Table of Content

11 March 1998, Volume 20 Issue 1

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Brief Report

FAULT GROWTH, DISPLACEMENT TRANSFER, AND EVOLUTION OF THE JIUGONGKOU FAULT OVERLAP ZONE IN WEIXIAN, HEBEI

Cheng Shaoping, Yang Guizhi

1998, 20(1):  1-8. 

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When normal faults overlap each other in extentional direction the overlapping faults and the overlapping area will compose a fault overlap zone. Fault overlap zones with a separation more than several kilometers are usually considered as segment boundaries in studying earthquake rupture segmtntation. The Jiugongkou fault overlap zone in Weixian, Hebei, however, presents a typical opposite example which can not be regarded as a segment boundary. The Jiugongkou fault overlap zone is located on the southern marginal fault zone of the Guangling Weixian Basin, with 7km of an overlapping length and 2.2～5.7km of an overlapping separation. The latest Pleistocene alluvial fan developed within the overlapping area. Geological and geomorphological investigations provide evidences for the fault growth: abandonment of the latest Pleistocene alluvial fan, offset of the fan gravels, doposition of the fault scarp derived colluvial deposits, instrusion of docite magma along the fault, occurrence of the antithetic faults, and fracture clevages developed in loesses. Throw analysis indicates that no displacement deficit has happened within the overlap zone, but interaction and displacement transfer have occurred between the two overlapping faults. They manifeste as asymmetric throw profiles for both the main overlapping fautls, symmetry of a composite throw distribution for all the fault traces characterized by a single fault, and a significant increasment of the throw within the overlap zone. Moreover, dating of the faulted materials shows that the fault growth and the displacement transfer occurred in a same time period, starting between (27400±2200) and (26000±2100)a B.P. and continuing to after (18700±1500)～(1300±1000)a B.P. For the overlap zone the mechanisms of displacement transfer may be associated with magmatic activity and tectonic tilting. The overlap zone experienced evolutionary stages offset and overlap, however, has not been breached or linked yet. The Jiugongkou fault overlap zone has evoluted and developed into a geometric and kinematic coherent unit, characterized by a single fault, in terms of fault growth, interaction and displacement between overlapping faults. In this way, a fault overlap zone on fault zones often occurs only as a temporary feature. These results can be of great importance to seismic hazards assessment, especially magnitudes of potential earthquakes, as well as earthquake probability predicition.

A STUDY ON SPATIAL TEMPORAL DISTRIBUTION OF THE FAULT SINISTRAL DISPLACEMENT IN THE ALTUN STRUCTURE SYSTEM SINCE OLIGOCENE MIOCENE

Guo Shunmin, Xiang Hongfa

1998, 20(1):  9-18. 

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The Altun structure system is a structure domain that consists of a set of monomict structures which have consistent strike, slimiar active character and origin connection. Many different geological units and a series of approximately parallel faults are involved in the Altun structure system. In the Altun structure system, all faults have sinistral displacement. Since Oligocene Miocene, sinistral displacement and slip rate for the the Altun structure system are 450～700km (average displacement is 580km) and 15mm/a, respectively. In the same period, displacement in the Altun fault zone is about 225～375km (average value is 300km), the left lateral slip rate is about 7mm/a. The displacement and slip rate for other faults in the Altun structure system are 150～250km (average displacement is about 200km) and 5mm/a, respectively. Since Pliocene, sinistral displacement and slip rate are 90～130km (average displacement is about 110km) and 11mm/a, respectively. For the Altun fault zone, left lateral displacement and slip rate are 50～80km (average is about 65km) and 6～7mm/a, respectively. For the other faults in the Altun structure system, sinistral displacement and slip rate are 40～50km (average is 45km) and 4mm/a, respectively. Since Quaternary, sinistral displacement and slip rate are 20～27km (average is 23.5km) and 9～12mm/a, respectively,. For the Altun fault zone, left lateral displacement and slip rate are 7～17km (average is 12km) and 5～7mm/a, respectively. For the other faults in the Altun structure system, left lateral displacement and slip rate are about 10km and 4～5mm/a, respectively. From Oligocene to Quaternary, the left lateral slip rate of the Altun structure system and the faults in system became slow down. The left slip rates get smaller from south to north. For the Altun fault zone, the displacment in the middle segment is larger than the two sides in Quaternary. It means that the left displacement reduced from the middle to two sides of the Altun fault zone.

CHARACTERISTICS OF GROUND RUPTURES CAUSED BY 1927 GULANG M 8 EARTHQUAKE AND THEIR CAUSATIVE MECHANISMS

Hou Kangming

1998, 20(1):  19-26. 

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The results of field investigations demonstrate that the 1927 Gulang M 8 earthquake caused six segments of ground ruptures which have different scales, strikes, mechanisms and grades. They composed a complex seismic fracture zone with plane shape distribtuion which is different from ruptures caused by strike slip faulting in western China. Among them four segements of rupture have the coincident locations with the epicentral and aftershock area and are distributed along the north rim of the Dongqingding mountain and the eastern section of the Huangcheng Shuangta fault and the north section of the Wuwei Tianzhu fault, forming the main rupture zone of the earthquake. The other two segments, as the secondary rupture zones are located in the middle section of the Huangcheng Shuangta fault and the Xishanpu Dishuiya fault, respectively. The four segments of the main rupture are the resluts from the uniform tectonic stress field and have consistent deformation mechanism. The Dongqingding extensional rupture zone was developed on the hanging wall of the Xiafangzhai Yanjiaxinzhuang reverse fault as a secondary fracture, while the main rupture was caused by the reverse faulting on the piedmont fluvial terrace.

A STUDY ON ACTIVITY OF THE SEISMOGENIC FAULT FOR THE TANSHAN EARTHQUAKE OF 1976

Li Jianhua, Hao Shujian, Hu Yutai, Yu Zhishui, Zhu Baogui

1998, 20(1):  27-33. 

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A ground fissure zone was found by field observation and aerial survey after the Tangshan M 7.8 earthquake of 1976. It indicates that the Tangshan fault is associated with the seismogenic structure of the event. We have made a new study to the activity of this fault by using the information of the satellite remote sensing and the results of seismic exploration to the shallow layers. Explanations of the satellite pictures show that the NE trending Tangshan fault was cut by the NNW trending fault,while the fault of NWW strike was trailed,therefore they represent right lateral strike slip fauting. Seismic exploration for shallow layers indicates that the Tangshan fault is a right lateral strike slip normal fault with NW dipping and a high dip angle. This fault dislocates the strata of the Holocene (Q₄),late Pleistocene (Q₃),middle Pleistocene (Q₂) and early Pleistocene Series (Q₁).

DEEP SOUNDING SURVEY BY CONVERTED WAVES OF EARTHQUAKES IN CENTRAL PART OF THE TARIM BASIN AND ITS INTERPRETATION

Zhang Jiaru, Shao Xuezhong, Fan Huiji

1998, 20(1):  34-42. 

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A deep sounding survey by the method of converted waves of earthquakes has been completed along the Kuche Tazhong Tanan profile in the central part of the Tarim Basin. The lithosphere of study region is characterized by the layered blocked structures with the crustal thickness about 40～50km. The crustal thickness is about 40km in the uplifted region and it is about 50km in the subsided region. The upper crust (the granitic gneiss layer) is obviosely thinning to the central part of the Tarim Basin. The velocity of the middle crust is relatively high( V_P= 6.4 ～6.5km/s, V_S=3.4～3.5km/s). There are some thin low velocity layers in the upper part of the lower crust. The average velocity of the crystalline crust of the Tarim basin is higher than the surrounding Qing Zhang plateau and Tianshan mountain regions. The velocity structures of the basin show its continental crust character. All the data of the investigation show that since the Cenozoic Era under the action of strong compression force from the India plate,the quasi synchronous bend deformation of all lithospheric layers and the adjusted movement of the rigid crust blocks along the deep sited faults may be the main deep dynamic processes in the lithosphere of the Tarim basin.

NUMERICAL MODELING ON STRESS FIELDS OF EN ECHELON FAULTS WITH DIFFERENT GEOMETRY

Liu Liqiang, Ma Jin, Ma Shengli

1998, 20(1):  43-53. 

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The stress fields of en echelon faults with different geometry and mechanical conditions were considered by dynamically numerical simulation. The effect of geometry on stress field pattern is discussed. The disturbed stress fields caused by fault propagation were calculated and their effects on the neighboring fault were investigated. Around the propagating fault segment the disturbed stress field of maximum shear stress displays an eight petal pattern and that of mean stress displays a four quadrant pattern. Interest is that when there are several propagating segments in a narrow area,the disturbed stress fields remain the same patterns.

INFLUENCE OF LOADING RATE VARIATION ON THE STABILITY OF FAULT MOTION

He Changrong, Ma Shengli, Huang Jianguo

1998, 20(1):  54-62. 

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This work elucidates the stability boundary for small perturbation and dynamic motion of an inclined spring slider fault system in which normal stress is coupled to shear stress on the sliding surface. With rate and state dependent friction,our anlysis shows that critical stiffness on the stability boundary decreases with increase of sliding rate. This means that a positive jump of the load point velocity may lead to a transition from cyclic stick slip motion to stable sliding; and a decrease of the load point velocity may lead to a transition from stable sliding to cyclic stick slip motion. Positive velocity jump greater than a certain amount may also trigger a single instability,but no transition between cyclic stick slip and stable sliding modes occurs in this situation. Experimental evidences of triaxial friction tests from references and our own experimental study support these theoretical results.

PHYSICAL MODELLING OF PLASTIC FLOW NETWORK CONTROLLED TECTONIC DEFORMATION IN THE CENTRAL EASTERN ASIAN CONTINENT

Li Jianguo, Zhou Yongsheng, Wang Shengzu

1998, 20(1):  63-72. 

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To model tectonic deformation of the central eastern Asian continent compressed along the plate boundaries, plasticized rosin and solidified talc powder slurry are used respectively as analog material of ductile lower layer (i.e. the lower crust and lithospheric mantle) and brittle upper layer (i.e. the upper crust) of the lithosphere. The preliminary results of the experiments indicate that two plastic flow network systems, i.e. those named “Central Eastern Asia” and “Southeast China”, are formed under the compression of the Indian and Philippine sea plates along the Himalayan and Taiwan driving boundaries, respectively. They control the tectonic deformation of the brittle upper layer and are coincident roughly in general pattern with the netlike distribution of earthquakes in this region. The third driving boundary with weaker action and smaller scale and a network system corresponding to it may exist in the northeastern part of this region, which may result in the tectonic and seismic belts to be kinked in the northern area of North China. The experiments also show that the origin of large scale compressional basins are associated with the existence of stable blocks in the lower lithosphere.

ROCK WAVE VELOCITY TYPES IN THE CRUST OF NORTH CHINA AND ITS GEOLOGICAL IMPLICATONS

Zhang Younan, Sun Junxiu

1998, 20(1):  73-81. 

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Seven types of typical rock wave velocity are classified according to the relationship of mineral composition,origin of the crystalline rocks with absolute values of the wave velocities at high pressures and room temperatures, and the relationship of the wave velocities with phase assembly transformation at high pressures and high temperatures. And at a certain structural envirement, in the light of the relation of variational wave velocity of the rocks with crustal depth, we suggest the concept of rock grade which reflects acidity or basicity of rocks. Its meaning is that with the values from seismic refraction sounding at a certain depth we can have rock grades of the comprehensive rocks at the same depth. On the basis of above principles, we study the composition of the crust of North China, the generation and exsisting conditions in the crust for the amphibolite, the phase of the quartzite and generating cause of the low velocity layers in the crust.

SOIL DEVELOPMENT PROPERTIES AND THEIR EVOLUTION WITH TIME (AGE) IN THE FORELAND REGION OF THE YUMUSHAN MOUNTAINS OF HEXI CORRIDOR

Xing Chengqi, Lu Dehui, Dong Zhiping, Sun Zhong, Chen Junving, Jia Zheng

1998, 20(1):  82-90. 

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In this paper, the primary physical and chemical characteristics of the soil development related to Quaternary faults in the foreland region of the Yumushan mountains of the Hexi corridor, and their evolution with age have been investigated and analyzed. In six well drained soils formed in loess and alluvial proluvial parent materials of various ages (their formation ages are 5000 to 38000a ago) in this region, calcium carbonate content obviously increases with age, and so does calcium carbonate accumulation index value which represents the degree of calcic accumulation horizon development in the soil profiles. The data of the ages, the calcium carbonate contents and calcium carbonate accumlation index values of these six soil profiles were used to derive four regression equations as follows: for CaCO₃ content: Y=-37.36+3.254X(1) and for CaCO₃ accmulation index: Y=-7.19+0.298X(2) Y=-91.689+58.01 log X(3) log Y=-1.748+1.545 log X(4)Where Y is the age in 10³ years, and X is the calcium carbonate content or its accumulation index value. Test and comparative analysis indicate that the three equation (1),(2) and (4) of the above four equations fit best to the data of the study region, and their coefficients of correlation are 0.997, 0.998 and 0.992, respectively. These three quantitative relational expressions can be used tentatively in practical dating work of active faults.

CONSTRUCTION OF THE COMPUTER NETWORK IN INSTITUTE OF GEOLOGY SSB AND ITS APPLICATION TO RESEARCH OF EARTHQUAKE PREVENTION AND DISASTER REDUCTION

Liu Zhi, Liu Qiyuan, Zhang Peizhen, Dai Lisi, Wang Lihong, Gao Weian, Nie Gaozhong, Zhang Chongli, Zhu Yanming

1998, 20(1):  91-96. 

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