Table of Content

06 June 1979, Volume 1 Issue 2

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Brief Report

CURRENT TENSILE STRESS FIELD IN THE BLOCK- FAULTING TECTONIC AREAS OF NORTH CHINA

Gao Mingxiu

1979, 1(2):  1-12. 

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As is well known the regional tectonic stress field is of great importance to the study of seismogenesis, earthquake prediction and earth dynamics. In this paper a synthetic analysis of current tensile stress field existing in the block-faulting tectonic areas of North China is given on the basis of observations and investigations of various phenomena such as seismogenetic structure, ground failure and seismic fault caused by a great earthquake, focal mechanism, the pattern of elastic rebound, the triggering of earthquake by tidal force and so on.In addition, the influence of the Pacific plate and the Indian plate on the tensile stress field in North China as a result of underthrusting of both plates under the Asian continent, is also discussed here, and it should be emphasized that the process occurring in the deep-seated portion of the earth crust must be of great importance to the tensile stress field present in North China.

CHARACTERISTICS OF CRUSTAL STRUCTURES IN NORTHEAST CHINA AND THEIR RELATIONS TO THE DISTRIBUTION OF STRONG EARTHQUAKES

Chung Yichang, Lu Zaoxun, Liu Tiange

1979, 1(2):  13-21. 

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Using the various data of geophysical fields combined with the interpretations of satellite photographs (images), this paper is written with the following conclusions:[1] The deep-seated crustal structures revealed by data of geophysical fields are basically in conformity with the surface structures inferred from the satellite images. The combination of these two methods would provide a powerful approach for studying the crustal structures.[2] The geotectonic framework of Northeast China is composed of several great fault blocks and characterized by the alternative arrangement and parallel distribution of the striped fault blocks.[3] The distribution of strong earthquakes epicenters is closely associated with the large regional structures, and most of them occurred around the edge of the uplifted area in the upper mantle or along the interference portion of the differential zones of deep-seated structures and/or at the intersection of fault zones in different directions.

ON THE NUMERICAL PROCESSING OF AEROMAGNETIC DATA-TOTAL FIELD REDUCED TO THE POLE, ITS SECOND DERIVATIVE AND THE CONTINUATION

Tao Guobao, Wang Qiming

1979, 1(2):  22-37. 

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Basing upon the even function (the cosine series) in this paper is introduced a method including the total field reduced to the pole and its second derivative and the continuation for numerical processing of aeromagnetic data. The method presented here is much more accurate than that of the Fourier method, while the boundary effect is depressed to a certain degree. In addition, an example utilizing the method also shows it to be useful for the exploration of mineral deposits and study of geologic structures of the deep-seated crust. Finally, the computer program of the method is presented. By combining the Fast Fourier Transforms (FFT) with the recurrence process applied in the program, the computation time is shortened and the amount of data is less restricted.

A PRELIMINARY STUDY ON RECENT CRUSTAL MOVEMENT AND SEISMIC ACTIVITY IN THE BEIJING REGION

Wang Tingmei

1979, 1(2):  38-45. 

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Based on archaeological, geological and geomorphological studies as well as seismic activities recorded in the Beijing region this paper discusses recent crustal movements and gives the following conclusions.[1] Recent crustal movements are characterized by the block-differential movement controlled by the NWW sinistrally trending fault and the NNE dextrally trending fault. [2] The stress field in the Beijing region coincides with the recent regional stress fieldof North China. [3] There has been marked changes in the faulting since Late Holocene, namely, beforethe Warring States Period (475-221 B.C.) the NNE trending fault was predominantwhile beginning from the Liao-Jing dynasties (907-1125A.D.) the NWW trending fault was prevailing. [4] Pleistoseismic activities are related with the NWW running structures.

NEOTECTONICS AND SEISMICITY OF YAN-HUAI BASIN IN HEBEI PROVINCE

Wu Zirong, Yuan Baoyin, Sun Jianzhong, Liu Zengsheng

1979, 1(2):  46-56. 

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The Yan-Huai basin consists of four minor basins, namely, Yanqing, Huailai, Zhuo-lu and Fanshan basins. This region has been subjected to a progressive subsidence since Neogene. It may be regarded as a typical region for seismogeological study owing to great development of Quaternary sediments, sharp contrast in relief, concentration of historic earthquakes and distinct neotectonic movements.Six neotectonic categories may be recognized in the region under study basing upon the sedimentary history, geomorphological features and geophysical fields. In addition, a preliminary analysis of the seismic trend of the whole region is also made in the light of the different neotectonic activities, both young and old structural frameworks, and historic earthquakes.

DEEP-SEATED ECLOGITES IN JUNAN AND RIZHAO, SHANDONG PROVINCE, CHINA

Xu Huaiji, Wang Wenhu, Go Jindi

1979, 1(2):  57-66. 

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This paper deals with the data of chemical analysis of 27 samples collected from the region of Junan and Rizhao, Shandong province, among them 8 are eclogites, 2 pyro-xenites, 11 clinopyroxenes and 6 garnets.Based upon their mineralogical and petrochemical characteristics, two types of eclogites can be recognized, e. g. basic and ultrabasic. They are closely related to the ult-rabasic intrusions emplaced in Precambrian metamorphics.Judging from the P-T conditions estimated by Fe⁺⁺-Mg being distributed between coexisting clinopyroxenes and garnets in eclogites as well as the average thermal gradient of Precambrian shield it is concluded that the basic eclogite might have originatedat a depth of 100km. whereas the ultrabasic eclogite at a depth of 40-60km.

THE GEOLOGICAL ASPECT OF EARTHQUAKE HAZARD CAUSED BY LIQUEFACTION OF SAND IN THE LOWER LIAOHE RIVER REGION

Zhu Haizhi, Wang Ligong, Gao Qingwu, Wang Dengdi

1979, 1(2):  67-73. 

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The Haicheng earthquake (Feb. 4, 1975, M=7.3) has caused the extensive liquefaction of sand in the Lower Liaohe River region, which expressed itself as ejected water and sandflows, sundken lands and sandy holes on ground surface, ground drumlings, landslides and ground fissures.According to the degree of ejection and ground failure the liquefied region may be classified into three areas different in hazard, i. e. intensive(I), moderate(Ⅱ) and weak (IE). The liquefied region in the Lower Liaohe River has been a regression area in Holocene period, covered by a layer of clayey soil with a thickness of 2-5m. This layer is thinner in the intensive hazard area with a higher aquifer head, but thicker in the weak hazard area with a lower head.The monograin structure of ejected sands shows the loosening nature of liquefied sandy layer. In the intensive hazard area the sand grains ejected are found to be coarser. According to the present analysis of sand grains it is concluded that the roundness of grains is of less importance in determining whether a sandy soil can be readily liquefied or not.

ON DISTRIBUTION OF INTENSITY ANOMALIES IN THE BEIJING REGION CAUSED BY THE TANGSHAN EARTHQUAKE

Gao Zhenhuan

1979, 1(2):  74-82. 

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This paper deals with the geological factors affecting the distribution of seismic intensity in the Beijing region during the Tangshan earthquake. It is suggested that the geological factors leading to the intensity anomalies in the Beijing region include mainly liquefaction of sandy soil, faults and buried hills. As the liquefaction of saturated sandy soil caused by the earthquake is related with the permeability of groundwater, earthquakes tend to build up easily the pressure in the region with less permeability due to the showness of dissipation. In general, the buried hills would play a role in the amplification of ground shock owing to its various relief.The data of historical earthquakes in Beijing and its adjacent regions indicate that the effect of several earthquakes on a given region appeared to have some kind of synchronous intensity anomalies. Therefore, this repetition anomalies may be regarded as a reflection of the geological factors having a long-term effect on the region under investigation. Hence, we are led to the conclusion that the geological conditions of the areas of intensity anomalies, especially the areas of frequent historical earthquakes, should be studied in detail so that they could offer an important approach to estimate seismic intensities and to prevent and resist future earthquake hazards.

ON THE WOBBLE OF GEOMAGNETIC POLE FOR LAST 2000 YEARS

Lin Jinlu, Wang Qiming, Cheng Guoliang

1979, 1(2):  83-85. 

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A NOTE ON CARBON-DIOXIDE DISCHARGING ZONES AND THEIR SEISMICITIES IN CHINA

Shi Huixin

1979, 1(2):  86-93. 

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This paper deals with the distribution of carbon-dioxide discharging localities in China (where CO₂ gas content is >90% and water with dissolved CO₂ >1g/l or with bicarbonate more than 1g/l), characteristics of water quality and the seismic activities of various carbon-dioxide discharging zones. The source of CO₂ fluids is also studied herein. It is suggested that the highly seismic activities are mostly associated with the zones of warm and hot springs rich in carbon-dioxide, but scarcely with the zones of cold carbon-dioxide springs. Warm and hot carbon-dioxide springs occur mainly in seis-mically active zones with strong neotectonic movements, such as in southern Xizang (Tibet) where the highly seismic activities are accompanied with regional metamorphism. Moreover, in area of heat flow anomaly probably there also occurred magmatic intrusion which brought about contact metamorphism and through all of these processes carbon-dioxide may be released. Cold carbon-dioxide springs are discharged in seismically less active zones, such as in Northeast China, where tectonic movements and magmatic activities became gradually quiescent. Carbon-dioxide appears to be released slowly from magma which cooled down under a relatively closed condition in the post-volcanic period, and it miaght be the released gases conserved in the altered surrounding rocks affected by volcanic magma. Meanwhile, the cold carbon-dioxide springs originated by mixing CO₂ fluid with shallow groundwater.In the seismically active zones within depressions subsides since Cenozoic time, such as in North China, the warm and hot carbon-dioxide springs have not been found at the ground surface, owing to the thick covers, but in the case of deep-seated ground-water hot carbon-dioxide water was discovered. Intensive studies of the origin and distribution of CO₂ fluids will be significant to understanding the recent stress field, seis-micity and regional metamorphism. It must be stressed that great effort must be made in the studies of mountainous areas, such as the Kunlun-Szechuan-Yunnan, Tienshan, Chilien. Moreover, the analysis of stable carbon isotopes from discharges of CO₂ fluids should also be carried out.

The Second Chinese Symposium of Structural Geology, Beijing, March, 13-23, 1979

1979, 1(2):  94-94. 

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