Table of Content

04 September 2002, Volume 24 Issue 3

Previous Issue    Next Issue

Brief Report

LATE QUATERNARY SEGMENTATION MODEL OF THE ZHONGTIAOSHAN FAULT, SHANXI PROVINCE

CHENG Shao-ping, YANG Gui-zhi

2002, 24(3):  289-302. 

Asbtract ( )   PDF (4051KB) ( )  

Related Articles | Metrics

The Zhongtiaoshan Fault, southern Shanxi is an active fault in the southeastern part of the active fault system around Ordos Massif, being the boundary between the Yuncheng Faulted basin and the Zhongtiaoshan Faulted Mountain. The recent study about fault activity indicates that it has a lower level of activity and a relatively long recurrence interval of faulting. On the basis of this understanding, a further study is carried out on the segmentation of the Zhongtiaoshan Fault, while the late Quaternary segmentation model and its quantitative parameters are provided in the light of evidence from tectonic discontinuity, tectonic geomorphology, trench paleoseismology, and fault kinematics. The Zhongtiaoshan Fault runs from the northern piedmont to the western piedmont of the Zhongtiaoshan Mountains, with a length of 137km, a general strike of NE-NEE, and a dip angle of 58°～75°, being a high angle normal fault. On one hand, the northern piedmont part of the fault exhibits a big curve convex southeast; and on the other hand, the western piedmont part of the fault intersects with its northern piedmont part at an angle of 120°. By these two major tectonic discontinuities, the Zhongtiaoshan Fault can be divided into three geometric segments from northeast to southwest: the Xiaxian segment, the Xiezhou segment, and the Hanyang segment. The major tectonic discontinuities persistently control the Cenozoic sedimentation in the Yuncheng Faulted Basin. The Xiezhou segment of the Yuncheng Faulted Basin was the depocenter not only of Tertiary system but also of Quaternary system. Morphometric analysis has indicated that the elevation profile of divides and geomorphic characteristics of drainage system in the Zhongtiaoshan Mountains, such as length and gradient index of stream, as well as evolution of its mountain front, have apparent segmented variations, showing an along strike tectonic inhomogeneity caused by long term faulting. The quantitative parameters, such as long term slip rate, displacement per event, age of the last event, and average recurrence interval obtained from the studies of trench paleoseismology and fault kinematics are indicative of segmented differences in late Quaternary fault behavior of the Zhongtiaoshan Fault. In summary, the Zhongtiaoshan Fault is a typical fault exhibiting relatively long recurrence intervals. The surface rupturing events revealed in the trenches correspond to strong earthquakes of magnitude 7.7～8.1, which were separated by a quiescence period of 12,000 years for the Xiezhou segment, and 50,000～85,000 years for the Xiaxian and Hanyang segments. For the Zhongtiaoshan Fault, the only segment that was active during the Holocene epoch is the Xiezhou segment, which has the fastest long term slip rate, the biggest displacement per event, and the shortest average recurrence interval among the three segments. The Holocene activity of this segment might be attributed to the growth and strong activity of the secondary faults of the Zhongtiaoshan Fault since at latest the middle Pleistocene, inducing the frequent rupturing on the main fault. The occurrence of such a type of fault as the Zhongtiaoshan Fault with a relatively long recurrence interval(10⁴ years)in the active fault system around the Ordos Massif implies the inhomogeneity of the movement of the Ordos Massif as a whole. The segmentation model and its quantitative parameters proposed in this paper may provide basic materials for seismic risk evaluation.

MULTILAYERED REVERSE FAULTS AND DEEP STRUCTURES IN THE MANAS EARTHQUAKE AREA, NORTHERN TIANSHAN

YANG Xiao-ping, GU Meng-lin, SUN Zhen-guo, ZHAO Cheng-bin, ZHOU Qing

2002, 24(3):  303-314. 

Asbtract ( )   PDF (4830KB) ( )  

Related Articles | Metrics

The Tianshan Mountains are a late Cenozoic rejuvenated mountain range in central Asia. The Urumqi Range front Depression is located along the northern margin of the Tianshan Mountains, consisting of a series of reverse fault-and-fold zones that form a typical thin skinned structural system. The southern Junggar Fault separates the Tianshan Mountains from the Urumqi Depression, in which three rows of reverse fault-and-fold zones are developed. From south to north, the three fault-and-fold zones are named Qigu reverse fault-and-fold zone, Manas reverse fault-and-fold zone and Dushanzi reverse fault-and-fold zone, respectively. Except for the anticlines in Qigu reverse fault-and-fold zone, the other anticlines in the Manas and Dushanzi reverse fault-and-fold zones are fault propagation fold. The shallow seismic exploration profiles show that the anticline consists of an overthrust fault zone, backward reverse fault and two partial anticlines. Four oil seismic exploration profiles show that the lower detachment fault exists in the Jurassic coal bearing strata, and the upper detachment fault exists in Paleogene strata. Some active folds are only formed on the ramps at the front of detachment fault. 2D electrical structure, deep seismic reflection profiling and crustal velocity structure across the northern Tianshan piedmont indicate that the active multilayered thrust tectonic system in the Urumqi Depression joins to a low-velocity(low resistance)layer through a brittle ductile transition zone in the crust of the Tianshan. The low-velocity layer in the upper crust of the Tianshan may be an active ductile shear zone. The brittle ductile transition zone under the Qigu reverse fault fold belt is the key link between the deep-seated active ductile shear zone and the shallow brittle fracture, and it is also the place of strong earthquake generation. The active surface structures in Manas earthquake region recorded only a part of the information of the activities of the deep-seated ductile shear zone.

PRINCIPAL FEATURES OF RECENT ACTIVITY OF THE ACTIVE NORTHERN MARGINAL FAULT ZONE OF MAXIANSHAN MOUNTAINS, LANZHOU, GANSU PROVINCE

YUAN Dao-yang, LIU Bai-chi, CAI Shu-hua, LIU Xiao-feng, WANG Yong-cheng

2002, 24(3):  315-323. 

Asbtract ( )   PDF (3725KB) ( )  

Related Articles | Metrics

In southern mountainous area of Lanzhou city, there are 4 large-scale regional fault zones, which have been active since Late Pleistocene. They include the active Northern marginal fault zone of Maxianshan Mountains(F₁), the active Southern marginal fault zone of Maxianshan Mountains(F₂), the active Southern marginal fault zone of Xinglongshan Mountains(F₃)and the active Northern marginal fault zone of Xinglongshan Mountains(F₄). They are assigned to the Maxianshan Xinglongshan Mountains active fault system. Among the 4 fault zones, the Northern marginal fault zone of Maxianshan Mountains has the strongest activity. The fault initiates from Neiguan~ying faulted basin in the east, passing through Miaowan, Yangzhai, Yinshan, and after converging with the Southern marginal fault zone of Xinglongshan Mountains at Moyunguan it runs along Tianjiagou, Hutan, Guanshan, Xianshuigou, and terminates at Bapanxia Gorge of the Yellow River in the west. The fault is generally striking N60°W, having a total length of about 115 5km.The nearest distance from Lanzhou city to the fault is only about 4km, so that the recent activity and the seismic potential of the fault zone play an important role in the seismic design and the safety measures for the Lanzhou City. The geometric feature of the Northern marginal fault zone of Maxianshan Mountains is relatively simple, and it can be divided into 4 subsidiary segments according to the branching, bending, and discontinuous step over of the fault, as well as the difference of recent activity. These subsidiary fault segments are called the Neiguanying(F_1-1), Maxianshan(F_1-2), Qidaoliang(F_1-3)and Wusushan(F_1-4)segments, respectively. The results of geological mapping of the active faults on the scale of 1/50000 indicate that the Northern marginal fault zone of Maxianshan Mountains is a long standing, south dipping thrust fault, which has become a left lateral strike slip fault with dip slip component since Middle Pleistocene. Apart from this segment, the other segments are Holocene active fault zones. Among them, the Wusushan segment is also dominated by thrust movement but is dipping to the north. It is only the Maxianshan segment that displays obvious left lateral strike slip movement with normal component, resulting in a series of offset landforms, such as offset ridge, hills, valley and terrace. The largest offset may reach up to several hundred meters, while the smallest only about several meters. The amounts of horizontal displacements along the fault are concentrated mainly at 10～ 30m, 95～ 105m and 140～ 160m, reflecting that the faults are dominated mainly by stick-slip movement. At the same time, a series of fault scarps are developed along the fault zone, and the height of the scarp at the first level terrace is about 1～1.5m. The standard offset of the second level terrace is observed at Quanshenmiao gully as about 49m, and that of the first level terrace is observed at the eastern branch of Shitougou gully as about 25m. According to the ages of the terraces, it is estimated that the average horizontal slip rate of the fault since Late Pleistocene is about 3.73mm/a.

TECTONIC STRESS FIELD AND SEISMIC TECTONICS CONTROLLED BY PLASTIC-FLOW NETWORK IN THE SICHUAN-YUNNAN RHOMBIC BLOCK AND ITS ADJACENT AREAS

WANG Sheng-zu, ZHANG Liu

2002, 24(3):  324-334. 

Asbtract ( )   PDF (9393KB) ( )  

Related Articles | Metrics

The tectonic stress field and seismic tectonics in the Sichuan-Yunnan rhombic block and its adjacent areas are investigated in terms of the "plastic-flow network and multi-layered tectonic deformation" model for continental dynamics(for short, called "netlike plastic-flow" model or NPF model). According to the model, it is presented that in consideration of the transition of deformation regime from brittle to ductile with increasing depth, the lithosphere is divided into several tectonic deformation layers, i.e. the ductile lower lithosphere(including the lower crust and lithospheric mantle)with netlike plastic-flow and the brittle upper lithosphere(including seismogenic layer and the shallow crust)with shear and tensile fractures, as well as the transitional layers(i.e. low-velocity / high conductivity layers)discontinuously distributed between the two layers; the long range transmission of driving force applying at continental plate boundary is carried out mainly by the netlike plastic-flow in the lower lithosphere, which controls intraplate tectonic stress field, tectonic deformation and seismic activities; the plastic-flow network, which is composed of two families of plastic-flow belts(i.e. ductile shear belts)intersecting each other, can be referred to as a "slip line" network developed under large deformation condition. The previous study indicates that the central eastern Asia plastic-flow network system(CEAs)developed under the compression of the Himalayan driving boundary spreads over a broad area in central eastern Asia. The Sichuan-Yunnan rhombic block is located in the southeastern part of CEAs and includes 6 right handed and 16 left handed plastic-flow belts. Under the control of these plastic-flow belts the large-scale seismic belts and the seismotectonic zones consisting of seismic faults are formed within the overlying seismogenic layer. Most of the seismic faults in this region intersect the relevant seismotectonic zones at angles within the range of -15°～+30°, being longitudinal type faults. The directions of the maximum compressive stresses in the lower lithosphere can be estimated from the lines bisecting conjugate angles of the network. Comparing these directions with those in the seismogenic layer inferred using focal mechanism solutions, it is shown that both of them are identical with each other in general tendency, indicating that the stress field in the upper layer is controlled by that in the lower layer; meanwhile, the directions of the stress vectors in the seismogenic layer tend to deviate to some extent from the general tendency owing to the influences of local factors and estimation errors. The horizontal sliding angle of seismic faults(i.e. the angle included between the directions of the maximum compressive stress and the normal to fault strike)ranges from 36.0° to 78.5°, which are suitable for strike slip. Most of the right handed seismotectonic zones in this region have matured with the apparent maturities Λ ≥0.8(Λ is the ratio of the total length of the segments within seismic fault to that of the related seismic belt), while the Λ values of most of the left handed zones are considerably less than 0.8, called quasi seismotectonic zone, except the Dali Tonghai and Tengchong Jinghong zones which approach to maturing. It is worth noting that the previously active faults may transit to be quiescent, such as the southeastern segment of the Honghe Fault zone, since they deviate from the plastic-flow belts, whereas the new seismotectonic zones may be developed along the plastic-flow belts, such as the Dali Tonghai and Tengchong Jinghong seismotectonic zones. Thus, some boundaries of the rhombic block, such as the Honghe Fault zone, have been probably replaced by new boundaries associated with the plastic-flow belts, and therefore the contemporary "Sichuan-Yunnan rhombic block" can be considered as a tectonic unit defined by some plastic-flow belts of CEAs as its boundaries.

SEQUENTIAL MIGRATION PATTERN OF SEISMICITY IN YUNNAN REGION

CAI Jing-guan, LI Yong-li, LIU Li-fang, XU Zhao-yong, QIN Jia-zheng, SU You-jin, FU Hong

2002, 24(3):  335-345. 

Asbtract ( )   PDF (5152KB) ( )  

Related Articles | Metrics

The dynamic pattern of moderate-strong earthquakes in Yunnan region has been traced in this paper. It is discovered that a sequential migration of moderate-strong earthquakes occurs 1～4 years before the active period of strong earthquakes in Yunnan region; the sequentially migrated moderate-strong earthquakes indicate the general layout of M ≥6.7 strong earthquakes during the active period, while about 80% of the strong earthquakes occur within a range of 150km from the epicenters of these moderate-strong earthquakes. The first strong earthquake of the active period occurs within 2 days to 11 months after the termination of sequential migration of moderate-strong earthquakes, and about 75% of the first strong earthquakes occur within 3 months. During the active periods Ⅰ and Ⅲ in Yunnan region, 8 of 10 strong earthquakes occur within the range of 190km from the apexes of tetragon formed by linking the sequentially migrated epicenters of moderate-strong earthquakes(such as the epicenters of Huaning, Ludian, Jianchuan, Longling, Qiaojia, and Jinning earthquakes, as well as those of the Jiangcheng, Ruili, Zhongdian, Yongshan, and Jinghong earthquakes), while about 60% of the strong earthquakes occur within a range of 50～90km from the epicenter locations of moderate-strong earthquakes. The M 7.7 Tonghai earthquake of January 5, 1970 occurred on the eastern side of the tetragon mentioned above, showing an equidistant layout of 230km from Yongshan in the north to Dongchuan, Tonghai and Jiangcheng in the south. The relationship between sequential migration of moderate-strong earthquake and the occurrence of strong earthquake for the active periods Ⅱ and Ⅳ has a little difference from that for the periods Ⅰ and Ⅲ. Sequential migrations of moderate-strong earthquakes from Lancang to Lufeng, Shiping and Tengchong, as well as from Shidian to Ninglang, Huidong and Lancang make up tetragons, while the strong earthquakes occur at the starting or ending positions of the migration, i.e. within a range of 140km from both ends of the diagonal lines of the tetragons, such as Lancang and Lufeng, as well as Lancang and Ninglang. About 75% of the strong earthquakes occur within a range of 100km from the epicenters of moderate-strong earthquakes. The strong earthquakes densely occur in the southwest Yunnan region, where is the starting or ending places of sequential migrations of moderate-strong earthquakes. The Tonghai and Longling earthquakes of magnitude 7 did not occur until the active period Ⅲ, located at the other ends of the diagonal line, 50km from Shiping and Yuxi earthquake epicenters, and 150km from Tengchong earthquake epicenter that occurred before the active period Ⅱ. The starting moderate-strong earthquakes of the migration sequence during the active periods Ⅰ and Ⅲ were M 6.0 double shocks, which are rarely seen in Yunnan. The ending moderate-strong earthquake occurred within a range of 50～70km from the epicenter of a certain moderate-strong earthquake from the migration sequence. The starting moderate-strong earthquake before the active period Ⅱ was M 6.0, and both the starting and ending moderate-strong earthquakes before the active period Ⅳ were all M 5.0 double shocks, possessing a certain peculiarity. Some earthquake examples, such as equidistant migration of strong earthquakes, sequential migration of moderate-strong earthquakes during the later stage of the active period, sequential migration before and after a great earthquake and oriented migration of moderate-strong earthquakes are also introduced in this paper. However, further study is needed for understanding the mechanism of sequential migration of seismicity.

PRIMARY STUDY ON SEISMOGENIC CONDITIONS OF THE KUQA DEPRESSION, XINJIANG

JIANG Jing-xiang, YIN Guang-hua, WEN He-ping, YU Jian-he

2002, 24(3):  346-354. 

Asbtract ( )   PDF (2841KB) ( )  

Related Articles | Metrics

Previous studies show that in the Kuqa Depression no significant activity of small earthquakes is observed before the occurrence of moderate-strong earthquake, but the results of deformation measurement across fault and mobile gravity measurement display distinct anomalies. We believe that this fact may reflect the seismogenic condition in the Kuqa Depression. The Kuqa Depression is a compressive crushed zone located at the southern edge of Tianshan Mountains and the northern margin of Tarim Basin. It comprises 4 lines of active thrust and anticline belts, among which the Qiulitage belt is the most active earthquake controlling and generating structure. The Kuqa Depression is a Mesozoic Neozoic sedimentary area with Precambrian basement. The upper part of the depression consists of relatively loose fluviolacustrine sediments of more than 10,000m thickness, while the lower part consists of Precambrian basement rocks. The crustal structure of the Kuqa Depression displays both vertical and horizontal inhomogeneities. Seismic tomography reveals that the depth of the depression may reach up to 15km. Low velocity layer occurs at a depth of 20～30km, while the velocity of the layers from the lower crust to Moho is still relatively low. Magnetotelluric sounding also reveals that there is a high conductivity layer at the corresponding depth of the crust. The hypocentral distribution section across the Tianshan Mountains shows that most of the hypocenters are located at a depth range of 20～40km, indicating that the southern marginal fault of the Tianshan Mountains has cut the Moho. The upper crust in central Tianshan has higher velocity and appears as uplift, but in the Kuqa Depression it has relatively low-velocity and appears as depression. A gradient zone occurs at a depth of 25～35km between the depression and uplift. Most earthquakes occurred at this gradient zone nearby the mountain front, and few earthquakes occurred at the other positions. The hypocenter distribution section in the Kuqa Depression(EW direction)shows that most earthquakes are distributed unevenly at a depth range of 18～40km. The seismic sources locate in the transition zone from high-velocity to low-velocity belts. It seems that the focal depth bears a close relation to the buried depth of the low-velocity layer, and most earthquakes are distributed at the transition zone between the low and high-velocity layers. Earthquakes of magnitude above 7 occurred around the Tarim Basin possess a quasi periodicity of 11 years. The activity of moderate-strong earthquakes in the Kuqa Depression has the same quasi periodicity, but lags behind for 18 months. The earthquake of magnitude above 7 in Tarim may cause the occurrence of moderate-strong earthquake in the Kuqa Depression. The seismic activity in the Kuqa Depression is characterized by earthquake cluster, and 3 earthquake clusters have occurred in the past 30 years. The result of deformation measurement on the middle segment of the Kuqa Depression shows that several years before the occurrence of moderate-strong earthquake or during the process of earthquake occurrence, the deformation across the fault has increased. This anomaly has been observed for 3 times in the past 28 years. The anomaly appears as the rapid drop of observation value and the distortion of annual variation. It indicates the intensification of the thrusting of the upper wall over the lower wall of the fault. The result of gravity measurement also indicates that before or during the occurrence of earthquake, the gravity displays a positive anomaly. This anomaly may last 4 years, and then turn into negative anomaly rapidly. The earthquake usually takes place by the end of positive anomaly or during the rapid drop.

A NUMERICAL SIMULATION OF SEISMOGENIC PROCESS OF LOW VELOCITY BODY AND DEEP AND SHALLOW ACTIVE FAULTS

LIU Jie, SONG Hui-zhen

2002, 24(3):  355-367. 

Asbtract ( )   PDF (4159KB) ( )  

Related Articles | Metrics

The seismogenic factors of the physical model for strong earthquake generation in North China established from the results of geophysical exploration are numerically analyzed by using the 3-D finite element method for visco elastic medium with split node. The results demonstrate that an area of stress accumulation will exist when fault activity occurs around the low-velocity body. In addition, the distribution of slip rates at depth for the major active faults in Capital Circle are inversed from GPS measurement data. Fault slip rate is usually about 10mm/a in this region, and it is higher for the faults in the western part than in eastern part of the region. Moreover, the slip rates of buried deep faults are higher than those of the shallow faults. These slip rates are treated as inner boundary conditions for the 3-D visco elastic finite element calculation. Then the variation of stress field and energy distribution with time in Capital Circle are simulated by considering a geologic body of 50km depth, 300km length and 220km width, with compressive stress of -9MPa in East West direction and -3MPa in North South direction, regardless of gravity. The distributions of initial effective stress and strain energy density are influenced mainly by the inhomogeneity of medium, especially the low-velocity body. Faulting process may cause the variation of effective stress and strain energy density. The variation rates of effective stress and strain energy density for each element can be obtained through several steps of visco elastic calculation. According to the physical structure of strong earthquake generation, the distribution of initial stress field and the increment of additional stress field, we have determined 3 potential risk areas of strong earthquake. They are the Yan Huai Basin, the Southeast margin of Yangyuan Basin and the Daxing Zhuozhou area. Moreover, the maximum potential earthquakes for these 3 risk areas are estimated to be M7, M6.5 and M6, respectively on the basis of different accumulation rates of strain energy, the volumes of seismic source bodies, and the Gutenberg-Rithter relation. As compared with historic earthquakes in these areas, the current accumulation rate of strain energy in Yan Huai Basin coincides well with that for the recurrence of historic earthquakes, but the coincidence is not good for the Southeast margin of Yangyuan Basin and the Daxing Zhuozhou area.

AN EXPLANATION OF THE RAPID ACCUMULATION OF STRESS IN CONTINENTAL PLATES

TAO Wei, HONG Han-jing, LIU Pei-xun, YU Yong, ZHENG Xiu-zhen

2002, 24(3):  368-376. 

Asbtract ( )   PDF (4085KB) ( )  

Related Articles | Metrics

According to the researches of strong earthquake distribution in China's continent and its vicinity, many researchers proposed that the strong earthquake activities in China and its neighboring areas may concentrate mainly in a certain seismic region during a certain time period. Hong Han-jing(1997)suggested that the seismic activities in China and its surrounding areas in the past century can be divided into several micro dynamic periods, and in different periods there should be a relatively stable region of seismicity. Moreover, in different periods the features of strong earthquake distributions are different in space and the main seismic regions may migrate for a long distance. In the research of this large-scale seimogenic model, the transfer of stress in various layers of the lithosphere has attracted more and more attention of most researchers. The dynamic analy~ses demonstrate that it is difficult to explain the long distance migration of the main seismic regions in the micro dynamic periods by considering only the brittle layer of the crust. Therefore, the layered rheological structures of the whole lithosphere and the process of stress accumulation must be taken into consideration. The layered structure of the lithosphere has been given in many previous studies, and the rupture in brittle layer of the crust was proved to initiate at deeper layer of greater strength and propagate to the shallow layer of lower strength. Kusznir & Bott(1977)have introduced a simplified model of two layers visco elastic lithosphere, which comprises an upper layer with higher viscosity coefficient and a lower layer with lower coefficient. As a boundary condition, they applied uniform horizontal boundary stress at one side of the model and maintain the other side to be fixed. The simulation proved that after the application of stress at one side of the model the stress relaxes in lower part of the model and concentrates in the upper part as a function of loading time. However, there are various kinds of boundary conditions in the nature, and the movement of Plate boundaries is a dynamic process. Sometime plates move at a certain rate(such as the squeezing rate of the Indian Plate), but do not maintain a constant force on the adjacent plate. In this case, the stress on the boundaries of the plate is not a constant stress, but within a certain time scale and precision scale the velocity can be considered as a constant velocity. We modify, therefore, the boundary condition of the lithospheric model of Kusznir & Bott to constant velocity boundary condition, and give the analytical resolution. It is proved that if the thickness of high viscosity coefficient layer, the Young's modulus and viscosity coefficients of each layer are given, then the rate of stress accumulation in the high viscosity coefficient layer increase with increasing thickness of the high viscosity coefficient layer. According to the result, under the same boundary condition, if the accumulation of stress for one layer homogenous model needs 950 years, then for two layers visco elastic model(with the ratio of the two layers of 1/100 and the viscosities of 10¹⁸ Pa·s and 10²² Pa·s, respectively), it needs only 38 years. Obviously, the accumulation time of stress is significantly shortened. The above discussions prove that the low viscosity coefficient layer plays an important role in the transfer and concentration of stress in different media. This result may provide a new insight into the understanding of the short term geological problems. In a word, by analyzing the transfer condition of stress in lithosphere and the dynamic boundary conditions surrounding the continent, we suggest that under the effect of dynamic boundary conditions, the accumulation time of stress in lithosphere can be shortened significantly.

2D-NUMERICAL SIMULATION OF DISPERSION OF TEPHRA FALLOUT

ZHAO Yi, ZHANG Cheng-yuan, XI Dao-ying

2002, 24(3):  377-386. 

Asbtract ( )   PDF (2653KB) ( )  

Related Articles | Metrics

Tephra falls is one of the most serious volcanic hazards. The purpose of the quantitative simulation of this complex dynamic process is to build up a model for real time prediction of this hazard of volcanic eruption. Tephra falls are controlled by many factors, so that it is difficult to build up a quantitative model, and possible only to build up a semiquantitative model. Another purpose of the simulation is to get quantitative information of eruption source based on the survey of tephra fallout. The results can also be applied to the assessment of volcanic hazards. The dispersion of tephra is a function of many factors: total mass, median diameter and standard deviation of material erupted, height of the eruption column, wind velocity, and the nature of particle diffusion from the eruption column. The relationship between these factors and the dispersion of tephra can be described by using a model of two dimensional diffusion in atmosphere. We adopt here the mathematic model for dispersion of tephra proposed by Suzuki(1983). Basing on Suzuki's formula, we compile a practical program for simulating the dispersion of tephra fallout from a single volcano at one event of eruption. Combined with the dynamic parameters of the eruption of Tianchi volcano obtained from physical volcanologic research, this program is applied to simulating the dispersion of tephra from Tianchi volcano. One outstanding characteristics of volcanic tephra dispersion is that the size and thickness of particles demonstrate an exponential attenuation with increasing distance from the source area. This relationship had been used as a base for calculating the total mass of tephra fallout. Unfortunately, no overall theory at present can be used to predict this kind of behavior. The following opinions are proposed for improving the mathematical model:(1)The physical units used in the empirical formulation are not uniform: some are in m/kg·s^-1, and some are in cm/g·s^-1. This may cause a deviation of several orders of magnitude from the real result. A coordination of the units has been made in this paper.(2)The integrations of particle size and the height are not an independent but a coupling process, so that the double integration of particle size and height can not be separated as the time of two single integrations.(3)No boundary condition is assumed in this model, so we need not to limit the model for hazard assessment of iso mass distribution larger than 1g/cm², but to use also the model for iso mass distribution of less than 1g/cm² for the purpose of environmental protection. The practice has demonstrated that the tephra of smaller size is more dangerous to the engine of airplane and may cause breathing difficulties for humans. Therefore, the iso mass distribution contours should be divided into two types: one for the purpose of hazard assessment and the other for environmental protection.

THE EFFECT OF WATER ON STABILITY OF FRICTIONAL SLIDING OF FAULT

HUANG Jian-guo, ZHANG Liu

2002, 24(3):  387-399. 

Asbtract ( )   PDF (3825KB) ( )  

Related Articles | Metrics

The friction behavior of faulted granite samples, with size of Φ 20 mm×40mm and saw-cut at an angle of 35° to σ₁ axis, was experimentally studied under various water containing conditions and tri axial compression. The stability during stable sliding or stick-slip was emphasized. Three types of polished fault surface are considered:(1)two surfaces contact directly without any gouge in between, used to simulate the friction behavior of brittle fault;(2)the preset fault is filled with 0.25g quartz powder with mean grain size of 10μm, used to simulate the friction behavior of a fault containing brittle gouge;(3)the preset fault is filled with 0.25g clay fault gouge with grain size of <10μm, used to simulate the friction behavior of a fault containing ductile gouge. The water content ranges from dry to water saturated, and the pore pressure ranges from 20 to 150MPa. The following conclusions can be drawn from the results of the experiment:(1)The mode of faulting is strongly influenced by the existence of water. For dry polished surfaces without any gouge, there is a certain range of confining pressure in which stick-slip behavior exists. This range of confining pressure may shrink quickly when water enters and forms pore pressure on sliding surface, i.e. the friction behavior turn to stable sliding. For samples with quartz powder, frictional behavior changes from stable sliding to stick-slip when dry gouge becomes wet, and return to stable sliding again when gouge is saturated and undrained. In this case, there exists also a certain range of confining pressure in which stick-slip behavior exists. For samples with clay gouge, frictional sliding keeps stable under any humidity and pore water pressure.(2)Experimental study also shows that stress drop of stick-slip is dependent on the water content. For samples without gouge, stress drop decreases with increasing water content in rock. For samples with quartz powder gouge, however, stress drop increases quickly at first and then slowly with increasing of water content in gouge under drained condition; under undrained condition, stress drop is small at first, then gets larger, and becomes small again when the content of water in gouge increases. When the samples are loaded with pore water pressure, frictional sliding turns to stable and the stress drop tends to zero.(3)Water influences the velocity dependence of frictional sliding of the tested rock samples. The velocity dependence of samples without gouge is velocity weakening at any water content. For the samples filled with quartz powder, it is velocity weakening when the water content ranges from dry to wet, i.e. at different humidity, and becomes velocity strengthening when pore pressure exists on the fault; for the samples filled with clay fault gauge, it is always velocity strengthening.(4)Based on velocity weakening model of friction instability and by considering the order of fault zone from no any gouge, to containing quartz powder gouge and clay gouge as a transition from brittle to ductile regimes, a conceptional model has been established for the relationship between the stability of faulting and the influential factors including fault property, water content and confining pressure(depth). Brittle fault displays velocity weakening in most cases of various water contents, and stick-slip appears in the region of low water content, while stable sliding appears when the water content becomes high(high pore pressure). Semi brittle or semi ductile fault displays velocity weakening when water content is relatively low, and stick-slip may exist; but it will displays velocity strengthening when high pore pressure appears, and the sliding will be stable. Ductile fault always displays velocity strengthening at any water content, and the sliding is always stable.(5)A conclusion can be drawn from the experimental results of this study in the light of pore pressure theory: Entering of water will decrease the stability of the fault system but the existence of water may increase its stability; the former effect can be called“mechanical effet of waster” and the latter can be called“chemical effet of waster”.These are two important aspects when we con-cider the influence of water or fluids on the stability of fault or rock mass. The ranges of confining pressure and pore pressure adopted in this study are comparable with the geological condition from ground surface to a depth of about 6km.Wester appears as free waster in this depth range. Therefore, these experimental results may provide an insight into the geological phenomena in shallow part of the crust the mechanisms and prediction of earthquakes and geological engineering problems etc. For example, it can be used as the criteria of the effect of water on fault stability in numerical simulation of seismic activity, and can also used for analyzing the mechanism and activity of reservoi,induced earthquake. Some reservoi,induced earthquakes show that earthquake activity occurs only at the beginning of waster storage or during the storaging period, then it becomes less and less. Accarding to the results of this study, the former one is dueto the“mechanical effect of water”,and the latter is dueto the“chemical effect of waster”.

AN ANALYSIS OF TECTONIC ENVIRONMENT AND CONTEMPORARY SEISMICITY OF FRONTAL OROGENY IN CENTRAL TAIWAN AREA

CHENG Kuei-hsiang

2002, 24(3):  400-411. 

Asbtract ( )   PDF (4054KB) ( )  

Related Articles | Metrics

The central part of Taiwan area shows an arcuate thrust tectonics. Every arcuate belt is composed of a series of varying scales of sub arcs, which appear as a separate fold and thrust belt with independent characteristics of geometry, kinematics and mechanics and a self similar system as well. The kinematics of arc changes from thrusting on the top of the arc to the oblique thrusting and wrenching at the sections, and finally thrusts to the bottom of the arc. The arcuate thrust tectonics is composed of frontal, lateral and oblique ramps at depth. Each earthquake deformation belt is related to the independent arc. Strong earthquakes correspond to the large-scale arcs, and medium earthquakes to the small ones, and even to the smaller sub arcs. Finally, the mapping analysis is carried out between the contemporary seismicity and the space area of the arcuate thrusting tectonic system in the central part of Taiwan area, especially on strong Chi-Chi aftershock series.

PREDICTED MODELS OF ECONOMIC AND LIFE LOSSES CAUSED BY EARTHQUAKE FOR TIANJIN CITY

CUI Yu-hong, WU Guo-you, NIE Yong-an, YAO Lan-yu, QIU Hu

2002, 24(3):  412-422. 

Asbtract ( )   PDF (5576KB) ( )  

Related Articles | Metrics

This paper discusses mainly the problem of the prediction of economic and life losses caused by earthquake disaster for Tianjin City. Five background factors for earthquake hazard prediction for Tianjin City are analyzed in detail. These factors include the features of buildings, the geological condition, the construction site, the basic intensity of seismic design and the attenuation of seismic intensity. According to these factors, for the purpose of seismic hazard prediction, Tianjin City can be divided into 4 areas with 7 structural styles of buildings. Furthermore, the models of economic and life losses caused by earthquake for Tianjin City are analyzed by taking account of the time factor. The models are compared with the actual hazards caused by the 1976 Tangshan earthquake, and some modifications have been made according to the result of comparison. The economic loss ratio DR(I_i), the ratio of public wealth loss f(I_i)and the ratio of collapsed buildings CR(I_i)caused by earthquake for the four areas of Tianjin City are provided in this paper, and the formulas used for predicting the economic losses of buildings, the economic losses of the general dominion production(GDP)and the life losses caused by earthquake are provided as well. Basing on these results, we divide Tianjin City into 2 858 units for seismici hazard assessment. And by supposing the occurrence of an earthquake of magnitude equal to that of the maximum earthquake that had occurred in Tianjin City in the past hundred years, we provide the distribution maps of the possible economic and life losses within these 2 858 units of Tianjin City. The result of this study is of practical significance to seismic hazard prediction and reduction for Tianjin City.

ARRANGEMENT OF WELL NETWORK AND ESTABLISHMENT OF OBSERVATION WELL AT THREE-GORGES OF THE YANGTZE RIVER

CHE Yong-tai, YU Jin-zi, LIU Wu-zhou, YI Li-xin, XU Feng, LI Jie-cheng, SUN Tian-lin

2002, 24(3):  423-431. 

Asbtract ( )   PDF (2480KB) ( )  

Related Articles | Metrics

The Three-Gorges well network is the first well network in China established specially for the purpose of monitoring the groundwater precursors of reservoir-induced earthquake. The basic consideration of the establishment of this well network is based on the fact that the seismic event of certain magnitude induced by the storage of reservoir may cause the changes of stress and strain states of the aquifer, as well as the corresponding pore water pressures. The number and spacing of the wells are determined according to the magnitude of earthquake(M_S), the length of seismogenic fault(L)and the variation range of stress strain field(R). The selection of well locations is based on the groundwater behaviors sensitive to earthquakes. Such peculiar tectonic sites as the termination, inflection and junction of fault zones are selected for well location. The depth and structure of the wells are decided according to the requirement that the well may reveal well enclosed confined aquifer and minimize the influence of surface water and hydro atmospheric factors on the premise of limit funds. According to the afore mentioned considerations, 4 wells are drilled at the dam site and 4 wells at the head of the Three-Gorges Reservoir. The depths of the wells are between 100～ 200m, and all the wells expose well enclosed confined aquifer. Primary observations show that the arrangement of the Three-Gorges well network and the establishment of the observation wells are relatively successful. They form a good base for monitoring the precursor of reservoir-induced earthquake.

TECHNICAL SYSTEM OF THE RESEARCH CENTER OF EARTHQUAKE RELATED SUBSURFACE FLUIDS IN CHINA

LIU Chun-duo, ZHAO Honk-li, GU Yuan-zhu, WANG Guang-cai, KONG Link-chang, CHE Yon-tai

2002, 24(3):  432-434. 

Asbtract ( )   PDF (1140KB) ( )  

Related Articles | Metrics

TECHNICAL SYSTEM OF GROUNDWATER OBSERVATION IN THREE-GORGES WELL NETWORK

CHE Yong-tai, YU Jin-zi, LIU Wu-zhou, LI Yi-quan, NING Li-ran, SUN Tian-lin, LIU Bei-shun, XIE Yue-qin, YAO Yun-sheng, YAN Ping

2002, 24(3):  435-444. 

Asbtract ( )   PDF (3089KB) ( )  

Related Articles | Metrics

The Three-Gorges well network is the first professional network in China established for the purpose of monitoring the groundwater precursors of reservoir-induced earthquake. It is composed of 8 observation wells. In addition to observe the change of water level, 4 wells are enabled to measure water temperature and soil Radon, 3 wells are enabled to measure rainfall, atmospheric pressure and temperature, and 1 well can be used to observe the water level of the reservoir. The observation items are totaled 26. The present paper introduces in detail the technical system of the observations used in the Three-Gorges well network, including the technical system of the whole network and each well, the technical index, installation and adjustment of every sort of instruments and equipments, as well as the corresponding supporting facilities such as the power supply, communication and lighting arrester. In addition, the running of the technical system of these observations and its preliminary results are also introduced briefly.

PRELIMINARY STUDY OF EARTHQUAKE-INDUCED URBAN FIRE SPREADING——A CASE STUDY IN FUZHOU CITY, FUJIAN PROVINCE

XU Jian-dong, WANG Xin-ru, LIN Chien-te, ZHANG Ning

2002, 24(3):  445-452. 

Asbtract ( )   PDF (4242KB) ( )  

Related Articles | Metrics

In the light of traditional fire spreading theory, we develop an Arcview/GIS based visual model to simulate the spreading process of urban earthquake-induced fire. The basic constitutional law is from traditional equation: ρ_ν0 ΔH=Q, in which ρ = density of combustible material; ν0 = fire spreading velocity;ΔH = entropy increment from initial temperature T₀ to final temperature T i for unit mass; Q = fire thermal velocity. There are also three other factors involved in this model: 1)seasonal dominant wind direction; 2)possible explosive site; 3)results of seismic vulnerability evaluation. Basing on the previous studies of the relation between damage degree and induced fire probability, we calculate the possible number of fire events under different seismic hazards(seismic intensity of ⅩⅠ, ⅩⅡ, ⅩⅢ, and Ⅸ). If the hazard vulnerability analysis indicates that the buildings have "no major damage", then the fire probability is taken as the annual average numbers of normal fire. The width of the fire lane is determined in accordance to the specific case of Fuzhou City as follows: 13m for distance between high storied buildings; 9m for distance between high storied building and 1^st class or 2^nd class private house; 11m for distance between high storied building and 3^rd class or 4^th class private house. Taking Fuzhou City as an example, we propose a method to classify the types of building combustibility based on different constructions of the buildings, and realize the visual and dynamic computer simulation under different damage conditions caused by earthquake. Under consideration of possible fire source, wind direction, wind speed, and spatial distribution of combustible materials, this model can be utilized to estimate the fire extent and damage degree at a specific time after induced fire caused by a strong earthquake, and is useful for economic loss assessment and hazard mitigation decision making. In normal times, this model can also be used as a tool to provide valuable information for the study of urban fire spreading, and for urban firefighting countermeasures.

TEST OF EPICENTER DETERMINATION OF MICROEARTHQUAKES RECORDED BY THE DIGITAL SEISMIC NETWORK IN CAPITAL CIRCLE

GUO Biao, LIU Qi-yuan, CHEN Jiu-hui, LI Shun-cheng

2002, 24(3):  453-460. 

Asbtract ( )   PDF (4229KB) ( )  

Related Articles | Metrics

The newly developed Digital Seismic Network in Capital Circle consists of 107 stations, the seismic monitoring and epicenter determination capabilities of which need to be verified. In epicenter determination, we should bring the superiority of digital seismic record into full play, and improve the reliability and precision of epicenter determination by using the corresponding digital processing technique. In this study, several digital processing techniques, including digital filtering, polarization analysis, station scanning and phase tracing, are used for improving the phase identification in the epicenter determination. Totally, 7 microearthquakes(M_L<2.0)recorded by the Digital Seismic Networks in Capital Circle in 2,000 were located precisely by using the Geiger method. In addition, the reliability and precision of the method and program have been verified by determining the epicenters of artificial events. It is shown that the error of epicenter location is less than 2.0km and the error of focal depth determination is less than 3.0km. The results of this study can be used for evaluating the seismic monitoring capability of the Digital Seismic Network in Capital Circle.

APPLICATION OF SARMA METHOD TO STABILITY EVALUATION OF JIPAZI SLOPE, YUNYANG COUNTY, SICHUAN PROVINCE

MA Zhao-ting, LIANG Hai-hua

2002, 24(3):  461-468. 

Asbtract ( )   PDF (2134KB) ( )  

Related Articles | Metrics

Several landslide events have successively occurred at Baota slope area of Yunyang County, Sichuan Province, among which the Jipazi landslide is the latest one. The Jipazi landslide represents partial revival of an old landslide. The total volume of the sliding mass is up to 1.5?10⁷m³ or more, of which about 2.3?10⁵m³ were detached from the slide channel and slided down below the flood level of the Yangtze River, causing a serious danger to the navigation on this river segment and enormous economic losses. Once the Three-Gorges Reservoir stores water, the water level of the Yangtze River will be much higher than the current level. As a result, new landslide may revive on the old one. Based on the research on the geometry, material component, structural and hydrogeologic features of the sliding mass, it is preliminarily concluded that the Jipazi landslide was caused by the increasing of pore pressure in the sliding mass due to excessive rainfall infiltration and impeded drainage during the storm. In the light of the limit equilibrium theory, a program of Sarma method is designed to evaluate the stability of the Jipazi sliding mass after the Three-Gorges reservoir storing water. The result shows that the critical value of the slope stability is that the water level of the Yangtze River reaches 145m high and the drainage rate of the slope is 50%. The dynamic sensitivity analyses of the factors affecting slope stability are carried out through adjusting the values of some affecting parameters of slope stability. The results show that the rainfall infiltration, the water level of the Yangtze River after the Three-Gorges reservoir storing water and the drainage rate of sliding mass are the crucial causes affecting stability of the Jipazi landslide. Little rainfall infiltration, low water level of the Yangtze River and unimpeded drainage in the sliding mass will enhance the stability of the slope. Therefore, corresponding prevention and control measures must be taken before the Three-Gorges reservoir stores water.