Table of Content

04 March 1980, Volume 2 Issue 1

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Brief Report

WARM CONGRATULATIONS UPON THE FOUNDATION OF SEISMOLOGICAL SOCIETY OF CHINA

Board of Editors

1980, 2(1):  1-2. 

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THREE-DIMENSIONAL’FINITE ELEMENT ANALYSIS OF RELATIONSHIP BETWEEN STRESS STATE OF A ROCK MASS AND DRIVING FORCE

Loo Huanyen, Gao Weian, Wang Qiming

1980, 2(1):  3-10. 

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The deformation and failure of rock masses are in general governed by the preexisting discontinuities and the local fissures induced by the state of stress, because these weak planes increase the deformability and reduce the strength of the rock mass. The mechanical properties of a rock system are thus determined by its interior structure and texture, as well as by the boundary conditions, such as the loading path, the constrained displacement and the geometry of the system.This paper describes a three-dimensional finite element model of the rock mass, where a fault underlain closely with or without a solf layer is dipping an angle of 60°or 90°between the fault plane and the horizontal axis. The external forces different in magnitude and direction are imposed at the boundaries, the effect of the intermediate principal stress, σ₂, on the concentrated stress level at the fault tip is studied as well.Numerical results similar to that of both the field and the laboratory tests confirm that present model provides physically measurful simulation and thus can be used to study the stress distribution of a geological body under different boundary conditions and thus to evaluate the geological background causing a huge earthquake.

EXPERIMENTAL STUDIES ON THE PROCESSES OF DEFORMATION AND FAILURE OF A TYPICAL ROCK MASS AND THEIR RELATIONSHIP TO BOTH THE SEISMIC PRECURSORS AND THE FORESHOCKS

1980, 2(1):  11-17. 

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This paper deals especially with the propagation of a 2mm thick saw-cut inclined 45? to the axis of the rock specimen of 5?5?12.5cm and filled with epoxy so as to simulate the development of an earthquake of continental interior. The specimen was loaded in uniaxial compression and attached with a measuring system consisting of strain gages, acoustic emission and wave travelling.Characteristics of failure modes observed in this experimental studies were as follows:1. Creep along the fault face occurred at a very low compressive load without any foreshocks, of course, it would generate precursors such as groundsurface displacements and groundwater level variations.2. As the load increased, a tearing-off crack started to develop at the existing fault tip nearly perpendicular to the fault plane, and appeared to arise initially from the creep process. The propagation of this primary crack was stable and its path was curvilinear. After a considerable primary crack propagation, a second tension crack developed again at the same tip and extended downward to the edge of the specimen, but without any stress drop before overall rupture. During cracking, both the number of acoustic emissions and the wave travelling time increased as time going on. The former phenomenon is equivalent to and the latter is caused by the foreshocks.3. As the compressive load reached a critical level, shear rupture, behaving like mainshock, took place suddenly at the fault end" zone under intensively compressive stress concentration and directed relatively to the tension cracks with angles of varing about from >30? to<90?. Obviously, this was due to the distribution of both the unadmis-sible shear stress from the sliding plane and the tensile stress from the cracked zone to the neighbouring region capable of sustaining an increase of stress.Finally, our results in applying to the prediction of continental earthquakes suggest that large shock should be preceeded first by a fault creep and then by a number of small shocks in tensile cracked regions trending in different directions from the main shear failure plane. In other words, the processes of deformation and fracture of a rock mass would vary in time and in space, which may be used as a guide how to estimate as well as possible the relative locations of precursors and foreshocks to the mainshocks.

NEWLY OBSERVED DATA IN OPHIOLITIC ZONE ALONG YARLUNG ZANGBO RIVER, CHINA

Chang Chengfa

1980, 2(1):  18-18. 

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THE IMPLICATION OF CONJUGATE FAULTING IN THE EARTHQUAKE BREWING AND ORIGINATING PROCESS

L.M. Jones, Deng Qidong, Jiangpy

1980, 2(1):  19-26. 

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The earthquake sequence, precursory and geologo-structural background of Haicheng, Tangshan, Songpan-Pingwu earthquakes are discussed in this, article. As we know that all these earthquakes occurred in a seismic zone controlled by the main boundary faults of a intraplate fault block. But the fault plane of a main earthquake is not the same faults rather a related secondary fault. They formed altogether a conjugate shearing rupture zone under the action of regional tectonic stress field. As to the earthquake sequence the foreshocks and aftershocks might be occurred on the conjugate fault planes within an epicentral region rather than limited in the fault plane of a main earthquake, such as the distribution of foreshocks and aftershocks of Haicheng earthquake. The characteristics of the long-, medium- and imminent term earthquake precursory anomalies of the three mentioned earthquakes, especially the character of well-studies anomaly phenomena in the electrical resistivity, radon emission, groundwater and animal behavior, have been investigated. The studies of these earthquake precursors show that they were distributed in an area rather extensive than the epicentral region. Some fault zones in the conjugate fault network appeared usually as distributed belts or concentrated zones of earthquake precursory anomalies, they can be traced in the medium-long term precursory field, but seem more distinct in the short-imminent term precursory anomalous field. These characteristics can be explained by the rupture and sliding originated along the conjugate shear network and the concentration of stress in regional stress field. Moreover, it is worthy to notice the migration of these precursory anomalies along the conjugate rupture zone in the earthquake-brewing process. The macroscopic precursory anomalies of the Songpan-Pingwu and Haicheng earthquakes did not appear first in the epicentral region, but emerged in a set of fault zone conjugated with the fault plane of main earthquake, or in a conjugate fault zone and their intersection region. These phenomena may be originated from the migration of stress-concentrated region and continuous concentration of stress in the earthquake source area. It is also possible that they were progressively initiated at a definite direction along the rupture zone and sliding caused by conjugate faulting.It can be supposed that the effect of conjugate fault on the earthquake brewing and originating process is significant in theoretical studies of seismology and in predicting practice, it should be drawn public attention.

THE CHARACTERISTICS OF INTRAPLATE EARTHQUAKE FAULTS IN NORTH CHINA AND THEIR RELATIONSHIP TO THE DYNAMICAL PROCESSES IN EARTH’S CRUST AND UPPERMOST MANTLE

Ye Hong, Zhang Wenyu

1980, 2(1):  27-38. 

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The mechanical nature of 121 earthquake faults, the strikes of 40 events and the directions of fracture propagation of 15 events are discussed in this paper.On this background,some relationships between the strikes of intraplate earthquake faults in North China and the distributions of uplifted zones of uppermost mantle have been found, e. g. within these zones the earthquake faults run nearly parallel tp their elongation, while on their opposing sides the earthquake faults are mainly perpendicular to them. However, it seems to be that the mechanical nature and the direction of fracture propagation are not controlled by the pattern of rising of uppermost mantle, but coincide with the general characteristic feature in the eastern part of China continent.The authors hold that there exist two types of geodynamical processes in North China, the one is, the North China fault blocks are subjected to a compression trending NEE-SWW due to the eastward movement of the Xizang (Tibet) block, the other is, the diapirism of the upper most mantle and the gravity isostasy make the continental crust of North China thinning and its certain parts having lense-shape. The characteristics mentioned above and the cause of the high seismicity in North China could be explained by the superimposition of these two dynamical processes.

MULTIPOINTS (CONCENTRATED) STRESS FIELD AND EARTHQUAKES IN NORTH CHINA CRUST

Ma Zhongjin

1980, 2(1):  39-47. 

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It is assumed that the continental crust, such as North China crust behaved as a elastic plate cutted by faults with various trends. Due to the complexity of crustal structure in the course of crustal movement would be formed a lot of stress-concentrated points where are clustered the most precursor phenomena. Some of those points would become sources of future earthquakes. In this paper an attempt is made to explain two kinds of phenomena by concept of multipoints stress field, that is, 1) there exist an isolated precursory-concentrated area long before the earthquake occurrence and 2) the strong earthquakes take place continuously and frequently in a short period, for instance the series of strong earthquakes in North China.

ABOUT THE DISCOVERY OF OPHIOLITE-MIGMATITE IN THE BAINANG COUNTY, XIZANG

Pan Yusheng

1980, 2(1):  48-48. 

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ON THE ORIGIN AND DEVELOPMENT OF MIGMATIZATION

Ying Sihuai, Yu Libao, Yang Zhuen

1980, 2(1):  49-63. 

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Basing upon the petrological analysis under a microscope, the thermodynamic method and the data of anatexis, some mineral reaction formulae and their paragenesis assemblages are presented in this paper.

ROCK ACOUSTIC EMISSION METER SF-1 MODEL

Chu Baogui

1980, 2(1):  64-64. 

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THE CHARACTERISTICS OF FORERUNNER FIELD OF UNDERGROUND WATER REGIME DURING TANGSHAN EARTHQUAKE AND ITS FOCAL EVOLUTION PROCESS

Wu Jinxiu, Wang Yaling, Li Shengyong

1980, 2(1):  65-78. 

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Basing upon the data obtained from long-term observations of ground water regime and taking the Tangshan earthquake as an example, the authors proposed first the principles and approaches to recognize the precursory anomalies of earthquakes and classified the subsurface water regime into underground water, interlayer water and confined water. For the Tangshan earthquake the forerunner field of underground water is characterized by its spatial distribution (limited to a certain quadrant), its stages of development in time and the synchronism of anomaly-changes during and after a shock.Furthermore, this study stresses the importance of these principles mentioned above in predicting the space, time and magnitude of future earthquakes and discusses the relationship between the focal stress field and regional stress field and the indicator to distinguish them. It is suggested that the former develops progressively and its principal axis of compression stress changes in direction just before an earthquake, thus enabling us to divide the focal process into two basically different stages-brewing and originating stages.

SHORT REPORT ON PALEOMAGNETIC MEASUREMENTS OF LACUSTRINE ARGILITE FROM GALA LAKE, KANGMAR COUNTY, XIZANG

Li Suling, Cheng Guoliang

1980, 2(1):  79-80. 

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