Table of Content

05 September 1990, Volume 12 Issue 3

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Brief Report

PRELIMINARY STUDY ON THE CONDUCTIVITY STRUCTURE OF THE CRUST AND UPPER MANTLE:THIRD REPORT ON THE GEOSCIENCE TRANSECT FROM XIANGSHUI,JIANGSU, TO MANDAL,NEI MONGOL

Jiang Zhao, Sun Jie, Xu Chang-fang, Wang Ji-jun, Shi Shu-lin

1990, 12(3):  193-206. 

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The features of deep conductivity structure along Xiangshui-Mandal Geoscience Transect are described in this paper basing on newly obtained magnetotelluric data. Large resistivity contrasts can be found on both borders of North China platform. The depth of the crustal high conductivity layer changes abruptly from 21km to 34km beneath Nei Mongol fold system on the northwestern end of the transect. While it is absent beneath Subei-Jiaonan terrane on the southeastern end of the transect,where two high conductivity layers are detected in the upper mantle. The crustal high conductivity layer is well developed in the Cenozoic basins within North China Platform with a relatively shallow depth. The depth of the upper mantle high conductivity layer along the transect ranges from 60 to 127kin,with a general tendancy of decreasing in depth from northwest to southeast,forming local uplifts beneath Ho-Bao basin,Central Hebei depression and Tan-Lu fault zone. It reflects the characteristics of the regional geology and the deep tectonic movements.

CHARACTERISTICS OF THE STRUCTURE OF MAGNETIC LAYER IN THE NORTH-SOUTH SEISMIC BELT IN CHINA

Wu Gang, Yu Qin-fan

1990, 12(3):  207-214. 

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An equivalent model of the bottom of magnetic layer and a two-dimensional inversion method with variable magnetic inclination in polar coordinate system are developed in this paper. By using the model and method,the structure of the magnetic layer and its relation to seismicity in North-South Seismic Belt are discussed in detail on the basis of the magnetic data in the region. The following conclusions can be drawn from the result:1. Both the northern and southern parts of the magnetic layer in the belt are thicker than the central part with their demarcati,on lines through 28癗 and 34癗,respectively;2. The bottom of the magnetic layer appears as a N-S-trending broad uplift zone in an area from 100癊 to 106癊,getting shallower from south to north,with its shallowest part at 103癊;3. The P,T condition of the magnetic,layer is suitable for the accumulation of stress energy,and thus the magnetic layer is a seismogenic layer. Most of the earthquakes occurred within the magnetic layer,especially in the transition zone from its thick to thin parts. The seismicity is usually accompanied by faulting,hot spring and magmatism.

PRELIMINARY STUDY ON CARBON AND OXYGEN ISOTOPIC COMPOSITIONS IN CARBONATE-MYLONITE AND CARBONATE-PSEUDOTACHYLITE,BEIJING AREA

Shangguan Zhi-guan

1990, 12(3):  215-220. 

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Isotopic compositions of carbon and oxygen in some carbonate-mylonites and carbonate-pseudotachylites distributed in the Shuiyu ductile shear zone and Beishicheng fault zone,Beijing area have been measured. The result shows that the δ¹⁸O and δ¹³C values in the carbonates all decreased to different degree during their ductile deformation. The data of carbon isotope geothermometer show that the temperature for formation of the carbonate-mylonite is about 260—270℃,and it was formed in a slowly cooling geological environment. The temperature for formation of the carbonate-pseudotachylite is about 420—450℃,and it was formed in a geological environment with higher temperature and rapid cooling. The formation of carbonate-pseudotachylite may be related to a rapid rupture process of the fault during earthquake. The study of stable isotope compositions of the carbonates distributed on the fault planes may be useful for demonstrating paleoseismic events.

FINITE ELEMENT SIMULATION OF TENSITE AND COMPRESSIVE EFFECTS AT THE ENDS OF SHEAR RUPTURE ZONE AND THEIR STRUCTURAL FEATURES

Xu Xi-Wei, Deng Qi-Dong

1990, 12(3):  221-228. 

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En echelon shear rupture zone is of a common type of ruptures in the crust. Several models which contain one,two and three fanlts arranged in en echelon arrays are calculated by twe-dimensional finite element method. The result indicates that a shear rupture zone acts just as a single shear fault in kinematics. There exist tensile and compressive areas at both ends of a shear rupture zone. They are the products of the relative movement of two blocks along the shear rupture zone. The structure of rock bridge (pullapart basin or push-upward) and the meterial mechanical coefficient are two main factors affecting the concentration of the tensile and compressive stresses at the ends of a shear rupture zone. Moreover,the structural features and faulting types in tensile and compressible areas are discussed in the paper.

THE SPATIAL SCANNING OF THE FREQUENCY-MAGNITUDE DISTRIBUTION OF EARTHQUAKES IN BEIJING-TIANJIN-TANGSHAN-ZHANGJIAKOU AREA

Ma Jin, Lei Xing-lin, Jiao Wen-jie, Liu Li-qiang

1990, 12(3):  229-236. 

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By the spatial scanning of the frequency-magnitude distribution of earthquakes in Beijing-Tianjin-Tangshan-Zhangjiakou area,it is found that there are three types of the distribution of earthquakes in this area. Type Ⅰ is consistant with a linear b-value model and types Ⅱ and Ⅲ are consistant with a characteristic earthquake model. The area,in which large earthquakes have never occurred in last 22 years,often has a distribution of type Ⅰ,and the area,in which large earthquakes have occurred,often has a distribution of type Ⅱ and Ⅲ. Result of the rock mechanical experiments shows that the frequency-magnitude distribution of acoustic emission events in rocks has a high b-value in the process of fracturing and fracture joining,and has a lower b-value in the process of frictional slip accompanied with stick-slip. But the frequency-magnitude distribution in a whole process of rupturing and slipping of intact rocks is consistant with the characteristic earthquake model. Therefore,it is evident that the frequency-magnitude relationship may represent a stage of structural deformation for an area. The nonlinearity of frequency-magnitude distribution might give us more information about the geometrical characteristics of the rigional structure. The emergence of nonlinearity of F-M distribution in area with moderate magnitudes but without large magnitudes in the considered period may indicate that the area is in a state much closer to the critical point for the unstable sliding.

STRESS DROP,SLIDE VELOCITY AND SUDDEN-INSTABILITY CRITERION FOR EARTHQUAKES

Wang Sheng-zu

1990, 12(3):  237-242. 

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The analysis of hypocenter parameters by the tectonophysical method from data of earthquake cases shows that the stress drop,Δτ_sei is proportional to the slide velocity along a fault,V_sei,and the ratio (Ω=V_sei/Δτ_sei) is affected by the acting direction of principal stress,expressed by the slide angle θ. The frequency distribution of ratio Ω for a large amount of earthquake cases approximates to the Gaussian distribution with a mean value of Ω equal to about 0.092,which corresponds to the dominant sliding angle of about 60?. The proportional relationship between V_sei and Δτ_sei proves further the reasonableness of selecting stress drop to be an index of the sudden-instability criterion for earthquakes. In addition,the dependence of Ω on θ provides a new possible approach to estimating the direction of the principal stress.

KNOWLEDGE GAME ON NATURAL DISASTERS AND DISASTER REDUCTION WILL BE HELD IN CHINA

Li Zheng

1990, 12(3):  242-242. 

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SUDDENLY INCREASING TEMPERATURE ON GROUND SURFACE AS ONE IMMINENT EARTHQUAKE PRECURSOR IN CASE OF1988 LANCANG-GENGMA (M7.6, 7.2) EARTHQUAKES

Xu Xiu-deng, Qiang Zu-ji, Dian Chang-gong

1990, 12(3):  243-250. 

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In this paper an analysis of many data of temperature on ground surface suggests that the suddenly increasing abnormal temperature on ground surface in seismic area is a kind of imminent earthquake precursor,which was found before two Lancang-Gengma earthquakes on Nov. 6,1988. Here we discuss the relationship between the regularities of the increasing temperature in time and space and earthquake activity,and the mechanism of the in creasing temperature on ground surface.

ESTABLISHMENT OF A CATION-LEACHING CURVE OF ROCK VARNISH AND ITS APPLICATION TO THE BOUNDARY REGION OF GANSU AND XINJIANG,WESTERN CHINA

Zhang Yu-ming, Liu Tan-zhuo, Li Shu-wei

1990, 12(3):  251-261. 

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Rock varnish dating method is a new chronologic method which has been widely used in dating the Quaternary deposits. A cation-leaching curve of rock varnish from the boundary area between Gansu Province and Xinjiang Region,Western China,is established in this study. In this paper the ages of middle-late Quaternary deposits and fault activity is also discussed from the varnish dating data.

CURRENT STATE OF RESEARCH ON CONTINENTAL SEISMOGENIC LAYER——INTERNATIONAL WORKSHOP HELD IN BEIJING,APRIL 27-30, 1990

Ma Zong-jin, Zhang Jia-sheng, Liu Cuo-dong, Liu Ruo-xin

1990, 12(3):  262-264. 

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ON THE RECENT TECTONIC MOVEMENT IN NORTH CHINA

Zhang Cun-de, Xiang Jia-cui

1990, 12(3):  265-271. 

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The recent vertical movement in North China region inherits generally the development of neotectonic framework. Eight zones in the region can be distinguished,such as Ordos uplift zone,Fen-Wei subsidence zone,Taihangshan uplift zone,Bohai subsidence zone,Heibei-Henan subsidence zone,Hebei-Liaoning uplift zone,Qilu uplift zone and Ding xian-Dezhou transition zone. In addition,some small subsidence zones also exist,such as Yinchuan,Zhongwei,Wuyuan-Hohhot and Xialiaohe subsidence zones.The recent horizontal movement is not intense in this region,annual strain rate is an order of 10^-6. Orientation of principal compression axis is NE 50?,and orientation of principal extention axis is SE predominant. Annual strain value of the latter is more than that of the former.

FRACTURE TRACES BY HISTORIC EARTHQUAKES IN SHANGGUANG-PANQU ZONE OF DALI AREA,YUNNAN PROVINCE

Ran Yong-kang, Wang Jing-bo, You Hui-chuan, Zhang Wan-xia, Li Ru-cheng

1990, 12(3):  272-274. 

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RECENT TECTONIC MOVEMENT ALONG THE NORTH-FRONTAL QILIANSHAN FAULT ZONE

Ye Chen-zhi

1990, 12(3):  275-281. 

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North-frontal Qilianshan fault zone is located in the northern part of QinghaiXizang Plateau,being a large active tectonic belt in West China. A lot of data has been obtained from precise levelling and displacement measurements across the faults in this region over 20 years. Based on these data the tectonic actiyity of the fault zone is analysed.On eight levelling arrays over the region,the largest displacement gradient R_m is found to be perpendicular to the Hexi Corridor. Due to the effect of the eastern extension of Altun fault,the variation in displacement along the Hexi corridor is large (up to 0.17),with exception of Chejipu-Xingmipu section,where R ranges in general from 0.01 to 0.05,and the variation is small.In the northwestern part of Yumen area,however,R reaches up to 0.20,and the mountain body has a tendency to uplift. In Zhangye-Mile section,the southeastern part of the corridor,R is relatively small (0.06),and the mountain body has a little crossfault subsidence relative to the basin. Cross-fault trilateration has revealed a shortenning rate of 0.2—0.5mm/a. The fault is characterized by compressional shearing,accompanied by antisymmetric elevation and subsidence. In addition,local uplift and subsidence as well as faulting have occurred in the inner part of the Hexi Corricor by the effect of secondary tectonic movement.

SECOND SYMPOSIUM OF CHINESE YOUTH GEOLOGISTS WILL BE IN BEIJING IN 1991

Xu Xi-Wei

1990, 12(3):  282-282. 

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THE SPORO-POLLEN ANALYSIS OF LATE PLEISTOCENE DEPOSITS IN HO-BAO BASIN OF NEI MONGOL

Dou Su-qin, Nie Zong-sheng, Mai Xue-shun

1990, 12(3):  283-289. 

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This paper deals with the results of sporo-pollen analysis of the sediments from second-step platform in the Ho-Bao Basin,Nei Mongol.On the basis of analysis and study of spores and pollens,it is convinced that the vegetation related to the sediments of the second-step platform was transitional one from forest steppe to desert vegetations,and the climate changed from humid and cool to dry and cool climate during the formation of the sediments. Comparison of the sporo-pollen assemblages shows that the second-step platform was formed in Late Pleistocene,which corresponds to the time of formation of Salawusu Group.