Table of Content

05 September 1979, Volume 1 Issue 3

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Brief Report

INTRAPLATE EARTHQUAKE AND MESO-CENOZOIC STRESS FIELD IN CHINA

Wang Yipeng

1979, 1(3):  1-11. 

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Except for earthquakes occurred in Taiwan and Himalayan region as well as the deep-seated swarm earthquakes in Northeast China, all of earthquakes in China are attributed to intraplate earthquake. Then, what are their origins? In the present paper, the emphasis is placed on the crustal stress field related closely with the earthquakes in recent and the youngest geological time in China. Data from geology, seismology, and geodesy demonstrate that in North China there is a significant change of stress field in middle Tertiary, while in the rest areas of China the crustal stress field remains principally unchanged since middle Mesozoic. It is suggested that the pattern and variation of the crustal stress field in China are caused by the action of plates surrounding the mainland of China.However, intraplate earthquakes in China are not only connected with the plate tectonics, they also depend upon the remarkable features of fault block tectonics in the mainland of China. It is noted that some deep geological features found in the seismic areas are a very important clue for studying the origins of intraplate earthquake.

RELATIONSHIP BETWEEN INTRAPLATE EARTHQUAKES AND INTERPLATE EARTHQUAKES IN CHINA AND ADJACENT REGIONS

Zhang Zhi-meng

1979, 1(3):  12-23. 

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The intraplate earthquakes and the interplate earthquakes are quite different in depth of seismic foci, mechanism and other characteristics. From this consideration, China and its adjacent regions may be divided into the Marginal Pacific Seismic Domain and the Tethys-Himalayan Seismic Domain, both of them subdivided into inner belt and outer belt respectively. The two domains act upon each other to make up an "intervening belt", i. e. the North-South Seismic Belt.Basing upon the statistical analyses of accumulative energy released from strong earthquakes (M≥6-7) in China and the adjacent regions during 1904-1977, it is demonstrated that seismic activity of inner belt is in its higher level when seismic activity of outer belt is in its lower level and vice versa. This kind of phenomena can also be referred as "shielding" of seismic belt. The function of shielding may be better or worse in different sections of inner seismic belt to permit seismic activity weaker or stronger in the corresponding sections of outer seismic belt. For example, shielding in Nakha-Arakan Yoma-Andaman is better than in the Himalayas, so that seismic activity is weaker in the western Yunnan and the Indochinese Peninsula than in the Qinghai-Xizang Plateau. Likewise, Taiwan, Northeast Japan and Chishima shield better than Ryukyu and Southwest Japan do, therefore seismic activity is weaker in South China and Northeast than in North China.Intraplate earthquakes may primarily be attributed to strengthening horizontal stress resulting from the interaction between plates on the margins. The other causes such as ascenting plume, mantle uplifting, earth-tide,Chandler wobble and variation of rotation velocity of the earth might be of the secondary importance.

DYNAMICAL MODEL OF THE STRESS ADJUSTMENT OF THE CONTINENTAL EARTHQUAKES IN CHINA

Liu Bochi

1979, 1(3):  24-40. 

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Basing on the following aspects: 1) the correlation of the macroreiiei and distribution characteristics of the great continental earthquakes in china; 2) the fault plane solution of 127 events; 3) data on 18 great earthquakes of fractured zone; 4) the rate of slip motion along the earthquake fault zones, a dynamical cause of the great continental earthquakes in China was studied, a dynamical model of their stress field adjustment has been presented and is also used to explain the continental seismicity of china.

THE GREAT EARTHQUAKE AND THE CONJUGATE TECTONIC MOTION

Zhang Sichang

1979, 1(3):  41-46. 

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Taking six great earthquakes occurred recently in China as examples a kind of rupture mode of great continental earthquake with shallow focus is discussed. By analysing the data from ground rupture zone, the solution of the P wave initial motion, the direction of mojor axes of the isoseismal lines, the time-space distribution of aftershocks and so on, the great earthquake sequences show a characteristic of 2-way rupture with conjugate nature.The development of two conjugate rupture plane shows itself always asymmetric nature depending on the tectonic feature in a certain seismic area. It is suggested that the seismic activities before some great major shocks are distributed along two nodal plane of P wave initial motion. The study of geological feature of the conjugate structures would be significant to predict major shock and strong aftershock.

PRELIMINARY STUDY ON THE EVOLUTION OF STRAIN FIELD NEAR LOCKED REGION OF A FAULT

Ma Jin, Zhang Botao, Yi Shurong

1979, 1(3):  47-55. 

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Combining the physical and mathematical simulations the strain field and its evolution near locked region are studied. The characteristics of pre-seismic strain fields and displacement fields around the bodies of seismic source with different shapes were discussed, and the strain changes before, just before, during and after earthquakes were also analyzed. Taking notice of the difference around an epicenter from its surrounding permits us preliminarily to explain the strain process of each point relevant to the tectonic conditions, fracturing process and its position in the stress field.

THE ESSENCE OF THE PLATE-TECTONICS THEORY AND ITS QUESTIONABLE POINTS

Ma Zongjin

1979, 1(3):  56-56. 

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ON THE RECENT TECTONIC MOVEMENT AND STRAIN FIELD IN BEIJING PLAIN AREA BY THE GEODETIC DATA

Zhang Cunde, Xiang Jiacui

1979, 1(3):  57-67. 

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In Beijing plain area the tectonic settings during the Tertiary period having an upheaval in its northwestern part, a depression in its central part and Dai Xing upheaval suffered remarkable changes in the course of recent tectonic movement. From the repeated levelling data it is suggested that along the NW trending Nankou-Sunhe fault was generated a subsidence zone cutting across the NE trending Tertiary structure and destroying the original tectonic framework.In connection with this the recent strain field also shows an obvious change. From the deformation tendency plot compiled on the basis of repeated levelling data the long axis of contour lines takes the direction from nearly east-west to north-west-west reflecting a tensile direction on the earth’s surface. Further triangulation surveys indicate that the tensile direction is south-east, the compression direction is north-east in the Beijing plain area. The tensile strain amounts to 2.5/10^-6, while the compression strain is 0.8/10^-6, much lesser than the former.Some faults in this area have showed anomalous activities before the Tangshan earthquake. During the period of 1973-1976, six faults have a tendency of increasing or reversing activities, but in 1976, four of them rebounded to the same situation as in 1973. It is preliminarily belived that the activities of these faults may be related to the 1976's Tangshan earthquake.

THE QUATERNARY TECTONIC STRESS STATES OVER THE UP-STREAM AREA OF MIN JIANG RIVER IN SICHUAN AND ITS RELATIONS TO EARTHQUAKES

Yang Jingchun, Deng Tiangang, Wan Yuanhai, Wen Xueze

1979, 1(3):  68-75. 

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The Quaternary tectonic stress states over the mentioned area and its relations to earthquakes are studied by combining the methods of geology, geomorphology. seismology and repeated levelling. Under the action of NEE-SWW trending horizontal principal compressional stress during the Quaternary period, the NW trending structure of this area was intensively upwarded and the EW trending faults suffered a tension-shear displacement leading to release the energy and to cause a series of smaller earthquakes, while the NS trending faults were characteristics of compression-shear displacement leading to release less energy and to cause scarcely minor earthquakes. At the intersection of the two faults mentioned above where the differential tectonic movement reached the maximum, has occurred an earthquake of 6 3/4 magnitude.

PALEOMAGNETIC STUDIES FOR SOME QUATERNARY SECTIONS AND DISCUSSION ON THE TWO BOUNDARIES OF N₂-Q₁ and Q₁-Q₂ IN CHINA

Cheng Guoliang, Li Suling, Lin Jinlu

1979, 1(3):  76-83. 

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It has been known that the geomagnetic field reversed several times during the past 4.5m.y. and so it is possible to correlate some Quaternary formations basing upon the geomagnetic polarity time scale. It may then be considered as a new approach to the Quaternary stratigraphic study.In this paper the basic theory, sampling and experimental procedures of paleomag-netism in the Quaternary stratigraphic research have also been introduced on the results of 11 sections and 5 cores during the last years in China. It is tentatively proposed that the boundary of N₂-Q₁ in China has an age of about 3m.y. while the boundary of Q₁-Q₂ gives age of 0.69m.y..

CHINESE GEOLOGO-SEISMOGEOLOGICAL SCIENTIFIC INVESTIGATION GROUP VISITING IN PAKISTAN IN 1978

1979, 1(3):  84-84. 

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THE TAISHAN METAMORPHIC COMPLEX

1979, (3):  84-84. 

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