Table of Content

09 December 1998, Volume 20 Issue 4

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Brief Report

THE CRITERIA FOR ACTIVE FAULT SEGMENTATION IN SEISMIC SAFETY ASSESSMENT OF MAJOR ENGINEERING

Zhang Peizhen, Mao Fengying, Chang Xiangdong

1998, 20(4):  289-301. 

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Fault segmentation has been playing an important role in seismic safety assessment of major engineering. There are,however,many problems either in the methods of fault segmentation themselves or in the applications to the engineering practice. This paper uses results of segmentation along three faults that have been regarded internationally as the most reliable fault segmentation to analyse the segmentation methods,segmentation features,and their uncertainties. The purpose is to establish several criteria for fault segmentation that can be used in seismic safety assessment. Following criteria are proposed: (1) Sufficient and concrete evidence is necessary for fault segmentation; (2) Uncertainties associated with fault segmentation require comprehensive segmentation features. It seems that more than three methods and more than four features may reliably delineate a fault segment; (3) In order to increase reliability of fault segmentation,reliable features should be used as much as possible; (4) It is important to pay attention to persistency of fault segmentation boundaries.

THE TL CHRONOLOGICAL EVIDENCE OF THE RECENT ERUPTION IN WUDALIANCHI VOLCANIC CLUSTER

Ji Fengju, Li Qi

1998, 20(4):  302-304. 

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The ages of bake layers of xenolith within the volcanic lava associated with the recent eruption in Wudalianchi volcanic cluster have been dated by TL dating technique for the first time. The results ((264±19)a B.P. and (273±19)a B.P.) are consistent with historical record (1719～1721) of the volcanic eruption in the error range. It shows that the lava made of these volcanoes is the eruptive materials with the corresponding period. And it also indicates that the very young ages of volcanic eruption can be dated accurately by TL technique which provides a reliable dating method for resuming recent eruptive history.

A STUDY OF THE YAOHE PLUTON AND ITS NORTHERN DUCTILE SHEAR ZONE AT NORTHEASTERN MARGIN OF THE DABIE MOUNTAIN

Yang Kunguang, Ma Changqian, Xu Changhai, Yang Weiran

1998, 20(4):  305-311. 

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The Yaohe pluton,found at the northeastern margin of the Dabie mountain recently,is a quartz diorite intrusive body. A precise ⁴⁰Ar/ ³⁹Ar hornblende dating age demonstrates that it had emplaecd in middle Jurassic (155.8Ma). The aluminum in hornblende geobarometer analysis indicates the crystallization pressure of this pluton is about (2.65±0.5)×10⁵Pa,and its emplaced depth is about 9km. Mylonite zone in the northern margin of the Yaohe pluton,those foliations dipping to NE direction,is the resultant of ductile shearing by the Mozitan Xiaotian fault. Macrostructures and micro ultramicrostructures in this ductile shear zone show the hanging wall of this normal fault moves down to NE direction,and the vertical difference uplift between two fault walls reaches to 9km. Combined with regional and deep structures,our study shows that eastern Diabie orogen does not possess“flowered”structure.

NEOTECTONIC FEATURE OF THE NUJIANG FAULT ZONE IN WESTERN YUNNAN

Li Jingchang

1998, 20(4):  312-320. 

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The Nujiang fault zone can be divided into two sections of SN and NE strike and is arc in plane. Its deformation is mainly dextral shear of Himalayan age which could be divided into transpression deformation in early stage and transtension deformation in late stage and has different characteristics in two sections. The transpression structure in early stage has developed mylonite zone and its plunge angle of lineation was different in two sections. In the section of NE strike,both doxtral shear and thrusting are quite clear. The transtension structure in late stage developed mainly in southern end of two sections and shaved progressive trend from south to north since Miocene epoch according to distribution characteristic of the oldest stratum in basin. The complexity of Nujiang fault zone in Himalayan age was caused by the inhomogeneity of deformation in time and space. These deformation features are described and analysed in detail in the paper.

ACTIVE FEATURES OF THE EASTERN LIUPANSHAN PIEDMONT REVERSE FAULT ZONE SINCE LATE QUATERNARY

Xiang Hongfa, Guo Shunmin, Zhang Bingliang, Zhang Wanxia, Yasutaka Ikeda, He Honglin

1998, 20(4):  321-327. 

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New results of field investigations show that the eastern Liupanshan piedmond fault is an active reverse fault zone since late Quaternary. This fault zone can be divided into north,middle,and south three segments in the light of the changes of its strike,active ages and active natures. The north segment is dominated by left lateral strike slip,while the middle and the south segments are characterized by thrust movement. The active age of the north segment is youger than the south and larger horizontal dislocations occurred on the north. Such changes of fault activities since late Quaterary are associated with obstruction for the eastward slide of the Xihuashan Liupanshan block as well as the variation of the fault strike.

RELATIVE CREEP RATE AND CHARACTERISTIC EARTHQUAKE RECURRENCE INTERVAL EXAMPLE FROM THE XIAOJIANG FAULT ZONE IN YUNNAN, CHINA

Shen Jun, Wang Yipeng, Song Fangmin, Li Yingzhen

1998, 20(4):  328-331. 

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The displacement of all earthquakes with all ranges of magnitude except to the characteristic earthquakes was termed as“relative creep”in this paper. Affection of the relative creep to the recurrence interval of the characteristic earthquakes is analysed. A function is deduced to calculate the relative creep rate. More reliable estimations of seismic interval can be made by comparing the calculated results with the paleoearthquake data.

VARIATIONS OF THE COMPOSITIONS AND STRUCTURAL STATE OF FELDSPARS DURING MYLONITIZATION IN THE DUCTILE SHEAR ZONES OF THE SOUTHERN TANCHENG LUJIANG FAULT

Yang Xiaoyong, Yang Xueming, Liu Deliang, Dong Shuwen, Dai Jinxing

1998, 20(4):  332-342. 

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On the basis of observations of the mineral assemblages and microstructures in the tectonic rocks from the ductile shear zones in the southern part of the Tancheng Lujiang fault,the feldspar was analyzed by means of EPMA,XRD and infrared spectrometry. It has been found that there are close correlations between the intensity of the rock deformation and the composition and structural state of the feldspars in the rocks. The relations are as follows: the positive correlation between the intensity of shearing deformation and the parameters of the feldspar structural state,the deformed index of feldspar(I_ss) and An values of plagioclase,and the negative correlation between intensity of shearing deformation and the unit cell volume of the feldspars and Si/Al value of the plagioclase. These results provide simple and effective methods for studies of the deformation metamorphism in structural strain field.

PRELIMINARY STUDY OF THE INNER SHEAR STRUCTURES IN THE COMPRESSIONAL STEPOVER OF STRIKE SLIP FAULT ZONES

Song Fangmin, Yu Weixian

1998, 20(4):  343-348. 

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Field investigation and airphoto interpretation have revealed that shear structures were developed in two compressional stepovers at Menggu on the northern segment of Xiaojiang fault zone and Dayingpan on the eastern branch of the fault zone. The shear structures appear on the earth surface mainly as fault troughes,fault scarps,surface fissures,beaded ponds and depressions. The inner shear structures in Menggu stepover extend about 5km,while those in Dayingpan stepover for about 3km. The results of high pressure and high temperature triaxial compression experiment,finite element analysis of fault dynamics of en echelon fault system and the deformation modeling of en echelon faults,have also indicated that the inner shear structures in compressional stepover are the same as those in the extensional stepover. They are newest active structures that are formed in stepover due to the action of local shear stress during the long term activity of the strike slip fault zone. The innner shear structures are intermittently outcropped on the earth surface. However,they are geomorphically less distinct than the shear faults controlling the boundary of the stepover. The inner shear structures in stepover display obvious recent activity. Historical and modern earthquakes often produced surface fractures along these structures. The surface fractures produced by the Dongchuan M7³/4 earthquake of 1733 were distributed just along the inner shear structures of Menggu and Dayingpan stepovers.

MINING-INDUCED EARTHQUAKES IN THE AREA AROUND THE YANGTZE GORGE PROJECT

Hu Yuliang, Yang Qingyuan, Chen Xiancheng

1998, 20(4):  349-360. 

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The results of observation by intensified seismic network and field investigation indicate that mining induced earthquakes are widely distributed around the Yangtze Gorge Project. To monitor and predict the reservoir induced earthquakes after filling in the future the seismic background should be made clear in the reservoir area before impoundment. Results of investigation show that mining induced earthqaukes are caused by the presence of underground mined out area which makes the stress difference increased and the rock mass failed. These events are quite different from the natural earthquakes. Seismologists and geologists should be careful to distinguish these two types of seismicity in analysing the regional seismic regime and predicting earthquakes as well as in studying the tectonic stress field and active faults.

A MODEL OF WATER INDUCED EARTHQUAKE RISK ASSESSMENT IN RESERVOIR AREA AND ITS NEIGHBOURING AREAS

Zhang Qiuwen, Li Anran, Wang Qingyun, Gao Shijun

1998, 20(4):  361-369. 

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We have made counting,calculating and analyzing to the entire combining environments (conditions) and its corresponding induced earthquake activity (results) of Hubei Qingjiang Geheyan reservoir area and its around region which is adjacent to the Yangtze Three Gorge reservoir and has accumulated water in 1993,and has seismological network. We propose a model of induced earthquake risk assessment in reservoir area and its around region. By using this model,the induced earthquake risk of Yangtze Three Gorge reservoir area and its around region is predicted and assessed.

GRAVITY FIELD AND CRUST UPPER MANTLE STRUCTURE OVER THE TARIM BAISIN

Yin Xiuhua, Li Yishi, Liu Zhanpo

1998, 20(4):  370-378. 

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In terms of gravity and magenetic anomaly data,we have studied the features of the Moho and Conrad discontinuit as well as the structures of density and magnetism over the Tarim baisin. The results show that the depth of Moho and Conrad is 37～44km and 15～38km,respectively. These two interfaces rise and fall in a roughly consisitent manner. Their spatial distributions have two dominant directions of NW and NE. The structure of the crust and upper mantle is characterized by consistent variations with the shallow geology as well as the basement. The structures of density and magentism in the cover and the crystalline crust are quite inhomogeneous along the vertical direction and between tectonic zones of uplift and depression.

MANTLE VORTEX CONVECTION IN CENTRAL EASTERN ASIA AND THE “VORTEX/NETWORK” CONTINENTAL DYNAMICS MODEL

Wang Shengzu

1998, 20(4):  379-390. 

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It is suggested on the basis of the vortical seismicity pattern and the distribution of large scale basins that a giant vortex tectonics exists in central eastern Asia,which is composed of the principal ring with diameter of about 2000km and some inner and outer branches,and the seismic activities migrate wavily along the vortex ring belts showing the propagation of vortex waves. The principal and inner rings rotate counter clockwise surrounding the Sichuan basin and the outer branches clockwise surrounding some large scale basins such as the Tarim basin. The evidence from seismic tomography indicates that the vortex tectonics is a consequence of the effect of upward mantle vortex on the base of lithospheric plate. The plate base driving force resulting from mantle vortex and the plate boundary driving force transmitted by plastic flow network,as two basic sources of driving forces,control the tectonic deformation and seismicities in the central eastern Asian continent. The“Vortex/Network”(Vor/Net) continental dynamic model provides a new approach to the studies of intraplate deformation,seismicity,and other geological geophysical processes.

THE CRUST MANTLE VELOCITY STRUCTURE AND DEEP TECTONICS IN THE DATONG YANGGAO SEISMIC REGION AND ITS ADJACENT AREA

Zhang Chengke, Zhang Xiankang, Gai Yujie, Zhao Jinren, Nie Wenying, Ren Qingfang

1998, 20(4):  391-398. 

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By using seismic data obtained from six wide angle reflection and refraction profiles passing through the area,the velocity structures and tectonics of the crust and upper mantle in the Datong Yanggao seismic region and its surrounding areas have been studied in detail. The results show that the crust and upper mantle velocity structures and tectonics have obviously both longitudinal and lateral heterogeneities. A great number of basement faults exist in the shallow crust which correspond to the deep structures. According to the characteristics of wave groups,the interfaces of crust interior and the undulations of the velocity contours as well as the boundaries of the low velocity abnormal blocks,three deep crustal faults are inferred. The most obvious low velocity block in the upper crust lies in the neighbourhood of Datong Yangyuan and in its south boundary exists a deep crustal fault. The Datong Yanggao earthquake swarm is associated this low velocity abnormal block as well as the deep faults.

SPATIAL DIFFERENCES OF THE EARTHQUAKE DISTRIBUTION ALONG THE ISLAND ARCS IN THE WESTERN PACIFIC AND THEIR CAUSES

Shi Xuejian

1998, 20(4):  399-404. 

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The seismic activity shows a distribution in narrow belt shape along the island arcs of the western Pacific,but the activity frequency differs greatly along island arc stretch or between island arcs. It is indicated that the activity frequency at both ends of island arcs or the cross points of island arcs and submarine ridges is more intense than in the middle,along the island arcs in the west (near the continent) of the Philippine Plate is more intense than the east (near the Pacific Ocean),along the parts in NNE and NE direction of Marianas arc is less intense than the other parts.These facts are closely related to the landform pattern,the forearc stress and island arc movement. It is suggested that these do not support that the Pacific plate moves and extrudes to the continent on its own initiative. The stress causing earthquakes may come from the movement of continent (or island-arc) to the ocean.

UPPER MANTLE STRUCTURE IN THE TIANSHAN AND ITS GEODYNAMIC ROLE TO CRUSTAL TECTONIC MOVEMENT

Xu Yi, Liu Futian, Liu Jianhua

1998, 20(4):  405-412. 

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Based on deep geophysical data,the geodynamic features of the upper mantle and possible driving mechanism are dicussed through a description of the basic structure of the upper mantle in the Tianshan seismic belt. It is supposed that horizontal squeezing deformation is one of main causes to make the upper mantle lithosphere shortened and thickened in the western Tianshan and the southern Tianshan adjacent to the western Kunlun; whereas in the central Tianshan and parts of the eastern Tianshan next to the southern edge of the Zhungeer basin,besides horizontal squeezing force caused by the plate movement,hot materials in the upper mantle perhaps flow upward to uppermost mantle,even intrude into the lower crust. As horizontal movement,they will influence the teconic activity of brittle medium in crust,in particular,a detachment layer with low velocity near the Moho under lower crust has become an important feature of the deep tectonics of the seismic region. In addition,since the India-Asia continental collision,the cool and solid root in parts of the Tianshan probably departed from original position and fell into upper mantle due to multiple squeezing deformation and thermal convection of small scales.

EARTHQUAKE ACTIVITIES AND SEISMIC SOURCE CHARACTERISTICS IN THE NEIGHBORING ZONE OF SICHUAN-QINGHAI-XIZANG(TIBET), CHINA

Gao Yuan, Zhou Huilan, Liu Zhen, Ma Lin, Wu Zhongliang

1998, 20(4):  413-422. 

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On the basis of analyzing regional earthquake activities and tectonic surroundings and analyzing seismic source rupture characteristics from GDSN broadband waveform data,combining the aftershock data,this paper discusses the relation of source rupture with aftershock distribution for nine earthquakes from 1988 to 1990 in Sichuan and Qinghai region,analyzes the migration of seismic radiated energy,studies the general pattern of earthquake activities since 1970 by Single link cluster (SLC) method,and discusses the tectonic movement background and geodynamic meaning in Sichuan Qinghai Xizang zone. The results show that the source features of these earthquakes are correspnding to the tectonic stress background. Sources of Qinghai earthquakes,however,all have apparent reverse slip components which are different from the strike slip source results of the researches in the past. And what those P axes of sources of Sichuan earthquakes basically are not near to horizontal direction is related to the local“Y”style of complicated tectonics.

GPS MEASUREMENT (1995～1996) IN THE NORTHERN AREA OF NORTH CHINA AND ITS EXPLANATION

Huang Liren, Guo Liangqian

1998, 20(4):  423-430. 

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Based on two periods of GPS remeasurement data of 1995 and 1996 in North China,the tectonic block movement of this area in present period is calculated and analyzed. The results indicate that the movement can be described by a four block movement model. The model consists of four blocks:Erdos block,Jin Ji Lu block,Jiaoliao block and Yinshan Yanshan block. As to the differential movement between the blocks,it is rather intense between Erdos block and Jin Ji Lu block,rather weak between Yinshan Yanshan block and Jin Ji Lu block and Tanlu fault zone is the moderate. The results also indicate that Bohai Sea may still have some extensional movement. In this area,if Erdos block is used as a reference,all the blocks in the eastern part are shifting toward the east.

THE RELATIONSHIP BETWEEN CRUSTAL FLUIDS AND MAJOR EARTHQUAKES AND ITS IMPLICATIONS FOR EARTHQUAKE PREDICTION

Che Yongtai, Liu Wuzhou, Yu Jinzi

1998, 20(4):  431-436. 

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Based on the results of recent studies of earth sciences,the relationship between crustal fluids and major earthquakes is discussed,including the influence of crustal hot fluids on earthquakes,the control of mid crustal fluids to major events,the ralationship between ancient fluids activities in fault zones and seismic recycles. New ways to determine the risk of strong earthquakes are proposed according to the features of distributions and activities of crustal fluids.

BUILDING BASIC GEOLOGICAL INFORMATION SYSTEM FOR ENGINEERING SEISMOLOGY USING THE TECHNIQUE OF GIS

Qu Chunyan, Ye Hong

1998, 20(4):  437-442. 

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Geographic Information System (GIS) is a new technique for spatial data processing which has been developed in recent years.This paper gives a presentation on application of GIS to the engineering seismological studies of the southeast coast regions. On the basis of software development by using the tool softwares Mapinfo and Arc/info,a basic geological information system of engineering seismology for the southeast coast regions has been established.It has many functions such as spatial inquiry and correlation analysis of spatial features. This system provides convenience for the effective management and synthetical use of all data in the engineering seismological studies.

ADVANCES OF RESEARCH ON THE INTRACRUSTAL SEISMOGENIC LAYER

Zhou Zhenheng, Deng Wanming, Xiang Caiying

1998, 20(4):  443-453. 

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The structural media,internal and external environments of intracrustal seismogenic layer are the important factors which constrain the generation and occurrence of earthquakes,especially strong ones in this layer. Till now,the main advances of studies on the earthquake generating environments of seismogenic layer are as follows: Seismogenic layer is located at the lower part of upper crust to mid crust with the assemblage of amphibolite facies (greenschist facies at upper part of this layer) metamorphic and granitic rocks. It is a medium layer with the relatively high seismic velocity,high density and high resistivity and its lower part corresponds to the transition zone of brittle ductile deformation and high shear stress zone. Intracrustal low velocity high conductivity layers exist below this zone or strong seismic sources. The stress field in this layer is characterized by regional variation and the cut off temperatures for intracrustal seismicity range 300℃～400℃. Seismic source faults are characterized by regional variation and may have different earthquake generating mechanisms for different region. To a certain extent,the strong body earthquake generating model may be useful for the efforts of earthquake prediction.

THE AFFECTION AND CORRECTION ON MAGNETOTELLURIC RESPONSE DATA FOR INCLINATION TWO-DIMENSION TERRAIN

Jin Guangwen, Zhao Guoze, Xu Changfang, Sun Jie

1998, 20(4):  454-458. 

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The affection of a two dimension inclination terrain mainly causes the distortion of the TM mode data,that decreases apparent resistivity curves of the TM mode. The affection could be ignored if the inclination angle is less than 10 degrees. The affection becomes more severe,as the inclination angle increases. The affection are also related with periods,the longer the period,the more severe the distortion causes,and remarkably affected periods become shorter as the inclination angle increases. In order to reduce these affections,two kinds of models which are homogenous half space with a flat surface and with a slope surface are respectively considered,magnetotelluric (MT) response functions have been computed and correction coefficients have been obtained.