Table of Content

06 June 1984, Volume 6 Issue 2

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Brief Report

STUDY ON ELECTRIC CONDUCTIVITY IN THE DEEP EARTH FOR THE TANGSHAN AREA

Magnetotelluric Sounding Group

1984, 6(2):  1-5. 

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Measurements of magnetotelluric sounding indicate two highly conductive layers in the crust and upper mantle in the Tangshan area.They lie at a depth of about 20km and 100km,respectively.And they are found deep at southern section and shallow at northern section.The results show that both of the Tangshan earthquake and the Ninghe one occurred along a variable depth-gradient zone for the two layers,with their focal depths no more than those for highly conductive layers in the crust.It has been found in the south of the Tangshan area that a highly conductive layer may be conjectured to be at a depth of 45km,an intermediate one within upper mantle.

LOADING STRESS IN THE EARTH'S CRUST

Chi Shunliang

1984, 6(2):  6-8,62. 

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DOES“YANGTZE FAULT ZONE”EXIST ALONG THE YANGTZE VALLEY BENEATH THE ZHENJIANG AREA?

Li Qitong, Nan Jinshen, He Dongpei, Guo Jiangning

1984, 6(2):  9-16. 

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Some debets,about some structures in the Yangtze valley to the east of the Zhe-njiang area,have been often focused on these two problems: 1) does"Yangtze fault zone" exist along the Yangtze valley? and 2) is it a contemporary continental rift valley? By means of field investigations of the tectonics of the lower Yangtze valley and research of various materials,the authors have concluded the scarps along the Yangtze banks beneath the Zhenjiang area are not real and the large faults do not go through Yangtze valley,instead of being a syncline in the Yangtze valley along the Jiangyin-Shazhou-Jingjiang area.Besides,the physiographic history of the Yangtze delta and characteristics of the geophysical fields,especially gravity field and the characterristics of volcanic and seismic activities,all illustrate that "Yangtze fault zone" does not exist along Yangtze valley beneath the Zhenjiang area nor the contemporary continental rift valley.

A STUDT OF THE TECTONIC BACKGROUND FOR MODERATE OR STRONG EARTHQUAKES IN THE NORTHEN PART OF HENAN PROVINCE

Zhao Jingzhen, Liu Yongxuan, Zhang Xingrong

1984, 6(2):  17-24. 

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On the basis of data from the field invesigation alongwith geophysical exploration and focal mechanism solutions,the authors have illustrated the tectonic framework of the crust and the characteristics of the tectonic stress field that had remained in the northern part of recent Henan area since Late Tertiary.A model has been developed for this crust segment that has been in a process of being deformed and the model has been put to test in many a way.The potential risk regions have been discussed for moderate and strong shocks.

SITE-RELATED EFFECTS ON INTENSITY FOR LIYANG EARTHQUAKES

Sun Shoucheng

1984, 6(2):  25-32. 

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On the basis of comparision of the distribution of intensity for two earthquakes occur ring in the Liyang area with a magnitude of 5.5 (Feb.22,1974) and of 6.0 (Sept.9,1979),it has been found that the principal axis of their magistoseismic area is in a WNW-ESE direction and the shapes they take are not in agreement with the isoseismal lines.In this paper,the writer has suggested that the disagreement of this sort may be explained by the widespread distribution of the basement soil characteristic of muddy quality mild clay in the eastern part of the epicentral area.Four categories of the basement soil have been presented according to characterized soil.Quaternary sediments and underground water table and the like.The writer has attempted to investigate the distribution of intensity and to reconstruct the map for intensity distribution for the purpose of elimination of site-related effect on intensity distribution and of emphasis on underlying role of structures when putting various intensities that are situated in four categories of basement soil under the same classification (Ⅲ).It has been concluded that the distribution in point is strictly dependent on those faults where earthquakes may attend and that there is a tendency to the same direction shown by intensity isolines for highly harzard area and magistoseismic one.

ON THE SEISMOGEOLOGICAL SETTING AND CONDITIONS OF SEISMOGENIC STRUCTURES OF 1981 DAOFU EARTHQUAKE

Tang Rongchang, Qian Hong, Chang Wenfu, Chang Chengqui, Cao Yangguo, Liu Shengli

1984, 6(2):  33-40. 

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A M 6.9 earthquake of 1981 occurred in the Daofu region in western Sichuan Province,an epicetre being siruated about 2km southeast of Daofu with an intensity of 8.On the basis of field investigations,including structural conditions,recent crustal movement and tectonic stress fields of the seismic region alongwith ground fractures,isoseis-mals,ground motions,aftershocks,focal mechanism solutions and geodetic observations,the Authors incline to consider that the event in question is caused by sinistral motion of the Xianshuihe fault under the action of nearly EW-trending tectonic stress field.Also discussed in this paper are locking of the fault and migrating of earthquakes along the northern segment of the fault.

GEOLOGICAL EFFECTS OF THE MAGNITUDE 5 EARTHQUAKE IN PINGGUO

You Xiangzhao

1984, 6(2):  41-43. 

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Rolling of rocks and stones down the hillside and changes of groundwater were principal geological effects associated with magnitude 5 Pingguo shock of 1977,which made worse disaster to the seismic region.

OPHIOLITE SUITE WAS DISCOVERED IN TONTIAN RIVER QINGHAl PROVINCE

Pan Yusheng

1984, 6(2):  44-44. 

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DISCUSSION ON THE MECHANISM OF CONTINENTAL STRONG SEISMS IN CHINA:DIFFERENTIAL DEFORMATION INDUCED THE GENERATION OF STRONG SEISMS

Ying Sihuai, Lin Chuanyong, Yu Libao

1984, 6(2):  45-52. 

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Based on the study of the genetic features of mineral associations and metamorphic facies in Miyun metamorphic rocks,the characters of repeated-seismic layers have been proposed in this paper.It is suggested that the differential deformation has a close relation to the generation of strong seisms.Taking the principal elements of flow cleavage and fracture cleavage as well as their dynamic implications into account,a generation process of strong earthquakes has been proposed: the mechanism of differential deformation causes an initiation of strong earthquakes.Briefly speaking,distinct rock types in a dynamically forced system are deformed asynchronously,so there should be a mutually-restricted force in the system.When the increase of the force has attained the critical limit,the initial steady state of the system is not maintained,leading to a change of the distance in the relating moment of force.During this process,the potential energy is transformed into kinetic energy,forming a shear-compressive thrust that will cause an initiation of strong earthquakes.Therefore,the force of differential deformation is a driving force of strong-seismic event.This mechanism is proposed in a qualitative consideration without any scale restriction.It is also considered that the equilibrium of distinct energies has been involved in this process.Based on the proposed mechanism,the relationship between slow deformation and quick earthquake generation,seismostructures and petrology have been demonstrated in this paper as well.

SEISMOTECTONIC ANALYSIS OF THE GRAVITY ANOMALY GRADIENT BELT IN EASTERN CHINA

Li Anran, Cheng Fuyuan, Gu Chengzhi, Xu Yongjian

1984, 6(2):  53-62,8. 

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The gravity gradient belt studied in this paper is very distinct in Eastern China continent.The distribution of seismicity along the belt is clearly different in various sections.From researching the seismicity and the gravity field character in the belt,the paper analyses the tectophysical environment of strong earthquakes that occurred along the front edge of Taihangshan Mountain.The authors discuss the cause of the difference of the recent seismicity along the belt and the problem about the gravity field indicator in strong earthquake zone,and the like.Finally,the study stresses the point that the relationship between gravity field and seismicity is actually a problem of seismotectonic relationship.If the regional gravity anomaly appears in the mobile geotectonic region and major deep-seated rift in the uplifted edge of upper mantle,and if the regional gravity anomaly accompanies sharp change of deep-seated crustal boundary,geomorphology,crustal deformation and so on,it can be regarded as an indicator to distinguish continental strong earthquake zones.

SHEAR FRACTURE AND STICK-SLIP——A STUDY ON SHOCK-GENERATION MECHANISM OF STRONG SHALLOW EARTHQUAKES

Wang Shengzu, Zhang Liu

1984, 6(2):  63-73. 

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The triaxial compression experiments of dry Zoukoudian granodiorite at temperature up to 700℃ and confining pressure up to 7 kb with solid confining media,the microscopic observations to the tested specimens,and the theoretical analyses indicate that both shear fracture and frictional sliding have a similar pregnancy process and generation mechanism.The pregnancy process of major shear fracture in a specimen can be divided into three stages: (1) microfracturing with plastic deformation of soft mineral grains; (2) appearance and propagation of macroscopic shear fracture and formation of rock bridges between them; (3) failure of rock bridges and final occurance of the major shear fracture running obliquely through the specimen.Fracturing-through shear strength is a special kind of frictional strength,which characterizes the friction behavior of the nei sliding surface in the originally intact rock specimen.Both shear fracture and frictional sliding can be classified into two types: the gradual and sudden one.Sudden frictional sliding is the stick-slip along pre-existing fault while sudden shear fracture is the "first stick-slip" of intact rock.The confining pressure condition neccessary for the "first stick-slip" and the closely subsequent one is basically the same,which is about 5 kb or higher at the room temperature and increases with temperature.However,it is possible to induce stick-slip under confining pressure far less than 5 kb for pre-existing flat faults which have undergone a quite long sliding distance or a certain change in the orientation of stress field.The upper temperature limit of stick-slip is about 300-350℃ and the lower confining pressure limit of stick-slip is less than 1 kb for dry Zoukoudian granodiorite.In consideration of little possibility for strong earthquakes caused by shear fracture of intact rock owing to the increase of crustal temperature with depth,the principal shock-generation mode for most of strong shallow earthquakes may be the stick-slip along pre-existing faults.

NATIONAL SYMPOSIUM ON THE RELARIONSHIP BETWEEN ASTRONOMY GEOLOGY SEISMOLOGY AND METEOLOGY HELD IN BEIJING

Xu Daoyi

1984, (2):  74-74. 

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Brief Report

EXPERIMENTAL STUDIES ON FAILURE PROCESSES OF SAW-CUT ROCKS UNDER CONFINING PRESSURE

Li Jianguo, Wu Xiuquan, Zhang Botao, Liu Tianchang

1984, 6(2):  75-80. 

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According the viewpoint of the earthquake caused by the faulting,the failure of such saw-cut rock specimens as granodiorite,gabbro and mable was studied under room temperature and confining pressure,σ₂=σ₃= 0.6—1.2kb.During the increase of differential stress,shear strains and fractures started to occur at soft material filling tne saw-cut rock,then the tension crack appeared at the tip of the saw-cut and finally the main rupture,shear/tensile rupture occurred at the same tip nearly parallal to the sawcut strike.Extending tensional cracks were restricted by confining pressure.The distribution of cracks and mechanisms of main ruptrue were influenced by making of the sawcut and properties of rocks.Significance of these results was discussed in the preparation and prediction of the earthquake.