Dehydration melting of micas in the Chilka Lake Khondalites: The link between the metapelites and granitoids

Garnet-sillimanite gneisses, locally known as khondalites, occur abundantly in the Chilka Lake granulite terrane belonging to the Eastern Ghats Proterozoic belt of India. Though their chemistry has been modified by partial melting, it is evident that the majority of these rocks are metapelitic, with some tending to be metapsammitic. Five petrographically distinct groups are present within the khondalites of which the most abundant group is characteristically low in Mg:Fe ratios — the main chemical discriminant separating the five groups. The variations in Mg:Fe ratios of the garnets, biotites, cordierites, orthopyroxenes and spinels from the metapelites are compatible with those in the bulk rocks. A suite of granitoids containing garnet, K-feldspar, plagioclase and quartz, commonly referred to as leptynites in Indian granulite terranes, are interlayered with khondalites on the scale of exposures; in a few spots, the intercalated layers are thin. The peraluminous character of the leptynites and presence of sillimanite trails within garnets in some of them suggest derivation of leptynites by partial melting of khondalites. Here we examine this connection in the light of results derived from dehydration melting experiments of micas in pelitic and psammitic rocks. The plots of leptynites of different chemical compositions in a (MgO + FeO)-Na₂O-K₂O projection match the composition of liquids derived by biotite and muscovite dehydration melting, when corrected for co-products of melting reactions constrained by mass balance and modal considerations. The melt components of the leptynites describe four clusters in the M-N-K diagram. One of them matches melts produced dominantly by muscovite dehydration melting, while three clusters correspond to melting of biotite. The relative disposition of the clusters suggests two trends, which can be correlated with different paths that pelitic and psammitic protoliths are expected to generate during dehydration melting. Thus the leptynites evidently represent granitoids which were produced by dehydration melting in metapelites of different compositions. The contents of Ti, Y, Nb, Zr and Th in several leptynites indicate departures from equilibrium melt compositions, and entrainment of restites is considered to be the main causative factor. Disequilibrium in terms of major elements is illustrated by leucosomes within migmatites developed in a group of metapelites. But the discrete leptynites that have been compared with experimental melts approach equilibrium melt compositions closely.     

Synthetic high-pressure micas compositionally intermediate between the dioctahedral and trioctahedral mica series

Intermediate-composition micas with octahedral occupancy 2.5 have been crystallized experimentally from natural phengite, 50% phengite+50% biotite, and synthetic basalt compositions in the pressure range 20–35 kb and temperatures of 800–1,000° C. Their compositions suggest a complete range of micas with octahedral occupancy between 3.0 and 2.5, but a very restricted range between 2.0 and 2.5. These 2.5-octahedral micas lie close to the new mica series proposed by Seifert and Schreyer (1965, 1971), with one end-member composition of K Mg_2.5 (Si₄O₁₀) (OH₂) which is extended by the present results into alumina-bearing members of the series (e. g. K Mg_1.5 Al_1.0 (Si₃Al_1.0O₁₀) (OH)₂). However, the possibility of interlayering of dioctahedral and trioctahedral micas to give an apparently intermediate composition cannot be ruled out. X-ray powder diffraction data on the critical 060 reflection for the phengite mix suggest a transitional change from a single phengite field, through a 2-phase phengite — 2.5-octahedral mica field to a single phase 2.5-octahedral mica field.Natural micas of similar composition have not so far been identified, due probably to the unlikelihood of obtaining a mineralogical record of an appropriate composition at the restricted pressure and temperature conditions apparently needed to stabilize the 2.5-octahedral mica phase. Nevertheless, such a phase may have an important role in mineral assemblages and melting reactions in the deep continental crust, subducted oceanic crust and in the upper mantle; evidence of its existence may be removed by later, lower-pressure reactions.     

The nature of short-periodic climate variation and its relation with water exchange

The climate effect of the solar energy flux variation in the annual cycle is discussed. The effect is considered to be the main cause of short-periodic climate change observed permanently in nature. Analytical approach to solve this problem based on application of the theory of perturbation is described.     

Distribution of Al between coexisting micas in metamorphic rocks from the Central Alps

In 61 pairs of coexisting biotites and muscovites from the Central Alps total Al scatters considerably, but in both series a gradual increase is noticed with increasing metamorphic grade. The ratio Al _Mu ^tot /Al _Bi ^tot remains virtually constant (1.61 average for greenschist facies, 1.57 for amphibolite facies). Tetrahedral Al varies little in biotites and increases in muscovites-phengites with rising metamorphic grade; accordingly the ratio Al _Mu ^IV /Al _Bi ^IV increases slightly with grade. Far the best control of metamorphism is evidenced by octahedral Al. In the muscovite series, and still more pronounced in the biotite series, Al^VI increases with increasing metamorphic grade. Consequently 1 $$K_D = \frac{{Al_{Mu}^{VI} }}{{Al_{Bl}^{VI} }}$$ decreases from 14 to 3. A map (Fig. 6) representing the regional distribution of the K_D values locates a 100 km long and 23 km broad central zone with low K_D. The outline of this central core almost coincides with the isograds anorthite-diopside-calcite and labradorite-pyroxene-hornblende of the Tertiary regional metamorphism; with some deviations this core also agrees with the zone in which phenomena of partial anatexis are observed. The K_D values of micas from anateotic pegmatites agree with those of associated gneisses and schists. The study demonstrates that in the course of progressive regional metamorphism equilibrium has been approached to an unexpected extent and that the two micas coexisted in a strict sense.     

Dehydration and diffusion of radiogenic argon in micas

There is a close analogy between dehydration and loss of argon curves obtained by laboratory heating of muscovite (figs. 2 and 3). Potassium atoms lie in potential cleavage planes of the muscovite. When K⁴⁰ atoms disintegrate the resulting argon atoms are driven into the mica structure by recoil, except those whose paths make a small angle with the cleavage plane. These remain in the "unstable zone" and may escape from the muscovite. This introduces an element of uncertainty in K/A absolute age figures from mica. -- F. E. Ingerson.     

The relation between glacial ages and terrestrial magnetism

An attempt is made to show that the change of angular momentum of the earth, caused by glaciations, may compensate the slowing down effect from tidal friction forces and even reverse the polarity in the terrestrial magnetic field.     

浅谈文书处理与档案质量的关系

文书档案对地勘工作起着非常重要的作用。文书档案的质量如何是每个地勘单位应该重视的问题，而档案质量又与文书处理有着密切联系，应该提倡文书处理到档案立卷一体化，提高文书档案质量，使其在地勘建设、改革与发展中发挥作用，加大文书档案的利用率，为振兴地勘经济服务。     

粗粒土的分数阶应变率及其与分形维度的关系

在波浪荷载、潮汐作用下砂土等粗粒土常常经受长期动力变形。运用分数阶微积分理论,分析了5种不同粗粒土在不同加载条件下的累积变形特性及粗粒土的分数阶应变率,传统的整数阶应变率随着加载次数的变化而变化,而粗粒土的分数阶应变率在同一加载条件下保持为常数。通过粗粒土颗粒破碎的分形理论,尝试建立分数阶应变率与土颗粒分布的分形维度之间的关系,分析土体分形维度对分数阶应变率大小的影响,发现随着分形维度的增加,分数阶应变率的数值降低。     

The relation between Rb,Ba and Sr in granitic rocks

The ternary relation between the elements Rb, Ba and Sr in granitoid rocks is discussed. This relation proved to be useful in tracing differentiation trends in acidic suites. Moreover, it has a potentiality for being used as a criterion in genetic problems of granites, i.e. in distinguishing magmatic from metasomatic or granitized granites. The averages of these three elements in the various types of granites are calculated. The Turekian and Wedepohl (1961) two-fold subdivision of granites into high- and low-Ca rocks is elaborated so as to comprise four distinctive types: quartz diorites and granodiorites, normal granites, strongly differentiated granites beside the group of anomalous granites.     

边墩纵向宽度对回流长度的影响研究

采用试验和三维紊流模型相结合,对长方体边墩在不同纵向宽度条件下的回流长度变化规律进行了研究。三维模型采用大系数法与壁函数法相结合对边墩区域边界进行了处理。研究结果表明:在模拟条件下,边墩后的回流长度随着边墩宽长比的增大而减小,在边墩宽长比较小时,边墩的宽长比对边墩后回流长度的影响较为敏感;当边墩宽长比大于4左右后,回流长度变化较小,此时已相当于河道的突扩问题。以边墩宽长比为0.02(丁坝)时的回流长度L₀为基准,利用实测资料建立了不同边墩宽长比条件下的回流长度计算公式。     

An experimental study of alkali metal distributions in feldspars and micas

K and Rb distributions between aqueous alkali chloride vapour phase (0.7 molar) and coexisting phlogopites and sanidines have been investigated in the range 500 to 800°C at 2000 kg/cm² total pressure.Complete solid solution of RbMg₃AlSi₃O₁₀(OH)₂ in KMg₃AlSi₃O₁₀(OH)₂ exists at and above 700°C. At 500°C a possible miscibility gap between approximately 0.2 and 0.6 mole fraction of the Rb end-member is indicated.Only limited solid solution of Rb AlSi₃O₈ in KAlSi₃O₈ has been found at all temperatures investigated.Distribution coefficients, expressed as $\text{K}{}_{\mathrm{d}}\text{= (}\text{Rb/K}\text{)}$ in solid/(Rb/K) in vapour, are appreciably temperature-dependent but at each temperature are independent of composition for low Rb end-member mole fractions in the solids. The determined $\text{K}{}_{\mathrm{D}}$ values and their approximate Rb end-member mole fraction ($\text{X}{}_{\mathrm{RM}}$) ranges of constancy are summarized as follows: (°C)$\text{T}$$\text{K}{}_{\mathrm{D}}{}^{\mathrm{Phlog/Vap.}}$$\text{X}{}_{\mathrm{RM}}$$\text{K}{}_{\mathrm{D}}{}^{\mathrm{Sandi/Vap.}}$$\text{X}{}_{\mathrm{rm}}$

     

13.

The relation between dilatancy,effective stress and dispersive pressure in granular avalanches     

Perry Bartelt  Othmar Buser 《Acta Geotechnica》2016,11(3):549-557

Here we investigate three long-standing principles of granular mechanics and avalanche science: dilatancy, effective stress and dispersive pressure. We first show how the three principles are mechanically interrelated: Shearing of a particle ensemble creates a mechanical energy flux associated with random particle movements (scattering). Because the particle scattering is inhibited at the basal boundary, there is a spontaneous rise in the center of mass of the particle ensemble (dilatancy). This rise is connected to a change in potential energy. When the center of mass rises, there is a corresponding reaction at the base of the flow that is coupled to the vertical acceleration of the ensemble. This inertial stress is the dispersive pressure. Dilatancy is therefore not well connected to effective-stress-type relations, rather the energy fluxes describing the configurational changes of the particle ensemble. The strict application of energy principles has far-reaching implications for the modeling of avalanches and debris flows and other dangerous geophysical hazards.     

14.

Evolution of zoned micas and associated silicates in the Oka carbonatite   总被引：1，自引：0，他引：1  

Jadvyga Rimsaite 《Contributions to Mineralogy and Petrology》1969,23(4):340-360

Thin section study of the Oka carbonatite revealed the presence of zoned pyroxene, amphibole, mica and carbonate which were studied by means of electron probe line scans to establish the relationship between optical zoning and chemical composition. The zoned micas were studied in more detail by means of optical, X-ray, classical chemical, spectrographic and electron probe point counting analyses. The results indicated two- to ten-fold variations in ionic proportions of titanium, iron and aluminium, and lesser variations in concentrations of magnesium, silicon and potassium in six different mica zones.Distribution coefficients of Si, Al, Fe, Fe, Ti and Mg in adjacent mica zones indicate that, with the exception of ferrous and ferric iron which show a straight line relation, the relationship between the major constituents is more complex. Iron and magnesium exhibit opposite trends, modified by prominent variations in aluminium and titanium. Gradual variations within the zones are accounted for by gradual differentiation of the magma. The abrupt changes in chemical composition between the zones, coinciding with optical boundaries, indicate sudden changes in the environmental conditions, resulting from crystallization of associated minerals and periodic emplacement of certain elements into magma.High Fe and high Ti-Al zones are used as markers for correlating cogenetic zones in pyroxene, amphibole and mica, and for establishing periods of crystallization of Fe-Ti ore minerals.     

15.

Noble gas components in planetary atmospheres and interiors in relation to solar wind and meteorites     

K. Marti  K. J. Mathew 《Journal of Earth System Science》1998,107(4):425-431

We discuss observed xenon isotopic signatures in solar system reservoirs and possible relationships. The predominant trapped xenon component in ordinary chondrites (OC) is OC-Xe and its isotopic signature differs from Xe in ureilites, in carbonaceous chondrites, in the atmospheres of Earth and Mars, and in the solar wind. Additional minor Xe components were identified in type 3 chondrites and in the metal phase of chondrites. The OC-Xe and ureilite signatures are both consistent with varying mixtures of HL-Xe and slightly mass fractionated solar-type Xe. Xenon in the Martian atmosphere is found to be strongly mass fractionated by 37.7‰ per amu, relative to solar Xe, favoring the heavy isotopes. Xenon in SNC’s from the Martian mantle show admixture of solar-type Xe, which belongs to an elementally strongly fractionated component. The origin of the isotopic signatures of Ne and Xe in the terrestrial atmosphere are discussed in the light of evidence that the Xe isotopic fractionations in the Martian and terrestrial atmospheres are consistent. However, in the terrestrial atmospheric Xe component excesses are observed for¹³²Xe and also for^129,131Xe, relative to fractionated solar Xe. The suggested chemically fractionated fission Xe component (CFF-Xe) seems to closely match the above excesses. We discuss models of origin for planetary volatiles and possible processes driving their evolution to present day compositions.     

16.

中国癌症与土壤环境中Er元素的关系   总被引：2，自引：2，他引：2  

曾昭华  曾雪萍 《吉林地质》2000,19(1):72-75

利用土壤环境中铒元素资料７６６个数据,癌死亡调查资料７８７ ０８０例,研究了胃癌 食管癌、肝癌、宫颈癌、肺癌、大肠癌、白血病、鼻咽癌、乳腺癌死亡率与人群生存区土壤环境中铒元素的关系。结果表明,仅癌死亡率与铒元素有相关性,等级相关系数为－０．５０６９（Ｐ〈０．００５）。     

17.

中国癌症与土壤环境中钒元素的关系   总被引：11，自引：1，他引：11  

曾昭华  廖苏平  曾雪萍 《吉林地质》2002,21(3):93-98

利用土壤环境中钒元素资料 32 335个数据和癌症死亡调查资料 7870 80例 ,研究了癌症死亡率与人群生存区土壤环境中钒元素的关系。结果表明 ,仅胃癌、大肠癌死亡率与钒元素有相关性 ,等级相关系数分别为 - 0 476 9(P <0 0 0 5) ,- 0 31 79(P <0 0 5)。     

18.

阿尔金断裂与周缘新生代盆地关系   总被引：1，自引：0，他引：1  

李建星  王永和  校佩曦  孙南一  张汉文 《西北地质》2005,38(2):19-23

通过野外填图工作,在阿尔金断裂附近填绘出一系列的新生代地层,对这些地层目前的展布与阿尔金活动断裂的关系分析认为,始新世以来阿尔金断裂对沉积盆地表现出明显的控制作用,表现为明显的左行走滑,并兼具向北西方向掀斜的逆冲活动。乌尊硝尔盆地(索尔库里盆地西部)中的主体沉积是第四系。盆地的东南边界是阿尔金南缘主断裂;盆地西南缘,第四系沉积受东西向北倾正断层控制;盆地北缘为近东西向北倾右行逆冲脆性断裂。盆地的西部边缘超覆在基岩之上,不受断裂控制。     

19.

The extent of oxygen isotope exchange between clay minerals and sea water     

Hsueh-Wen Yeh  Samuel M. Savin 《Geochimica et cosmochimica acta》1976,40(7):743-748

The extent of oxygen isotopic exchange between detrital clay minerals and sea water was investigated by analyzing $\text{O}{}^{18}\text{O}{}^{16}$ ratios of separated fine-grained size fractions of deep-sea sediments from three North Pacific ocean cores. Isotopic results were interpreted according to models based on the assumption that the extent of isotopic exchange should increase with decreasing particle size and increasing time of exchange between the sediment and sea water. The data indicate that information concerning the provenance and mode of formation of detrital clay minerals can be obtained from the $\text{O}{}^{18}\text{O}{}^{16}$ ratios of the coarser-than-0.1 μm fraction of deep-sea sediments younger than several million years and the finer-than-0.1 μm fraction of deep-sea sediments younger than several tens of thousands of years. Furthermore, if the extent of chemical reaction between detrital clays and sea water is similar to the extent of oxygen isotopic exchange, such reaction may be important in regulating the chemistry of sea water.     

20.

黑龙江省氢气释放与地震及断块构造关系的探讨     

邵济安  赵谊  陆永发  唐克东 《地学前缘》2010,17(5):271

通过对2009年安达M4.5级地震成功的预报,进一步认识到将H2浓度的异常变化与断层背景研究结合,并配合区域其他异常的检测可以作为地震预报的一种手段。结合不同深度的地球物理资料,讨论了黑龙江省的断块构造、中新生代断裂以及深部软流圈上涌的构造背景。将近年来地震分布、局部收集的土壤气体浓度资料与断层相结合,展示它们的内在联系。同时还引入GPS水平矢量显示中蒙东部构造区块现今向东的伸展蠕散运动的研究成果,用此解释研究区断裂活动的特征,并且指出这有别于中国西部地区地震活动的特征。     

(°C)T	$\text{K}{}_{\mathrm{D}}{}^{\mathrm{Phlog/Vap.}}$	$\text{X}{}_{\mathrm{RM}}$	$\text{K}{}_{\mathrm{D}}{}^{\mathrm{Sanid/Vap.}}$	$\text{X}{}_{\mathrm{RM}}$
500	$\text{0.64 ± 0.11}$	0–0.2	$\text{0.17 ± 0.04}$	0–0.07
700	$\text{1.11 ± 0.11}$	0–0.2	$\text{0.33 ± 0.04}$	0–0.1
800	$\text{1.28 ± 0.03}$	0–0.2	$\text{0.45 ± 0.06}$	0–0.1

The relation between dilatancy,effective stress and dispersive pressure in granular avalanches

Here we investigate three long-standing principles of granular mechanics and avalanche science: dilatancy, effective stress and dispersive pressure. We first show how the three principles are mechanically interrelated: Shearing of a particle ensemble creates a mechanical energy flux associated with random particle movements (scattering). Because the particle scattering is inhibited at the basal boundary, there is a spontaneous rise in the center of mass of the particle ensemble (dilatancy). This rise is connected to a change in potential energy. When the center of mass rises, there is a corresponding reaction at the base of the flow that is coupled to the vertical acceleration of the ensemble. This inertial stress is the dispersive pressure. Dilatancy is therefore not well connected to effective-stress-type relations, rather the energy fluxes describing the configurational changes of the particle ensemble. The strict application of energy principles has far-reaching implications for the modeling of avalanches and debris flows and other dangerous geophysical hazards.     

Evolution of zoned micas and associated silicates in the Oka carbonatite

Thin section study of the Oka carbonatite revealed the presence of zoned pyroxene, amphibole, mica and carbonate which were studied by means of electron probe line scans to establish the relationship between optical zoning and chemical composition. The zoned micas were studied in more detail by means of optical, X-ray, classical chemical, spectrographic and electron probe point counting analyses. The results indicated two- to ten-fold variations in ionic proportions of titanium, iron and aluminium, and lesser variations in concentrations of magnesium, silicon and potassium in six different mica zones.Distribution coefficients of Si, Al, Fe, Fe, Ti and Mg in adjacent mica zones indicate that, with the exception of ferrous and ferric iron which show a straight line relation, the relationship between the major constituents is more complex. Iron and magnesium exhibit opposite trends, modified by prominent variations in aluminium and titanium. Gradual variations within the zones are accounted for by gradual differentiation of the magma. The abrupt changes in chemical composition between the zones, coinciding with optical boundaries, indicate sudden changes in the environmental conditions, resulting from crystallization of associated minerals and periodic emplacement of certain elements into magma.High Fe and high Ti-Al zones are used as markers for correlating cogenetic zones in pyroxene, amphibole and mica, and for establishing periods of crystallization of Fe-Ti ore minerals.     

Noble gas components in planetary atmospheres and interiors in relation to solar wind and meteorites

We discuss observed xenon isotopic signatures in solar system reservoirs and possible relationships. The predominant trapped xenon component in ordinary chondrites (OC) is OC-Xe and its isotopic signature differs from Xe in ureilites, in carbonaceous chondrites, in the atmospheres of Earth and Mars, and in the solar wind. Additional minor Xe components were identified in type 3 chondrites and in the metal phase of chondrites. The OC-Xe and ureilite signatures are both consistent with varying mixtures of HL-Xe and slightly mass fractionated solar-type Xe. Xenon in the Martian atmosphere is found to be strongly mass fractionated by 37.7‰ per amu, relative to solar Xe, favoring the heavy isotopes. Xenon in SNC’s from the Martian mantle show admixture of solar-type Xe, which belongs to an elementally strongly fractionated component. The origin of the isotopic signatures of Ne and Xe in the terrestrial atmosphere are discussed in the light of evidence that the Xe isotopic fractionations in the Martian and terrestrial atmospheres are consistent. However, in the terrestrial atmospheric Xe component excesses are observed for¹³²Xe and also for^129,131Xe, relative to fractionated solar Xe. The suggested chemically fractionated fission Xe component (CFF-Xe) seems to closely match the above excesses. We discuss models of origin for planetary volatiles and possible processes driving their evolution to present day compositions.     

中国癌症与土壤环境中Er元素的关系

利用土壤环境中铒元素资料７６６个数据,癌死亡调查资料７８７ ０８０例,研究了胃癌 食管癌、肝癌、宫颈癌、肺癌、大肠癌、白血病、鼻咽癌、乳腺癌死亡率与人群生存区土壤环境中铒元素的关系。结果表明,仅癌死亡率与铒元素有相关性,等级相关系数为－０．５０６９（Ｐ〈０．００５）。     

中国癌症与土壤环境中钒元素的关系

利用土壤环境中钒元素资料 32 335个数据和癌症死亡调查资料 7870 80例 ,研究了癌症死亡率与人群生存区土壤环境中钒元素的关系。结果表明 ,仅胃癌、大肠癌死亡率与钒元素有相关性 ,等级相关系数分别为 - 0 476 9(P <0 0 0 5) ,- 0 31 79(P <0 0 5)。     

阿尔金断裂与周缘新生代盆地关系

通过野外填图工作,在阿尔金断裂附近填绘出一系列的新生代地层,对这些地层目前的展布与阿尔金活动断裂的关系分析认为,始新世以来阿尔金断裂对沉积盆地表现出明显的控制作用,表现为明显的左行走滑,并兼具向北西方向掀斜的逆冲活动。乌尊硝尔盆地(索尔库里盆地西部)中的主体沉积是第四系。盆地的东南边界是阿尔金南缘主断裂;盆地西南缘,第四系沉积受东西向北倾正断层控制;盆地北缘为近东西向北倾右行逆冲脆性断裂。盆地的西部边缘超覆在基岩之上,不受断裂控制。     

The extent of oxygen isotope exchange between clay minerals and sea water

The extent of oxygen isotopic exchange between detrital clay minerals and sea water was investigated by analyzing $\text{O}{}^{18}\text{O}{}^{16}$ ratios of separated fine-grained size fractions of deep-sea sediments from three North Pacific ocean cores. Isotopic results were interpreted according to models based on the assumption that the extent of isotopic exchange should increase with decreasing particle size and increasing time of exchange between the sediment and sea water. The data indicate that information concerning the provenance and mode of formation of detrital clay minerals can be obtained from the $\text{O}{}^{18}\text{O}{}^{16}$ ratios of the coarser-than-0.1 μm fraction of deep-sea sediments younger than several million years and the finer-than-0.1 μm fraction of deep-sea sediments younger than several tens of thousands of years. Furthermore, if the extent of chemical reaction between detrital clays and sea water is similar to the extent of oxygen isotopic exchange, such reaction may be important in regulating the chemistry of sea water.     

黑龙江省氢气释放与地震及断块构造关系的探讨

通过对2009年安达M4.5级地震成功的预报,进一步认识到将H2浓度的异常变化与断层背景研究结合,并配合区域其他异常的检测可以作为地震预报的一种手段。结合不同深度的地球物理资料,讨论了黑龙江省的断块构造、中新生代断裂以及深部软流圈上涌的构造背景。将近年来地震分布、局部收集的土壤气体浓度资料与断层相结合,展示它们的内在联系。同时还引入GPS水平矢量显示中蒙东部构造区块现今向东的伸展蠕散运动的研究成果,用此解释研究区断裂活动的特征,并且指出这有别于中国西部地区地震活动的特征。