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921.
Franois Paul 《Atmospheric Research》2001,56(1-4)
The analysis of 304 French tornadoes shows that the most struck areas are the northwestern part of the country, the south and the east. Tornadoes occur mainly in spring and in summer; August is the month when the frequency is maximum. Tornadoes move mainly from southwest to northeast, except in the south where the direction is sometimes south to north. Thirty-six killer tornadoes were listed (12% of the cases); most of them with an intensity greater or equal to F3. The occurrence is approximately 15–20 tornadoes each year in France, and the annual risk probability of significant tornadoes in France is 0.66×10−5. This value is probably underestimated because all the tornadoes are not listed. 相似文献
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924.
Paul A. Knapp 《The Professional geographer》1997,49(1):39-51
Understanding the environmental factors that influence spatial patterns of rangeland wildfires facilitates both fire and land management decisions. I used discriminant analysis to examine ignition frequency of lightning-initiated fires in grass-dominated communities in the Intermountain West between 1980 and 1994, then mapped regional fire frequencies to illustrate spatial patterns. Two canonical discriminant functions effectively separate groups of high, medium, and low ignition frequencies, based on climate conditions and fuel characteristics. Regions of highest frequency tend to have large elevational differences, more mesic climates, and less annual grass cover. Spatial patterns of ignition frequencies tend to reflect local topography, with higher frequencies west of 119° W. Expansion of exotic annual grasses throughout the region may be reducing differences in ignition frequency between regions of highest and lowest frequency. 相似文献
925.
Paul Docherty Ron Silva Satish Singh Zhong-Min Song Margaret Wood 《Geophysical Prospecting》1997,45(5):865-878
Migration velocity analysis, a method for determining long wavelength velocity structure, is a critical step in prestack imaging. Solution of this inverse problem is made difficult by a multimodal objective function; a parameter space often vast in extent; and an evaluation procedure for candidate solutions, involving the calculation of depth-migrated image gathers, that can be prohibitively expensive. Recognizing the global nature of the problem, we employ a genetic algorithm (GA) in the search for the optimum velocity model. In order to describe a model efficiently, regions of smooth variation are identified and sparsely parametrized. Region boundaries are obtained via map migration of events picked on the zero-offset time section. Within a region, which may contain several reflectors, separate components describe long and short wavelength variations, eliminating from the parameter space, models with large velocity fluctuations. Vital to the success of the method is rapid model evaluation, achieved by generating image gathers only in the neighbourhood of specific reflectors. Probability of a model, which we seek to maximize, is derived from the flatness of imaged events. Except for an initial interpretation of the zero-offset time section, our method is automatic in that it requires no picking of residual moveout on migrated gathers. Using an example data set from the North Sea, we show that it is feasible to solve for all velocity parameters in the model simultaneously: the method is global in this respect also. 相似文献
926.
Reactions occurring on the surfaces of biogenic carbonate minerals can have important consequences for the biogeochemical cycle of carbon. In this study, carbonate mineral surface reactions with ambient seawater were investigated by atomic force microscopy (AFM). A sampling method was developed in which calcite surfaces were hung at discrete depths on a sediment trap array line for a three-day deployment period in subtropical North Pacific waters. Changes in surface morphologies were examined at nanometer resolution and evaluated using as a constraint the depth profile of calcite saturation in these waters. Evidence suggests that: (1) organic films which develop on carbonate surfaces exposed to shallow seawater may be responsible for the oversaturated state of the upper oceanic water column, (2) dissolution of carbonate minerals within the shallow warm layer of the ocean could be responsible for part of the alkalinity anomaly observed in the North Pacific. 相似文献
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Paul F. McMillan George H. Wolf Phillipe Lambert 《Physics and Chemistry of Minerals》1992,19(2):71-79
We have carried out a Raman Spectroscopic study of single crystalline quartz samples shocked to peak pressures up to 31.4GPa. Samples shocked to above 22 GPa show shifts in peak positions consistent with the quartz being under tensile stress, and new broad bands associated with the formation of high density SiO2 glass appear in the spectra. These changes are accompanied by an increase in the lattice parameters of the quartz. Formation of the diaplectic glass could be due to a metastable melting event, or spinodal lattice collapse on attainment of a mechanical stability limit of crystalline quartz, as suggested by previous studies of pressure-induced amorphization in static pressurization experiments on SiO2 and GeO2 polymorphs. 相似文献
930.
Alkaline and calc-alkaline lavas near Los Volcanes,Jalisco, Mexico: geochemical diversity and its significance in volcanic arcs 总被引:1,自引:0,他引:1
A remarkably diverse suite of lavas erupted during the late-Pliocene at the volcanic front of the western Mexican Volcanic Belt near the town of Los Volcanes, Jalisco. This region is much closer to the Middle America Trench than the main axis of Quaternary andesite-dacite stratovolcanoes, and volcanism occurred in a complex tectonic regime involving both subduction of the young Rivera Plate and transverse crustal extension of the Jalisco structural block. The variety of lava types covers a wide spectrum from highly potassic minettes and leucitites to calc-alkaline basalts and andesites which are compositionally similar to those erupted elsewhere in the Mexican Volcanic Belt. Other alkaline varieties intermediate between these extremes include absarokites, trachybasalts and trachyandesites. Phlogopite, amphibole and apatite are common phenocryst phases; whole-rock compositions show a wide range of alkali contents (e.g. K2O of 1.0–8.6 wt.%), and typically contain >5 wt.% MgO. MgO, Ni, V and Cr show little systematic decrease with increasing SiO2, suggesting that these lavas have evolved from primitive, mantle derived magmas with a wide range of SiO2 contents. Strong enrichments in incompatible trace elements are observed in all of the lavas (Sr 700–5100 ppm, Ba 470–4800, Ce 22–325 ppm, Zr 90–700 ppm), as is the relative enrichment of large ion lithophile (Ba, Sr, Rb) and light rare-earth elements (La, Ce) over the high field strength elements (Ti, Zr) which is typical of magmas in volcanic arcs. This enrichment pattern suggests that these magmas come from source regions which contain incompatible element-rich phases such as phlogopite, amphibole and apatite. The petrological and geochemical features of the lavas which occur in the Los Volcanes region provide direct evidence of the extreme heterogeneity which may exist in magma source regions at convergent margins. The complex tectonic regime in western Mexico further suggests that rifting and crustal extension play an important role in the generation and successful ascent of melts from enriched regions of the sub-arc mantle. 相似文献