Table of Content

01 June 1983, Volume 5 Issue 2

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Brief Report

ANALYSES OF CONJUGATE EN-ECHELON VEIN ARRAYS

Song Honglin

1983, 5(2):  1-10. 

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Conjugate en-echelon vein arrays well-developed in the limestone of the Helvetic nappes of western Switzerland have been studied. They can be divided into two types: the primary and the surperimposed. The geometry of the veins and arrays have been described in detail. It may be concluded that the en-echelon veins discussed are synte-ctonic extension veins which were associated with the second order stress field due to the motion of the shear zone. The angle between the veins and array containing them and the conjugate angle between two shear zones have been measured. It is suggested that the former,varying from 20皌o 45?,depends on the normal stress acting on the shear zone. It is considered that the en-echelon veins were formed under the transitional condition from brittle fracture to ductile flow of the rocks. The generation of veins has been connected with the developmental history of the nappes.

THE BASEMENT DUCTILE SHEAR ZONE OF THE MIDDLE YIHE-SHUHE FAULT BOET

Zhang Jiasheng

1983, 5(2):  11-23. 

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The Yi-She fault system is located in Shandong province and lies on the middle section of the Tancheng-Lujiang fault zone. The ductile shear zone found on the crystalline basement of the middle Yi-Shu fault system provides evidence for its formation in Presinian period Ductile shear zones have a NE or NNE trending and show a pattern of sinistral movement that turned Nw trending gneissosity in the west of the fault into NNE one. This great ductile shear deformation zone is about 20 km. in width. The distribution of the fault system may be controlled by the major shearing zone.The east part of the zone consists of the ductile shear zones in the lack of mylonite and original gneiss remnants,which suggests that it may result from gneissosity under the shearing at a relatively high temperature. However,the west part contains the shear zonet with various kinds of mylonite. They cut through and made original gneissosity move in such a way as sinistral movement,which reflects the result of displacesment under the condition of lower greenschist facies metamorphism.In this paper,the texture,deformation rocks and strain measurements of the shear zones with the mylonite are discussed. The deformation behavour of minerals in the matrix and mortar of mylonite,the fabric pattern and the preferred orientation of c-axis of the plastically deformed and syntectonic recrystallized quarz grains,and their evolution with progressive increase of the shear strain are described. The relationship of grain size and variation in content of feldspar with increasing strain in felsic mylonite is analysed. It is inferred that mylonite was formed at the crustal depth of 10—15 km. and had undergone a long time shearing at a strain rate of about 10^-14/sec. and a stress of about 580—745 bars.Finally,the ideal model of the major fault zone and the synthetical classification of fault rocks are discussed.

HAPPY NEWS FOR EARLY SPRING——CONGRATULATION ON THE FOUNDING OF SUBORDINATE GEOLOGICAL SOCIETY OF BEIJING GEOLOGICAL SOCIETY IN THE INSTITUTE OF GEOLOGY STATE SEISMOLOGICAL BUREAU

Wang Xiaoyuan

1983, 5(2):  24-24. 

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CHARACTERISTICS OF SOME PYROXENES OF GRANULITE FACES IN MIYUN REGION OF BEIJING

Yu Libao

1983, 5(2):  25-30. 

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The orthopyroxenes of granulite facies in Miyun region,Beijing,are hypersthenes,the end members of which are Wo 4% En 52—59%,Fs 37—44%,and ferrous index 38.8—45.5%; while the clinopyroxenes in pyroxene granulite and hornblende granulit ubfacies are augites with end members of Wo 41—43%,En 39—43%,Fs 14—20%,and ferrous index 30.8—34.4%.According to Saxana's two pyroxenes geothermometer method,it has been calculated that metamorphism temperature in this region is 702—887℃,while the pressure 9—10Kb.

DISCUSSION ON CONCENTRATION AND DIFFUSION OF THE REGIONAL SEISMICITY BEFORE AND AFTER SOME GREAT EARTHQUAKES

Fu Zhengxiang

1983, 5(2):  31-38. 

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This paper is concerned with the process of concentration and diffusion of the regional seismicity before and after eight great earthquakes (M_s=7.0),Xingtai,Bohai sea,Tonghai,Luhuo,Yongshan,Haicheng,Longling and Tangshan,which took place in China during 1966—1976. The process might be divided into three stages: (1) seismicity was distributed mainly in the far-source area; (2) then it migrated and concentrated in the focal region and near-source area; (3) after the occurrence of great earthquake,seismicity diffused toward the far-source area gradually Finally,Rice and Rudni-cki's inclusion model of earthquake precursor and the idea of stress adjustment are used to understand the above process.

TIME-SPACE DISTRIBUTION OF MAXIMUM HORIZONTAL SHEARING STRAIN IN SOILS BEFORE THE 1976 TANGSHAN EARTHQUAKE

An Ou, Huang Shi-bin, Wang Wen-qing, Jiang Li-fang

1983, 5(2):  39-51. 

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The results observed network by of strainmeters in superficial soils show that the maximum horizontal shearing strain in soils had produced abnormal variation before the 1976 Tangshan earthquake and the principal earthquake would have happened after this abnormal variation. The contour lines of equal duration of maximum horizontal shearing strain abnormaly in the soils propagated outward in all directions from the epicenter of the earthquake at a velocity of 0.1—6.6 km per day. This velocity of propagation increases with distance from the epicenter and the epicenter is enclosed by the maximum contour line or situated near it.The epicentral distance of an earthquake can be determined from the abnormal duration curves recorded at the stations as well as the epicentral distance-abnormal duration curves which correspond to the earthquakes of same magnitude. To illustratethis,three or more circles are drawn. Each station is the center of a circle,and the radii are equal to their relative distances to the epicenter. These circles then intersect at the epicentral area.

THE GEOMAGNETIC ANOMALY AND THE CHARACTERISTICS OF THE RUPTURE PROCESS IN THE FAULT PLANE DURING THE LIYANG EARTHQUAKE (M^s=6.0)

He Churu, Peng Chunyi, Zhang Siwei, Xu Guoming, Guo Juanxiu, Zhang Zhitian, Lu Zhenfei

1983, 5(2):  52-58. 

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Before the 9th July 1979 Liyang earthquake,beginning from mid June the vertical component at the Liyang station about 20km from the epicentral area decreased gradually to a negative anomaly. After decreasing to the lowest level in the last ten days of June,it increased quickly to a positive anomaly. Just before the tectonic event,the observational value was brought back to its initial value. It is consigderd that the seismic fault perhaps responsible for the shock trends WNW or NNE. The shear stress piezomagnetic modol by Stacey has been used to calculate the theoretical values of the surface Z-component anomaly for this skock.A comparision of observations with the theoretical values shows that there have bsen the strong tectonic activities along both the faults. The shock was probably accompanied by conjugated offset on the faults.

THE EFFECT OF INFILTRATING OF WATER AND MECHANISM AND CHARACTERISTICS OF EARTHQUAKES IN XINFENGJIANG RESERVOIR

Zang Shaoxian

1983, 5(2):  59-69. 

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According to the effect of water infiltrating in porous rocks and the generalized classification of earthquake by Bell and Nur and Mogi,it is suggested that the process of infiltrating of water may be responsible for the unstable and inhomogeneous distributioneh of the effective stress over the reservoir area. The characteristics of the earthquake sequence in the reservoir are related to this kind of stress distribution. The effect of infiltrating water on the main shock is discussed. The permeability can be found to be 10^-4 darcy. It is suggested that the tectonic stress in the reservoir area is not in the critical state; water in the reservoir area collected along with underground water to form a new water system; and then water widely infiltrated into deeper parts with the pore pressure increasing and the tectonic stress concentrating to cause the main shock. On the basis of the Coulomb criterion,the transformation of the stress field is studied. The transformation of the stress field after the main shock may account for different stress state of earthquake sources between the foreshock and the aftershock.

RESEARCH ON THE EXTENDING TO THE EAST OF THE YANGTZE FAULT ZONE AND ITS CHARACTERISTICS OF RIFTING

Qin Dazheng, Liu Changsen, Ding Songhua, He Yuebao

1983, 5(2):  70-78. 

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According to a large amount of data,the authors have demonstrated the extending to the east of the Yangtze fault zone and indicated that its continuation passes through Jiangyin,Shazhou,Jingjiang and Nanton of Jiangsu province and sea waters of southern Yellow Sea. Nature of the Yangtze fault zone is also discussed. The authors consider that the Yangtze fault zone has been a great modern continental rift zone since the Late Mesozoic,1500 km long and 40—80km wide (the rift valley basin is 10—25km wide). The zone here called Yangtze rift valley zone is typical of features of rift valley; for example,the depth of the rift zone is an elevated one of the mantle current zone,and the like. This,therefore,may serve as a mirror of the modern continental rift zone in the world.

MODERN TECTONIC STRESS FIELD AND RELATIVE MOTION OF INTRAPLATE BLOCK IN SOUTHWESTERN CHINA

Kan Rongju, Wang Shaojin, Huang Kung, Sung Wen

1983, (2):  79-90. 

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The P-wave first motion solutions are given in this paper for 70 shocks of magnitude equal to and greater than 4 which occurred in Southwestern China and adjacent areas during 1973—1979. The relative motion of intraplate blocks including the Yun-nan-Xikang rhombic block,Sichuan-Qinghai block,and the Southwestern Yunnan block is studied on the basis of the focal mechanism,seismic fissures,horizontal deformation in the seismic areas and the trend of variations of the length of baselines across major faults.The direction of the lateral motion of crustal material of three blocks in question is SE-SSE.The Yunnan-Xikang rhombic block can be devided into several sub-blocks and their relative motion is also discussed in this paper.