Table of Content

02 June 2005, Volume 27 Issue 2

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Brief Report

OBSERVATION OF A GPS PROFILE ACROSS MAIN FAULTS ON NORTHEAST MARGIN OF TIBETAN PLATEAU AND DATA ANALYSIS

GAN Wei-jun, CHENG Peng-gen, ZHOU De-min, TANG Fang-tou, LI Jin-ping

2005, 27(2):  177-187. 

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In order to investigate the present-day movement characteristics of Haiyuan active fault zone and Xiangshan-Tianjingshan active fault zone on the northeast margin of Tibetan Plateau in detail, we established a GPS profile across the fault zones. The profile, extending from Lanzhou, Gansu to Zhongwei, Ningxia, is composed of 12 stations and locally reinforced the existing regional GPS network of Crustal Movement Observation Network of China (CMONOC). These new stations, together with the existing GPS stations, constructed a spatially dense profile whose average interval of the stauions is ~22km. Considering that there were two continuous GPS stations of CMONOC (i.e. XNIN and YANG) around the region, we tried a “Flexible Observation Method” in GPS observations. The method allows non-synchronal observations for all the GPS observation teams and makes the observation schedule rather flexible. In data processing, we used the advanced strategy of “Precise Point Positioning” of GIPSY software. Our result shows that with the support of CMONOC, especially the continuously observed fiducial GPS network of CMONOC, we can use the “Flexible Observation Method” and “Precise Point Positioning” data processing strategy to effectively observe local GPS networks to monitor crustal deformation with satisfying accuracy.

EXTENSIONAL STRUCTURES WITHIN THE COMPRESSIONAL OROGENIC BELT AND ITS MECHANISM: A CASE STUDY FOR THE LATE CENOZOIC DEFORMATION IN CENTRAL YUNNAN

WANG Gang, WANG Er-qi

2005, 27(2):  188-199. 

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Under continuous N-S convergence between the Indian and Eurasian continents, the Sichuan-Yunnan block on the southeastern margin of the Tibetan Plateau has undergone southeastward extrusion and clockwise rotation since late Cenozoic along the right-lateral Red River to the southwest and left-lateral Xianshuihe-Xiaojiang Fault systems to the northeast. However, the surface deformation along the front part of the block in central Yunnan, where the Red River and Xianshuihe-Xiaojiang Fault systems join each other, is characterized by transtension instead of transpression, as indicated by the presence of a series of grabens and horsts along the Xiaojiang Fault system, penetrating the central Yunnan antiform in N-S direction. To explain such contradiction between the field data and theoretical modeling, a mechanic model is constructed in this study, based on the geologic and geomorphic features of the central Yunnan antiform, and the results illustrate that the generation of the E-W extension occurring along the Xiaojiang Fault strands is predominated by stress and strain patterns across the antiform.

QUANTITATIVE ANALYSIS OF RECENT ACTIVITY OF THE XIAODIANZI-MAOBU SEGMENT OF THE ANQIU-JUXIAN FAULT, SHANDONG PROVINCE

SONG Fang-min, YANG Xiao-ping, HE Hong-lin, LI Chuan-you, ZHANG Lan-feng

2005, 27(2):  200-211. 

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The Xiaodianzi-Maobu segment is part of the Anqiu-Juzian Fault in the Tancheng-Lujiang Fault zone. It starts from northeast of Xiaodianzi village, Juxian County in the north and terminates at Maobu, Juxian County in the south. The fault segment has a general strike of 10°～20°, dipping northwest or southeast at an angle of 60°, and has a length of about 30km. The segment can further be divided into 5 sub-segments: the Xiaodianzi-Qijiazhuang, Yuanhe, Kushan-Xilianci, Qingfengling and Sanzhuang-Zhaike sub-segments from north to south successively. These 5 sub-segments are aligned in right or left-step en-echelon, appearing as a brush structure converging to the north and diverging to the south. The fault segment appears as distinct lineation on satellite image or aerial photo, and geomorphically, as distinct bedrock scarp. According to field observation on natural exposures or trench logs, as well as dating results of samples collected from the fault segment, it can be deduced that the latest faulting event occurred in early Holocene and was dominated by right-lateral strike-slip with compressional reverse faulting component. The distinct horizontal fault striate is well developed along the fault plane and the drainage system crossing the fault segment is right-laterally distorted. This corroborates the right-lateral strike-slip of the fault segment. Field measurement of displacement and dating of the relevant samples have indicated that the displacement amount of the fault is 64～73m, and the displacement rate is 0.91～1.04mm/a in the past 70ka, while in the past 11ka, the displacement is 5.5～7.8m and displacement rate is 0.46～0.65mm/a. The reverse faulting along the fault segment can also be recognized in exposures or trench logs. It can be observed that the Cretaceous or Paleocene system is thrusted over the Quaternary system, making a distinct fault scarp landform. Field measurement of fault scarp and laboratory dating of relevant samples have revealed that in the past 11ka the vertical displacement along the fault segment is 2.3～3.8m and the displacement rate is 0.17～0.32mm/a. In the same period, the right-lateral displacement is 2～3 times as large as the vertical displacement.

GEOMETRY AND ACTIVITY OF THE ANQIU-ZHULI SEGMENT OF THE ANQIU-JUXIAN FAULT IN THE YISHU FAULT ZONE

WANG Zhi-cai, JIA Rong-guang, SUN Zhao-min, SHI Rong-hui, CHAO Hong-tai

2005, 27(2):  212-220. 

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The Yishu Fault zone is a part of the Tanlu Fault zone that passes through Shandong Province and the northern area of Jiangsu. It is composed of 5 nearly parallel faults among which the Anqiu-Juxian Fault is the most important and prominent active fault since late Pleistocene and is usually considered to be the seismogenic fault of the 70 BC M 7 Anqiu earthquake and the 1668 AD M 8¹/₂ Tancheng earthquake. By detailed research in recent years, e.g. the active fault mapping implemented during the “Eighth Five-Year Plan” period, we have gained thorough knowledge of the middle segment of the Anqiu-Juxian Fault to the south of Juxian, while the knowledge of the segment to the north of Juxian has been poor, even no active fault profiles have ever been found. Recent discoveries about the geometry and activity of the Anqiu-Zhuli segment of the Anqiu-Juxian Fault are introduced in this paper. The segment consists of the Nanliu sub-segment, the Shuangguan-Meicun sub-segment and the Zhuli sub-segment. All these sub-segments are distributed along the linear boundary line between hilly area and plain, or locate in the eastern slope of the NNE-trending hills. These fault segments are featured with right-lateral strike-slip movement combined with dip component, and their latest activity age is late Pleistocene to early Holocene. In conclusion, The Anqiu-Juxian Fault is the predominant active structure between Juxian and Changyi in the north of Shandong, and it should have some connections with the occurrence of the 70 BC M 7 Anqiu earthquake. Since the elapsed time is nearly 2100 years, the future earthquake risk in this area shall be emphasized.

KINEMATICAL AND STRUCTURAL PATTERNS OF YINGJING-MABIAN-YANJIN THRUST FAULT ZONE, SOUTHEAST OF TIBETAN PLATEAU, AND ITS SEGMENTATION FROM EARTHQUAKES

ZHANG Shi-min, NIE Gao-zhong, LIU Xu-dong, REN Jun-jie, SU Gang

2005, 27(2):  221-233. 

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Segmentation of thrust fault zone is a basic problem for earthquake hazard evaluation. The Yingjing-Mabian-Yanjin thrust fault zone is an important seismic belt trending northwest in the southeast of Tibetan plateau. The longitudinal faults in the thrust zone are mainly of thrust slipping. The late Quaternary motion modes and displacement rates are quite different from north to south. Investigation on valleys across fault shows that the transverse faults are mainly of dextral strike-slipping with a bit dip displacement. Based on their connections with longitudinal faults, three types of transverse faults are generalized, namely, the separate fault, the transform fault and the tear fault, and their functions in the segmentation of the thrust fault zone are compared. As the result, the Yingjing-Mabian-Yanjin thrust fault zone is divided into three segments, and earthquakes of the three segments are compared. The trisection of Yingjing-Mabian-Yanjin thrust fault zone identified by transverse faults reflects, on one hand, the differences in slip rate, earthquake intensity and pace from each segment, and the coherence of earthquake rupturing pace on the other hand. It demonstrates that the transverse faults control the segmentation of thrust fault zone to a certain degree, and each type of the transverse faults plays a different role.

LATE QUATERNARY SLIP RATE IN THE CENTRAL PART OF THE LONGMENSHAN FAULT ZONE FROM TERRACE DEFORMATION ALONG THE MINJIANG RIVER

MA Bao-qi, SU Gang, HOU Zhi-hua, SHU Sai-bing

2005, 27(2):  234-242. 

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We estimate the slip rates of three faults in the central segment of Longmenshan Fault zone, namely, the Maowen-Wenchuan Fault, the Beichuan-Yingxiu Fault and the Jiangyou-Guanxian Fault, based on the measurement and dating of deformation of Late Quaternary terraces along the Minjiang River and its tributaries. The three-level Late Quaternary terraces, T₁, T₂ and T₃, are well developed along the Dujiangyan-Wenchuan reach of the Minjiang River. The cross profiles show that T₃ and T₁ have undergone a complete development process. The thermoluminescence ages of the upper deposits on T₁, T₂ and T₃ are 9～13ka BP, 19～30(ka BP) and 51～58ka BP, respectively. Therefore, we can constrain the ages of T₁, T₂ and T₃ surfaces to 10ka BP, 20ka BP and 50ka BP, respectively. The synchronous three-level terraces also exist in the valleys of the Baishahe River and other tributaries of the Minjiang River. These river terraces are laterally and vertically dislocated by the Longmenshan Fault zone. According to the dating and vertical displacements of river terraces, the Late Quaternary reverse slip rates of the Maowen-Wenchuan Fault, the Beichuan-Yingxiu Fault and the Jiangyou-Guanxian Fault are estimated to be 0.5mm/a, 0.6～0.3mm/a, 0.2mm/a, respectively. According to the lateral displacements of river terraces, the Late Quaternary dextral strike-slip rate of the Maowen-Wenchuan Fault and the Beichuan-Yingxiu Fault averages 1mm/a each. The late Quaternary faulting of the Longmenshan Fault zone is characterized with intermittency. Three faulting stages can be identified. The first one is between 50ka BP to 20ka BP, the second one between 20ka BP to 10ka BP, and the latest starts at 10ka BP. The existence of thick alluvial deposits in the present channel indicates that the uplift of Longmenshan Mountains is more complex than previously estimated.

STUDY ON THE ACTIVITY AND CHRONOLOGY OF THE YINWASHAN FAULT DURING HOLOCENE EPOCH

CHEN Bai-lin, LIU Jian-min, LIU Jian-sheng, ZHANG Yong-shuang

2005, 27(2):  243-251. 

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The Yinwashan Fault situates in the western part of Hexi Corridor at the northern side of the Yinwashan Mountains, about 18km to the west of Jiayuguan city. The fault is NW-trending, 25km long. The activity age of the fault becomes younger from north to south along the fault. It offsets the Cretaceous system and Pliocene series at the northwestern part, the upper Pleistocene series(the age of the faulted talus is 22.4ka)at the middle part and the Holocene series at the southeastern part (the age of fault wedge deposits is about 4.8～5.1ka). The Yinwashan Fault exhibits a vertical displacement with uplifting on its southwestern side and subsiding on the northeastern side, and a horizontal displacement with a right-lateral slip. Obviously the fault has experienced two Neotectonic events, one occurring at the end of late Pleistocene (about 10ka) and the other at the middle of Holocene epoch (about 5ka).

ANALYSIS OF CHARACTERISTICS OF BODY-WAVE VELOCITY WITH AZIMUTH VARIATION FOR ARBITRARY SPATIAL ORIENTATION TIMEDIA

HAO Chong-tao, YAO Chen, WANG Xun

2005, 27(2):  252-259. 

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Based on the analytic solution of body-wave velocities and polarizations for arbitrary spatial orientation TI media (ATI), the characteristics of body-wave velocities with azimuth variation in ATI media can be obtained by using simulating calculation. The result shows that the body-wave propagation in ATI media is related to not only the five elastic constants of TI media, but also the dip angle of the symmetrical axis of TI and the azimuth between the symmetrical axis of TI and the survey line. That is, the spatial orientation of TI symmetry axis and surveying line azimuth have an effect on body-wave velocities. The patterns of body-wave group-velocities and phase-velocities are fixed relative to TI symmetry axis and exhibit certain symmetry and repetition of azimuth variation. For complex structural geology of ATI areas, this research can provide the patterns of body-wave group-velocities and phase-velocities to benefit the theoretical research. The result can also be used directly to analyze VSP and cross-well seismic data.

MULTI-LAYER STRAIN RATE FIELD CONTROLLED BY NETLIKE PLASTIC-FLOW IN THE LITHOSPHERE IN CENTRAL-EASTERN ASIA

WANG Sheng-zu

2005, 27(2):  260-272. 

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According to the “Netlike Plastic-Flow” continental dynamics, the transition of deformation regime from brittle in shallow layer to ductile in deep layer in the lithosphere and the controlling effect of netlike plastic-flow result in the intraplate multi-layer tectonic deformation. What have been measured using the GPS method or fault-slip-inversion method can deal only with the surface and shallow crust. The strain rates in the seismogenic layer can be estimated using the “earthquake-recurrence-interval method”, in which the strain rate is calculated in terms of the recurrence interval of two adjacent earthquakes occurring one after another and the seismic probability of the posterior one, and the strains of the plastic-flow network in the lower lithosphere using the “conjugate-angle method” on the basis of the relationship between the conjugate angles and the compressive deformation of the network, and then the characteristic strain rates in that layer are calculated from the strains and the durations of deformation inferred. The contour map of characteristic maximum compressive strain rates in the lower lithosphere in central-eastern Asia given in the paper shows the strain rates with the magnitude orders of 10^-15～10^-14/s in this region. The strain rates in the plastic-flow belts, which control the seismic activity in the seismogenic layer, are greater than the characteristic strain rates of the network. In addition, the strain rates in the seismogenic layer are influenced also by the factors, involving the directive action of driving boundary along the upper crust, the plastic-flow waves and the existence of the transitional weak layer distributed discontinuously between the upper and lower layers. The comparison between the strain rates in the seismogenic layer and the characteristic strain rates in the lower lithosphere for 11 potential hypocenter areas in the region from the Qinghai-Tibet plateau to North China plain indicates that, except the considerable deviation for the Linfen basin in Shanxi province, the significant linear correlation between the strain rates in the upper and lower layers is shown for all the rest 10 areas by the values of β the ratios which are distributed within the range of 1.25～2.25 with an average of 1.75. It is suggested that the contour map of characteristic strain rates together with the introduction of the ratio β can be used approximately for estimating the strain rates of potential hypocenters in medium- and long-term earthquake prediction and the effects of driving boundaries and relative stable blocks on the hypocentral areas should be considered as sufficiently as possible when taking the values of the ratio β.

CONTEMPORARY CRUSTAL STRESS FIELD AND FEATURE OF EARTHQUAKE FAULT SLIP IN EAST CHINA

ZHOU Cui-ying, WANG Zheng-zheng, JIANG Hai-kun, LI Yong-hong, WU Yan-he

2005, 27(2):  273-288. 

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Statistic analysis has been made on the characteristics of the contemporary tectonic stress field of East China based on 143 sets of single focal mechanical solutions of moderate and small earthquakes and on 17 sets of composite focal mechanism solutions. Results show that the stress field of East China exhibits a NNE (about 80?) oriented principal compression and NNW (about 350?) principal tension, and the principal stresses are mainly horizontal or near-horizontal. In the context of basically consistent orientation and action mode of stress field, there are some variations in the stress field among different seismotectonic regions, which are possibly related to the distribution of major active faults in the respective regions. This indicates that fault slip may be dominated or affected by existing tectonics. The feature of earthquake fracture and the orientation of fault slip of contemporary earthquakes in East China have been analyzed with data of focal mechanism solutions of contemporary moderate and small events, and of the orientations of major axis for innermost isoseismal of historical and contemporary events and so on. It indicates that the earthquake fractures of East China extend at large in NE and NW directions, and sometimes with NNE, NEE, NWW, or near EW directions. The movements of earthquake faults are mainly of strike-slip or near strike-slip with oblique-slip component. There exist some regional variations in the movement of earthquake faults. The historical moderate and strong events in East China exhibit mainly a NE trending slip, while the modern events show the NW-SE trending slip on the land and NE and NW trend slip in the sea area.

A MATHEMATICAL SIMULATION FOR THE TECTONIC STRESS FIELD OF THE BOHAI SEA AREA

CHEN Xiao-li, CHEN Guo-guang, YE Hong

2005, 27(2):  289-297. 

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The Bohai Sea, located in the northern part of North China, is an inland basin about 73 thousands km². Tectonically, it is one of the most active areas in eastern China since Late Tertiary. Meanwhile, it is also an area rich in oil and gas deposits. Previous studies have shown that faults and tectonic stress are important for oil and gas migration and accumulation. Based on the in-depth study on the tectonics of the Bohai Sea area and in the light of the geological and evolutional history, the authors simulate the stress field of the area using 2-D finite element method. In the simulation, the studied area is assumed as a rigid block that has been divided into 1347 finite elements, 1339 nodes. The tectonic field stress is calculated using SAP5 procedure. Some significant results are achieved. In light of the calculations and the evolutional history of the area, we firstly divide the Bohai sea tectonic stress field into four regions, namely the east Liaoning Bay, the central Bohai Sea, the east Bohai Sea and the west Bohai Sea. The stress characteristics are different in different regions, e.g. the central Bohai Sea has the maximum average normal stress value and shear stress value compared with other regions,while its minimum principal stress is lower. Secondly, the stress distribution of the whole study area shows that the faulting in the west is more intense than that in the east since Late Tertiary. The faults in the east Bohai Sea region have a higher degree of development and gaping along the Tanlu Fault belt, while in the middle Bohai Sea area, the faults have a low degree of displacement and development.

THE REGIONAL CHARACTERISTICS OF FOCAL MECHANISM SOLUTIONS IN CHINA AND ITS ADJACENT AREAS

CUI Xiao-feng, XIE Fu-ren, ZHAO Jian-tao

2005, 27(2):  298-307. 

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Based on analysis of distribution of 2 660 earthquake focal mechanism solutions from “Database of Crustal Stress of China and Adjacent Area” (updated to December, 2003), the regional characteristics of focal mechanism solutions are studied. In the Northeast and North China stress regions, the orientations of maximum horizontal stress mostly are in the near EW direction and NEE-SWW direction. The strike-slip focal mechanism is predominant in Northeast and North China stress regions. In the South China stress region, the predominant orientation of maximum horizontal stress is in NE-SW direction. The focal mechanisms mainly include thrust faulting and strike-slip faulting. In the Xinjiang stress region, the predominant orientation of maximum horizontal stress is in near N-S direction. There is a significant number of thrust faulting events and strike-slip faulting events in Xinjiang stress region. In the south Tibetan stress region, the predominant orientation of maximum horizontal stress is in near N-S direction, and the focal mechanism solutions are of strike-slip and normal faulting. In the stress region of north Tibet plateau and in the northeastern edge, the predominant orientation of maximum horizontal stress is not concentrated, and the focal mechanisms mostly are of strike-slip faulting, with some normal faulting and thrust faulting.

SIMULATION OF HIGH-RESOLUTION IMPEDANCE USING SEISMIC DATA, LOG DATA AND RELATIVE STABILITY OF RESERVOIR SPACE

SONG Wei-qi, LIU Shi-you

2005, 27(2):  308-315. 

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The seismic impedance inversion is bandlimited. Because the breadth of frequency band of the seismic inversion constrained by well log is limited. The research in this paper is focused on how to expand the breadth of frequency band in seismic impedance inversion and to improve the resolution of seismic data. By using the logging constrained seismic impedance inversion data, the seismic data and the log data, we studied the relative stability of reservoir space and estimated and inversed the seismic impedance data with higher resolutions based on correlation function analysis and geological modeling technique used for reference. Considering the spatial correlation and distinguishability of the geologic body, we select the correlation radius in the range from the maximum point of fitted correlation function to the end of smoother section behind the point of inflexion. When calculating the objective function, we consider both the current weight coefficient and the one of extrapolating point to obtain more stable and reliable results. By synthesis simulation, we can distinguish the smaller sand body.

THE DETECTABILITY OF TRANSIENT ELECTROMAGNETIC METHOD TO MULTIPLE 3D BODIES WITH VALLEY TOPOGRAPHY

TANG Xin-gong, HU Wen-bao, YAN Liang-jun

2005, 27(2):  316-323. 

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The delectability of transient electromagnetic method (TEM) to multiple 3D conductive sheets with valley topography is discussed in detail in the paper. An anomalous body with a resistivity of 10⁶ohm.m is designed to simulate the valley topography. The results of the modeling illustrate that the total anomalous fields are the summation of anomalous fields produced by the valley and the conductive sheets. The delectability of TEM to the conductive sheets buried shallower beneath the receivers is quite good without valley topography and TEM is much more sensitive to anomalies near the source than those far from the source. It is interesting that the total field will enlarge the anomalies fields of those conductive sheets placed in far source when the valley topography exists and, in some sense, this will be helpful for TEM to detect the anomalies from far source. The delectability of TEM will become weaker when the distance between two anomalous bodies is too small. In general, the delectability of TEM to multiple 3D conductive sheets is still good when valley topography exists.

A PRELIMINARY ANALYSIS OF SEISMOTECTONICS FOR THE M 8.7 INDONESIA EARTHQUAKE ON DECEMBER 26, 2004

XU Jie GAO, Xiang-lin, CHEN Guo-guang, ZHOU Ben-gang

2005, 27(2):  324-331. 

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The Indonesia region is one of the most seismically active zones of the earth. On December 26, 2004 an M_S 8.7 (given by China Seismograph Network, M_W=9.3 given by USGS) earthquake occurred off the west coast of northern Sumatra, Indonesia. Its magnitude classifies it as the fourth largest earthquake in the world since 1900 and the largest since the 1964 Alaska earthquake. The spatial distribution of the relocation of larger aftershocks (M＞4.5) following the main shock suggests a length and width of the rupture of about 1200km and 200km, respectively. The shock triggered massive tsunamis that affected several countries throughout South and Southeast Asia. It is a shallow interplate event of thrust type in the trench. Its epicenter is located at the northwestern end of the Indonesia-Melanesia plate boundary tectonic zone. In 2004, eight events of M≥7 happened in this zone, showing a migration from east to west. It implies that these shocks represent a correlated and consistent dynamic process along this subduction zone. These interplate events are associated with convergence of several plates and their fast motion in this region, which result in strong and complex structures and deformation. The India-Australia plate is underthrusting toward the Sunda continental block or Burma plate at a low angle, producing a great locked area on the shallow portion of the subduction zone where enormous strain is accumulated. Interseismic uplift recorded by coral growth and horizontal velocities measured by GPS show the geometry of the locked portion of the Sumatra subduction zone. The vertical and horizontal data are reasonably well reconciled with a model in which the plate interface is fully locked over a significant width. This locked fault zone extends to a horizontal distance of 132km from the trench, which corresponds to a depth of 50km. The sudden ruptures and large-scale slip of this locked area as the release of stress are the direct cause of the M 8.7 earthquake near Indonesia in 2004.

TIME SERIES OF VERTICAL DISPLACEMENTS OBTAINED FROM CONTINUOUS GPS MEARUREMENT AT FIDUCIAL STATIONS IN THE CRUSTAL MOVEMENT OBSERVATION NETWORK OF CHINA

GU Guo-hua

2005, 27(2):  332-340. 

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Time series of vertical displacements obtained from continuous GPS observation at fiducial stations in Crustal Movement Observation Network of China, from September 1999 to March 2004 are introduced with discussions on the selection of datum definition and analysis of the general characteristics of vertical displacements at fiducial stations. A group of 6 stable stations in the east part of China with the same variation pattern and small annual vertical displacement velocities are used in datum definition for vertical displacement time series. Results of calculation show that the accuracy for weekly solutions at a station is about 3mm. Compared to the annual horizontal displacement velocities at the fiducial stations, the annual vertical displacement velocities are rather low almost at all stations. In comparison to the time series of horizontal displacement, the time series of vertical displacement show very significant variations of annual period with magnitudes of 20～50mm. The annual variations in vertical displacement at most fiducial stations in west China show significant correlation with earth rotation and are in agreement with conservation of the momentum of earth rotation. It would be worthwhile to make comprehensive studies on the vertical displacement at fiducial stations and related subjects.

DISCUSSION ABOUT GROUPING OCCURRENCES OF RECENT STRONG EARTHQUAKES IN CHINA FROM BLOCK VIEWPOINT

HU Xin-liang, DIAO Gui-ling, MA Jin, WANG Jun-guo, ZHAO Ming-chun, MIAO Ju-ling

2005, 27(2):  341-351. 

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The thinking of earthquake research in China should be shifted from the viewpoint of fault to active block (MA Jin, 1999). ZHANG Pei-zhen et al. divide the active blocks in Chinese mainland and its adjacent area into two degrees: the first degree refers to active-block regions and the second degree refers to active blocks. The former contains 6 block regions, e.g. the Qingzang (Qinghai-Tibet) region, etc. and the latter contains more than 20 active blocks, e.g. the Lhasa block and so on. We attempt to analyze the characteristics of geological structure and focal mechanism of group strong earthquakes that occurred recently in Chinese mainland from the block viewpoint on the basis of the two-degree active blocks. The strong earthquakes (M≥7 in the west and M≥6 in the east) occurring in China of 1977—2003 can be roughly divided into 9 groups. In summary, the strong earthquakes occurring in the recent 10 years still have the grouping feature and most of them are located in the boundary zones between active-block regions or active blocks. Moreover, their focalmechanism solutions are quite similar to each other, except for the earthquakes in the 4th group (the earthquakes that occurred in the Beibu Gulf and the Taiwan Straits can be considered as an individual case) and in the 5th group (the earthquakes that occurred in Mandalay-Diannan block near the plate boundary are not regarded as intraplate earthquakes). Based on the study of horizontal strain field in Chinese mainland and its surroundings with GPS data, we point out in the paper that group strong earthquakes have their own genesis for the similar motion pattern and dynamic origin. From the above analysis, we conclude: (1) The rule of strong earthquake occurrences in groups is still effective after more than 10 years practice, and it is an applicable method for locating earthquakes in the medium and short-term earthquake predictions. (2) The regional characteristics of group strong earthquakes enable us to predict the location of earthquakes in a smaller range on the boundary zone between the second-degree blocks in a first-degree block region or between two first-degree block regions. (3) Except plate-margin area, the group strong earthquakes have consistent focalmechanism solutions. This indicates that they have similar kinetic mechanisms and dynamic processes, or perhaps, we can say, that they develop monolithically and occur successively.