Table of Content

14 June 2000, Volume 22 Issue 2

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Brief Report

CHARACTERISTICS OF GROUND RUPTURES CAUSED BY THE 1999 M 7.3 EARTHQUAKE OF JIJI,TAIWAN

Wang Yanbin, Wang Yong, Lee Jiancheng, Zhang Yuzhan

2000, 22(2):  97-103. 

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We have investigated the ground rupture zone caused by the M 7.3 earthquake of Jiji,Taiwan in 1999 after occurrence of the event.The results indicate that the seismogenic structure of this event is associated with the Chelongpu fault which is an active fault with dominant reverse-dip-slip and a sinistral strike-slip component.The total length of the ground rupture zone is about 80km.The vertical displacement on the ground along the rupture zone is mostly 1.5～3.5m and the hori-zontal displacement 3.0m.It is suggested that decollement between the basement and the cover of the Taiwan foothills has played an important role to the seismogenic fault.

BACKGROUND OF SEISMOTECTONICS OF MAJOR EARTHQUAKES IN THE SEA AREA SOUTH-EAST OF JINMEN

Lu Haojiang

2000, 22(2):  104-110. 

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The sea area south-east of Jinmen is active in major earthquakes located in the Quanzhou-Shantou seismic zone. This area is a faulting depressed gulf which developed in late Neogen. Occurrence of major events is associated with the structural activities of the gulf. In this paper, fractures and geophysical fields of this area are presented. Some geological and geophysical marks of seismotectonics are also suggested.

THE STRUCTURAL CHARACTERS OF THE PIEDMONT FAULT ZONE OF TAIHANG MOUNTAIN

Xu Jie, Gao Zhanwu, Song Changqing, Sun Jianbao

2000, 22(2):  111-122. 

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We have made a study to the piedmont fault zone of Taihang Mountain based on geological and geophysical data of recent years. Our new insights suggest that this fault zone began to emerge in Mesozoic and formed mainly in Early Tertiary. It consists of a series of NE-NNE strike faults, which were arranged obligue in a left-lateral manner. Its structures and activities have evident segmentation. Along the middle and north segment, the faults, such as the Baoding-Shijiazhuang fualt, are of large detachment faults. They stretch about 70km horizontally in the tilt direction with horizontal offsets of 17km and vertical offset of 5000-6000m during Early Tertiary. This fault zone belongs to the detachment structure in the upper crust rather than a deep fault. It has weak activities since Quaternary and has no direct cause relation with major earthquakes. It is, however, still somewhat of importance to studies of regional seismotectonics and earthquake prediction, because both the south and north section of this fault zone intersect with other seismic zones which have different strikes.

ANALYSES FOR ACTIVITY OF NORTH SECTION OF THE SHUNYI LIANGXIANG FAULT FROM THE SHUNYI GROUND RUPTURE ZONE

Hu Ping, Luo Huachun, Meng Yongqi, Bai Lixin, Wang Xiangnan

2000, 22(2):  123-128. 

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There exists a ground rupture zone along north section of the Shunyi Liangxiang fault zone near Beijing. We have made field investigation, chemical exploration and trough exploration to study its cause. It is suggested that this rupture zone reflects recent activity of north section of the Shunyi-Liangxiang fault. Seismicity along this fault section as well as potential hazards caused by it should be taken into consideration for construction planning.

RIVER OFFSET AND SLIP RATE OF THE EAST SEGMENT OF ALTUN TAGH FAULT ZONE SINCE QUATERNARY

Xiang Hongfa, Guo Shunmin, Zhang Wanxia, Zhang Bingliang

2000, 22(2):  129-138. 

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Based on the gradation of stream and ridge offset and the dating of terrace sediments in a distance of 150km from Hongyaza in the west of Akesai eastward to Subei and Hongliuxiakou, the river offsets in Quaternary can be divided into 8 grades and their formation dates are acquired. The fault slip rates in different segments range 4.7～6.7mm/a. The average value is 6.0mm/a. As to the local section in a short period, the maximum slip rate can be 7.7mm/a. The distribution of offsets and slip rates shows that they are large at the middle and small at the ends, which may demonstrates that rupture initiates from the middle and develops towards the ends. The slip rate in Pliocene is larger than other epoches, which may be related to the quick lifting of Qinghai-Tibetan block since 5Ma B.P. and the large scale of escaping. Data show that fault rupture begins at the main fault, then extends to the vicinity and goes back the main fault.

QUATERNARY TECTONIC STRESS FIELD IN THE REGION OF HAIYUAN-LIUPANSHAN FAULT ZONE TO YINCHUAN FAULT-DEPRESSION

Xie Furen, Shu Saibin, Dou Suqin, Zhang Shiming

2000, 22(2):  139-146. 

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By inversion of fault slip data, it is suggested that stress field of the region from the Haiyuan Liupanshan fault zone to the Yingchuan fault-depression has two major stages. Before early Pleistocene the maximum principal compression stress direction was NE-SW, the stress regime was reverse type, witch resulted in dominating reverse faults. After early to middle Pleistocene the tectonic stress field had a readjustment, the maximum principal compression stress direction was changed from NE-SW to NEE-SWW, the stress regime from reverse type to strike-slip type too, and the former reverse faults became strike-slip faulting, which continues much the same today. The recent tectonic stress field of the studied area can be divided into strike-slip region of Haiyuan fault zone, mixed region of reverse and strike-slip of Liupanshan and extension region of Yinchuan fault-depression.

CHARACTERISTICS OF MICROSTRUCTURES OF THE INVISIBLE FAULTS IN QUATERNARY UNCONSOLIDATED SEDIMENTS

Chao Hongtai, Deng Qidong, Li Jialing, Wang Zhicai, Man Hongmin

2000, 22(2):  147-154. 

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Invisible faults often developed in Quaternary unconsolidated sediments including concealed faults and die-out faults, which lead to complexity and multi-solution for paleoearthquake research. In this paper, original-state directional samples related to invisible or die-out faults are collected from some typical paleoearthquake profiles and solidified in laboratory. Microstructures have been observed after grinding slides in three-dimension coordinate system. Some indicators of invisible faults have been found in micrscopic field. The mechanisms of die-out faults have also been studied. It is suggested that according to microstructural observation, to confirm or negate the existence of concealed faults or to trace the terminated levels of die-out faults, can help us determine the age and periods of paleoearthquake events more accurately combining with macroscopic observation and age dating.

EXPERIMENTAL STUDY OF MANTLE CONVECTION: NON-COLUMNAR PLUME AND MANTLE VORTEX

Wang Shengzu, Li Jianguo, Zhang Zongchun

2000, 22(2):  155-166. 

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The physical modeling of mantle convection based on the theory of similarity provides an approach to understanding the origin and mechanism of mantle vortex. The results of the experiments indicate that: thermal plumes in the mantle (i.e. mantle plumes) should be divided into two types, vertical plumes, as mentioned traditionally, and non-vertical plumes, including those of inclined-columns, vortexes, etc., and the former may be only a particular case of the latter; under the condition of the Coriolis force to be ignored. Vortex-like plume can result from a variety of flow guides, such as those formed by the subduction, stagnancy and flush of lithospheric slab, the existence of lithospheric roots, and the inhomogeneous viscosity distribution in the mantle; the mantle vortex in the asthenosphere flows along a logarithmic-spiral trajectory with its rotating radius and linear velocity decreasing gradually while its angular velocity roughly to be constant.

DEFORMATION BEHAVIOR TRANSITION OF CRUSTAL BOCKS AND ITS TEMPERATURE-PRESSURE CONDITIONS

Zhou Yongsheng, He Changrong

2000, 22(2):  167-178. 

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Brittle-ductile and brittle-plastic transition in rocks are two different concepts. The brittle ductile transition generally refers to a change from localized failure (by macroscopic fracture or faulting) to macroscopic homogeneous flow (by any deformation mechanism, including cataclastic flow, semi-brittle \ semi-plastic flow and plastic flow). This change is often associated with macroscopic texture and mechanical behavior. The brittle-plastic transition means the change from brittle (including fracture or faulting, cataclasis and fraction) to crystal-plastic deformation. With increasing temperature and pressure, the transition undergoes from localized brittle fracture, semi brittle deformation (cataclastic flow or semi-brittle faulting, semi-brittle flow) to homegeneous crystal-plastic flow. This change is assoiated with mechanical behavior and micro-mechanism. Such transitions are important to understanding earthquake source mechanics, the strength of the lithosphere, and the style of deformation. Through comparing experimental T-P condition and natural environment, and their effects on deformation of quartz and feldspar (which are the most important rock-forming minerals in the crust), it is found that the same deformation features in experiment and nature require different temperature and pressure. It is possible to resolve the conflict through setting up deformation mechanism map. In application, the mechanical behaviors and deformation mechanisms should be determined first, then the temperature and pressure are found according to isomechanical group. Unfortunately, the deformation mechanism maps of major crustal rocks are not available because of insufficient experimental data. So, It is a useful way for extrapolating experimental results to geological scales that comparing common deformation character and mechanisms in experimental results and that in nature, looking for the different temperature and pressure from experiment and nature condition, then determining physical mode of deformation by experiments.

EFFECT OF UNDERGROUND WATER ON THE STRESS AND STRAIN FIELD IN EARTHQUAKE FOCI --A CASE OF TANGSHAN EARTHQUAKE

Sun Junxiu, Liu Wuzhou, Che Yongtai

2000, 22(2):  179-186. 

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We have made numerical simulation by the finite element method to study interaction between vadose field and stress field and to understand the effect of underground water to deformation in the seismic source area. The resalts indicate that the shear strength and frictional resistance of rocks decrease with increasing pore pressure, while shear stress becomes greater around the fault. The pore pressure also causes changes of strain distribution. These mechanical effect of underground water may be associated with source processes of earthquakes.

MODERN ERUPTION SCALE OF THE TIANCHI VOLCANO IN CHANGBAI MOUNTAIN OBTAINED BY DIGITAL REMOTE SENSING IMAGES

Shan Xinjian, Chen Guoguang, Ye Hong, Wang Changlin

2000, 22(2):  187-194. 

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We have used some new techniques to study the modern eruption scale of the Tianchi Volcano in Changbaishan Mountain. They include analysing the remote sensing data, GIS, multisource data fusion, orthogonal cross-course wavelet transform and the BP neural network method. The geological investigation and dating data are incorporated in the analyses. The results will be helpful for the volcanic hazard assessment.

TERRACE DATING OF UPPER YONGDING RIVER DRAINAGE BASIN SINCE LATE PLEISTOCENE

Yin Jinhui, Ji Fengju, Zheng Rongzhang, Zheng Yonggang, Liu Yuexia

2000, 22(2):  195-199. 

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There are three levels of alluvial terrace in the upper Yongding River drainage basin since late Pleistocene. We obtained the timescale of terraces by radiocarbon dating method and luminescence dating method. It shows that the degradational episodes of these terraces occurred at the same period and the climate is the primary control factor and plays a greater role on formation of the terrace. The T₃, T₂ and T₁ began to degrade about 30ka B.P., 10 ka B.P., 4ka B.P., respectively.

GIS ON SEISMIC ENVIRONMENT AND POTENTIAL SEISMIC SOURCE ZONE AND ITS PRELIMINARY APPLICATION

Zhou Qing, Ye Hong, Chen Guoguang

2000, 22(2):  200-204. 

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Being a sub-system of Information System on Seismic Zonation in China, GIS on seismic environment and potential seismic source zone includes data of seismology, geophysics, earthquake records, seismic zonation and etc. It can provide service for earthquake research by querying, computer-aid mapping and statistic analysing. Furthermore, extra subroutines give ways to seismicity and obtaining seismic parameters for different seismic areas.

A CONVENIENT AND QUICK METHOD FOR DETERMINATION OF EARTHQUAKE OCCURRENCE YEAR-RATE

Yang Zhixian, Zhang Peizhen

2000, 22(2):  205-208. 

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