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51.
Application of the weighted residual method to diffraction by 2-D canyons of arbitrary shape: I. Incident SH waves 总被引:1,自引:0,他引:1
A solution for the two-dimensional scattering and diffraction of plane SH waves by canyons of arbitrary shape in an elastic half space is presented. The wave field for arbitrary geometry in this paper is computed numerically by the method of weighted residues (moment method). The wave displacement field computed by the present residual method for the case of a semi-circular canyon was shown to agree analytically and numerically with that computed by the exact closed form series solution. The same observations about ground amplifications, their dependence on frequencies and orientations of the incident waves, can be stated here for canyons of arbitrary shape as previously made for circular canyons. 相似文献
52.
53.
Vincent S. Cronin 《Tectonophysics》1994,230(3-4):151-159
Intuition suggests that all points on the same mid-ocean ridge should rotate around the relative pole of the two-plate system at the same instantaneous angular velocity. Contrary to intuition, the instantaneous angular velocity of a ridge varies from one point to another along the ridge, given the general case in which two plates move around different plate-specific poles of rotation. The variation in the instantaneous angular velocity of a ridge is a function of the motion characteristics of the plates and the position of the ridge relative to the poles of plate motion. The length or orientation of individual ridge segments is predicted to vary over time, leading to local changes in the shape of the ridge. The gradient in instantaneous angular velocity for the fast-spreading East Pacific Ridge, between the Cocos and Pacific plates, is an order of magnitude greater than the gradient along the Mid-Atlantic Ridge, between the North American and African plates. This great contrast in ridge instantaneous velocity gradients may be reflected in the contrasting ridge geometries of the East Pacific and Mid-Atlantic Ridges. 相似文献
54.
Eight box cores from the tropical Atlantic were studied in detail with regard to foraminiferal oxygen isotopes, radiocarbon, and Globorotalia menardii abundance. A standard Atlantic oxygen-isotope signal was reconstructed for the last 20,000 yr. It is quite similar to the west-equatorial Pacific signal published previously. Deglaciation is seen to occur in two steps which are separated by a pause. Onset of deglaciation is after 15,000 yr B.P. The pause is centered between 11,000 and 12,000 yr B.P., but may be correlative with the Younger Dryas (10,500 yr B.P.) if allowance is made for a scale shift due to mixing processes on the sea floor. Step 2 is centered near 10,000 yr B.P. and is followed by a brief excursion toward light oxygen values. This excursion (the M event) may correlate with the Gulf of Mexico meltwater spike. 相似文献
55.
Equilibrium state of diopside-bearing harzburgites from ophiolites: Geobarometric and geodynamic implications 总被引:5,自引:0,他引:5
Jean -Claude C. Mercier Vincent Benoit Jacques Girardeau 《Contributions to Mineralogy and Petrology》1984,85(4):391-403
The bulk compositions of coexisting enstatite and diopside in basal lherzolites and clinopyroxene-bearing harzburgites from ophiolitic complexes are typical of solidus/subsolidus equilibria, but for a few texturally distinct magmatic diopsides. They would presumably reflect the state of equilibrium at the time they last coexisted with liquid as the rocks reentered subsolidus conditions. The total lack of correlation between Al and Ca concentrations shows that the compositional scatter observed for any given massif, results from analytical errors related to extensive exsolution and serpentinization, rather than from differences in equilibrium conditions. However, significant differences are found between the residual ophiolitic lherzolites from Hare Bay, Newfoundland, and from Xigaze, Tibet, two massifs selected for their distinct structural and textural features. As for thermobarometry techniques relevant to these rocks, the best barometer found is an empirical relation for the expression of pressure as a virtually temperature-independent function of the ratioK
f=(X
Di
opx
)/(1 –X
Di
cpx
), in agreement with semi-quantitative models based on natural solid solutions. Temperatures are then simply derived from a surface-fitting expression relating pressure, temperature and diopside-solvus compositions, according to a regularX
En
cpx
solution model (CMS) corrected for the effect of Al in the spinel facies. Application of these techniques yield pressures of 0.4 and 1.4 GPa, i.e. depths from sea-bottom of about 13 and 43 km, for temperatures of 1,170 and 1,300° C for the ophiolitic lherzolites of Tibet and New-foundland, respectively, in good agreement with dry-solidus data by radioactive tracing and with geothermal-model estimates for ridges. 相似文献
56.
Claude Robin Philippe Mossand Guy Camus Jean-Marie Cantagrel Alain Gourgaud Pierre M. Vincent 《Journal of Volcanology and Geothermal Research》1987,31(1-2)
The evolution of the Colima volcanic complex can be divided into successive periods characterized by different dynamic and magmatic processes: emission of andesitic to dacitic lava flows, acid-ash and pumice-flow deposits, fallback nuées ardentes leading to pyroclastic flows with heterogeneous magma, plinian air-fall deposits, scoriae cones of alkaline and calc-alkaline nature. Four caldera-forming events, resulting either from major ignimbrite outbursts or Mount St. Helens-type eruptions, separate the main stages of development of the complex from the building of an ancient shield volcano (25 × 30 km wide) up to two summit cones, Nevado and Fuego.The oldest caldera, C1 (7–8 km wide), related to the pouring out of dacitic ash flows, marks the transition between two periods of activity in the primitive edifice called Nevado I: the first one, which is at least 0.6 m.y. old, was mainly andesitic and effusive, whereas the second one was characterized by extrusion of domes and related pyroclastic products. A small summit caldera, C2 (3–3.5 km wide), ended the evolution of Nevado I.Two modern volcanoes then began to grow. The building of the Nevado II started about 200,000 y. ago. It settled into the C2 caldera and partially overflowed it. The other volcano, here called Paleofuego, was progressively built on the southern side of the former Nevado I. Some of its flows are 50,000 y. old, but the age of its first outbursts is not known. However, it is younger than Nevado II. These two modern volcanoes had similar evolutions. Each of them was affected by a huge Mount St. Helens-type (or Bezymianny-type) event, 10,000 y. ago for the Paleofuego, and hardly older for the Nevado II. The landslides were responsible for two horseshoe-shaped avalanche calderas, C3 (Nevado) and C4 (Paleofuego), each 4–5 km wide, opening towards the east and the south. In both cases, the activity following these events was highly explosive and produced thick air-fall deposits around the summit craters.The Nevado III, formed by thick andesitic flows, is located close to the southwestern rim of the C3 caldera. It was a small and short-lived cone. Volcan de Fuego, located at the center of the C4 caldera, is nearly 1500 m high. Its activity is characterized by an alternation of long stages of growth by flows and short destructive episodes related to violent outbursts producing pyroclastic flows with heterogeneous magma and plinian air falls.The evolution of the primitive volcano followed a similar pattern leading to formation of C1 and then C2. The analogy between the evolutions of the two modern volcanoes (Nevado II–III; Paleofuego-Fuego) is described. Their vicinity and their contemporaneous growth pose the problem of the existence of a single reservoir, or two independent magmatic chambers, after the evolution of a common structure represented by the primitive volcano. 相似文献
57.
Multiple geographies of the Arab Internet 总被引:1,自引:0,他引:1
The Arab world plays a relatively minor role in the rapidly changing geographies of global cyberspace. This paper explores the multiple geographies of the Arab Internet. First, it addresses Internet penetration rates, which averaged 7.8 per cent in 2006, although these varied widely among and within the region's countries. Between 2000 and 2006, the number of users jumped by 830 per cent, indicating these geographies are in rapid flux. It then examines the telecommunications infrastructure of the Middle East and North Africa, including fixed and mobile telephone networks and Internet cafes. Third, it turns to the reasons why the Internet has experienced relatively late adoption among Arab countries, including the dominance of the Latin alphabet, high access costs reflecting state-owned telecommunications monopolies, low Arab literacy rates, and restrictive gender relations that keep the proportion of female users low. The paper pays special attention both to government censorship of the Arab Internet as well as resistance to such controls and attempts to utilize the Internet counter-hegemonically. Finally, it explores the impacts of the Internet on some Arab societies, including the opening of discursive communities of politics, the Palestinian–Israeli conflict, Iraq and electronic commerce. 相似文献
58.
59.
A new mesh refinement strategy for generating high quality unstructured meshes of the Northwestern European continental shelf, the continental slope and the neighbouring ocean is presented. Our objective is to demonstrate the ability of anisotropic unstructured meshes to adequately address the challenge of simulating the hydrodynamics occurring in these three regions within a unique mesh. The refinement criteria blend several hydrodynamic considerations as the tidal wave propagation on the continental shelf and the hydrostatic consistency condition in steep areas. Several meshes illustrate both the validity and the efficiency of the refinement strategy. The selection of the refinement parameters is discussed. Finally, an attempt is made to take into account tidal ellipses, providing another cause for anisotropy in the mesh. 相似文献
60.
利用Fourier Bessel级数展开法,给出了表面具有覆盖层的圆弧形凹陷地形对入射平面SV波散射问题的一个解析解,并利用该解分析了不同深宽比凹陷地形表面覆盖层刚度和厚度对入射SV波的影响.结果表明,凹陷地形表面覆盖层的存在,即使厚度很薄,也会显著增加对入射SV波的放大作用,该放大作用可达到单一凹陷地形的2.5倍以上.覆盖层刚度和厚度的变化对入射平面SV波也具有很大影响. 相似文献