Table of Content

01 December 1983, Volume 5 Issue 4

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Brief Report

PRELIMINARY INVESTIGATION OF DEFORMATION BELTS CAUSED BY EARTHQUAKES IN 1951—1952 IN THE DANGXIONG-BENGCUO AREA OF TIBET

Han Tonglin

1983, 5(4):  1-11. 

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In 1951—1952 some earthquakes occurred in the studied area. Preliminary investigation shows that the Bengcuo earthquake deformation belt is well developed, in which deformation structural types are various. The most strong deformation occurred near Dagza southeast of Bengcuo, of which the location coincides basically with that of the main shock epicentre. The belt is straight, trending N310°W, about 100km long and several to 20m wide or more and characteristic of a dextral strike-slip with the greatest offset of about 12m. The NW-trending one is dextral, dominated by strike-slip displacement and having dip-slip faults which are mostly obscure. And the NE-trending one is sinistral, dominated by dip-slip displacement and having strike-slip faults with small displacement. The nearly NS-trending one is characteristic of a tensile normal fault. The above mentioned belts also basically extend along the boundary fractures or secondary fractures. Naturally, the NW/NE-trending belts should be considered as a set of conjugate fractures.Viewed from the directions of the large-scale deformation belts associated with some aftershocks, strictly controlled by the boundary fractures of the active tectonic zones. It is most likely that the aftershocks in the studied area are of tectonic shocks responding to successive releasing of ground stresses following the main shock, although no data on focal depth are available at present.In summary, earthquake-controlling structures in the seismic area should be the active tectonic zone (No.2) of an active tectonic system in Southern Tibet, controlling stress concentration and migration in the seismic area. The earthquake-generating structures should be on NW/NS-trending boundary fractures and at their intersection of active tectonic zones, where are favorable to the regional stress release.Analysis of the earthquake-controlling structures and seismostructures indicates that their stress fields show more or less consistent characteristics, which are similar to that in the Tibet plateau, the orientation of maximum principal compression stress trending NS and the maximum tension, approximately EW.

PALAEOMAGNETISM OF LOWER PERMIAN IN THE WUSHI-AKSU AREA OF XINJIANG

1983, 5(4):  12-12. 

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IDENTIFICATION OF MAIN DISTURBANCE FACTORS FOR GROUND WATER LEVEL AND EXTRACTION OF SEISMIC INFORMATION

Groundwater Research Group of Scientific

1983, 5(4):  13-22. 

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The mechanisms of two main disturbance factors which would cause the change of groundwater level are discussed. The disturbance factors are quantitatively and semiquantitatively eliminated by means of mathematical methods. Some seismic information is extracted and some new ideas about the anomaly of groundwater level appearing in Tangshan region before the Tangshan earthquake are put forward.

PROCESS OF BOUNDARY CONDITION DURING COMPUTING CRUST STRESS FIELD WITH FINITE ELEMENT METHOD

Son Huizhen, Pan Shande, Sun Junxiu

1983, 5(4):  23-28. 

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This papper has processed a boundary problem of an undetermined boundary by means of elastic constraint. The greatest difficulty was an undetermined boundary condition during computing crust stress field with finite element method. We processed especially the boundary condition with method of mirror image in physics. We have transformed the distribution stress on surface of structure body into the point force of the element on boundary and then into point displacement on boundary. Through an example we proved that the method of elastic constraint is available.

THE MAXIMUM HORIZONTAL DISPLACEMENT IN THE HAIYUAN EARTHQUAKE OF 1920 AND THE ESTIMATION OF THE EARTHQUAKE RECURRENCE ALONG THE NORTH ERN MARGINAL FAULT OF THE XIHUASHAN

Song Fangmin, Zhu Shilong, Wang Yipeng, Deng Qidong, Zhang Weiqi

1983, 5(4):  29-38. 

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Through the mapping on the large scale and measurement in the epicentral area of the Haiyuan earthquake, left-lateral displacements of the gullies of various kinds which cross the earthquake fault are found. The data show that the larger the gullies are, the larger the horizontal displacements. Among these displacements, there are 73 offsets of small gullies and farming banks which were formed in the earthquake of 1920 (M= 8.5) with the maximum of about 10m.Displacements of various gullies are relatively large between the Yanghuzhang valley and the Gudunzi valley.On the basis of displacements of gullies on the second terrace, initial time of offseting as well as the average and maximal displacements associated with a great earthquake, we have calculated the slip rate and seismic recurrence for 8.5 earthquakes along the studied fault.

PRELIMINARY STUDY OF STRUCTURES OF EARTH'S CRUST AND UPPER MANTLE AROUND DAYAWAN GULF,GUANGDONG PROVINCE

Shao Xuezhong, Zhang Jiaru, Li Zhenjiang, Sun Weigou, Zhang Siya

1983, 5(4):  39-51. 

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This paper discribes the results of preliminary study on the structures of earth's crust and upper mantle surrounding the Dayawan gulf inferred from converted waves. Three observed profiles have been completed, with a distance of about 5—10km between observed points by use of 3-component short period seismograph, type DD-1 made in China. During about two months 60 distant earthquakes have been recorded to provide a number of available data for analysis of P to SV converted waves.Two stable groups of PS waves can be recognized in this region: PS_C and PS_M from the boundary of earth's crust. 1-2 groups of PS converted waves from upper mantle have been picked up as well, although they were shown unstable.Three cross sections of deep-seated structures have been made up along profiles. The Moho discontinuity and the boundary C in the crust can be determined and average depth of boundary C and M ranges from 12 to 14 km and 28 to 30km, respectively.Based on above-mentioned seismic cross sections we can outline the isobathy line map of boundary C and M for this region. On the map in question , evident is major feature of the local uplift in the central of the region, with a trend of NE and an amplitude of 2-3km. In correspondence with the Moho uplift, there is a local depression in isobathy line map of boundary C.The following conclusions can be drawn:1. Method of PS converted waves can be used efficiently for investigating the deep-seated structures in South China as well as in North China;2. The deep-seated structures in seismically passive zones of the Dayawan gulf are very different from those of the active zones in North China. The deep-seated structures in the region of the Dayawan gulf are characterized by smooth distribution of deep-seated boundaries, less change of layer thickness and a few faults on the boundary.

DISCUSSION ON CHARACTERIZED DISTRIBUTIONS OF PRECURSORY ANOMALOUS AMPLITUDES IN/NEAR SOURCE AREA

Peng Wanli, Zhou Ruiqi, Xu Zhaoyong

1983, 5(4):  52-60. 

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Based on the observations of four major shocks, nonlinear correlations between the anomalous amplitude of precursory information and epicentral distance are discussed in this paper. It is found that the maximal anomalies are not often at epicentre but on the edge of the source area. It is suggested that the gaps enclosed by the anomalous peaks can be used to predict occurrence of an earthquake.

PRELIMINARY STUDY OF WNW-TRENDING CHANGPING-FENGNAN BURIED ACTIVE FAULT ON THE SOUTHERN MARGIN OF YANSHAN MOUNTAIN

Li Jianhua

1983, 5(4):  61-66. 

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The WNW-trending Changping-Fengnan buried active fault on the southern margin of Yanshan Mountain is clearly identified on the Landsat images as a dark bandlike linear imagery several kilometers wide. This imagery was produced under a specific climatic condition due to the slight difference of loose sediment of watery zone and its adjacent area in density represented by reflective electromagnetic wave spectra on the imeges. And the watery zone was formed with the migration of ground water along the active fault and ascension on ground surface.By analyzing geological and geophysical data, it has been proposed that Changping-Fengnan buried active fault might be formed during the late Mesozoic time. This was a crustal fault, characteristic of dextral slip, that has dissected the Conrad and the Moho discontinuity since Quaternary.

QUANTITATIVE ANALYSIS OF RECENT TECTONIC STRESS FIELDS AND SEISMICITIES IN NORTH CHINA IN TERMS OF HOLOINTERFEROMETRY

Wang Chunhua, Liao Suqiong, Qian Ruihua, Xie Weijiang

1983, 5(4):  67-73. 

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This paper is an application of laser holographic photoelasticity and quantitative analysis in the study of recent tectonic stress fields and seismicities in North china. Several points can be covered:1) In the area of North china the principal compressional stress trends ENE—NE;2) In this region right-lateral motion can be found on those faults trending ENE—NE and left-lateral one, on those trending EW and NE;3) Regions of stress concentration are Haicheng, Weifang, Linyi, Shijiazhuang, Yi-xian, Linfen, Huayin and Pingluo in North China.

SYMPOSIUM ON SEISMOGENESIS,SPONSORED BY CHINESE GEOPHYSICAL SOCIETY,WAS HELD IN LANZHOU

1983, 5(4):  74-74. 

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THE SPORO-POLLEN ASSEMBLAGE IN THE LUO 4 DRILLING OF LOP LAKE IN UYGUR AUTONOMUS REGION OF XINJIANG AND ITS SIGNIFICANCE

Yan Fuhua, Ye Yongying, Mai Xueshun

1983, 5(4):  75-80. 

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The Luo 4 drilling is situated south east of the Lop Lake, Xinjiang. Many pollen grains of shrub and herb plant have been found in the sediments from this drilling. They mainly belong to Ephedra sp,, Artemisia sp., Chenopodiaceae and Gramineae.On the basis of the results obtained it is believed that the vegetation from sediments of the Luo 4 drilling is mainly typical of a desert vegetation.The result of sporo-pollen analysis shows that there was a climatic fluctuation during the formation of the sediments in the Luo 4 drilling, of which the lower part (4m below) reflected a cool and dry climate and the upper part (4m above), a temperate and quite dry one.Based on sporo-pollen and ¹⁴C dating, the geological age of the Luo 4 drilling sediments can be inferred to be Late Pleistocene for the lower part and Holocene for the upper part.