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51.
根据2001~2007年期间在菲律宾境内测量的74个点的地磁场数据,利用球冠谐和、泰勒多项式两种方法建立了2005.0菲律宾及周边地区地磁基本场模型,得到了菲律宾地区X、y、Z、D、I、H、F七个分量的分布.两种模型反映的同一地区的地磁场分布形态基本类似,只在拐点的舒缓度、等值线平滑度等方面存在区别.在8°N左右,X、... 相似文献
52.
Massimo Chiaradia Othmar Müntener Bernardo Beate Denis Fontignie 《Contributions to Mineralogy and Petrology》2009,158(5):563-588
In the Northern Andes of Ecuador, a broad Quaternary volcanic arc with significant across-arc geochemical changes sits upon
continental crust consisting of accreted oceanic and continental terranes. Quaternary volcanic centers occur, from west to
east, along the Western Cordillera (frontal arc), in the Inter-Andean Depression and along the Eastern Cordillera (main arc),
and in the Sub-Andean Zone (back-arc). The adakite-like signatures of the frontal and main arc volcanoes have been interpreted
either as the result of slab melting plus subsequent slab melt–mantle interactions or of lower crustal melting, fractional
crystallization, and assimilation processes. In this paper, we present petrographic, geochemical, and isotopic (Sr, Nd, Pb)
data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic
centers (Pululagua, Pichincha, Ilalo, Chacana, Sumaco) forming a NW–SE transect at about 0° latitude and encompassing the
frontal (Pululagua, Pichincha), main (Ilalo, Chacana), and back-arc (Sumaco) chains. All rocks display typical subduction-related
geochemical signatures, such as Nb and Ta negative anomalies and LILE enrichment. They show a relative depletion of fluid-mobile
elements and a general increase in incompatible elements from the front to the back-arc suggesting derivation from progressively
lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab. We observe
widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene- and/or
amphibole-bearing cumulates and of magma mixing at all frontal and main arc volcanic centers. Within each volcanic center,
rocks display correlations between evolution indices and radiogenic isotopes, although absolute variations of radiogenic isotopes
are small and their values are overall rather primitive (e.g., εNd = +1.5 to +6, 87Sr/86Sr = 0.7040–0.70435). Rare earth element patterns are characterized by variably fractionated light to heavy REE (La/YbN = 5.7–34) and by the absence of Eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation.
We interpret the petrographic, geochemical, and isotopic data as indicating open-system evolution at all volcanic centers
characterized by fractional crystallization and magma mixing processes at different lower- to mid-crustal levels as well as
by assimilation of mafic lower crust and/or its partial melts. Thus, we propose that the adakite-like signatures of Ecuadorian
rocks (e.g., high Sr/Y and La/Yb values) are primarily the result of lower- to mid-crustal processing of mantle-derived melts,
rather than of slab melts and slab melt–mantle interactions. The isotopic signatures of the least evolved adakite-like rocks
of the active and recent volcanoes are the same as those of Tertiary ”normal” calc-alkaline magmatic rocks of Ecuador suggesting
that the source of the magma did not change through time. What changed was the depth of magmatic evolution, probably as a
consequence of increased compression induced by the stronger coupling between the subducting and overriding plates associated
with subduction of the aseismic Carnegie Ridge. 相似文献
53.
In this paper, we show how to couple the local discontinuous Galerkin method and the Raviart–Thomas mixed finite element method for elliptic equations modeling flow problems. We then show that the approximation of the velocity converges with the optimal order of k when we take the local discontinuous Galerkin that uses polynomials of degree k and the Raviart–Thomas space of polynomials of degree k?1. 相似文献
54.
55.
The seismic response of the Mexico City Cathedral built of very soft soil deposits is evaluated by using motions recorded in various parts of the structure during several moderate earthquakes. This unique set of records provides significant insight into the seismic response of this and other similar historic stone masonry structures. Free‐field ground motions are carefully compared in time and frequency domains with motions recorded at building basement. The dynamic characteristics of the structure are inferred from the earthquake records by using system identification techniques. Variation of seismic response for different seismic intensities is discussed. It is shown that, due to the soil–structure interaction, due to large differences between dominant frequencies of earthquake ground motions at the site and modal frequencies of vibration of the structure, and due to a particularly high viscous damping, seismic amplifications of ground motion in this and similar historic buildings erected on soft soil deposits are much smaller than that induced in most modern constructions. Nevertheless, earthquake records and analytical results show that several components of the structure such as its central dome and the bell towers may be subjected to local vibrations that significantly amplify ground motions. Overall, results indicate that in its present state the structure has an acceptable level of seismic safety. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
56.
Retrieval of volcanic ash particle size, mass and optical depth from a ground-based thermal infrared camera 总被引:2,自引:2,他引:0
Volcanoes can emit fine-sized ash particles (1–10 μm radii) into the atmosphere and if they reach the upper troposphere or lower stratosphere, these particles can have deleterious effects on the atmosphere and climate. If they remain within the lowest few kilometers of the atmosphere, the particles can lead to health effects in humans and animals and also affect vegetation. It is therefore of some interest to be able to measure the particle size distribution, mass and other optical properties of fine ash once suspended in the atmosphere. A new imaging camera working in the infrared region between 7–14 μm has been developed to detect and quantify volcanic ash. The camera uses passive infrared radiation measured in up to five spectral channels to discriminate ash from other atmospheric absorbers (e.g. water molecules) and a microphysical ash model is used to invert the measurements into three retrievable quantities: the particle size distribution, the infrared optical depth and the total mass of fine particles. In this study we describe the salient characteristics of the thermal infrared imaging camera and present the first retrievals from field studies at an erupting volcano. An automated ash alarm algorithm has been devised and tested and a quantitative ash retrieval scheme developed to infer particle sizes, infrared optical depths and mass in a developing ash column. The results suggest that the camera is a useful quantitative tool for monitoring volcanic particulates in the size range 1–10 μm and because it can operate during the night, it may be a very useful complement to other instruments (e.g. ultra-violet spectrometers) that only operate during daylight. 相似文献