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91.
Repetitive gravity surveys at Pacaya Volcano from 1975 to 1979 revealed time-dependent changes in the gravity field, which although related to volcanic activity, could not be uniquely attributed to elevation changes or mass redistributions because elevation control was lacking. Elevation control was established in July 1979 using precision leveling. Relative elevation and gravity measurements in June and July of 1979, January 1980 and June 1980 indicate concurrent gravity and elevation changes contemporaneous with variations in eruptive activity. From June 1979 to January 1980, while fumarolic activity was dominant, relative to the most remote station, the volcano deflated by at least 195 mm and the gravity field increased by up to 221 μgal. From January 1980 to June 1980, preceding a Strombolian eruption beginning about June 1980, the volcano inflated by at least 19 mm and the gravity field decreased by up to 231 μgal. Gravity change maps for the intervals of January 1978 to June 1979, June 1979 to January 1980, and January 1980 to June 1980 show areas subject to repeated positive and negative gravity change. Some of those areas coincide with areas of maximum elevation change observed in the June 1979–January 1980 and January 1980–June 1980 intervals; however, gravity changes were observed in areas lacking elevation changes. Adjusting observed gravity changes for elevation changes using a free-air value of −3.086 μgal/cm does not substantially alter the pattern, position, or amplitude of the gravity changes. The relationship between gravity changes, elevation changes, and volcanic activity requires a mechanism producing gravity decreases with little inflation during times of increased eruptive activity, and producing gravity increases with subsidence during times of declining eruptive activity. Such a pattern of changes could be produced by a near-surface magma body in which high-density degassed magma is displaced volume for volume by low-density vesiculated magma during time of increased eruptive activity, and in which loss of gasses by fumarolic activity produces a density increase and a reduction in volume of the magma body during periods of declining eruptive activity. Such a pattern of changes could also be induced by a low-density, vesiculated magma body moving upward in the volcanic pile by piecemeal stoping where the high-density rocks of the volcano are replaced on a volume for volume basis by low-density magma during periods of increasing eruptive activity; and by later density increases and magma body volume reductions accompanying devolatilization and devesiculation during periods of declining eruptive activity. Simple density change and density contrast models involving shallow magma bodies at depths of 100 to 200 m indicate density changes or contrasts of about 0.4 g/cm3 could produce the gravity changes. 相似文献
92.
Based on petrological and geochemical arguments, it is possible that arc magma is derived from subducted oceanic crust. In this paper, regional thermal models have been constructed to study the feasibility of melting cold subducted oceanic crusts at shallow depth (i.e. at depths of about 100 km) by a dynamic mantle. Calculated results suggest that plate subduction will generate an induced flow in the wedge above the subducting slab. This current continuously feeds hot mantle material into the corner and onto the slab surface. A high temperature thermal environment can be maintained in the vicinity of the wedge corner, immediately beneath the over-riding plate. Our regional models further demonstrate quantitatively that production of local melting is possible just about 30 km down dip from the asthenosphere wedge corner. Additional geological processes such as reasonable amounts of shear heating and minor dehydration (which will lower the local melting temperature) will further increase the probability of melting a cold subducted oceanic crust at shallow depth. 相似文献
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95.
Batrice Ledsert Janie Joffre Andr Ambls Paul Sardini Albert Genter Alain Meunier 《Journal of Volcanology and Geothermal Research》1996,70(3-4)
Organic matter has been observed in cores of the Soultz-sous-Forêts granite (Alsace, France) at depths between 2158 and 2160 m, in a highly fractured and altered zone. The granite is overlain by a 1400-m-thick sedimentary cover containing petroleum (Pechelbronn oil field). The Soultz area is devoted to Hot Dry Rock geothermics thanks to a high geothermal gradient (up to 100 °C/km). During drilling operations, an artesian source produced oil in a fractured zone of the sedimentary cover (Buntsandstein). Its gas-chromatography and mass-spectrometry (GC-MS) analysis revealed the presence of normal alkanes (n-alkanes, i.e. linear hydrocarbons) centered on C17, branched alkanes with a major C19 peak, and few unsaturated hydrocarbons. The aromatic fraction is present in small amounts. In the Soultz granite, where it is altered, organic matter is exclusively associated with tosudite (interstratified clay mineral) which crystallized in plagioclase sites during a hydrothermal alteration episode. Organic matter has been later displaced and concentrated along veinlets in which illite and carbonates have crystallized during another hydrothermal alteration stage. The soluble organic matter analyzed by GC-MS is composed of aliphatic acids, n-alkanes with a bimodal C18 and C24-C25 centered distribution, alkylbenzenes and aromatic acids. Organic compounds in the granite would either originate from a single source (immature sediments) or from two sources (immature sediments and migration of the Pechelbronn oil). No real evidence was found to prove which hypothesis is the best one. The presence of organic matter in the granite shows the importance of fluid flows between the sedimentary cover and the granitic basement through major fractures. In addition, the impregnation of plagioclase pseudomorphs with organic matter is made possible due to their high interconnection degree and to the intergranular microfracturation of the granite. The succession of several hydrothermal events with different physico-chemical characteris- tics may also be inferred from the occurrence of organic matter found in association with neoformed clay minerals in the granite. 相似文献
96.
Wang Zonghao Mao Jianping Huang Jihong Arnold Gruber Albert Thomasell Tan Sun Chen 《Acta Meteorologica Sinica》1992,6(4):421-432
This paper is to examine the impact of satellite data on the systematic error of operational B-model in China.Em-phasis is put on the study of the impact of satellite sounding data on forecasts of the sea level pressure field and 500 hPaheight.The major findings are as follows.(1)The B-model usually underforecasts the strength of features in the sea level pressure(SLP)field,i.e.pressuresare too low near high pressure systems and too high near low pressure systems.(2)The nature of the systematic errors found in the 500 hPa height forecasts is not as clear cut as that of the SLPforecasts,but most often the same type of pattern is seen,i.e.,the heights in troughs are not low enough and those inridges are not high enough.(3)The use of satellite data in the B-model analysis/forecast system is found to have an impact upon the model'sforecast of SLP and 500 hPa height.Systematic errors in the vicinity of surface lows/500 hPa troughs over the oceansare usually found to be significantly reduced.A less conclusive mix of positive and negative impacts was found for allother types of features. 相似文献
97.
R. O. Davis 《地震工程与结构动力学》1992,21(3):253-274
Pounding of adjacent buildings is modelled by an impact oscillator subjected to harmonic excitation. Non-linear impact stiffness is represented by a Hertz-type relationship. Spectra of impact velocity are presented for a range of model parameters. These spectra are characterized by a strong peak near a period equal to one half the natural period of a similar non-impacting oscillator. Bands of response are found in which periodic multiple impacts and non-periodic or chaotic impacts occur. 相似文献
98.
A synoptic climatology is developed for Virginia using 21 years of late spring and summer surface and upper air observations. The climatology is produced by applying a combination of principal components analysis and cluster analysis such that each day is classified into one of a distinct number of synoptic situations. Days on which at least one severe storm occurred in Virginia are merged with the synoptic climatology. A majority of severe storms are associated with one synoptic situation distinguished by moderate instability and a high moisture content. 相似文献
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100.
A regionalized composition is a random vector function whose components are positive and sum to a constant at every point of the sampling region. Consequently, the components of a regionalized composition are necessarily spatially correlated. This spatial dependence—induced by the constant sum constraint—is a spurious spatial correlation and may lead to misinterpretations of statistical analyses. Furthermore, the cross-covariance matrices of the regionalized composition are singular, as is the coefficient matrix of the cokriging system of equations. Three methods of performing estimation or prediction of a regionalized composition at unsampled points are discussed: (1) the direct approach of estimating each variable separately; (2) the basis method, which is applicable only when a random function is available that can he regarded as the size of the regionalized composition under study; (3) the logratio approach, using the additive-log-ratio transformation proposed by J. Aitchison, which allows statistical analysis of compositional data. We present a brief theoretical review of these three methods and compare them using compositional data from the Lyons West Oil Field in Kansas (USA). It is shown that, although there are no important numerical differences, the direct approach leads to invalid results, whereas the basis method and the additive-log-ratio approach are comparable. 相似文献