Table of Content

03 January 1979, Volume 1 Issue 1

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Brief Report

DISCUSSION ON DRIVING FORCES AND MECHANISMS OF EARTHQUAKES IN CHINA FROM THE VIEWPOINT OF EARTH'S CRUST DYNAMICS

Loo Huanyen

1979, 1(1):  1-10. 

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This paper considers mainly the contradiction between the gravitational force and heat energy as a guidence dealing with the occurring of earthquakes in China and their mechanisms from the angle of both energy transformation and material transportation. Although earth rotation, gravity and heat energy are major driving forces to produce tectonic activities and seismicities, but the first one is just regarded as a long-term acting force for slow motion of its interior materials, the second acts as a force to keep the earth in equilibrium, and the third may cause the crust break up abruptly and may destroy the gravity isostasy accompanied with lateral sliding due to gravitational rheology which is only responsible for shallow earthquakes and superficial tectonic activities. Therefore, both time-and space-dependent movements of earth material may be horizontal or vertical in phenomena controlled by the characteristics of acting forces.Generally, the state of tectonic stress can directly be influenced by the subduction of oceanic plate at island arc structures or of one continent underneath the another only within relatively narrow zones adjacent to the subduction belts, and the earthquake mechanisms along them may be subjected to frictional sliding or stick-slippage. Most of sedimentary basins and grabens in China with characteristics of abnormally high heat flow, positive gravity isostasy and thin crust appear to have developed since Mesozoic and Cenozoic, it reflects that large-scale vertical motions, strong in the past and weak nowadays, are driven by the upward flow of materials in upper mantle beneath these basins and grabens. This sort of vertical motion are responsible to compensate the materials imposed by the lateral motions which are induced indirectly by descending plate into upper mantle. Thus the driving forces and mechanisms of earthquakes occurred at these edges of the basins and grabens should be different form those at subduction belts.

ON THE TECTONIC STRESS FIELD IN CHINA AND ITS RELATION TO PLATE MOVEMENT

Deng Qidong, Zhang Yuming, Xu Guilin, Fan Futian

1979, 1(1):  11-22. 

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Basing upon the data of geological and seismological observations and crustal deformation this paper deals mainly with the characteristics of the tectonic stress field in China since Late Tertiary. Such a tectonic stress field is characterized first of all by obvious zonations. And it is especially distinct in three broad regions, namely West China, North China-Northeast China and South China-Taiwan. The tectonic stress field in these regions seems to be stable.The mechanical behavior of tectonic belt in S-N direction in Central China varies significantly with different sections, since it should neither be considered as a simple shearing belt nor a tensile fault zone. On the basis of quantitative data obtained it may be suggested that it is predominatly horizontal, but the vertically applied stress also exists in certain regions due to the motion of the Earth's deep-seated materials.It is to be noted also that the tectonic stress field in China is controlled by the joint action of the Indian and the Pacific plates.

DISCUSSION ON THE CHANGES OF CENOZOIC TECTONIC STRESS FIELD IN THE EASTERN PART OF NORTH CHINA

Zhang Yuming, Wang Liangmu, Dong Ruishu

1979, 1(1):  23-28. 

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On the basis of analysis of the development of grabens, seismogenic faults and focal mechanism, the nature of changes of tectonic stress field in the eastern part of North China during Early-Late Tertiary is studied in this paper. The region under investigation was subjected to a strong NW-SE tension in Early Tertiary. Since Late Tertiary it is mainly controlled by the NE-NEE compressive stress field with the NW-SE running tensile stress field in subordinate place. The stress field here has been affected by the action of the Pacific plate in Early Tertiary and by the action of the Indian plate since Late Tertiary.

A GIANTIC TECTONIC BELT RUNNING ACROSS EASTERN CHINA

Liu Xingsong

1979, 1(1):  29-38. 

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On the basis of geomorphological, geological and geophysical studies as well as observations of seismic activities this report tries to show the existence of a giantic active deep-fault belt consisting of a system of horsts and grabens and stretching over eastern China.From the analysis of the structures, structural surfaces and elongation of this tectonic belt, as well as the studies of deposits, the tectonic belt under investigation may be considered as a result of the action of isostatic adjusment occurred after Yanshan movement, namely, a tracking fault belt of intense differential vertical movement predominated by tensional shear.As the belt exerts its control on sedimentary fades, draingage system and igneous activities-especially the existence of the ultrabasic rock belt, it may serve as an indicator of ore-searching and a reminder of the presence of a tectono-seismic belt as well.

PRELIMINARY RESULTS OF GRAVIMETRIC EARTH-TIDE MEASUREMENTS IN BEIJING

Chen Yihui, Du Pinren, Zhu Hanyun, Guo Ziqiang, Gau Weian

1979, 1(1):  39-55. 

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This work introduces results of gravimetric earth-tide measurements from October 10, 1977 to May 3, 1978 in Beijing. The gravimeter used in the measurement is Askania GS-15 No. 231 and is installed in thermostatized underground room （at a depth of 4m） with an annual temperature variation of less 1℃ and moisture of less 70%. The station is located at 39.97°N lat., 116.42°E long, with an elevation of 45m. The chart speed is about 30mm per hour. The sensitivity of the recorder scale is about 2μgal/mm. Hourly reading errors are less than 1μgal. The sensitivity calibration of the instrument is checked every fortnight by means of the electromagnetic calibration equipment. The instrument time constant is 54 sec. The scale value of the gravimeter was calibrated at the National Gravimetric Datum Level. The harmonic analysis of the records is made by the method of Venedikov with the Cartwright-Tayler-Edden complete tidaldevelopment. The results of amplitude coefficient δ and phase lags κ for main tidal wave in Beijing are as follows: δ（O₁）= 1.1639 ± 0.0086, κ（O₁） = 0.44°±0.42°,δ（M₂） = 1.1593±0.0027, κ（M₂）=-0.40°±0.13°.

NEO-TECTONIC MOVEMENTS IN THE NORTHERN SECTION OF THE YUANMOU FAULT ZONE, YUNNAN PROVINCE AND THE EARTHQUAKE RISK INVOLVED

Han Mukang, Chai Tianjun

1979, 1(1):  56-65. 

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Neo-tectonic movements in the northern section of the Yuanmou fault zone, Yunnan Prov. are very well reflected by the modification of landforms and the deformation of the early Pleistocene lacustrine strata.1. A narrow, deep trough-basin (the Yuanmou Basin) formed on the western side of the major Yuanmou fault was at first filled with Q₁ lacustrine deposits of more than 400m thick, and then uplifted into a westward tilting high lake terrace. Subsequently 5 lower and lower river terraces appeared one after another only along the eastern margin of the trough-basin. The Longchuanjiang River, which has carved out the terraces, now runs along the western margin of the basin.2. Jurassic sandstones and shales were thrust westward on Q₁ lacustrine deposits, which was affected by a series of young folds and faults. These folds and faults trend with a sharp angle toward the major Yuanmou fault and constitute some Quaternary tectonic structures of "λ" pattern.3. A series of cross tectonic structures such as downwarp-faulted basins with an arched uplift between them is found in the trough-basin.All the features above mentioned suggest that the northern section of the Yuanmou fault zone forming the middle part of the Sichuan-Yunnan N-S tectonoseismic belt, has been affected not only by vertical, but also horizontal (either sinistral or dextral) multi-cyclic movements.Nevertheless, from the seismo-geological analysis we consider that the risk of a destructive earthquake here would not be so serious as that in the northern and southern terminal areas of the Sichuan-Yunnan N-S tectonoseismic belt.

EMPLOYMENT OF PATTERN RECOGNITION FOR LOCATING STRONG EARTHQUAKE ZONES IN BEIJING-TIANJIN AREA AND ITS ADJACENT REGIONS

Xiao Yiyue, Li Ping

1979, 1(1):  66-78. 

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Adaptive pattern recognition has been applied to predict possible location of earthquakes and their magnitude in the Beijing-Tianjin area and its adjacent regions. It consists of discrimination between two pattern classes on the basis of binary attributes. A trial set of coefficients are progressively modified until maximum discrimination is obtained. The basic steps in the adaptive pattern recognition algorithm are as follows:1. Set power vector W to zero;2. Set a counter K to zero;3. In the epicentres of known earthquakes order first of all, compute the discriminant score for first epicentre area, using equation D₁ = WY₁;4. If this epicentre area belongs to group A go to 5, otherwise go to step 8;5. If D₁>0 go to step 11;6. Increase K by 1;7. Recompute the W from the formula W'=W+αY, and go to step 11;8. If D₁<0 go to step 11;9. Increase K by 1;10. Recompute the W from the formula W'=W -αY, and go to stcp 11;11. Repeat step 3-10 for second epicentre area and others;12. If K is zero then none of the epicentres areas are misclassified namely D_i = WY_i correctly discriminates between two groups;If K is positive, go to step 2.First, analysis is made of the geological features of historical and recent earthquake epicentres and are then grouped to 16 factors.Strong earthquake epicentres in Beijing-Tianjin area are classified into two pattern classes: epicentres of earthquakes with magnitude of 6.0-7.0 and those above 7.0.Then previously uncatagorized or unknown earthquake areas are classified into one of the two pattern classes, based on the discriminant function produced by the computer. The results then obtained from pattern recognition are noteworthy. The Tangshan earthquake occurred just between the two areas which after the calculation are shown to be the most dangerous zones.After the Tangshan earthquake another prediction was made of future earthquake location and their magnitude. No matter whether this prediction will be accurate or not, we still consider that adaptive pattern recognition may be a promising method for prediction of earthquake location and their time of occurrance and for the study of numerical prediction of earthquake or say predictive geology.

THE OPTIMUM NUMERICAL FILTER AND ITS APPLICATION

Li Hongji, Xue En, Wu Yongjiang

1979, 1(1):  79-89. 

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The optimum numerical filter is a useful mathematical tool for separating waves with a given period or eliminating short period fluctuations in a time series, including three types: low-pass filter, band-pass filter and high-pass filter.In this paper, a new formula for calculation of sinusoidal integral is presented and the optimum numerical filter is already applied in different fields of geology.

DISCUSSION ON THE ANOMALOUS AREAS OF HIGH INTENSITY NEARBY BEIJING CAUSED BY THE TANGSHAN EARTHQUAKE

Jiang Pu, Wang Qiming, Xu Feng, Cui Jinlong

1979, 1(1):  90-98. 

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According to the features of litho-geologic body and its response to earthquake the study of anomalous areas of high intensity nearby Beijing caused by the Tangshan earthquake is carried out in this report. It is suggested that the "anomalous areas" may be regarded as a loose geologic unit formed under the conditions of lithologic alternation and mainly consisted of fine sand and silt interbedding structure or of clayey fine sand intercalated with silt.The coefficient of rigidity of each unit is less than 2.0 and it has a significant long-period component of microseism. Under the action of great and distant Tangshan earthquake the response of these geologic units to the ground motion has a distinct common features, in other words, it shows the great response to the earthquake and its duration in a range of broad band.The results and the method of analysis presented herein would be an effective approach for ground motion analysis and for zonation of seismic response.