Table of Content

09 March 1983, Volume 5 Issue 1

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Brief Report

THE INVERSE ESTIMATION OF FAULT MOVEMENT IN GEOLOGICAL TIME WITH BOUGUER GRAVITY ANOMALY NEARBY THE FAULT

Zhang Chao, Zhao Guoguang

1983, 5(1):  1-8. 

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In this paper,using the integral expression of additional gravity field due to displacement of point mass within the deformed medium,the relation between the Bouguer anomaly on the ground and permanent deformation caused by fault movement for a long time in Maxwell half-space is quantitatively studied. A general method for inversely estimating the fault slip trend in geological time by the Bouguer anomaly distribution nearby the fault is obtained,which is principally valid in identifying the fault movement,especially for strike-slip mode in a plain region.By means of a numerical integration,the authors have calculated gravity anomalies on the ground associated with the fault motions in Luannan region. By comparing the theoretical results with the practical gravity measurements,the authors come to a conclusion that the right-lateral strike-slip and dip-slip of a fault may occur in the geological time.

DEFORMATION MICROSTRUCTURES WITHIN A SHEAR ZONE IN GRANODIORITE

He Yongnian, S. H. Kirby, P. Segall

1983, 5(1):  9-14. 

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An E-W striking,subvertical shear zone in the granodiorite of Lake Edison,Central Sierra Nevada,California,U. S. A. was studied to determine the variation in deformation microstructures with distance from the shear zone center. Field and thin-onsecti relations demonstrate that the shear zone nucleated on a pre-existing quartz-filled fracture. Selective chemical etching was used to reveal dynamically recrystallized grain-boundaries of quartz in the granodiorite,offset aplite dike and deformed fracture-filling. The recrystllized grain-size of quartz decreased with increasing shear strain toward the center of the shear zone. This would suggest significant variation in the apparent magnitude of tectonic stress. Important effects of strain on the microstructures are suggested by our observations. This phenomenon is contrary to that usually reported in experimental studies,which found no dependence of recrystallized grain size on strain.

SEISMOLOGICAL STUDY OF DYNAMIC PROBLEMS OF RECENT FRACTURE IN THE NORTH CHINA FAULT BLOCK REGION

Jiang Pu, Dai Lisi, Xu Feng, Gao Qingshan

1983, 5(1):  15-28. 

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Some dynamic problems of recent tectonic fracture were studied by analysing seismi-city and seismic mechanism parameters and the rate of slip and apparent stresses for majcr shear fracture belts in the North China fault block region.Several points can be covered as:1. The recent crust fracture system formed by seismic activity and its distribution look like stable shear fracture network and the direction of the line dividing equally an acute angle of the network points to nearly east-west,which shows that principal stress field would be from nearly east-west horizontal compressive stress field in whole region.2. The strike slip and near strik slip faulting among seismic faults and horizontal and nearly horizontal maximum and minimum stress in soucrce region are dominant,but some oblique and dip slip faulting,and oblique and vertical stress exist and can not be neglected,according to the results of the mechanism solution for 92 events in this region,which shows the characteristics of combinatory action of horizontal or nearly horizontal force with local oblique or vertical force applied on the North China fault block region.3. The rate and grade of seismic slip along major shear fracture belts computed from seismic moment are smaller than 0.1 cm/yr. which implys that the regional crust faulting may be formed and local shear faulting may be a main way of releasing stress from earth's crust.4. The average seismic tectonic stress of medium-sized events and the shear stress around small circular crack seem to be up to from several tens of to several hundreds of bars in magnitade.

TECTONIC STRESS FIELD AND SEISMICITY IN THE THREE GORGE REGION ON THE CHANGJIANG RIVER

Liu Suowang, Gan Jiasi, Xue Hongjiao

1983, 5(1):  29-41. 

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In this paper,according to the horizontal dislocation patterns of the active fault and the composed features of deformation and fracture having taken place in the Cretaceous-Tertiary and Quaternary strata,related parameters of main stress axes (σ₁,σ₂ and σ₃) at the observed sites are obtained by means of the stereographic projection; and by combining with the data of focal mechanism,macroscopic earthquake phenomenon,crustal deformation and others,the tectonic stress field from the time of Himalayan movement is remoulded. It is show that,the axis direction (σ₁) of principal compressive stress in various tectonic zones of the Changjiang River and their related regions may have some tendency to evolve progressively,and may appear to be intersected state with time and space. During early and late neotectonic movement σ₁-axis of central faulting zone (A) in Hubei province is NNE-SSW and ENE-WSW,and at this same time,this axis in the southern segment of southwest tectonic zone (B) is NW-SE,but the northern position it is dominated by NNW-SSE In the eastern part of northwest tectonic zone (C),σ₁-axis direction is mainly NE-SW,while in its southwestern and northern segments this directron is under the control of NW-WNW-SE-ESE. In the recent stress field,σ₁-axis in the eastern portion of the (A) zone is NNE-SSW,in the western portion it is ENE-WSW,and the included angle between them is 40皁r so; σ₁-axis in the southwest (B)zone is NE-SW,but in the northeast part it is nearly N-S; in the whole (Ozone,ranges from NW-SE to WNW-ESE. with the obvious NE-SW principal compressive stress,It is quite evident that σ₁-axis distribution seems to be in the lack of uniform direction between or in the tectonic zones.According to "elastic rebound hypothesis" and "Boushinger effect",the relation shup between the alternation of σ₁-axis direction and seismicity of the Danjiang area is dealt with in the paper. Finally,the authors suggest that,in the tectonic stress field the alternation of σ₁-axis may be regarded as one of the seismogeological marks in the intraplate weak shock regions (or zones),based on the comparison of abovementioned features with the cases similar to relative stable segments of strong earthouake regions in North China and South-west China (for example Honghe River area).

GEOPHYSICAL INVESTIGATION OF STRUCTURES BENEATH THE OKINAWA TROUGH

Jin Xianglong, Tang Baojue, Zhuang Jiezao, Yu Puzhi

1983, 5(1):  42-42. 

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THE DEEP STRUCTURAL EVIDENCE ON INTENSE SEISMICITY FOR THE TANCHENG-LUJIANG FAULT BELT

Zhan Chuanfu

1983, 5(1):  43-51. 

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This paper deals with tectonic features and seismicity for the Tancheng-Lujiang fault belt in terms of aeromagnetic data.Some hidden segments of this fault belt,south of Tancheng,are discussed by use of aeromagnetic data in large and moderate scale. The Changyi-Daidian fault and An-qiu-Juxian fault are considered to be a main fault on the studied fault and they have extended down to the upper mantle.The author tries to find out the law for strong seismic activity. It may provide another base for modifying the location of the epicenter of 1668 earthquake of magnitude 8.5.

TRACE OF THE ANCIENT EARTHQUAKES FOUND IN THE DALI REGION OF YUNNAN PROVINCE

The Seismo-geological Study Group, Experimental Site of Western Yunnan Province

1983, 5(1):  52-52. 

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EXPERIMENTAL STUDIES OF THE FRACTURE MODE AND VELOCITYFFOR THE CENTER OF MARBLE SLAB UNDER THE STRESS LOADING

Xu Zhaoyong, Peng Wanli

1983, 5(1):  53-58. 

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The fracture mode and velocity for the center of marble slab under the stress loading were studied in the experiment. The results are as follows.1. Most cracks cross veins at an angle of about 45° and some nearly 0°. Failures of the rock usually occurred at different time,with one of the cracks first going through the slab. There was shear strain before failure. The direction of the main fracture was in disagreement with the surface of minimum tensile strength. The hydraulic fracturing direction of main fracture is probably in disagreement with the direction of maximum horizontal principal compressive stress.2. logⅤ-L diagram (Ⅴ,velocitv of propagating cracks; L,ratio of crack's length L to (R₂-R₁):R₁ R₂,radius of the indenter and the marble slab,respectively) shows that three stages for the failure of the slab: in the initial,Ⅴ was relatively large (10mm/s) when cracks occurred and then exponentially decreased (0.05mm/s) with increasing L; in slowly fracturing,Ⅴ alternatively increased with increasing L(0.1-2mm/s); and in rapidly fracturing,Ⅴ increased abruptly and a noticed turning might be found,over which log Ⅴ increased exponentially (to 1 m/s) and then fractures occurred.

THE EARTHQUAKE DISASTER FOR THE EARTHQUAKES IN CHINA AND ITS IMPLICATIONS

Wang Kelu, Song Huzhen, Liu Huimin

1983, 5(1):  59-69. 

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It is concluded that the surface manifestation of the earthquake disaster was associated with 620 of 2054 shocks that appeared in China in a span of time from 1177 B. C. to 1969. The earthquake disaster is represented by landslip,ground fissure,ejected water and sandblow and the like.Viewed from data in this regard,it can be found that the distribution of the earthquake disaster is characterized by regionalization,segmentation and reproducibility. Where an intensity is up to Ⅵ,the earthquake disaster may be distinguished. With an intensity more than Ⅸ,there would be very a possibility for an earthquake to lead to a severe damage. An earthquake magnitude can be shown to be proportional to a distance of the segment where the earthquake damage was noticed from the epicenter. This can be written as a formula and this is a significant reference to prediction of the earthquake disaster,seismic mierozoning and earthquake resistance and the like.

GEOLOGICAL EVIDENCE FOR THE RECENT ACTIVITY OF HUANGZHUANG FAULT IN BEIJING

1983, 5(1):  70-70. 

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THREE ESSENTIAL SIGNS OF CRUSTAL DEFORMATION DURING THE PERIOD OF THE STRONG EARTHQUAKE PREPARATION IN THE CASE OF TANGSHAN

Zhang Cunde, Xiang Jiacui

1983, 5(1):  71-77. 

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Analyses of data for the crustal deformation before and after the Tangshan earthquake indicate an evolution of the crustal deformation. Three signs may have emerged from the process of the preparation of a strong shock:1. rapidly increasing regional stress field;2. occurrence of intensively active structures and change of the regional stress field and;3. formation of concentrated deformation points on the seismogenic structures. From the signs of crustal deformation,it is believed that east of Daofu may be asegment of the potential earthquake.

RECOVERY OF REMAINS OF THE ANCIENT EARTHQUAKE ALONG THE DONGLING AREA OF THE LIYANG DISTRICT

Ji Youting, Ding Hao, Wang Qingqing

1983, 5(1):  78-78. 

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PALYNOLOGICAL EVIDENCE OF LATE PLEISTOCENE SEDIMENTS OF PINGHAI TOWN,GUONGDONG PROVINCE

Yan Fuhua, Ye Yongying

1983, 5(1):  79-79. 

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HOLOCENE STRATIGRAPHY OF XIANGYANG BOREHOLE,QIDONG COUNTY,JIANGSU PROVINCE

1983, 5(1):  80-80. 

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