Table of Content

11 June 1997, Volume 19 Issue 2

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Brief Report

3-D TRENCH EXCAVATION AND PALEOSEISMOLOGY AT GAOWANZI OF THE HAIYUAN FAULT

Ran Yongkang, Duan Ruitao, Deng Qidong, Jiao Decheng, Min Wei

1997, 19(2):  97-107. 

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The 240km long Haiyuan fracture zone is a typical active fault in the inland of China. The 1920 Haiyuan earthquake (M_S8.6) has made the whole fault offset sinistrally. Our recent research on the large scale geological-geomorphological mapping and 3-D trench excavation at Gaowanzi,Haiyuan fault has revealed that at least 7 paleoearthquakes have been occurred since Holocene. And we have also studied the time intensity distribution of these events. Except the earthquake of 1920,these events have their ages about (10004±3196),(6689±169),(6120±505),(4208±577),(2763±372),(1005±465) and 30a,respectively,and the recurrence interval between two adjacent events is (3315±3200),(561±532),(1920±766),(1425±686),(1578±595) and (980±465)a,respectively. It shows that recurrence of these events is not uniform. At least 4 events comply with the Quasi periodic occurrence model and their average recurrence interval is (1641±207)a while the former 3 events and the later 2 events have the possibility of complying with the nonquasi-periodic occurrence model. This implies that the fault activity has the stage feature in the time distribution,that is,it complies with different occurrence models in different period. We have obtained the horizontal displacements of events Ⅲ,Ⅳ,Ⅴ,Ⅵ,Ⅶ. They are (5.6±2.3),(1.5±1.1),(1.5±1.1),(2±1) and (7±0.5)m,respectively. It indicates that the intensity distribution is also not uniform. The intensity of event Ⅲ is nearly the same as that of event Ⅶ while that of other events is much smaller. This suggests that the characteristic slip of the Haiyuan fault is graded,that is,large characteristic earthquakes occurred on the master segments while relatively small ones occurred on the secondary segments. The recurrence interval of the large earthquakes which have nearly the same intensity as that of 1920 earthquake is about 6000a since middle Holocene while that of small earthquakes with 1～2m horizontal displacement is 1000～2000a. There are 3 events whose displacement is 1～2m occurred between these two large earthquakes. So we estimate that the next earthquake is a relatively small one with a horizontal displacement of 1～2m.

CHARACTERISTICS OF TIME SPACE EVOLUTION OF ACTIVITY IN QUATERNARY ALONG THE HONGHE FAULT ZONE

Ji Fengju, Guo Shunming, Xiang Hongfa

1997, 19(2):  108-114. 

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Since Quaternary,intense activity has occurred along the Honghe fault zone. But its active features in intension,time and form are differential on the different segments of this fault zone. Based on the studies of predecessors,combined with features of tectonic geomorphology,composition of fault materials and a large amount of dating data of strata associated with fault activity,the activity characteristics of the fault and its time-space evolution since Quaternary have been studied synthetically for the whole fault zone. The results are: (1) As time goes on,the active center removed gradually from southern to northern segment of the Honghe fault zone,the acitve sites in later Holocene is located near Dali City and Midu county. (2) The active property of the fault zone is different in basins and mountain areas on the same secondary fault,the former is dominant with dip-slip and the later is dominant with right-lateral strike-slip movement. And their activity time is not synchronous too,the active age of former is often newer than the later. (3) On the same segment of the Honghe fault zone,the active ages of range front-fault of the Ailaoshan trend to be older than the Zhonggu fault and removes gradually from the former towards the later.

RECENT EARTHQUAKES AND ASSESSMENT OF SEISMIC TENDENCY ON THE GANZI-YUSHU FAULT ZONE

Zhou Rongjun, Wen Xueze, Chai Changxing, Ma Shenghao

1997, 19(2):  115-124. 

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We have studied the magnitude,epicentre of recent earthquakes,and spacial distribution of ground seismic ruptures on the Ganzi-Yushu fault. We have estimated the tendency of major earthquakes for each segment of the Ganzi-Yushu fault zone in the future 50 years by using the probability model of the general distribution for repeat time of characteristic earthquakes (Nishenko and Buland,1987). Our results show that the Manigange segment has high risk of recurrence of large earthquakes in the future 50 years. The possibility of major event recurrence on the Dangjiang segment also exists. These estimates can provide a basis for the long-and mid-term earthquake prediction for this region.

JIEXIU-XINXIANG-LIYANG NORTHWEST TRENDING NEWLY-GENERATED SEISMOTECTONIC ZONE IN THE SOUTHEAST AREA OF NORHT CHINA

Xu Jie, Wang Ruobai, Wang Chunhua, Song Changqing

1997, 19(2):  125-134. 

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On the basis of the data of seismicity,geological structures and geophsics,we have done an analysis on the seismogeological chracteristics of the Jiexiu-Xinxiang-Liyang northwest trending seismic zone located in the southeast area of North China. This seismic zone is a recent crust shear rupture zone under the uniform modern tectonic stress field in a large scale; and along it there are a series of newly-generating northwest trending faults which continued or began to develop since the Neogene period. They make up an active fault zone which extends through different tectonic units. Therefore,it is a newly formed seismic zone.

DYNAMIC CHANGES OF STRESS FIELD BEFORE THE CANGSHAN M5.2 EARTHQUAKE IN 1995

Zhou Cuiying, Hua Aijun, Wang Mei, Zhang Lairong, Ma Yuxiang

1997, 19(2):  135-140. 

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An earthquake of M5.2 occurred in Cangshan,Shandong Province on September 20,1995. Abnormal changes and their time-space evolutionary features of the azimuth and plunge of the principle stress axes and ratio of their contrary symbols before this event are analysed. Possible causes of these changes are briefly discussed which can provide background evidence for studies of other kinds of precursorary anomalies.

MECHANICAL ANALYSIS OF SOURCE ENVIRONMENT FOR THE HAICHENG M7.3 EARTHQUAKE

Lu Liangyu, Li Ping, Pan Ke, Lu Zaoxun, Xu Xintong

1997, 19(2):  141-147. 

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The crustal structure model is established on the basis of the geoscience transect data. The precise leveling data are used as the criterion to test models. Two-dimensional numerical calculations are performed by using the finite element method to model the tectonic stress field of crust interior before the Haicheng M7.3 earthquake. Boundary conditions include both horizontal and vertical forces. Three schemes of loading are designed. Based on analyses of calculation results and considering focal mechanism solutions,the best model with proper loading scheme is chosen. With this model the mechanical and tectonic environment before the Haicheng earthquake and the evolution feature of the stress field are discussed.

NEW CHARACTERISTICS OF REGIONAL CRUSTAL ACTIVITES SHOWN BY SURVEYING ACROSS FAULTS IN THE AREAS OF JIANGSU,SHANDONG AND ANHUI PROVINCE

Bo Wanju, Guo Liangqian, Xie Juemin

1997, 19(2):  148-154. 

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Mobile leveling data measured across faults in three provinces Jiangsu,Shandong and Anhui are analyzed systematically and completely time and space,and all the observation curves removing the long-time linear components are gmen to show abnormal features.The abnormal activities of the faults are analyzed one by one with the knowledge of geological structure.The result shows that the fault activites have a trend of being strengthened from 1991 in the Tanlu fault zone,that is,the Tanlu fault zone is being active.The activity of the south segment of the Tanlu fault zone is extensional,in the middle and north part of it,the fault activity behaves compressional.While in the east to Maoshan mountain,the fault activity is mainly compressional.All the anomalies occurred to mainly in 1991 and 1995,their distribution in the time-sapce is relatively concentrated.These facts show that the stress field has had comparatively more variation in this area.These results and conclusions have important materta value and reference value for further researches on geoaynamics,crustal deformation monitoring earthquake prediction and recent tectonic movement.

GEOELECTRICAL STRUCTURE OF THE CRUST AND UPPLE MANTLE IN THE XINGTAI EARTHQUAKE AREA AND ITS TECTONIC IMPLICATIONS

Deng Qianhui, Zhang Musheng, Zhan Yan, Tang Ji, Zhao Guoze, Liu Tiesheng, Zhu Zuoquan, Liu Baoqin, Jia Zheng, Li Zhuguo, Qu Jianpeng

1997, 19(2):  155-163. 

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Magnetotelluric (MT) sounding observations have been carried out along four profiles in the Xingtai earthquake area and its neighbouring regions. The results show characteristics of the geoelectrical structure of the crust and upper mantle for this region. The focal area looks like complex,while the surrounding area seems to be simple. The range of seismogenic depth is more complicated than the other depth range. Sharp changes of the electrical structure of the focal area indicate the hidden faults with steep angles. High conductive layers may represent the detachments of 10～20km depth. Upper mantle uplift is likely associated with upward migration of mantle material. These features are of great importance to understanding the tectonic background of the study region,the seismogenic structure,and relation between shallow and deep structure.

PRELIMINARY OBSERVATIONS OF THE TIANCHI VOLCANO AREA IN CHANGBAISHAN MOUNTAIN BY MT METHOD

Tang Ji, Liu Tiesheng, Jiang Zhao, Zhao Guoze, Wang Jijun, Zhan Yan

1997, 19(2):  164-170. 

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Observations by magnetotelluric technique have been carried out at 15 sites along two profiles in Tianchi volcano area of Changbaishan Mountain northeast China. The impedance tensor decomposition and a new two-dimensional inversion method (RRI) are used for data analysis and interpretation. The results show that the conductivity structure beneath the volcano and its neighbouring area is of two-dimensional character and a local three-dimensional low resistivity body exists at depth of about 12km under the volcano. It is inferred that this anomaly body may be a magma chamber.

FRICTIONAL STRENGTH AND VELOCITY-DEPENDENCE OF SERPENTINE GOUGES UNDER HYDROTHERMAL CONDITIONS AND THEIR SEISMOGEOLOGICAL IMPLICATIONS

Ma Shengli, D. Lockner, D. Moore, R. Summers, J. Byerlee

1997, 19(2):  171-178. 

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The velocity-dependence of frictional strength of serpentine gouges has been measured at constant normal stress of 110 MPa,pore pressure of 10 MPa,temperature 25,100 and 200℃,and at sliding rate ranging from 0.001 to 10μm/s. At 25℃,the coefficient of friction of chrysotile gouge is very low (μ≈0.2～0.25),while lizardite and antigorite gouge are much stronger,with μ≈0.39 and 0.45,respectively. The frictional strengths of chrysotile and antigorite gouges change little with a temperature increase to 200℃,whereas the strength of lizardite gouge increases substantially with increasing temperature. At 25℃,all three gouges show a transition from weak velocity weakening at high slip rates to velocity strengthening at low slip rates. With increasing temperature,the velocity dependence of each gouge shifts towards more positive values,especially at high slip rates. Based on this study and previous results,we suggest that the presence of serpentine in the fault zone may contribute to the occurrence of stable creep rather than earthquakes,but this effect may be limited to shallow depths. Although chrysotile is one of the weakest rock-forming minerals,it is still too strong to explain the weakness of the San Andreas fault deduced from heat flow data.

ON THE SCIENTIFIC THINKING-AND EXPLORING WAYS TO MEDIUM AND SHORT-TERM EARTHQUAKE PREDICTION

Zhang Chongli, Deng Zhihui, Che yongtai

1997, 19(2):  179-184. 

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The scientific thinking for medium-and short-term earthquake prediction research is described in detail. It is that the static geological or crustal structure is given dynamic property and the precursorary anomaly is given tectonic implication. Under guidance of this thinking,the ways and methods of medium-and short-term earthquake prediction are briefly discussed. Three problems are pointed out which should be given attention in the studies of medium-and short-term earthquake predicition.

FORMATION TIME AND DYNAMIC CHARACTERISTICS OF THE NORTHERN PART OF THE TANLU FAULT ZONE IN EAST CHINA

Wang Yu, Dou Lirong

1997, 19(2):  185-192. 

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The Tanlu fault zone extending toward northeast China is branched into three subdivison parts. They are Yilan-Yitong,Mishan-Dunhua and Siping-Changchun faults. According to the isotopic chronological determination,the Rb-Sr isochron age of the diabase whole rock within the fault zone is (100±5.9)Ma,the ⁴⁰Ar/³⁹Ar plateau age of the augite is 105Ma. The ⁴⁰Ar/³⁹Ar plateau age of biotite mineral selected from the fault breccia and schists (mica quartz schists) within the Yilan Yitong fault zone is (100±1)Ma and (105.5±1.2)Ma～(95.7±1.8)Ma,respectively. In this region,in view of the outcrop investigation and comparison of different strata,strain calculation,the fault displacement is about 4km,or not more than 10km which is far less than the general estimation that the Tanlu fault zone is displaced more than 500km (Xu,1984). In this part,deformation of the fault is not very intensive,only cutting the Quantou formation (122Ma) of the lower Cretaceous system. Within the fault zone,the fault gouges developed less than 0.5m width. All of the evidence illustrates that the sinistral strike slip occurred during the time of 100Ma,i.e.,the end of early Cretaceous early of late Cretaceous. Activities of this part of the Tanlu fault zone affect the neighbouring basins where inverted structures occur. It is suggested that the strike-slip movement along the Tanlu fault of this part occurred on the basis of counterclockwise rotation of the eastern Asia continent and is associated with the subduction of the Pacific plate which induced upwelling of the upper mantle.