Table of Content

05 September 1984, Volume 6 Issue 3

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Brief Report

MICROSTRUCTURES OF FAULT ROCKS IN BEISHICHENG AND HEFANGKOU FAULT ZONES,BEIJING

Shi Lanbin, Chen Xiaode

1984, 6(3):  1-13. 

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The Beishicheng and Hefangkou fault zones in Miyun and Huairou area have been studied.A mylonite belt tens to hundreds meters in width was developed on the foot wall of the former fault and a fracture belt 300—500 meters in width on the hanging wall.A mylonitized granite belt occurred on the foot wall of the latter fault and cataclasites or cataclastic granite on the hanging wall.Microscopically quartz and mica ,within mylonites exhibit their features of strong plastic deformation indicated by apparent flow structure and grain size reduction due to dynamic recrystallization.All minerals in the cataclastic rocks,however,display brittle behaviour.It is clear that the two kinds of rocks were formed under different physical conditions.Based on the distribution pattern of the fault rocks and dynamic analysis of twin lamellae of dolomite,it can be inferred that there were two phases during the developement of the fault zones: a ductile shearing occurred at the deeper level of the crust leading to the formation of mylonite belts,and then the mylonite belts gradually were uplifted along the south-eastern sides of the fault zones onto the higher level.According to the mineralogical assemblage and the characteristic of the mylonites,it is deduced that the ductile shearing occurred at temperature of 450—500°,and confining pressure of about 4 kb,corresponding to the depth of 15 km.A differential stress value of 0.5—1 kb and a strain rate of 1.6×10^-9—10^-10 sec^-1 have been estimated from rescrytallized grain size,subgrain size,dislocation density and the spacing of deformation lamellae of quartz revealed in chemically etched mylonite samples.Finally,the possible relationship between ductile shear belt,the elastic-plastic transition zone within the crust and the layer feasible to shake in continent has been discussed.

A BRIEF REPORT ON PROFILES OF THE HAIYUAN PALEOEARTHQUAKE

Zhang Weigi, Jiao Decheng

1984, 6(3):  14-14. 

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AN ANALYSIS ON THE GEOTECTONIC BACKGROUND OF VERTICAL DEFORMATION IN THE JIANSU-SHANDONG-ANHUI-HENAN REGION

Gui Kunchang, Shen Yongjian

1984, 6(3):  15-21. 

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In this paper,a preliminary analysis on the geotectonic background of land deformation has been made using vertical-deformation contour map based on precice levelling that was carried out in 1953—1955 and 1976—1980 in the study region.It is suggested that there are two abnormal areas of deformation,one along the Tancheng-Lujiang fault zone ranging from Linyi to Jiashan; the other at the joint between Shandong and Henan Provinces.For this study region,the deformation is mainly controlled by the faults trending WNW,NW and NE.And recent tectonic movement for the fault zone is of distinct segmentation.

AN APPARATUS FOR ROCK CREEP TESTING UNDER UNIAXIAL COMPRESSION AND HIGH TEMPERATURE

Shi Liangqi, Chang Xiulan, Zhao Axing

1984, 6(3):  22-22. 

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CHARACTERISTICS OF THE EARTHQUAKE-GENERATING STRUCTURES FOR MAGNITUDE 8.0 PINGLUO EARTHQUAKE OF 1739 AND THE PROCESS OF ITS PREPARATION

Li Mengluan, Wan Zicheng

1984, 6(3):  23-28. 

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Following the discussion of regional-geological characteristics of the northern part of Ningsia,it is indicated that the earthquake-generating structures for the Pingluo quake (M=8.0) of 1739 is held to be NNE-striking Huanghe River fault lying at the Yinchuang-Pingluo area.This paper is an analysis of the characterized preparation the shock in terms of a generalized focal model and an elucidation of the spatial position and characteristics for the structural elements.

SMALL-SCALE MANTLE CONVECTION,SUBCRUSTAL STRESS FIELD AND SEISMICITY IN THE NORTH CHINA REGION

Zhu Yueqing, Qiang Zuji, Ma Li

1984, 6(3):  29-37. 

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In this paper the geopotential harmonic coeffients derived from satellite tracking by Wagner are studied and divided into two parts: one with n≤11,correlated with largescale mantle convection; the other with 12≤n≤25,correlated with small-scale convection.The subcrustal stress field caused by mantle convection under the study region has been obtained,which is a regular symmetrical compressional stress field,with a compressive field central at.the Linfen-Hondon area (111.5°E; 36°N).The magnitude of stress for this region is about 0.2—0.75×10⁸dyn·cm^-2.The pattern of the epicentral distribution for major shocks and the focal mechanism solutions are almost similar to those of the subcrustal stress field,especially for Tangshan and Haicheng earthquakes.It is concluded that the subcrustal stress field caused by small-scale mantle convection controls seismic activities,even those strong quakes that occurred in the study region at high tide,recently.

AN ALTERNITIVE CURRENT DEMAGNETIZER

1984, 6(3):  38-38. 

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EXPERIMENTLAL STUDIES ON THE RELATIONSHIP BETWEEN STRESS AND STRAIN OF ROCK MASS MODELS UNDER CONFINING PRESSURE

Zhang Botao, Li jianguo, Liu Tianchang, Wu Xiuquan

1984, 6(3):  39-48. 

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The characteristics of three typical models of rockmass have been studied under room temperature and confining pressure.The results suggest that the stress-strain curves can be divided,into four stages by the behaviours of linear and nonlinear elasticity.The deformation mechanisms are correspondent to the closing of preexisting cracks,elastic deformatiom of grains and pores,macroscope tension (tensile region),shear (compressive region) and main shearing or shearing-tension ruptures,respectively.The resulting processes of these curves stages in longitudinal and transverse direction near the tip of sawcut and in the region of homogeneous stress state were different in time.The fracture propogation related to the change of stress-strain stage was not considered in time and space.The characteristics of stress-strain curves and distributions and sequences of fracture propagation were controlled by structure features of models of sawcut rockmass.Finally the relationship between the developing stages of stress-strain in some points along the seismogenic fault and the preparation of earthquakes,has been discussed.

CORRELATION BETWEEN THE STRUCTURES OF CRUST AND UPPER MANTLE AND THE THERMAL EVENTS

Zhao Guoze, Zhao Yonggui

1984, 6(3):  49-58. 

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This paper deals with instable temperature field caused by thermal events and demonstrates that abnormal temperature distributions and heat flow values depend on the event parameters such as intensity,life-span and time it occurred.Comparing some theoretical results with observation data,it is found that the passive block and the active region are different in thermal processes and that the differential depth of low-velocity layers/low-resistivity layer between the crust and upper mantle is correlative with thermal-activation processes.The differential depth between the two layers ranges from 40km to 60km for the North China plain,suggesting a therml event of about 50—70m.y., and a duration of about 30—40m.y.,while for the Yin Chuan basin,it ranges from 36km to 40km,suggesting a thermal event of 30—40 m.y..Generally,the effects of thermal events which occurred more than 100m.y.ago can be neglected and the temperature field can be regarded as the stable.The method based on stable temperature field to determine structures of crust and upper mantle is only suitable to passive blocks.

GEOLOGICAL STRUCTURES AND THEIR ACTIVITY IN THE HANDAN AND TANGYIN GRABENS

Yang Chengxian

1984, 6(3):  59-66. 

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On the basis of seismic measurements and drilling data,the Handan and Tangyin faulted basin is of NNE direction on the eastern piedmont of Taihang Mt.The Handan graben was formed in Eogene,predominantly controlled by the listric normal fault along the eastern margin of the basin,while the Tangyin graben was formed in Tertiary,its northern part being controlled by the listric normal fault along the eastern margin of the basin.In the southern Tangyin basin lie the two listric normal faults on its eastern/western flank,alternatively characteristic of an intense tectonic activities in the context of geology.These three faults are of the Pre-Tertiary basement at a depth of 4000—5000 meters or more and are the continuations of the preexisting fractures.The two basins in question have their shallow hypocentral regions where small quakes have often attended in the recent past.No seismic activity has been found along the eastern Handan fault at present and one for the Handan faulted basin has been relatively in quiescence.The reverse case can be found along both the eastern fault and western fault in the Tangyin faulted basin,with the seismic activity different in the southern part and northern part of the basin.Great attention should be paid to the southern segment of the basin for its frequent activities.

DISTRIBUTION OF FOCAL DEPTHS FOR INTRAPLATE EARTHQUAKES IN NORTH CHINA AND ITS PHYSICAL EXPLANATION

Zang Shaoxian, Yang Junliang

1984, 6(3):  67-76. 

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Distributions of the focal depths in the Peking area and such provinces as Shanxi,Shandong,Lingxia and Yunnan for 1976 Tangshan,1975 Haicheng,1970 Tonghai,1966 Xingtai and 1976 Longling earthquake sequence were studied using more accurate earthquake data.The results show that the foci scatter within a range of depths from 4 km to 30 km which is called an active layer of the earthquake and are concentrated mainly in a layer of thickness of 10 km—15 km which is called a dense layer of the earthquake.For various region,different are the depths at which the seismically active dense layer remains A comparison between distributions of foci and crustal structures was made,showing that the foci are mainly located in upper part of the crust.The characteristics of focal distributions are determined by the depth at which a transition of brittle shearing fracture to ductile deformation occur.The differnce of the temperature and pressure as well as the type of rocks may account for that of depth foci between the dense layers for differsnt areas.It appears that some foci can be found to be contoured,represented by some lines on a section of crustal structures among which is C discontinuity,as a result that a part of a discontinuity serves as a surface along which stick slip may appear.The earthquake tend to attend those areas where the thickness of the crust changes sharply for the reason that there have been a lot of preexisting faults and concentrations of stresses in the gradient belt in quenstion,which would have made a favourable condition for occurring of earthquakes.

THE RECURRENCE INTERVALS OF THE STRONG EARTHQUAKES IN TANGSHAN

Wang Tingmei, Li Jianping

1984, 6(3):  77-83. 

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The traces of two paleo-seismological events,preserved in late Pleistocene sediments at a depth of 2m,were excaved by trenching technique at several sites along the surface fissure zone which was formed during 1976 Tangshan strong earthquake.The geological observation around the suberb of Tangshan city also found some similar evidences.By comparing and analysing surface fissure,seismic faults and liquification scale,seismic intensity for that event was estimated to more or less equal that for Tangshan quake.Based on the 14C dating and computation of sedimentory rate,two events may be estimated to have occurred 7665?105 years and 14865 years ago,respectively,suggesting a recurrence interval of about 7500 years for strong earthquakes in the study region.

ABOUT "PLATEAU-TYPE VOLCANOES" IN 《THE ALBUM OF TYPICAL SATELLITE IMAGERY OF ACTIVE STRUCTURES OF CHINA》

Han Tonglin

1984, 6(3):  84-84. 

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