Table of Content

03 December 2003, Volume 25 Issue 4

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Brief Report

SEISMOGENIC ENVIRONMENT AND ASSESSMENT OF THE MAXIMUM MAGNITUDE OF POTENTIAL EARTHQUAKES ON THE MAIN FAULTS IN FUZHOU BASIN

WEN Xue-ze, XU Xi-wei

2003, 25(4):  509-524. 

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Comprehensive prospecting in Fuzhou basin has revealed that all the main faults in the basin have not been active since Holocene time. Therefore, how to assess the long-term seismic risk of such a type of fault has become an important problem. Taking the Fuzhou basin as an example, we seek after a possible way to evaluate the long-term seismic risk on non-Holocene active faults in eastern China. Firstly this paper demonstrates the position of the Fuzhou basin in the regional earthquake environment by analyzing regional crustal dynamics and seismic background, and then finds out the fault-segments of relatively high seismic risk and estimate the maximum magnitude of potential earthquakes on these fault-segments through the comparison of seismotectonic conditions and the statistic analysis of seismic activity levels in individual seismotectonic zones. The main results are as follows: The Fuzhou basin lies in the area that has been influenced slightly by the action of the Taiwan Dynamic Wedge (TDW), but is close to the border between the areas that are slightly and strongly influenced by the TDW, and is located at the transition zone between regions with and without strong earthquake (of M_S≥6.0) occurrence. As compared with the seismotectonic background in Fujian northeast Guangdong areas, where strong earthquakes have occurred, the main faults in Fuzhou basin and its neighboring area have older ages of the latest activities, while the crust beneath the region contains less distinct low-velocity layers. Moreover, among 12 zones that are taken as statistic units for seismicity in Fujian, northeastern Guandong and the border region among Fujian, Guangdong and Jiangxi Provinces, the unit in Fuzhou basin shows the lowest level of seismicity. It is suggested that the Minhou-Nanyu Fault along the western border of the Fuzhou basin and the Wuhushan northern-foot fault along the southern border of the basin are the two fault-segments with relatively high potential of moderate to strong earthquakes. The maximum magnitudes of the potential earthquakes on the two fault segments have been estimated to be M_S6.0 and 5.6, respectively, on the basis of the worldwide empirical relationship between subsurface rupture scale and magnitude of earthquake.

COMPOSITION,THERMAL STRUCTURES AND RHEOLOGY OF THE UPPER MANTLE INFERRED FROM MANTLE XENOLITHS FROM HAOTI,DANGCHANG,GANSU PROVINCE, WESTERN CHINA

SHI Lan-bin, LIN Chuan, Yong, CHEN Xiao-de

2003, 25(4):  525-542. 

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Mantle xenoliths brought up by Cenozoic volcanic rocks onto the earth surface may provide direct information about the upper mantle beneath the volcanic region. In eastern China there are more than one hundred reported localities of mantle xenoliths, on which a lot of studies have been carried out. In western China, however, only a few mantle xenolith localities have been reported, where less work has been done. This paper presents the results of study on mantle xenoliths collected from Haoti village, Dangchang County, Gansu Province, western China. The main purpose of the study is to gain an insight into the composition, thermal structure and rheology of the upper mantle beneath the region, and then to compare the obtained results with those obtained in North China. The rock types of mantle xenoliths in this region have confirmed that the upper mantle of the region is composed mainly of spinel lherzolite at shallower depth (52～75km), and garnet lherzolite at greater depth (greater than 75km), rather than of harzburgite and dunite as proposed by some previous studies. The upper mantle geotherm derived from the equilibrium temperatures and pressures of xenoliths from the region is lower than that of the North China, and is somewhat closer to the Oceanic geotherm. The crust mantle boundary is determined from the geotherm to be at about 52km, and the Moho seems to be the transition zone of lower crust material with spinel lherzolite. If taken 1 280℃ as the lowest temperature of asthenosphere then the lithosphere asthenosphere boundary should be at about 120km depth. The differential stress of the upper mantle is determined by using recrystallized grain size piezometer, while the strain rate and equivalent viscosity are determined by using high temperature flow law of peridotite. The constructed differential stress, strain rate and viscosity profiles indicate that asthenospheric diapir occurred in this region during the Cenozoic time, resulting in the corresponding thinning of the lithosphere. However, their scale and intensity are less than those occurred in North China. Moreover, numerous small scale shear zones with localized deformation may occur in the lithospheric mantle, as evidenced by the extensive occurrence of xenoliths with tabular granuloblastic texture. All these results may indicate that the region is located at the transition between the East China continental rift zone and Central China cratonic block group of Deng jinfu et al.(1996).

CONTEMPORARY CRUSTAL DEFORMATION AND ACTIVE TECTONIC BLOCK MODEL OF THE SICHUAN-YUNNAN REGION,CHINA

L? Jiang-ning, SHEN Zheng-kang, WANG Min

2003, 25(4):  543-554. 

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We derive horizontal velocity field of the Sichuan-Yunnan region by analyzing GPS data collected from the Crustal Movement Observation Network of China between 1998 and 2001. Our result shows that the region is composed of 4 tectonic terrains: the Chuan-Dian block, the Longmen Shan region, the Red River shear zone, and the Ruili Ximeng region. The Chuan-Dian block is further decomposed into 3 sub-blocks: the Dianzhong block, Yajiang block and Zhongdian block. The Longmen Shan region is decomposed into the Minshan block and the Aba region. Viewed relative to the Eurasian plate, the Dianzhong block, Yajiang block and Zhongdian block are rotating clockwise at 0.37°±0.16°/Ma, 0.84°±0.39°/Ma and 0.90°±0.39°/Ma, respectively, resulting in active faulting between the sub-blocks: ～3 mm/yr north-south shortening across the Muliarcuate Fault, ～4 mm/a east-west extension across the Lijiang Fault, and ～6mm/a east-west extension across the Litang Fault. About 8～10mm/a left-lateral strike-slip is detected across the Xianshuihe Fault, and 5～6mm/a left-lateral strike-slip across the Anninghe-Zemuhe-Xiaojiang Fault. No obvious crustal shortening is observed across the Longmen Shan Fault. About 150km northwest of the Longmen Shan Fault lies a velocity gradient zone with a 4～5mm/a right-slip across. The Minshan block and the Aba region bounded by the gradient zone rotate counterclockwise at 0.13°±0.08°/Ma and 0.53°±0.19°/Ma relative to the Eurasian plate, respectively. Our results are consistent with a model attributing the crustal deformation of the Chuan Dian block in the Sichuan Yunnan region to be driven mainly by the eastward extrusion and gravitational collapse of the Tibetan Plateau. Being resisted by the relatively stable south China block to the east, the Chuan-Dian block turns from eastward to southward motion along the Xianshuihe-Xiaojiang Fault, resulting in a cluster of clockwise rotations of its internal sub-blocks.

CHARACTERISTICS OF LATE QUATERNARY FAULTING AND PALEOSEISMIC EVENTS ON THE EAST OF DELINGSHAN SEGMENT OF THE SERTENGSHAN PIEDMONT FAULT

CHEN Li-chun, RAN Yong-kang, CHANG Zeng-pei

2003, 25(4):  555-565. 

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The Sertengshan piedmont fault is a very important active fault. Along with the Langshan piedmont fault, it controls the northwest boundary of the Hetao fault-depression zone. We have carried out detailed investigation of fault activity and paleoseismology on east of Delingshan segment of this fault, on which little work has been done before. The result of investigation indicates that the geomorphic characteristics of late Quaternary activity on this segment of the Sertengshan piedmont fault are very remarkable, appearing as the offset and uplift of lacustrine deposits, alluvial fan and stream terraces, resulting in linear scarps and fault platforms. Taken the Xiaoshetai canal outlet as boundary, this fault segment can be subdivided into the Dashetai and Wulanhudong active fault segments characterized by different features. In the past 30ka BP,the average uplift rate of the upthrown side on the Dashetai segment is 0.19mm/a, while that of the upthrown side on the Wulanhudong segment is 0.20mm/a. One trench has been excavated on each active fault segment for paleoseismic investigation. As a result, the Hongming trench on the Dashetai segment has revealed that 4 paleoseismic events have occurred on this segment since about 32ka BP, the occurrence times of which are successively 31,690?1770, 23,000?1320, 15,420?870 and 7,440?440a BP. The corresponding displacements produced by these 4 events are 2.6, 1.6, 2.2 and 1.4m, respectively. The Wulanhudong trench on the Wulanhudong segment, has also revealed that 4 paleoseismic events have occurred on this segment since about 25ka BP, the occurrence times of which are successively 25,130?1430, 14,570?820, 11,660?650 and 7,220?400a BP. The corresponding displacements produced by these 4 events are 2.6, 1.8, 1.3, and 1.2m, respectively. The comparison of these paleoseismic events also indicates that the subdivision of east of Delingshan segment of the Sertengshan piedmont fault into 2 active fault subsegments is acceptable. Finally, according to the Displacement amount-constraining Method, it is confirmed that the acquired paleoseismic activity histories for the two segments are complete during the observed period of time.

THE EFFECT OF DIFFERENTIAL STRESS ON THE FORMATION OF ULTRA-HIGH PRESSURE METAMORPHIC ROCKS—Evidence from high temperature-high pressure experiments on quartz-coesite transition

ZHOU Yong-sheng, HE Chang-rong, MA Sheng-li, Ma Jin

2003, 25(4):  566-573. 

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In summarizing the metamorphic temperature and pressure at hydrostatic condition of ultra high pressure rocks obtained from both field geology and experiments, we find that this problem is needed to further discuss, because the collision tectonic zone is not under hydrostatic conditions but under the action of differential stress. In this paper, we reassess and analyze the data of high temperature-high pressure experiment on quartz-coesite transition at differential stress condition made by Hirth and Tullis (1994). The results show that coesite is observed in both the semibrittle faulting and semibrittle flow regimes under temperature condition of 500～700℃ and pressure condition of 1.20～1.25GPa. Coesite is present mainly at the top and bottom of the tested samples adjacent to the pistons, as well as along fracture zones and along grain boundaries oriented perpendicular to σ₁ within the sample. The confining pressure (1.20～1.25GPa) required for quartz coesite transition in the presence of large differential stress is much lower than that (2.5～3GPa) at hydrostatic pressure condition. Obviously, the effect of differential stress is of great significance in the experiments. It is found that garnet in eclogite might be plastically deformed, indicating that differential stress do exist in collision tectonic zone, while the upper limit of the tectonic differential stress is constrained by the strength of rocks. Accordingly, differential stress is of great significance to ultra high pressure metamorphism. It is suggested, therefore, that systematic high temperature high pressure experiments are the essential and effective way to further investigate this problem.

TIP STRUCTURES OF THE WESTERN SEGMENT OF SURFACE RUPTURE ZONE OF THE M_S 8.1 WEST OF KUNLUN MOUNTAIN PASS EARTHQUAKE

ZHAO Rui-bin, LI Jun, XIANG Zhi-yong, GE Ming, LUO Gang

2003, 25(4):  574-580. 

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On November 14, 2001, an earthquake of magnitude M_S8.1 occurred to the west of the Kunlun Mountain Pass. This event has produced a nearly east-west-striking surface rupture zone of more than 400 km in length. Macroscopically, this surface rupture zone can be distinctly divided into the western and eastern segments. Field investigation has revealed that the western segment of the rupture zone extends from the Kushuiwan Lake (35°57′36.2″N, 90°15′34.6″E) to the Taiyang Lake (35°55′34.0″ N, 90°32′52.2″E), having a total length of about 25km and a general strike of 285°～290°dominated by left-lateral strike-slip. The western segment of the rupture zone exhibits a typical tip effect of left-lateral strike-slipping. The western tip of this segment occurs on the gully bed to the west of the Kushuiwan Lake, where the general strike of the rupture zone turns from NNW to S60°W. The ruptures here consist of a series of 30°～40°-striking en-echelon tensile fractures of 5～15m in width, and NW-SE-striking compression ridges. The eastern tip of this segment is located on the terrace at the western bank of the Taiyang Lake. The strike of the rupture zone here turns from 105°～110°to about N50°E. The rupture zone is characterized by the crisscross arrangement of NE-trending tensile fractures and NW-trending compression ridges, appearing as a tessellated structure and ending at the shores of the Taiyang Lake. According to the tip structures of the western segment of the surface rupture zone, the distribution feature of the western segment and its relation to the eastern segment of the rupture zone, as well as the characteristics of the Eastern Kunlun Fault and available seismic observational data, it is suggested that the western segment of the rupture zone was produced by an independent seismic event, and that the M_S8.1 west of Kunlun Mountain Pass earthquake is characterized by multi spot fracturing.

SEISMOGEOLOGIC BACKGROUND OF THE CHENGDU PLAIN AS REVEALED BY SEISMIC PROSPECTING DATA OF PETROLEUM

HUANG Sheng-mu, HE Tian-hua, FAN Ming-xiang, LI Jia-pen, XIE Xiong-fei, FANG He-di, WU Zhi-shen

2003, 25(4):  581-594. 

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This paper presents the identification and delineation of buried faults in Chengdu and Deyang depression, based on seismogeologic and petroleum prospecting data accumulated in the past 20 years by Seismological Bureau and Petroleum Administration Bureau of Sichuan Province. A very important question about whether a potential or buried fault exists in Chengdu and Deyang depression basin is answered. Seismo-geological data show that a fault starting from Xinjin and terminating to the south of Pengzhen, Shuangliu County does exist in the region. Furthermore, the relationships between active faults and seismic activities, as well as the related characteristics of the faults are discussed. At the same time, a strict distinction between the shallow faults and deep buried faults is made, and then their distributions are delineated. Five new types of seismogenic faults are proposed. Based on the above mentioned recognition, the potential areas of strong earthquakes in Chengdu basin are determined. The results show that the Chengdu-Deyang area of hundred kilometers in length and 40km in width possesses stable seismo-geologic environment and good conditions for earthquake engineering. This conclusion is of great importance to the economic developments and constructions, and especially to the assessment of seismic risk of major engineering sites in Chengdu basin.

DEEP GEOLOGICAL FEATURE AND DYNAMIC EVOLUTION OF THE SONGLIAO BASIN

YUN Jin-biao, YIN Jin-yin, JIN Zhi-jun

2003, 25(4):  595-608. 

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The Songliao basin located in Northeast China is a large Mesozoic rift basin. A lot of work has been done on the geological characteristics, formation and evolution of the basin, as well as the accumulation regularity of oil and gas in this basin. However, the question about the formation of the basin is still a point at issue, and has attracted the great attention of many geologists. Various models have been proposed so far for the formation of the basin, such as continent rift, back arc rift, and pull-apart models. Basing on deep seismic reflection data, this paper discusses the type of the basin and the geodynamic cause of its formation. Deep seismic reflection profiling has revealed that the crust of the Songliao basin is characterized by both layered and fault-block structures. The crust can be divided into three layers: the upper crust, middle crust and lower crust. Parallel reflections in shallow layer and oblique reflections at above 6s twt represent the upper crust. The strong reflection belt below the upper crust represents the detachment zone between the upper and middle crust. Most of the faults in the upper crust terminate in this zone. The analysis shows that the faults in the upper crust are extensional syn sedimentary faults formed during late Jurassic to early Cretaceous and thrust system formed in late Paleozoic era. The middle crust at about 68s twt not only has a lot of discontinuous parallel reflections and rhombic reflections, but also a small quantity of "crocodile mouth" reflectors. The former represents extensional reformation formed in late Mesozoic era, and the later represents compressional structure formed in late Paleozoic era. The middle-crust is also a low-velocity and high conductivity layer. The lower crust is located at about 8～11s twt, and is characterized by rhombic reflection events. The Moho below the lower crust appears as a strong reflection belt, beneath which the layer is transparent. The reflection structure of the Songliao basin is similar to that of typical rift basin. Boundary faults between blocks were the passage of upward transferring of hot fluid from the middle lower crust or mantle to the basin. "Mushroom cloud" reflections at the deep part of the boundary faults might be the reflections of hot fluid diapir. Mirror symmetry between the down-warped basin and mantle uplift is the main evidence for the rift-origin of the Songliao basin. Deep seismic reflection data indicate that the main factor at depth that affected the subsidence of the basin was not only the uplift of the mantle, but also the "low velocity-high conductivity" layer in the upper-middle crust and hot fluid diapirs. The former caused the subsidence in later stage, while the later caused the subsidence in early stage. Regional geology and geophysical data indicate that this area is located in back-arc formed by the subduction of the Kula-pacific plate into the Eurasia plate in Mesozoic. Mesozoic volcanic rock system in this area is mainly of CA series, with a few exception of a small amount of A and Th type. Geochemical analysis reveals that the volcanic rocks here are characterized not only by high SiO₂ and K₂O contents and enrichment in LREE, but also by EMII type mantle, indicating back arc environment disturbed by plate subduction. However, seismic tomography data have revealed that the Songliao basin is located in disturbed area of continental mantle, rather than back-arc area. It is suggested, therefore, that formation and evolution of the Songliao basin was the result of the subduction of Kula-Pacific plate into the Eurasian plate. The plate subduction has led to the uplift and diapir of the mantle, and the detachment of the crust. Strong crust-mantle interaction has led to the delimitation and extension of the crust, resulting in the formation of sandwich structure and large scale subsidence of the crust. It was the afore mentioned factors that caused the formation of mirror symmetry between the downwarping part of the basin and the uplifting part of the mantle, as well as the dissymmetry between the fault depression and the uplifting part of the mantle. The former was the result of large scale hot subsidence of the crust caused by the mantle cooling, while the later was resulted from simple shearing deformation of the middl-upper crust caused by the detachment of middle crust from the upper crust. The intense crust-mantle interaction was the major factor controlling the formation of no-granite basin w ith sandw ich structure and hot fluid diapirs at depth, and it may play an important role in the accumulation of deep-seated fluid and oib gas in the basin.

NUMERICAL MODELING OF THE GENERATION PROCESS OF THE 1906 MANAS M_S7.7 EARTHQUAKE

ZHOU Wei-xin, YANG Zhu-en, SUN Jun-xiu, ZHAO Rui-bin

2003, 25(4):  609-616. 

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Numerical modeling of the relationship between tectonic stress and seismicity is one of the important subjects in seismogeology. In this paper, the generation process of the 1906 Manas M_S7.7 earthquake is simulated using finite element method on the basis of the integration of newly obtained geologic and geophysical data on this earthquake. The Manas strong earthquake region is composed of North Tianshan Mountain and South Marginal Depression of the Jungar basin separated by the South Marginal Fault of the basin. The tectonic deformation in this region is dominated by a thrust system developed at middle-upper crustal level, thrusting over from the south to the north. The southern part of the thrust system, the North Tianshan Mountain, belongs to a thick-skinned structure and the main detachment surface is located below the depth of brittle-ductile transition. There are three major fault zones in the North Tianshan Mountain; they are the Boloholo Fault, the Yamate Fault and the South Marginal Fault of the Jungar basin. The tectonic deformation in the South marginal Depression of the Jungar basin is dominated mainly by the brittle deformation of the sedimentary cover. The South Marginal Fault of the Jungar basin is situated at the transition zone from thick skinned to thin skinned structures. This transition zone is also the brittle-ductile transition zone, which has strong ability to store energy and hence is suitable for the generation of strong earthquake. The 1906 Manas M_S7.7 earthquake just occurred on this tectonic zone. According to the tectonic framework of the Manas M_S7.7 earthquake region, the distribution of tectonic stress in this region is simulated using three-dimensional finite element method. The results show that the hypocenter of the Manas earthquake is just located at the position of stress concentration, and the stress here increases as time goes on. These results reveal also the process of stress accumulation at the hypocenter of the Manas strong earthquake, and would help to understand the generation process of the Manas M_S7.7 earthquake.

THERMAL MODELING OF OCEANIC CRUST SUBDUCTION AND ADAKITE GENERATION

ZHANG Jian, SHI Yao-lin, WU Chun-ming

2003, 25(4):  617-624. 

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Adakitic rocks play an important role in the evolution of continental crust, and recently have attracted great attention of many scientists all over the world. Cenozoic adakites are exclusively distributed in circum-Pacific regions, and are mostly produced by partial melting of young and hot subducting oceanic crust at 70～90km depths. Previous kinematical model of Subducting oceanic plate shows that ridge would approach to trench and ridge subduction may occur if the velocity of ridge outspreading is significantly smaller than subducting velocity in actual geological process. In this condition, dehydration of some hydrous minerals in oceanic crust and melting in subduction zone will become crucial geodynamic condition for the generation of adakitic rocks. In this paper, we quantitatively model the thermal evolution of ridge subduction with the finite element method, and discuss the effects of dehydration and melting on arc volcanism during the process of active ridge subduction. The result of finite element modeling shows there is a distinctive cooler zone along the suducted slab. Dehydration of amphibolites occurs in the subducted oceanic crust at temperature range of 825～1 000℃. While the ridge arrived at trench and subducted with the oceanic plate, the dehydration temperature zone ascended to the depth of 75～85km. And, the eclogite in the dehydrated oceanic crust would melt and produce adakite. After ridge subduction, the dehydration temperature range of 825～1 000℃ would drop to the depth of 100km under the Arc. At this depth, oceanic crust could not melt due to high pressure. However, andesitic magmas might be formed when the released free water moves up to the mantle wedge. Thermal modeling reveals that the dehydration in the subducted oceanic crust may cause two possible cases of partial melting. One possible case is the melting of the subducted oceanic crust itself, which will produce adakitic magma activity in fore arc region. Another case is the partial melting of overriding mantle wedge due to dehydration of subducted slab, resulting in the formation of andesitic magma in the arc. Because of the differences in dehydration and melting depths, the formation places of adakite and andesite are different. Adakitic magma generates at the fore arc region, whereas andesitic magma forms in the Arc. Besides, the occurrence time of adakite or andesite is mutually exclusive. Andesitic magma would be produced after ridge subduction, but adakitic magma would be active only in the period of ridge subduction. In other words, the andesitic magma would not occur during Adakitic activity. Oppositely, the adakitic magma would disappear when andesite magmas are formed.

STRESS-FAILURE COUPLING EFFECT:A POSSIBLE EXPLANATION FOR GRAVITY VARIATION BEFORE EARTHQUAKE

CHEN Bing, JIANG Zai-sen, ZHU Yi-qing, ZHANG Xiao-liang, LOU Zhi-wen

2003, 25(4):  625-631. 

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Early in the 60s-70s of the 20th century, Japanese and American scientists have successfully observed the variation of gravity associated with earthquake process. In China, many researchers have studied the gravity field of some typical earthquakes. The results showed that observable variations of gravity do exist during the development process of medium-strong earthquakes, and that different earthquakes have different features of gravity variation. It is considered that the density variation in the crust may be an important factor. In this paper a model of stress-failure coupling effect is proposed in the light of failure mechanism. In addition, the theoretical relation between the stress failure coupling effect and variation of gravity in the crust is derived according to strain equivalent hypothesis. Furthermore, the different features of gravity evolution during the development processes of the Tangshan earthquake (M_S7.8) in 1976 and the Kunlun earthquake (M_S8.1) in 2001 are simulated through rock mechanical test with various loading paths. The results are something in good agreement with the actual facts and show that the stress failure coupling effect may be a physical explanation of gravity change before earthquake.

BEHAVIORS OF SUBSURFACE FLUID AND THEIR RESPONSIBILITY TO SEISMIC EVENT IN THREE-GORGE OBSERVATION WELL NETWORK

LIU Wu-zhou, YU Jin-zi, CHE Yong-tai, LIU Xi-lan, YAO Yun-sheng, YAN Ping

2003, 25(4):  632-639. 

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This paper presents the results of analysis of observed data of water levels from 8 observation wells, water temperature from 4 wells and soil radon from 4 well sites obtained during the year of 2001. Based on the work, the normal behaviors of each observation item are established and their responsibility to earthquake event are tested. The results indicate that all the groundwater levels from 8 wells are able to reflect the solid tide. The sensitivity of the response of groundwater to volumetric strain of the earth crust in eight wells ranges from 10^-9 to 10^-10 volumetric strain/mm. Groundwater levels in most wells within the well network have barometric effects and are influenced to some extent by the rainfall. The water temperature in 4 wells is relatively stable, the daily change of which is ranging within several degrees a thousandth. The soil radon in 4 well sites fluctuates dramatically, and the annual change ranges from ten to several tens Bq/L. The anomalies of water levels in 6 wells, water temperature in one well, and soil radon in one well site were observed before the M_L3.6 earthquake occurred at Meijiahe, Zigui County on October 16, 2001. This may indicate that the Three Gorge observation well network has a relatively high capability to respond earthquake event.

STUDY ON INTERFERENCE FACTORS OF THE DIGITAL MEASUREMENT OF ESCAPED RADON

ZHANG Chao-ming, CHEN Hua-jing, ZHU Fang-bao, LI Gang-feng, TAO Yen-chao, LUO Zhi-chun, SHU Ke-ming

2003, 25(4):  640-646. 

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In digital measurement of radon gas from subsurface fluid and the use of the obtained data in earthquake prediction, we found that some interference factors have different effects on digital measurement of radon gas. In this paper, the effects of some interference factors, such as atmospheric pressure, atmospheric temperature, discharge of escaped gases, impurity in escaped gases, discharge of well water, leakage of gas-pipe and different gas-extraction systems on digital measurement of escaped radon gas are discussed in detail. The study shows that under the present conditions of measurement devices and method, the influence coefficient of atmospheric temperature on digital measurement of escaped radon is -0.4%/℃, and that of the atmospheric pressure is 0.1%/hpa. Different types of gas-extracting devices have great influence on radon measurement. The change of the discharge of the observation well caused by short-term variation of the discharge of well or spring of the same aquifer has less effect on radon measurement, and the discharge of escaped gases has little influence on radon measurement. However, the impurity in the escaped gasses and the leakage of gas-pipe may cause the decrease of measured value of escaped radon. Taken the working principle of the measurement apparatus into account, the mechanism of the interferences and the eliminating method are discussed. The results of this study are of actual significance to the application of digital measurement of escaped radon to earthquake prediction.

CHARACTERISTICS OF GREAT EARTHQUAKE ACTIVITY IN WEST CHINA AND ITS INFLUENCE ON SEISMIC TENDENCY IN NORTH CHINA

CHEN Yu-wei, ZHANG Jun, YAN Su-ping, QING Mei

2003, 25(4):  647-654. 

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Based on the analysis of temporal and spatial distribution of great earthquakes (M_S≥7.5) in West China since 1900, the correlation between great earthquake activity in West China and strong earthquake (M_S≥6.0) activity in North China is studied. The result shows that the activity of great earthquakes in West China is controlled by the seismotectonics of the region. Seismic activities in each active period of great earthquake in West China almost occur successively or in cluster within the same sub plate, but other tectonic units in surrounding areas still keep relatively quiet. This may indicate that a seismically active block is an active tectonic unit confined by boundary faults of various strikes. The activation of strong earthquake on a boundary fault will affect the stability of the other boundary faults within the same tectonic unit, and it may trigger strong earthquake activities on those boundary faults. Seismic activities in China have the characteristics of oriented migration. In the first active period, strong earthquakes concentrated mostly in Xingjiang sub-plate, northwest China. In the early stage of the second active period, the earthquake site migrated to the southwest of Qinghai-Tibet sub-plate, while in the later stage it migrated to the southern part of the South-north seismic belt. In the third active period, the earthquake site migrated to the northwest of Qinghai Tibet sub plate. The earthquakes have no repeatability in spatial distributions. When a series of earthquakes occur in a tectonic block, the last earthquake always migrates from the block and occurs in another relatively stable block, and hence is useful for determining the occurrence site of the next series of earthquakes. The activity of strong earthquakes in North China is basically synchronous with the activity of great earthquakes in West China, but the beginning time of strong earthquake activity in North China is later than that in West China. Therefore, though the third active period of great earthquakes in West China has begun now, the probability of the successive occurrences of strong earthquakes (M_S≥6.0) in North China in the near future is very low.

DIFFERENCE OF CRUSTAL DEFORMATION IN ACTIVE BLOCKS CAUSED BY GREAT-EARTHQUAKES

TANG Fang-tou, ZHANG Pei-zhen, DENG Zhi-hui

2003, 25(4):  655-663. 

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The most remarkable feature of Cenozoic and present-day tectonic deformation of the continental lithosphere of China is that the crust has been cut by huge late Quaternary active faults, forming active crustal blocks of different orders. Various active crustal blocks exhibit different horizontal movement and different deformation styles. The inner part of the active crustal block is relatively stable. Deformation commonly takes places along their boundary structures, and most of the great earthquakes (M≥7) occur along these boundaries. In order to monitor crustal movement in China mainland, China Crustal Movement Observation Network has disposed 25 continuous GPS base stations in the main tectonic units all over the country. These stations had been run for 3 years from March 1999 to December 2001. On 14 Nov. 2001, an earthquake of M_S 8.1 occurred to the west of the Kunlun Mountain Pass. This event has produced a surface rupture zone of more than 350km in length with a general strike of 70°～90°. The rupture zone is dominated by left-lateral strike-slipping, and the largest horizontal displacement is about 6m. The observation data of continuous GPS measurement stations show that various GPS stations in different active blocks around this earthquake site had different responses to the earthquake. The GPS station within the active block where the earthquake occurred, such as the Delingha station, exhibited very obvious displacement. However, no obvious displacement was observed at the GPS stations located in the active blocks that are secluded by one active block from the earthquake site,such as the Lhasa GPS station. If the GPS stations are located on the boundary structures of the active blocks adjacent to the earthquake site, such as the Xiaguan GPS station, then they would record obvious displacements several days after the occurrence of the earthquake. If the stations are located within the active blocks, such as the Xining and Kunming GPS stations, no obvious displacement would be observed. However, no obvious displacements was observed at the Xiaguan GPS station after Burma earthquake (M=7.2) occurred in the north of Burma active block, although the epicentral distance of this earthquake (about 370km) is significantly less than that of the west of Kunlun Mountain Pass earthquake. This can be attributed to the relative small magnitude of the Burma earthquake, which did not cause the compression of the Sichuan-Yunnan active block. This fact may indicate that the deformation on the boundary zone of the active block is obviously stronger than that occurs within the block, and it is independent to the epicentral distance. The difference of the effects of great earthquakes on its adjacent active blocks depends mainly on the mode of action on the adjacent block by the movement of active block where the great earthquake occurs. If the movement of the block results in compression of the adjacent block, then the effect of the earthquake will be obvious, while the movement does not result in compression of the adjacent block, no obvious effect can be recorded by the GPS station in this block, because the effect may rapidly decrease when it passes through the boundary zone of the block. The observation data of the GPS stations in response to great earthquake-demonstrate that more effective monitoring of earthquake related crustal movement can be fulfilled, provided that the GPS stations are reasonably disposed within the active blocks and on their boundary zones.

NUMERICAL SIMULATIONS OF VERY-LATE TIME RESPONSE AND APPARENT RESISTIVITY IN LONG-OFFSET TEM SOUNDING

WENG Ai-hua, WANG Xue-qiu

2003, 25(4):  664-670. 

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In this paper, the numerical methods for improving the simulation accuracy of very late time response in Long-Offset TEM (LOTEM) sounding and its all-time apparent resistivity have been introduced. For the purpose, we use the direct numerical integration to get the low frequency response that fits in with the Hankel Transform. The integration is then recast into the form of the sum of the partial integration over the intervals constrained by the continuous roots of the Bessel Function of the first kind. And to accelerate the convergence speed of the sumation, continued fraction is adapted, and the coefficients of the continued fraction are evaluated from the partial integration over each integral subsection by a recursive method. In this way the direct integration of Hankel Transform can be fulfilled quickly and accurately. In consideration of the complexity of the electromagnetic response of LOTEM in half-space model, and basing on the idea that continued fraction has been utilized to solve nonlinear equations successfully, we try to adapt the method to obtain the all-time apparent resistivity from LOTEM responses, and to derive the iterative formulae for transforming the approximate solution into the continued fraction coefficients. Numerical simulation of theoretical models show that the combination of direct integration and continued fraction can greatly improve the calculation accuracy of rather low frequency response, and hence more accurate and stable late-time response of LOTEM can be obtained, while the range of the late time can be extended to about 100 seconds or more. A new method for estimating all-time apparent resistivity in LOTEM sounding is developed by using continued fraction expansion. The results of this study may provide scientific basis for the processing and interpretation of LOTEM data for understanding the electrical properties at greater depth.