Table of Content

03 March 1982, Volume 4 Issue 1

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Brief Report

BLOCK DIAGRAM OF EARTH CRUST AND UPPER MANTLE UNDER THE BEIJING-TIANJIN-TANGSHAN REGION

Shao Xuezhong, Zhang Jiaru, Zhang Siya, Li Zheijiang

1982, 4(1):  1-10. 

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This paper describes the structural profiles showing the main boundaries of earth crust and upper mantle, such as the boundary of granitic layer (G), the Conrad boundary (C), the Moho discontinuety (M) and the boundaries M₁ and M₃ in the upper mantle. They are constructed by using a method of converted seismic waves in the Be-ijing-Tianjin-Tangshan region.Three of them show the existence of a deep-seated structure zone of WNW trend in the study region. Along this zone uplifted boundary G, concaved boundaries of M₁ and M₃ in the upper mantle and thicked basaltic layer exist.The comparision of deep-seated structures with the distribution of earthquake epicenters in this region indicates that most of strong earthquakes occurred at the crossings of WNW trending and NE trending structural belts, where the depth of boundaries G, M₁ and M₃ has a sharp change. And the hypocenters of these strong earthquakes are located in granitic layer.

XINYANG SUBDUCTION COMPLEX, HENAN PROVINCE

Shi Quanzeng, Mu Yongji, Zhang Enhui, Zhang Kewei, Yao Ruizeng, Wang Jiade

1982, 4(1):  11-22. 

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In the eastern section of the Qinling geotectonic belts, the authors discovered an ancient subduction zone at the northern foot of the Mt. Dabie near Xinyang, Henan province. The subduction complex consists of ophiolites, melanges and low-T/high-P metamorphic rocks. The melanges are composed of native blocks (dolomite-marble, olisto-strome), exotic blocks (serpentinite, issite, hornblendite, granite, gabbro, chert, etc.) and argillo-arenaceous matrix. The suite of ophiolite includes metamorphosed basic volcanic rocks, diabase dykes, metamorphic pelites, basic and ultrabasic magmatic rocks. The essential characteristics of the suite are coincident with those of the island-arc ophiolites. 3T-phengites and C-type eclogites have been found in the low-T/high-P zone of meta-morphism. According to their general tectonic character the subduction complex was produced by a marginal sea floor underthrusting southwards beneath an island-arc. The acorites, sampled from the low-T/high-P zone, have been dated at 693-603 Ma by U-Pb age determination. The quartz-diorites, which intruded into the ophiolitic suite, have been dated at 413.5 Ma by K-Ar age determination. These ages imply that the subduction zone was formed in the end of Late Proterozoic.We suggest that the structural evolution of the ancient convergent plate margins should be divided into following three episodes.(1) During Late Proterozoic (about 700-800 m.y. B.P.), the oceanic crust of the East Qinling Sea underthrusting northwards beneath the continental block of North China, which gave rise to the Tongbai island, is associated with a marginal sea.(2) Nearly in the end of Late Proterozoic (about 700-600m.y. B.P.), a polarity reversal of this subduction zone occurred. The oceanic crust of the marginal sea was consumed beneath the Tongbai island arc, which resulting in the Xinyang subductio ncomplex.(3) During Early paleozoic (about 600-500m.y. B.P.), the oceanic crust of the East Qinling Sea was subducted northwards again beneath the accreted continental margin along a new Benioff zone.

THE AFTERSHOCK ACTIVITIES OF INDUCED EARTHQUAKES IN THE XINFENGJIANG RESERVOIR AREAS

Ding Yuanzhang, Zeng Xianze, Chen Yiming

1982, 4(1):  23-30. 

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According to stress field of the focal area, six types of earthquakes may be recognized, i. e. tension, compression, contortion and their transitional types. In the Xinfen-gjiang reservoir the main shock and most aftershocks of Ms>3 belong to the contortion type and are the same as most tectonic shocks outside the reservoir area. But in the last decade or so, the most microseisms belong to the tension type. The aftershocks have a more complicated driving mechanism than the main shock.In present, the seismic magnitude in Xinfengjiang areas correlates with the energy release of continental China but with a time lag. It has been a general view that a strong earthquake sequence in the neighboring regions may cause considerable events in the induced seismic reservoir.

FINITE ELEMENT ANALYSIS FOR RECENT QINGHAI-XIZANG PLATEAU UPLIFTING AND ITS RELATION TO SEISMICITIES

Loo Huanyen, Xu Yujian, Song Huizhen, Sun Junxiu, Guo Caihua, Zhang Ruze, Gu Zhijuan

1982, 4(1):  31-37. 

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Based on the available geological and geophysical evidences as well as the geodyna-mics of basin and range structure, the current deformation and stress state in a simplified tectonic profile crossing northeasternly the Xizang plateau, about 2,000km long and 80 km deep, have been analysed using finite element method. Numerical results show that the Indian plate motion is the driving force to disturb the gravity equilibrium leading to a lateral density contrast, but the horizontal compression resulting from the isostatic adjustment is the mechanism for plateau uplift and earthquake development, and that stresses often concentrate at the main fault ends near the low velocity layer.Electrical conductivity measurements of 2-mica gneiss under temperature and pressure conditions equivalent to about 40km deep, together with the optical determination and the X-ray diffraction analysis, indicate that the mechanical weakening due to the dehydration, accompanied with lattice breaking of muscovite and the thermal cracking of quartz may be the mechanisms of forming the low velocity-low resistivity layer in the plateau crust, but the shear movement existing along the low velocity layer seems to be the source of raising its temperature.

AN APPARATUS FOR TRIAXIAL COMPRESSION TESTS OF ROCKS AT HIGH TEMPERATURE AND IN HIGH PRESSURE

Zhang Liu, Wang Shengzu, Wang Guanggen, Liu Shushan

1982, 4(1):  38-38. 

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ON THE ANALYSIS OF CURIE ISOTHERMAL SURFACE OVER THE BOHAI SEA WATER AREA

Hao Shujian, Wang Chuhua

1982, 4(1):  39-43. 

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On the basis of aeromagnetic data the Curie depth over the Bohai Sea water area was calculated by using the method of spectral analysis of residual anomalies described by Prof. B. K. Bhattacharyya. The results obtained indicate that the crustal temperature under the sea is rather higher and in some places the Curie point is located only at a depth of 12km.The idea that the Bohai Sea area can be viewed as a high point for upwelling of materials in upper mantle beneath the North China region was formulated by many researchers in geoscience according to the geological and geophysical data from the Bohai Sea and the surrounding areas. And later it was confirmed to be correct by the calculated results mentioned above. Therefore, the high heat values that appeared in the Bohai depression and the surroundings are not occasional but have a deep-seated background. Thus the upper mantle u’pwarping from the North China block fault, leading to thinning out of crust and subsiding and squeezing of heated materials into crust along the faults, all promoted the thermal action, in which the Tancheng-Lujiang fault zone would play a specially important role. The upwelling mantle materials through this fault zone resulted in a gradual increase (from west to east) of the temperature field in the North China region and reached its maxium over the Bohai Sea area.This paper indicates that the earthquakes having occurred in the North China region including the Bohai Sea area cannot be fairly explained simply by the stress field formed in collision of some lithospheric plates around the Chinese mainland. The horizontal gradient of temperature field generated during the upward flow of deep-seated heat materials due to the non-homogeneity of earth crust is closely related to the seismicity in the North China region.

NEW ACTIVITY OF HUANGYANGUAN FAULT OFFSETTING THE GREAT WALL

Cao Zhongquan, Wang Jinduo, Ye Dinheng

1982, 4(1):  44-44. 

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EXPERIMENTAL STUDY FOR EARTHQUAKE PREDICTION BY RADON ANOMALIES

Fan Shuquan, Gao Qingwu

1982, 4(1):  45-56. 

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This paper deals with the results of pressure and solution tests on rock samples in order to provide an experimental basis for earthquake prediction using radon anomalies. Based upon the general determinations of radon concentration in ground water from the Tangshan area, the authors on the samples collected from the wall rocks and wells made a comprehensive study of the uranium (thorium) content in the rocks and the relationship between the radon concentrations which were escaping from the broken rocks by pressure ordis solving in water and existing in wells. The authors suggest that the higher the content of radioactive element in wall rocks of the wells is, the larger the escaping radon amount from the broken rocks; the more the radon amount solved in water of the broken rocks and the higher the radon concentrations in ground water of the wells will be.

SOME FEATURES OF RADON ANOMALOUS FIELD IN THE BEIJING-TIANJIN REGION BEFORE THE TANGSHAN EARTHQUAKE

Li Xuanhu

1982, 4(1):  57-66. 

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Analysing the radon data from several wells in Beijing-Tianjin region, it is believed that the main characteristics of radon anomalous field in this region before the Tangshan earthquake are as follows:1) There was a trend anomaly of the regional average variations in radon for nearly four years, and the anomalous duration was closely related to the Tangshan earthquake (including strong aftershocks). 2) The moderate-term anomaly of radon (1975.8-1976.4) showed a radiative transmission phenomenon from the epicentre to the surface at a speed of 35km/month.3) Just before the Tangshan earthquake (some smaller shocks had occurred in early July, 1976), the sharp radon variations showed itself an opposite pattern, converging from the surroundings towards the epicentre at a speed of about 25-30km/day.The characteristics mentioned above are discussed on the basis of premonitory creep together with the results of simulation tests. It is presumed that if the linear strain reachs 10^-6-10^-5 in the study region, the anomalous variations in radon can be observed distinctly.

SEISMIC FAULTS OF XIALATUO, LUHUO COUNTY, SICHUAN PROVINCE

Huang Shengmu, Liu Benpei, Jiang Zaixiong, Li Dinfu

1982, 4(1):  67-71. 

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The seismometric Brigade of Sichuan Seismological Bureau made a preliminary investigation on the digging trench at the Xialatuo site along the Luhuo fractured zone, where an earthquake (M = 7.9) occurred on Feb. 6, 1973. It was found that the house remains were buried at the depth of 0.4-2.2m and 1.9-3.58m respectively during the two historical events on Mar. 24, 1923 and Dec. 8, 1816. It is reported that there occurred three large earthquakes with an intensity scale of X within 158 years. The view that "the strong earthquake (intensity of X) cannot repeat at the same place" cannot be applied to the seismic zoning.Owing to the recurrence of great earthquakes, we have studied four groups of earthquake faults occurring in 1973, 1923, 1816 respectively.

COMING OUT OF“THE ALBUM OF TYPICAL SATTELITE IMAGERY OF ACTIVE STRUCTURES OF CHINA”

Yao Gan

1982, 4(1):  72-72. 

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ON THE ORIGIN AND DEVELOPMENT OF EARTH FROM THE ARCHAEAN GNEISSES

Yang Jie

1982, 4(1):  73-78. 

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In the area from Beijing to Chengde the extensive occurrences of the Archaean-Proterozoic crystalline rocks are composed mainly of two suits of metamorphic rock series; the Miyun Group in the lower member dominated by hypersthene-granulite and hornblende-granulite, being the type of orthogneisses and named "gneiss series" in the past: the Zhangjiafeng Group (or Huairou Group) in the upper member in predominance of crystalline schist, belonging to the paragneisses, and which used to be referred to as "crystalline schist series".In this paper the earth’s origin was presumed by virtue of the above-mentioned metamorphic rocks. It is assumed that the original earth may stem from the nebula. Originally the earth would have a layer of " a basic "scoria shell" after it had formed. And this may be the initial embryonic crust of igneous rocks. Because of its outer crust the transmission of heat was difficult, and then the rock differentiation of the inner crust took place, leading to the generation of the magmatic source in the upper crust.Geothermal and magmatic sources having an effect on the scoria shell would cause metamorphisms and then form the crust of basic orthogneiss, the lower crystallized crust.At that time the wavelike folding deformation due to the contraction happened over the crust. However, the deep-seated faults would never occur within the crust. Consequently, a large scale of intrusion and eruption of the magma was of little possibility and only the dissemination and injection of the magma appeared in dispersion form, leading to a general migmatization. The temperature of the ground surface had since decreased rapidly and the solid crust was under the action of the internal and external stresses deformed and fractured to cause uplifts and depressions, and followed by the erosion and sedimentation. The consolidated sedimentary rocks were accompanied by the magmatic activity and the metamorphism to form the paragneisses. i. e. the upper crystalized crust.On the basis of the composition and occurrences of the ancient rocks together with the characteristics of metamorphism, magmatism and tectonic movements, the earth’s origin discussed in this paper may be assumed to be a general rule for the accretion and evolution of the earth.

HAIFENG EARTHQUAKE SWARM(1981), GUANGDONG PROVINCE

Wei Bailin, Xue Jiamou, Xie Mingfu, Wang Zhengshang, Zhang Linyun, wu Xianji

1982, 4(1):  79-81. 

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