Table of Content

01 September 1982, Volume 4 Issue 3

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Brief Report

STUDIES ON THE GENERAL DISTRIBUTION OF CRUSTAL THICKNESS AND CHARACTERISTICS OF CRUSTAL STRUCTURE UNDER THE ASIAN CONTINENT

Wang Qianshen, Wu Chuanzhen, Liu Hongchen, Wei Ying

1982, 4(3):  1-9. 

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In this paper according to the data of Asian continental 1白1? mean gravity field of Bouguer anomaly and a monolayer, homogenous, calculative crustal model, the total 10456 gravity data belonging to 131 gravity profiles have been calculated for determining the depth of Moho discontinuity. Obtained from this has been an outline map showing the distribution of Asian continental crustal thickness. Based on these results gained, the features of distribution of the crustal thickness and outline of crustal structure under the Asian continent have been analyzed and studied. Finally, this paper advances the principal characteristics of the Asian continental crust. There exist four vast areas of relatively minor variations in the crustal thickness, namely, northern, eastern southern and central areas of Asian crust; three great comprehensive steep (progressive) variational zones of crustal thickness, i. e. Pamir-Altai-Trans-Khingan complex steep variational zone, Trans-Khingan and Da Hinggan-Taihang-Wuling-Miaoling-Hengduan mountains progressively variational zone, and Zagros-Pamir-Himalayas steep variational zone; two complex convergent "node" of steep variational zones of crustal thickness, Pamirs "node" and Hengduan mountains "node"; and one Qinghai-Xizang crustal block with great thickness.

A REVIEW ON"TECTONIC DEFORMATION OF THE SONGSHAN AREA,HENAN PROVINCE, CHINA"

1982, 4(3):  10-10. 

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THE DANGEROUS DEGREE OF FRACTURE CAUSED BY THE UPWARPING OF MANTLE

Xu Shouchun

1982, 4(3):  11-16. 

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A two dimensional non-linear finite element scheme was used in the study of stress field on a deep section of a typical graben system. The parameters of the specific section is determined according to the data given in ref,The programme employed is a transplantation of the programme employed by Wang Ren. The faults are represented by laminar material, transversely isotropic elasto-plas-tic material. The change in the degree of safety caused by the upwarping of mantle is estimated (See Figs 2 and 3).The concept of degree of safety has been employed in R. Wang's paper. This is a non-dimensional quantity, characterizing the degree of safety of a material under Coulomb shear criterion. The calculation equation is as follows: α=(μσ_n+|τ|)/(μ·σ_n) For the fractured zone, σn and τ are the normal stress (trensile as positive) and shear stress on the fault plane, respectively.μ is the coefficient of friction. For the elastic area outside the fault, the value of degree of safety α is calculated along the direction which forms an angle of intersection of 0.5Arctg (1/μ) with the maximum principal compression stress. It is easy to see thatwhen α≤0, shear fracture occurs in the material;when 0<α≤1, the larger α (nearer to 1), the higher safety of material;when 1<α, then σ_n>0, this plane is under tensile stressThe upper surface is a free boundary. The two sides are under compressive stresses which increase linearly with depth. For the base boundary two cases are computed: the displacement in the vertical direction is zero or non-zero. The upwarping mantle can be described by non-zero displacement in the vertical direction of base boundary. By comparing two results calculated, the danger of fracture caused by the upwarping of mantle is obtained. Let f and f₁ be the lithostatic pressure and compressive stress acting on two sides of the boundary, respectively. The Fig. 3 and 2 are the diagrams of the change of safety degree caused by upwarping of mantle when f₁ = f and f₁ = f/2, respectively. The safety degree increases in the dotted areas. The results of computation coincide with the following characteristics of seismicity in graben system: a) the graben system is an area of intense seismicity, but much intensive is its inner part as compared with the outer part; b) with only individual exception, the foci of strong shocks are shallow.

ON ACTIVITY OF THE NW-TRENDING FRACTURES ALONG THE FUJIAN-GUANGDONG REGION

Zhang Hunan

1982, 4(3):  17-25. 

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Along the Fujian-Guangdong region there are in general 3 sets of tectonic lines. The most striking ones, belonging to the NE-trending Neocathaysian system, represent the principal ranges and valleys. The Ew-trending lines are lesser in number. The Nw-trending ones, poorly known and represented often by fractures, will be briefly discussed in this paper.The NW-trending fractures are numerous in Southeast China, particularly in the coastal areas of Fujian, Guangdong and Guangxi provinces. They are equidistantly apart in a regular pattern, many of them are followed by rivers; some may measure be comparable in size with the Neocathaysian fracture; most of them are vigorously active in recent geological periods.A thorough investigation into the NW-trending fractures may be significant in solving (l)some geological problems not yet resolved theoretically, as well as those frequently met in Southeast China and (2)some practival problems regarding ore searching, delta conservancy and earthquake predition.

COMING OUT OF "THE PHOTO ALBUM OF EIGHT STRONG EARTHQUAKE HAZARDS IN CHINA"

Yao Gan

1982, 4(3):  26-26. 

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CHRONOLOGICAL STUDIES ON THE LATE QUATERNARY STRATIGRAPHY AND NEOTECTONIC MOVEMENT ALONG THE COASTAL AREA OF SOUTH FU JIAN AND EAST GUANGDONG

Zhang Jingwen, Li Guiying, Zhao Xitao

1982, 4(3):  27-36. 

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According to the field investigation along the coastal area of South Fujian and East Guangdong and the ¹⁴C age determinations, a number of stratigraphical profiles are described and ¹⁴C dates of 22 samples published. The subdivision and correlation of the late Quaternary stratigraphy, the Holocene sea level changes and the characteristics of neotectonic movement are preliminarily discussed in this paper.The late Quaternary stratigraphy in this area can be divided into Jiangtian peat and Jingzhou mud belonging to the late Holocene; Zhanglin chenier, Gongqian beachrock and Juewei mud to the Middle Holocene; Xianqian formation of the Early Holocene and Meipu calcareous tufa, Chishan peat, Jiazi buried soil and Houlou formation of the late stage of Late Pleistocene.From the altitudes and C¹⁴ dates of marine beds, beachrock, cheniers as well as sedimentological, geomorphological and biological evidences indicative of the positions of paleosea level, the authors come to the conclusions on the lower stands of sea level in the late stage of late Pleistocene, rising of sea level of the Early-Middle Holocene, and existence of several higher sea levels in the past 6000 years than the present-day's, that were further, proved to be correct. Correlation of altitudes and ages ¹⁴C of samples of this area with representative curves of the Holocene sea level changes, it can be seen that this area can be ascribed to a gentle to moderate uplift one, denying then the idea on the subsiding coast of the South China. Thereby it is also unfavourable to the view on the intense uplift of this area.

FINITE EIEMENT MODEIIING OF CRUST TEMPERATURE DISTRIBUTION IN THE NORTH CHINA BAY AREA

Loo Huanyen, Hong Hanjing, Song Huizhen, Sun Junxiu

1982, 4(3):  37-44. 

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This paper describes the application of the Galerkin finite element method and the treatment of unknown heat flow boundary condition by hybrid method to compute the crustal temperature distribution in North China Bay area. It has been shown that: (1) temperature along the Moho discontinuity is not a constant as generally assumed, but varies from a lower value of about 700℃ at its downwarping portion to a higher value around 810℃ at its upwarping portion; and (2) large shallow earthquakes occurred along a belt of steep thermal gradient just above a low velocity (i. e. low resistivity) zone where the temperature is about 600℃, this may be due to the high temperature below such a zone, weakening the materials of lower crust unable to concentrate stress causing large earthquakes.

PRELIMINARY STUDY ON THE CAUSE OF SEISMICITY IN THE HUNANZHEN RESERVOIR AREA, ZHEJIANG PROVINCE

Hu Yuliang, Chen Xiancheng

1982, 4(3):  45-49. 

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The Hunanzhen reservoir with a dam 129m high and a volume of 2.06 billion m³ is located in Quxian County, Zhejiang province. The seismic activity started along the bank 15km up stream from the dam, 5 months after impounding of this reservoir. The earthquake of magnitude 2.8(Ms)occurred on October 7, 1979 and caused a light damage in the central area. A mobile seismograph was installed in the epicentral area and from March 14 to May 26, 1980 1483 tremors were recorded. Most of them are ultramicroear-thquakes and have a regular distribution in space with extremely shallow depth. The seismic activities vary with the water level sensitively and lag in a short time. Based on the geological environment and macroscopic characteristics of seismic records it can be suggested that the seismicity is likely to be caused by the gravitational slip of the unstable rock masses on the banks beneath the reservoir studied.

THE RADIOCARBON AGE DETERMINATIONS (2)

Jiao Wenqiang, Li Guiying, Chen Yijian, Zhang Jingwen, Peng Gui

1982, 4(3):  50-52. 

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A PRELIMINARY DISCUSSION ON THE REGIONAL MAGNETIC ANOMALY AND CRUSTAL MAGNETIC LAYERS IN BEIJING-TIANJIN AND SURROUNDING AREAS

Tao Guobao, Chen Yihui

1982, 4(3):  53-62. 

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This paper discusses the method by which the regional anomaly can be abstracted.On the basis of making a series of upward continuation of the aeromagnetic data from Beijing-Tianjin and surrounding areas, it is proposed that the regional anomaly can be divided into two parts: the upper one and the lower one corresponding to the upper and lower crustal magnetic layers, respectively. According to the Pre-cambrian metamor-phic basement the study region can tectonically be divided into six blocks. Moreover, from the lower regional magnetic anomaly it can be derived that the lower magnetic layer may be largely affected by the thermal events related to the Mesozoic and Cenozoic magmatism. This paper also discusses the deep regional anomaly, presumably resulting from the rise of the Moho surface.

AN ANALYSIS OF EARTHQUAKE-RELATED VULNERABILITY FOR THE DONGKUANG DISTRICT IN TANGSHAN

Wang Ligong

1982, 4(3):  63-72. 

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A new approach to prediction of local quake damage is based on experience on recognition of the quake damage. Following effects (also expressed by number) on the quake damage are dealt with: foundation soils, Quaternary sediments, the burried ground water table, liquefaction of soil, slopes, faults and relief and the like.

THE FEATURE OF THE FAULT ACTIVITIES BY STAGES IN THE BEIJING REGION

Wang Songxian

1982, 4(3):  73-79. 

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The fault activities in Beijing region show a changing feature by stages. For instance, there obviously appeared two turns in mode of fault movements for the study region in 1973 and 1976 respectively. The first one caused a substantial change of regional stress pattern, while the second did not do so. It is suggested that this faulting feature by stages is generated due to development and occurrence of the 1976 Tang-shan earthquake(M=7.8).

BRIEF INFORMATION ON CHINA-NATIONAL SYMPOSIUM ON THE INTERFERENCE OF GROUND WATER MONITORING

Li Xuanhu, Zhang Yusong

1982, 4(3):  80-80. 

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THE CHANGE OF HYDROXYL IN HORNBLENDE AT CONFINING PRESSURE 3Kb AND TEMPERATURE 700℃

1982, 4(3):  81-81. 

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