Table of Content

10 December 1997, Volume 19 Issue 4

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Brief Report

BASIC CHARACTERISTICS OF THE FISSURE ZONES IN XI'AN AND THEIR CAUSTIVE MECHANISM

Yi Xuefa, Su Gang, Wang Weidong, Tang Junchang

1997, 19(4):  289-295. 

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Basic features of the ground fissure zones which have occurred in the Xi'an city area since late 1970's are described by using field observations and survey data. The focus is put on expounding the spatial-temporal relationship between the dynamic deep-well water level changes, inhomogeneous ground subsidence and ground fissure activities. It is suggested that the large-scale ground subsidence induced by overextraction of confined ground water is the primary cause of the ground fissure zones in the Xi'an city while the local geological conditions play a controlling role to the development of these fissure zones.

A SYNTHETIC ASSESSMENT ON ACTIVITY ALONG THE MIDDLE SEGMENT OF SUSONG-ZONGYANG FAULT,SOUTHWEST AREA OF ANHUI PROVICE

Yao Daquan, Tang Youbiao, Liu Jiacan, Li Jie, Liu Qingzhong

1997, 19(4):  296-300. 

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Since having historical records, there have been many times middle or small earthquakes occurred along the Susong-Zongyang fault, southwest area of Anhui province. This paper gives a synthetic study on the fault by means of macro-microscopic structural analysis, dating of the deformational materials, and combing with a series of seismic safety evaluation of the enginnering sites in this area. Based on the aforementined work, the authors revealed the geological background of middle or small earthquakes along the fault, and forecased initially the level of seismic acitvity nearby the fault in future. It offers an example to evaluating fault's active behaviour in the areas of lower seismicity.

ARCUATE THRUST TECTONICS AND ITS CONTEMPORARY SEISMICITY IN THE EASTERN SECTION OF THE EXTERNAL ZONE OF THE PAMIR

Chen Jie, Qu Guosheng, Hu Jun, Feng Xianyue

1997, 19(4):  301-312. 

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The external zone of the Pamir is the boundary between the Pamirs and the Tianshan which are two major Cenozoic mountain belts of Central Asia and a part of the most active portion of the India-Eurasia collisional plate boundary. Its eastern section consists of the large Aikaerte and Kazkeaerte arcuate fold-and-thrust belts, whose style ranges from tight folds and northward-verging thrusts in the Aikaerte arc gradually to open folding in the Kazkeaerte arc to the north. Every arcuate belt is composed of a series of varying scales of sub arcs that is a separate fold and thrust sheet with independent characteristics of geometry, kinematics and mechanics and is a self-similar system as well. The kinematics of arc changes from thrusting at the top of the arc(where thrusts have gently dipping planes and the largest slips) to oblique thrusting and wrenching at the sections (steeper faults with sinistral wrenching at the left section and dextral wrenching at the right section), and finally thrusting to the end of the arc. The arcuate thrust tectonics is composed of frontal, lateral and oblique ramps at depth, which merged downward into a series of decollements at different depths. The external zone of the Pamir is most active with shallow-focus seismicity. Each earthquake deformation belt is probably related to the independent arc. Strong earthquakes correspond to the large scale arcs, and middle to small earthquakes to smaller ones.

DEFORMATION FEATURES OF MANTLE XENOLITHS FROM QUATERNARY BASALTS IN DATONG,SHANXI PROVINCE AND THEIR RHEOLOGICAL IMPLICATIONS

Chen Xiaode, Lin Chuanyong, Zhang Xiao'ou, Shi Lanbin

1997, 19(4):  313-320. 

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The Quaternary basalts in Datong, Shanxi Province contain small mantle xenoliths which are generally only 2～3cm in size. These mantle xenoliths comprise mainly spinel lherzolite. The textures of the xenoliths are variable from protogranular to porphyroclastic. The dislocation substructures of olivine from the xenoliths are dominated mainly by dislocation walls representative of high temperature dislocation creep. In addition, some dislocation substructures typical of lower temperature plastic deformation and high strain rate events such as dislocation tangles and slip band can also be observed. All these indicate that at least two deformation events had occurred within the upper mantle of this area. The equilibrium temperature and pressure of these xenoliths are within the range of 900～1050℃ and 1.1～1.7Gpa, respectively, corresponding to a range of depth of 40～60km. The differential stress estimated by using microstructural piezometer is in the range of 21～31MPa, showing a tendency of decreasing with depth, while the estimated strain rate and equivalent viscosity are within the range of 10¹⁵～10¹⁷s¹ and 10²¹～10²³ Pa·s, respectively. All these results have shown that an upper mantle diapir had continued to occur during the Quaternary in this area.

COSEISMIC SURFACE-RUPTURE FEATURES OF THE RIKUU REVERSE FAULT SYSTEM ALONG THE EASTERN MARGIN OF THE YOKOTE BASIN,NORTHEAST JAPAN AND ITS SEGMENTATION

Xu Xiwei, Deng Qidong, Wang Yipeng, Suzuki Yasuhiro, Yonekura Nobuyuki, Wang Cunyu

1997, 19(4):  321-332. 

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The recognizable segment boundaries of the Rukuu reverse fault system are obtained through study of coseismic surface ruptures for the 1896 Rikuu earthquake, long-term faulting behaviour and offset geomorphology. They are the transition areas for permanent change of fault scarps, watershed altitudes most nearby the faults and geometrical structures of active reverse faults in cross section, and buried tranverse bedrock ridges. The stepovers and fault trace gaps in kilometers' order can not stop or slow down effectively coseismic-surface-rupture propagation along the reverse fault strike, thus, can not serve as segment boundaries for reverse faults. The seismic risk on the Senya and Yokote segments of the Rikuu reverse fault system along the eastern margin of the Yokote basin is also assessed briefly.

ACROSS-FAULT DEFORMATION MEASUREMENT ALONG THE XIANSHUIHE FAULT ZONE AND ITS SEISMOLOGICAL IMPLICATIONS

Lu Yipei, Li Tieming, Liao Hua

1997, 19(4):  333-340. 

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By analysizing the data of across fault deformation measurement along the Xianshuihe fault zone for many years, we have revealed the deformation features of different segments of the fault zone. The fracture areas are characterized by creep of higher rates which decay from the center to the both sides. The closed areas have features of motion stop and evident segmentation. The special form of fault deformation is related to earthquakes. For example, when “V” shape or “reverse V” shape of deformation occurs, there will be corresponding middle or small earthquakes. Reverse and other changes of fault motion are also associated with earthquakes.

TECTONIC STRESS ANALYSIS FOR ZHANGQIANG DEPRESSION AND ITS ADJACENT AREA

Qu Guosheng, Zhou Yongsheng, Xu Jie, Xiao Xiaohui, Niu Luanfang

1997, 19(4):  341-352. 

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By the analysis of deformation in outcrop area and fissure measurement of core rocks in Zhangqiang depression and its adjacent area, we divide the geological history of this region into eight tectonic stress phases and determine the tectonic stress states and deformation forms of each phase. We conclude that the tectonic stress fields of the late Jurassic and the Cretaceous resulted in the formation and development of faulted basins, and the stress state of the late Jurassic was nearly E-W extension. Between the end of Fuxin Group and the beginning of Quantou Group sedimentation stages, the stress fields resulted in the close of basin faulting stage and deformation of the late Jurassic caused by the change of stress states from E-W extension to E-W compression. In the Cretaceous phase, the principal stress direction was E-W compression which caused the regional depression as a whole in the early period and uplifting and erosion widely in the later time. The principal stress direction was NW-SE compression in the Eogene, and was NEE-SWW compression since the Neogene. Two phases of stress fields in Cenozoic only caused some reactive faults, the deformational strength was less than that in Jurassic and the Cretaceous.

THE HYDRODYNAMIC MECHANISM OF WATER RADON ANOMALY

Che Yongtai, Yu Jinzi, Liu Wuzhou

1997, 19(4):  353-357. 

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The rock dynamic mechanism is now overemphasied in the study of the mechanism for water radon anomalies as the precursor of earthquakes. This model suggests that the microfracturing of rock releases radon during the late period of earthquake pregnancy. A great deal of observations indicate, however, that water-radon anomalies can occur in the early time of earthquake pregnancy and wells far from epicentres, where rocks of water bearing layers can hardly be in the state of microfracture. So we propose the hydrodynamic mechanism for the water-radon anolmalies which suggests that the water-radon anolmalies can be casued by deformation of water-bearing rock mass and changes of its hydrodynamic state under the low stress level.

HYDRODYNAMIC DISPERSION AND TRANSMISSION CHARACTERISTIC OF HYDROGEOCHEMISTRY PRECURSOR INFORMATION

Liu Chunguo

1997, 19(4):  358-362. 

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On the basis of analyses to the many observational data, abnormal changes of the hydrogeochemistry precursor are classified into three basic types of curves. Comparing these curves with the hydrodynamic dispersion curves, it is suggested that the transmission mechanism of hydrogeochemistry is actually a hydrodynamic dispersion. Application of the theory on hydrodynamic dispersion in the study of transmission mechanism of hydrogeochemistry precursor information is also discussed in this paper.

EFFECT OF TOPOGRAPHY ON THE DATA OF MAGNETOTELLURIC SOUNDING (MTS)

Jin Guangwen, Sun Jie, Wang Jijun

1997, 19(4):  363-369. 

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Effect of topography is one of the main problems for the MTS work in mountain areas. To this problem we have made theoretical analyses and numerical modeling by using a two dimensional model in homogeneous halfspace. Our results indicate that effect of topography depends on the type of MTS data, mode of polarization, electromagnetic wave period of field source, survey site, as well as the form and scale of topography. In addition, we also give the estimates on the extent of topography effect in terms of calculations by a cliff model.

NUMERICAL MODELING AND ANALYSES OF TECTONIC DEFOMATION DURING MESOZOIC AND CENOZOIC IN THE TURPAN-HAMI BASIN OF XINJIANG

Liang Haihua, Ding Zhongyi, Pan Mao, Hou Jianjun, Zheng Wentao, Liu Xida

1997, 19(4):  370-378. 

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On the basis of data of geology, seismic profiling, gravity and field observations, we have analyszed the mechanical features and motion ways of the blocks which consist of the Turpan-Hami basin in Xinjiang. From this the development model of the basin is proposed. Considering the geological model as well as the factor of geological time, calculational models including regional plane model, whole-basin model, NS strike profile model and the Taibei depression 3D model are established. The finite element method is used to simulate the paleotectonic stress and deformation fields. The results show that the Turpan-Hami basin has been subjected a compressive force since the Jurassic period. The modeling results also indicate the asymmetric form of the basin as well as the variations with depth of the orientation of the principal stress.