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An average crust-mantle model has been derived for the East African Rift system based on a number of presently available seismic data. The inversion of experimentally determined spectral transfer ratios of long-period body waves recorded at stations AAE (Addis Ababa, Ethiopia), NAI (Nairobi, Kenya) and LWI (Lwiro, Zaïre) requires at least a two-layer crust. Except for station AAE. the observed P-wave delays can be accounted for by differences in the deduced crustal structure. Phase- and group-velocity measurements of Rayleigh waves along the path AAE-NAI provide additional information on the gross structure of the crust and upper mantle. Only a well-developed asthenosphere channel can explain the observed surface-wave dispersion. It is shown that the average model MS-71 permits a satisfactory interpretation of all the data presented in this paper. 相似文献
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Onno R. Pols Klaus-Peter Schro¨der Jarrod R. Hurley Christopher A. Tout & Peter P. Eggleton 《Monthly notices of the Royal Astronomical Society》1998,298(2):525-536
We have calculated a grid of empirically well tested evolutionary tracks with masses M between 0.5 and 50 M⊙, spaced by approximately 0.1 in log M , and with metallicities Z = 0.0001, 0.0003, 0.001, 0.004, 0.01, 0.02 and 0.03. We use a robust and fast evolution code with a self-adaptive non-Lagrangian mesh, which employs the mixing-length theory but treats convective mixing as a diffusion process, solving simultaneously for the structure and the chemical composition. The hydrogen and helium abundances are chosen as functions of the metallicity: X = 0.76 − 3.0 Z Y = 0.24 + 2.0 Z . Two sets of models were computed, one without and one with a certain amount of enhanced mixing or 'overshooting'. This amount has been empirically chosen by means of various sensitive tests for overshooting: (1) the luminosity of core helium burning (blue loop) giants of well-known mass, (2) the width of the main sequence as defined by double-lined eclipsing binaries with well-measured masses and radii, and (3) the shape and implied stellar distribution of isochrones of various open clusters. The first two tests have been the subject of previous papers, the third test is discussed in this paper. On the basis of these tests, we recommend the use of the overshooting models for masses above about 1.5M ⊙. We describe here the characteristics of the models, the procedure for constructing isochrones for arbitrary age and metallicity from the models, and the performance of these isochrones for several intermediate-age and old open clusters. All original models are available in electronic form and we describe the means by which they may be obtained. 相似文献
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Vladimir Sokolov Klaus-Peter Bonjer Friedemann Wenzel 《Soil Dynamics and Earthquake Engineering》2004,24(12):929-947
The paper presents recent achievements in evaluations of site-dependent seismic hazard in Romania and the capital city of Bucharest caused by the Vrancea focal zone (SE-Carpathians). The zone is characterized by a high rate of occurrence of large earthquakes in a narrow focal volume at depths 60–170 km. The database that was used for the hazard evaluation includes parameters of seismicity, ground-motion source scaling and attenuation models (Fourier amplitude spectra), and site-dependent spectral amplification functions. Ground-motion characteristics were evaluated on the basis of several hundred records from more than 120 small magnitude (M 3.5–5) earthquakes occurred in 1996–2001 and a few tens of acceleration records obtained during four large (M 7.4, 7.2, 6.9 and 6.3) earthquakes. The data provide a basis for probabilistic seismic hazard assessment in terms of peak ground acceleration, peak spectral acceleration and MSK intensity using Fourier amplitude spectra for various exceedance probabilities or average return periods. It has been shown that the influence of geological factors plays very important role in distribution of earthquake ground-motion parameters along the territory of Romania. 相似文献
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W. Wirth F. Wenzel V. Yu. Sokolov K. -P. Bonjer 《Soil Dynamics and Earthquake Engineering》2003,23(8):737-758
As a uniform approach to the assessment of ground motion variation within the Romanian capital Bucharest we analyze and compare strong motion records from analog recorders, weak motion data from a modern digital accelerometer network, and intensity observations of previous strong earthquakes. These different data sets allow to clearly characterize geographical trends in the distribution of ground shaking in the city for future earthquakes. Below 2 Hz the variability is small. Between 2 and 5 Hz, however, variations by a factor of 3–4 have to be expected. As the key source for the seismic hazard—the intermediate depth Vrancea earthquakes—remain at hypocentral distances in excess of 150 km from the city the ground motion variation must be predominantly attributed to site effects. This geometry of Vrancea sources to the site of Bucharest is ideal for the application of source-site separation techniques. However, despite this fact site effect amplification functions display a very large amount of aleatory uncertainty. In other words the standard source-site parameterization is too simple and we do not yet fully understand the cause and size of site effects. 相似文献
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