mountains and makes up a typical strain partitioning style of the oblique compressive belt. The strain in the oblique compressive belt is partitioned into pediment thrust belt and intermontane strike-slip fault. They transform the deformation laterally along the mountains and directly toward the foreland basin.

CHARACTERISTICS OF THE REMOTE SENSING IMAGES OF ACTIVE TECTONICS IN YANQI BASIN, SOUTHEASTERN TIAN SHAN, CHINA

GUO Jian-ming, FU Bi-hong, LIN Ai-ming, Ken-ichi Kano, Tadashi Maruyama

2003, 25(2):  195-202. 

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Tian Shan, an intracontinental orogenic belt formed in the late Paleozoic, has been reactivated since the India-Eurasia collision in the Eocene. Active tectonics are widespread in and around Tian Shan, and they are of great significance to the understanding of the effect of India-Eurasia collision and the seismic hazard assessment of this region. Conventional geological investigation is very difficult to be carried out in this region, since the most areas of the range are difficult of access. We employed satellite remote sensing data, therefore, to identify the distribution of active tectonics and their deformation features in these areas. In addition, a selected field study was carried out in Yanqi basin in order to verify the interpretation and analysis results of remote sensing data and to get some ground truths. Yanqi Basin is one of the Mesozoic-Cenozoic intermontane basins in the southeastern Tian Shan. The features of active tectonics in the western part of the basin can easily be recognized on remote sensing images, such as distinct linear structures, offset gullies, and left-stepping en echelon arrangement of fault scarps. The active tectonics are composed of the active folds in the northwestern part of the basin and active right lateral strike-slip faults in the southwestern part of the basin. The results of fieldwork are coincident with the imagery interpretation, indicating that the remote sensing technique is a very powerful, rapid and low-cost way to research active tectonics in the region with poor vegetation and traffic conditions. The maximum displacement of offset gullies along the fault is 210m with a vertical component of 20m, indicating that strike-slip is predominant. The well developed active tectonics in the Tian Shan reveals that the collision between the Indian and Eurasian continents is still continuing.

LATE QUATERNARY VERTICAL DISLOCATION RATE ON SEVERAL FAULTS IN THE ZHUJIANG DELTA AREA

SONG Fang-min, WANG Yi-peng, LI Chuan-you, CHEN Wei-guang, HUANG Ri-heng, ZHAO Hong-mei

2003, 25(2):  203-210. 

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Fault structures are well developed in the Zhujiang Delta area. They can be classified into NW-, NE- and E-W-trending groups, most of which display distinct features of vertical differential movement. Since most of the faults are buried and their activities are relatively weak, the evidence of vertical dislocation are difficult to be identified directly in the field, and hence the amount of displacement along the faults can not be measured. Fortunately, a large number of drill holes have been drilled during the construction of the Zhujiang Delta, and a lot of data concerning the present altitudes above sea level of late Quaternary deposits and their ages have been measured and determined. Basing on the analysis of the formation and evolution of the Zhujiang Delta, we have selected a pair or some pairs of bore holes, which are close in position and may reveal the deposition time and environment of the same strata for comparative analysis. Then the vertical displacements along the faults can be determined according to the differences between the present altitudes above sea level of the same strata on both sides of the faults, and hence the dislocation rates can be calculated based upon the age data. We have calculated the late Quaternary vertical dislocation rates for 5 faults. The results show that the dislocation rates are within the range of 0.14～0.47mm/a. Among them, the vertical dislocation rate of the Xijiang Fault in the past 29,400 years is 0.44mm/a. The vertical dislocation rate of the Baini-Shawan Fault in the past 15,000 years is 0.39mm/a, while that in the past 11,400 years is 0.38mm/a. The vertical dislocation rate of the Gulao-Conghua Fault in the past 29,530 years is 0.47mm/a. The vertical dislocation rate of the Wuguishan north piedmont fault in the past 6,390 years is 0.19mm/a. The vertical dislocation rate of the Shougouling Fault in the past 29,800 years is 0.14mm/a, and that in the past 17,480 years is 0.19～0.21mm/a. All these results are of great importance to the project of detecting the active faults in major cities that will be carried out in the near future.

SEISMIC ACTIVITIES AND STRUCTURES OF THE LIU'AN-HUOSHAN SEISMIC RISK AREA

YAO Da-quan, LIU Jia-can, LI Jie, ZAI Hong-tao

2003, 25(2):  211-219. 

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In this paper, the approach to the understanding of seismic tectonic environment in moderate and strong earthquake areas are discussed through the antithetic analysis of shallow and deep structures, macro-microstructural analysis, and analysis of both historical and recent seismic data. Taking the Liu'an-Huoshan seismic risk area as an example, the authors have carried out a three dimensional analysis of the area, and predicted the potential location of future earthquake and its corresponding parameters including focal depth, magnitude, and the attenuation direction of the isoseism. The results of this study indicate that the NE-, NW- and nearly E-W-trending faults in this area exhibit obvious activities. The junctures of these 3 sets of faults are prone to stress concentration, making up the seismogenic background for future moderate strong earthquake in this area. The most possible location for future earthquakes is the western side of the Funanshan Mountain, the juncture between the Luo erling-Tudiling, Meishan-Longhekou and Xiafuqiao-Hujiahe Faults. The seismogenic model for this area is constructed on the basis of analysis of seimotectonic environment by referring to the interpretation of remote sensing images, the results of seimogeolo~gic investigation and the Suizhou to Halqin Qi geoscience transect that obliquely passing through this area. Based on the focal depth, the features of isoseimal lines and the migration regularity of the past seismic events occurred in this area, the focal depth of the future earthquake is postulated to be 12～15km, and the isoseism will be equidimensional in shape. It is proposed that the Liu'an-Huoshan seismic risk area is obviously active. As the area is broken, however, energy is difficult to be accumulated and large earthquake is difficult to be generated. It is suggested, therefore, that the future earthquake will be a moderate-strong earthquake.m ,andtheisoseismw

GEOMETRICAL FEATURES OF SUBDUCTION BOUNDARIES

YAO Hua-jian, XU Guo-ming, XIAO Xiang, CHEN Min

2003, 25(2):  220-226. 

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At the convergent boundaries of plates, when one plate subducts into the another, the form of the subduction boundaries (trench, mountain belt) bears a close relation with the properties of the subducting and subducted plates. In this paper, we mainly use plate subduction model to calculate and simulate some major subduction boundaries all over the world, and discuss the problem of the dip angle. Here we consider the subducting plate as a plane plate and the subducted plate as a part of the spherical surface of the Earth. And we suppose that the dip angle calculated from the model is the initial dip angle of the plate. From the simulation we find that many large subducting plates can be approximately regarded as plane plates. Plate subdution model, however, is not fit for some reversed-arc subduction boundaries such as Izu trench, which is fit for spherical subduction model. From the simulation we get the positions of six large subduction boundaries, in good accordant with the positions of the present subduction boundaries. Besides, we calculate also the initial dip angle of the subducting plate. The recent studies show that the lithosphere exhibits universal westward drift. From our calculation, it is shown that in the east part of the Pacific the initial dip angle of the subducting plate is less than the recent dip angle of the Benioff zone, but it is not true for the west part of the Pacific where the initial dip angle is larger than the recent dip angle of the Benioff zone. This result directly tallies with the westward drift of the lithosphere, and with the eastward flow of the mantle relative to the lithosphere. Because the westward motion of the subducting plate in the Pacific is opposite to the eastward movement of the mantle, it will lead to an additive resistant force on the subducting plate, which makes the dip angle becomes larger and larger. In east pacific, however, the subduction direction of the subducting plate is the same as that of the flow of the mantle, so the current dip angle of the Benioff zone will be larger than the initial dip angle.

PLASTIC-FLOW WAVES AND EARTHQUAKE MIGRATION IN SOUTHEAST CHINA (Ⅰ)

WANG Sheng-zu, ZHANG Zong-chun

2003, 25(2):  227-236. 

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The studies of the phenomena about earthquake migration and ground deformation diffusion have long been carried out by many seismologists (Richter,1958; Bott et al .,1973; Kasahara, 1979; Mogi, 1986; Sato, 1989; Babazade et al ., 1991; Ma et al ., 1992; Zhao et al ., 1995), and show that the velocities of these migrations range roughly between 1～1,000 km/a. The suggestion of the idea about plastic-flow waves is also closely related with the study of earthquake migration (Wang et al ., 1994b, 1995). However, it differs from those mentioned above in the following aspects: it focuses on the migration of earthquakes along plastic-flow belts, considering that the large-scale seismic belts are controlled by the netlike plastic-flow in the lower lithosphere (including the lower crust and lithospheric mantle) and the earthquake migration is a manifestation of wave propagation in the lower layer; this kind of waves is different from the waves along elastic or viscoelastic layer and is mainly a kind of viscous gravity waves in the lower lithosphere, in short, it can be called network waves in consideration of the closed relation between the plastic-flow waves and the plastic-flow network. According to the viewpoints of plastic-flow network and plastic-flow wave in the lower lithosphere, the migration of earthquakes along the plastic-flow belts (Fig.2 and 3), velocity field (Fig.4 and 5) and wave-generating stages at driving boundary (Fig.7) are investigated by means of the method used previously for the network waves in the major area of the central eastern Asian continent. Moreover, the travel-time contour map of the network waves (Fig.6) and the map of wave-crest belts (Fig.8) for the southeastern region of China are compiled, preliminarily showing the seismic energy background controlled by the network waves and providing the basis for further studies and the physical prediction of seismic energy background in this region. The studies indicate that in addition to the network waves corresponding to the Central Eastern Asian plastic-flow network system stated previously (Wang et al ., 1994b), there are also the waves corresponding to the Southeastern China network system. They are originated from the Himalayan and Taiwan driving boundaries, respectively, and propagate into the continent at different periods and velocities. The former includes mainly the "centenary wave" with average period of 93.4 a and velocities of about 1～10 km/a and the "decadal wave" with 10.8 a and about 20～60 km/a. The latter (Fig.8) includes mainly the "twenty year wave" with average period of 23 a and velocities of about 10～20 km/a and the "decadal wave" with 12a and about 30～50 km/a. As indicated by the experimental models of plastic-flow waves in the lithosphere, the quasi periodicity of the network waves is mainly related to the pulsative or intermittent actions at driving boundaries. Note that some simplifications and assumptions have been made in this study, such as the linear assumption of the migration of earthquakes along plastic-flow belts, the rough estimation of the migration lines in the time-distance distribution charts of earthquakes, etc. The obtained results, therefore, should be considered only as an "initial approximate value" for further studies, and should be examined in the practice of earthquake prediction and revised progressively for approximating the "optimum value" of the seismic energy background. The network waves in the southeastern China region have been examined and revised by the actual seismic activity in 1999 and 2000. The much better understanding about network waves will be gained through a number of "prediction-feedback-revision-prediction" cycles.

PLASTIC-FLOW WAVES AND EARTHQUAKE MIGRATION IN SOUTHEAST CHINA (Ⅱ)

WANG Sheng-zu, ZHANG Zong-chun

2003, 25(2):  237-244. 

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It is known from the paper (Ⅰ) that earthquakes, controlled by the plastic-flow waves, or called network waves, occur mainly in the wave-crest belts in which energy concentrates relatively. Based on the distribution of earthquake-released energy (Fig.1), the distribution of energy along the direction normal to wave-crest belts (Fig.2) and the attenuation of energy with propagation distance (Fig.3) are analyzed further in this paper. In addition, the distribution of earthquake occurrence probabilities is estimated in consideration of its relation to the seismic background energy. As a result, the distribution map of earthquake occurrence probabilities associated with the seismic energy background in the southeastern China region (Fig.4) is given as one of the supports for earthquake prediction. The distribution of relative values of earthquake released energy along the direction normal to wave-crest belt, p , is approximately shown as the Gaussian distribution (eq.1 and Fig.2), where ΔT=T-T_av, T_av is the average travel-time for the wave-crest belt, corresponding to the position of the belt's mid-line, S is the standard error in travel-time. The total energy for each wave-crest belt, ΣE, attenuates as an exponential function of its travel-time (Fig.3) and the tendency of the decrease in earthquake magnitude with increasing propagation distance can be expressed by the attenuation coefficient α . The distribution maps of wave-crest belts given in the paper (Ⅰ) and the seismic-energy-background map expressed by earthquake occurrence probabilities given in this paper show the propagation characteristics of the network waves originated from the Taiwan driving boundary in the southeastern China region. The former can be used as a basis for preliminarily determining the seismic energy background zones and the latter is one of the bases for the quantitative prediction of seismic risk regions. Both of them will be examined in the practice of earthquake prediction and revised progressively to approach the actual situation of the propagation of plastic-flow waves. As the "cloud chart" used for weather forecasting, showing the distribution of water necessary for precipitation, the seismic energy background map (Fig.4) provides an "energy chart" for earthquake prediction, showing the distribution of energy necessary for earthquakes. It is noteworthy that the seismic background "energy chart" can be established and used for earthquake prediction on the basis of the ideas of plastic-flow network and plastic-flow waves through other approaches, such as the inversions of ground deformation or other geophysical fields, in addition to the time-space distribution of earthquakes stated in this paper. It may be more effective to synthesize the results obtained from different approaches for improving the reliability of the seismic "energy map".

A FUZZY METHOD FOR EVALUATING THE INFLUENCES OF SOME GEOLOGICAL FACTORS ON EARTHQUAKE DISASTER RISK

YAO Qing-lin

2003, 25(2):  245-259. 

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Risk analysis about earthquake involves many different aspects such as seismic intensity, site condition, distribution and antiseismic capability of buildings, supporting system of disaster precautions and rescue etc. However, little effort has been made to quantitatively research the influences of these factors on earthquake disaster risk so far. In this paper, the evaluation of the influence of geological factors on earthquake disaster risk is discussed in detail. There are many geological factors that influence significantly the degree of seismic hazard, such as stratum, groundwater, topography, seismogenic faults etc, among which the stratum plays an important role. The algorithm for evaluating the influence of geological factors on earthquake hazard risk is established here by considering mainly stratum, especially soft formation. The evaluating indexes are mainly lithology, buried depth and thickness of the stratum, and the lithology index is quantitated with shear wave velocity (v_s). In order to elementarily dispel the interference of earthquake magnitude and epicentral distance in the analysis of the relationship between earthquake hazard of the buildings and the site conditions, it is necessary to normalize the earthquake hazard data from different intensity background by timing a coefficient. After dividing by the maximum of the obtained data, the normalized data is simply called Relative Earthquake Disaster Coefficient of Buildings (RDC). The risk factors are very complicated, so it is extremely difficult to collect enough data and set an organic connection between the geological factors and earthquake hazard variables in statistic meanings. Therefore, the theory and methodology of fuzzy system are needed for the analysis. After giving the domains of the variables, a fuzzy relation matrix between lithology (L), thickness (T), depth (D) of the stratum and RDC can be established with Multi-dimension Information Distribution. On the basis of it, the Possibility-Probability Fuzzy Risk, which reflects better the nature of the risk, needs to be calculated but lacks an algorithm in the case of multi variables (here is 4). A universal algorithm with its corresponding definitions, formulas, and calculating steps is established in this paper, which is applicable to the similar analysis even if the input variables are different. When the values of lithology, thickness and depth of stratum samples are input into the calculation model of the Possibility-Probability Fuzzy Risk, at first the possibility-probability distribution of the combination of these stratum factors is calculated on their own variables space, while earthquake hazard risk analysis requires the evaluation of the contribution of those factors to earthquake disaster indexes in another space. Risk analysis on the basis of possibility-probability distribution of relevant factors in different spaces, however, has not been reported in the previous work. In this paper, the question is solved by means of fuzzy information transmission of risk possibility with a given formula, utilizing the fuzzy causality matrix between stratum factors and RDC. An example of how to use this method to evaluate the influence of stratum factors on earthquake disasters risk is illustrated by the data of the stratum and earthquake hazard of Xingtai large earthquake in 1966 and Haicheng large earthquake in 1975. Although lithology, thickness and depth of the stratum are taken as the basic input parameters in the above mentioned example, one may increase, decrease and change the input parameters according to practical needs and information of risk evaluation.

MAXIMUM ENTROPY PRINCIPLE AND SEISMIC MAGNITUDE-FREQUENCY RELATION

FENG Li-hua

2003, 25(2):  260-265. 

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Entropy is a state function. Entropy increasing principle shows that under isolated or adiabatic condition the process of a system developed spontaneously from non equilibrium state to equilibrium state is a process of entropy increasing. The equilibrium state corresponds to the maximum entropy. In equilibrium state, the state of the system is most chaotic and disorder. Earthquake is a random event, the occurrence of which possesses extremely great uncertainty, and hence can be expressed by entropy. Earthquake occurs disorderly in different areas, implying that the seismic entropy has reached a maximum value. Therefore, magnitude distribution of earthquakes in one region for a certain time period can be expressed by the principle of maximum entropy. Assuming that M₀ is starting magnitude and AM-U is average magnitude, through the deduction we can get lgn= lgN/AM-U-M₀+M₀/AM-U-M₀-1/AM-U-M₀M, where n is differential frequency, N is total number of earthquakes, and M is magnitude. The magnitude frequency relation proposed by Gutenberg and Richter according to seismic data and experience is expressed as: lg n=a-bM. Comparing the two equations gives a= lgN/AM-U-M₀+M₀/AM-U-M₀, b=1/AM-U-M₀. Obviously, the Gutenberg-Richter magnitude-frequency relation is essentially a negative exponent distribution obtained by taking the maximum value of seismic entropy under a given constrained conditions. In this way the cause of magnitude-frequency relation is theoretically explained.

DENSITY STRUCTURE OF THE TAIHANG MOUNTAINS GRAVITY ANOMALY ZONE AND ITS GEOLOGICAL INTERPRETATION

LIU Zhan-po, GAO Xiang-lin, LI Yi-shi

2003, 25(2):  266-273. 

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In eastern China, there is a great gravity anomaly zone which extends from the Daxinganling Mountains, through the Taihang Mountains to the Wuling Mountains. This zone is presumably associated with the structural variation in the deep crust. We select the Fuping Dingzhou profile, which is perpendicular to this gravity anomaly zone, to make an inversion analysis of the crust upper mantle density using the data of seismic sounding and Bouguer anomaly. The result shows that the density distribution of the crust and upper mantle along this profile is evidently heterogeneous. The general feature is that the density is relatively low in the west and high in the east. The lateral variation of the crustal thickness basically determines the generation of the Bouger gravity anomaly zone. Taken the Mesozoic and Cenozoic tectonic evolution of North China into account, it is suggested that this gravity anomaly zone was developed after the Yanshan period and associated with plate interaction and thermal mass transfer of the upper mantle.

DETERMINING RESISTIVITY ANISOTROPY OF GEOLOGICAL FORMATION BY JOINT INVERSION OF LATERAL AND INDUCTION LOGS

YANG Wei

2003, 25(2):  274-279. 

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Real geological formations usually exhibit resistivity macro anisotropy in two ways: the mean result of formation microstructures and the structures such as fracture and strike. Three-component induction well logging may be the best method to determine the resistivity anisotropy. However, this technique has not been used in China so far. The data available in China at present are normally lateral and dual induction logs. Neither lateral well logging nor induction well logging methods alone can resolve the anisotropy of a formation. However, a joint inversion of lateral and inductive data makes it possible to resolve the coefficient of anisotropy. In this paper, an analysis of the importance of taking anisotropy into account in inversion modeling is presented. It is shown how the combined use of lateral and inductive logs can resolve the coefficient of anisotropy of a formation. Synthetic 2D modeling shows that inductive methods will only sensitive to the horizontal resistivity of a layer, while the thickness is undistorted. That is to say, the horizontal resistivity and formation thickness can be determined by inductive methods, but the vertical resistivity cannot be defined, and hence the coefficient of anisotropy cannot be defined. As the apparent resistivity deduced from lateral methods can be approximately considered as the geometric mean of horizontal and vertical resistivities, then the apparent thickness deduced from lateral methods is the product of anisotropy coefficient and real thickness. Therefore, we cannot determine any parameters by lateral logs alone. However, a joint inversion of data from lateral and inductive logs may determine three parameters: the coefficient of anisotropy, horizontal resistivity and formation thickness. Synthetic data show that the joint inversion method is feasible.

ACTIVITIES OF VOLCANOES AND FAULTS IN NORTHERN HAINAN ISLAND-RADIOACTIVE TRACE OF RADON AND THORIUM GASSES

GAO Qing-wu, SHANGGUAN Zhi-guan, HU Jin-wen

2003, 25(2):  280-288. 

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Northern Hainan Island (Qiongbei) volcanoes are one of the major volcanic clusters in China. A Rn/Th measurement was carried out in April, 2001, in order to get better understanding of the activities of volcanoes and the relevant faults in Qiongbei area. Three traverses were arranged for the measurement. Traverse I for Rn and Th measurement was arranged along the northwest and southeast sides of the Leihuling. The measurement on this traverse focused also on the Leihuling Fault, which is a part of the Rongshan-Lingnan Fault. Traverse Ⅱ for Rn and Th measurement was located on the lip of the Yangnan crater. Traverse Ⅲ for the measurement of gas of earth interior was arranged on both sides of the Leihuling, Maanling and Yangnan craters, respectively. The main purpose of the measurement is to reveal the feature of gas release in crater. The results show that the variations of Rn and Th are closely related to fault structures. The anomalies of Th and Rn observed on traverse Ⅰ and traverse Ⅱ are the distinct reflection of the Rongshan Lingnan Fault. The activity of the fault is analyzed in detail according to the distribution feature and variation of Th and Rn. In addition, the activity of the volcanoes in the studied area is analyzed and discussed as well.

PRELIMINARY STUDY ON LATE PLEISTOCENE PHREATO-MAGMATIC ERUPTIONS IN NORTHERN HAINAN ISLAND

SUN Qian, FAN Qi-cheng, WEI Hai-quan, SUI Jian-li, BAI Zhi-da, XU De-bin, SHI Lan-bin, ZHANG Bing-liang, HONG Han-jing

2003, 25(2):  289-297. 

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This study focuses on the northern Hainan volcanoes. The volcanic region belongs to the southern part of the Leiqiong depression, which experienced extensive subsidence during the Cenozoic. The surface of this area is covered with basalts and Quaternary sediments. Cenozoic volcanic rocks cover about 12 percent of the whole Hainan Island. In general, NW-trending fractures have controlled the volcanic activities in this area since Holocene. According to the published data, the volcanic activity in northern Hainan Island since Late Pleistocene was divided into Daotang period (Late Pleistocene) and Shishan period (Holocene). Our field investigation on volcanic activities has revealed that after the effusive eruption in early Pleistocene, considerable scale phreatomagmatic eruptions occurred during late Pleistocene, and then the typical central eruptions occurred at the beginning of Holocene. Late Pleistocene is the most active period of phreatomagmatic eruptions in Northern Hainan Island, while phreatomagmatic eruptions were the important eruptive character before the Holocene volcanic activity in this area. Phreatomagmatic eruptions may produce base-surges, and base-surges are usually accompanied with "maar". Maar is originally the name that the local people of Rhine, Germany called the lake and swamp filled with water in the region. In 1921, Steininger, a Germanic scientist, defined maar as a kind of volcanoes when he studied a Quaternary small round crater at Eifel, western Germany. Chinese scholar Liu Jiaqi had researched some maars within Chinese territory during the past several years, he pointed out that maar is something caused by phreatomagmatic eruptions, and its shape is similar to some other volcanic apparatus. Maar is a kind of familiar volcanic physiognomy besides volcanic cone. Some typical maars concentrate in Longgang (Northeast China) and Leiqiong (South China) regions, which are Cenozoic volcanic area. In Longgang region, there are eight maars caused by phreatomagmatic eruptions. Local people call these maars as "Longwan", such as big Longwan, triangle Longwan, south Longwan, east Longwan and so on. There are also several maars located in Leizhou Peninsula, such as Tianyang, Jiu Touyang, Qing Tongyang and Hu Guangyan. Some native geologists have begun to study maar and have made remarkable progress. The study of phreatomagmatic eruptions and its products is an important branch of volcanology, while the northern Hainan Island provides a good position to study this phenomenon. A number of maars with typical base-surges are caused by late Pleistocene phreatomagmatic eruptions in northern Hainan Island. Typical phenomena caused by phreatomagmatic eruptions can be observed around these maars, such as large-scale and low-angle cross-bedding, slaty-bedding, current-bedding and distal facies accretionary lapilli. If basaltic red-hot magma ascends and meets with the ground water near the earth's surface or surface water, explosion must occur, resulting in the formation of base-surge deposits. This process creates Base surge. In order to get better understanding of the modern volcanic activity and tectonic environment in the northern Hainan Island and southern Leizhou Peninsula, great attention should be paid to the study of base-surge in this area. Moreover, the study of base-surge may provide significant evidence about future volcanic hazard.

RESEARCH ON THE ANOMALIES OF ANNUAL WELL WATER LEVEL VARIATION IN TIANJIN AREA

LIU Xi-lan, ZHENG Wen-jun, JIN Yan, SUN Bei-zhu, CHEN Hua-ran

2003, 25(2):  298-306. 

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In recent years, more and more attention has been paid to the anomaly of annual well level variation and its implication to mid-term and short-term earthquake prediction. However, the qualitative comparison of dynamic pattern of the anomaly of annual well level variation displays an obvious uncertainty. To solve this problem, the random process and time series analysis techniques from probability theory and mathematical statistics are used to establish "the method of dynamic annual period" and "the method of comparative rate at comparative periods" for quantitative analysis of the anomalies of annual dynamic variation of well water levels. These methods are applied to the analysis of the observation data since 1985 till now from 21 wells of Tianjin well networks. The result has proved the feasibility and effectiveness of these methods for earthquake prediction. The analysis results demonstrate that the anomalies of well level occurred 17 times in 7 wells of Tianjin network one months and a half to six months before the occurrence of 4 moderate-strong (M_S≥5.8) earthquakes in this region. This fact may indicate again the applicability of the anomaly of annual well level variation to mid-term and short-term earthquake prediction.

THE RELATIONSHIP BETWEEN VERTICAL DEFORMATION AND TECTONIC STRESS FIELD IN NORTH QILIAN MOUNTAIN-HEXI CORRIDOR REGION-AN ANALYSIS BASED ON 3-D FINITE ELEMENT METHOD

LIU Xia, HUANG Li-ren, YANG Guo-hua, SUN Dong-ping, ZHANG Si-xin, HAN Yue-ping

2003, 25(2):  307-316. 

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Based on the seismic and geologic information available, North Qilian and Hexi Corridor area is divided into a number of 3-D finite elements. On the assumption of linear elastic model, the general tendency of vertical deformation spanning about 20 years in this area is simulated. The simulated results of this area bear a close relation with boundary condition and variation of background stress field. Among them, the result obtained by taking the increment of tectonic stress field as loading condition and stable block as displacement boundary condition, is in good consistence with the general distribution of vertical deformation in this area in the past 20 years. Based on this acknowledgement, the increment of the background stress field is estimated. The pressure change along N-S direction is +0.11MPa, and that along E-W direction is +0.04MPa (compressive pressure is positive). The quantity of the pressure change along E-W direction is three times of that along N-S direction. In addition, by comparing the distribution character of the obtained incremental shear stress (Δγ_xy) field with that of the foci of mid strong earthquakes, we find that there is a certain relationship between the incremental shear stress and the location of these earthquake foci. The fact that the earthquake focus is usually located in the area of severe variation of shear stress increment proves that the simulated tectonic stress field is reasonable to some extent.

FEATURES OF ACOUSTIC EMISSION DURING FRACTURING PROCESS OF GRANITE BLOCK WITH V-SHAPED STRUCTURE UNDER BIAXIAL COMPRESSION

CHEN Shun-yun, XU Zhao-yong, YANG Run-hai, ZHAO Jin-ming

2003, 25(2):  317-326. 

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Using the coarse-grained granite from Dali, Yunnan, as the test material, a sample of 200mm?200mm?15mm in size was prepared. On the sample, V-shaped block boundaries consisting of en echelon faults (slots or prefabricated cracks) were cut, and the slots were filled with gypsum. According to the structural characteristics, the sample was divided into a few zones: zone A where faults converge, zone B where faults form left-lateral and left-stepping rock bridges, and zone C where faults form right-lateral and left-stepping rock bridges. Observational study was made by means of biaxial loading. The result of microcrack location shows that acoustic emission (AE) has the features of grouped occurrence in a region and grouped migration. Microcracks migrated from zone A to zone B and finally to zone C. In the initial stage of migration, AE events appeared alternatively in the place of original occurrence and the place where they migrate. In each stage, relatively large AE events occurred in each zone, but they concentrated relatively in zone C where they finally occur, and the sample ruptured finally when rock bridges in zone C broke down. The result of fixed strain showed 3 times sudden change, sudden rise or sudden fall, in accordance with the movement characteristics of AE of the zones A, B, and C. Analysis of the local b-value by the "accumulated frequency-energy level" relation showed that the b-value of the zone C reduced obviously before the main fracture. When examining from the structural characteristics, it can be seen that the three zones are the places where different structures meet each other and therefore are sensitive sections for events to occur. This has led to different orders of fracture or different precursor phenomena.

SATELLITE INFRARED ANOMALY OF SEVERAL STRONG EARTHQUAKES IN CHINA MAINLAND

DENG Zhi-hui, WANG Yu, CHEN Mei-hua, TANG Fang-tou, CHU Quan-zhi, XU Hao-min

2003, 25(2):  327-337. 

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This paper is aimed at the relationship between satellite infrared anomaly and tectonic activities by analysis of anomaly images of 3 earthquake cases (M>6) occurred in 2000 in China. Yao'an earthquake (M_S6.5) occurred on January 15, 2000 in Yunnan Province, southwest China. The infrared anomaly of the earthquake began 20 days before the mainshock and disappeared after the event. It developed along the Honghe Fault, which is the most important fault near the epicenter. The brightness temperature along the fault is always higher than that away from the fault during the anomalous period. The anomaly of temperature difference along the fault and away from the fault exists not only when weather temperature increases but also when it falls down. It seems that the anomaly is more active along the fault than in the other area. Jingtai earthquake (M_L6.2) occurred on June 6,2000 in Gansu Province, northwest China. Satellite Infrared anomaly appeared in a peculiar geometrical pattern before the earthquake. The monthly average infrared brightness temperature increased abnormally along 3 NE-trending tectonics which intersect with the NW-trending faults. Near the epicenter, the infrared anomaly developed in "Z" configuration that seems to extend along conjugated structures. Xinghai earthquake (M_S6.6) took place on Sept. 12, 2000 in east Qinghai Province, northwest China. The epicenter is located near the NNW-trending Ngola Shan Fault. Before the earthquake, the monthly average infrared brightness temperature was different on both sides of the Ngola Shan Fault and its extension. It was higher than 10℃ on the eastern side, and less then 10℃ on the western side. From the above-mentioned 3 cases, the primary conclusions can be drawn as fellows: (1) The distribution of satellite infrared anomaly is closely related to geologic structures, especially active faults; (2) The earthquake epicenter is often located at the margin of relatively high infrared temperature anomaly area or its vicinity. Infrared anomaly develops asymmetrically, propagating from one side toward the epicenter. (3) The spatial and temporal distribution of infrared anomalies may vary in different areas. In the analysis of infrared anomaly information, therefore, it is necessary to take the tectonic conditions, geographical features and meteorological interferences into consideration.

VISUAL DATABASE FOR INDUCED EARTHQUAKE PREDICTION OF YANGTZE THREE-GORGE RESERVOIR BASED ON GIS TECHNIQUE

ZHANG Qiu-wen, LI An-ran, WANG Cheng, ZHANG Guo-an

2003, 25(2):  338-347. 

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Reservoir-induced earthquake has been one of the most important problems affecting the Yangtze Three-gorge project at all times. A large number of data and achievements on this aspect have been accumulated and obtained. However, most of them are stored with paper medium by different institutions and individuals, lacking exchange and sharing. In fact, all these data are of great importance to the assessment and prediction of reservoir-induced earthquake. Obviously, the construction of the database system of basic information necessary for the prediction of induced earthquake in the Yangtze Three-gorge reservoir area is an urgent task at present. It not only will provide scientific basis for the theoretical research of reservoir-induced earthquake, but also can be directly used for monitoring and prediction of induced earthquake in the Yangtze Three-gorge area, as well as provide information service for the command of earthquake relief work. In this paper, the contents and technology of visual database for induced earthquake prediction of Yangtze Three-gorge reservoir are determined on the basis of the principal requirements for the database system. The basic data and information for the prediction of reservoir-induced earthquake are divided into four main categories: samples of domestic and foreign typical reservoir-induced earthquake, basic geographical information of the Yangtze Three-gorge reservoir area, information about the geological environment and condition of induced-earthquake and earthquake precursory information. Referring to the national standards and criterions, and basing upon the practical characteristics of various kinds of information obtained in the Yangtze Three-gorge reservoir area, we customize separately the structure and format of data for various types of information. All the basic map data are digitally captured using digital scanning technique, and the visual raster map database is created. Furthermore, most of the raster maps are vectored through screen hand traced, and the visual vector map database is constructed. In addition, the related attribution information are entered and converted, and the corresponding attribution databases of space features are built up respectively. By taking advantage of the mixed data model of GIS, the map data and corresponding attribution data are linked and integrated without seal. Therefore, on the visual database platform, users can not only interactively browse, query, find, identify, display and output the corresponding attribution information of each feature on the map, but also can locate the coordinates of the feature according to its attribution data, which can satisfy the information needs of reservoir-induced earthquake prediction. In addition, because all the map data are digitized, a lot of special manipulation, management and analysis, such as map layers overlaying and integration can be carried out visually and easily. Of course, general functions and maintenances of database such as statistics, sorting and report can also be offered by the database system.