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71.
72.
We have examined the history of the elevated primary productivity associated with the Benguela Current upwelling system off southwest Africa using sediments from 7.5 to 4.8 Ma at Ocean Drilling Program Site 1085 in the middle Cape Basin. Sedimentation rates are low until 6.9 Ma. Low accumulation rates of benthic foraminifers and organic carbon indicate that biological productivity was also low. Paleoproductivity dramatically increased at 6.7–6.5 Ma and was highly variable until 4.8 Ma with productivity maxima during cooler periods. The presence of radiolarian opal only between 5.8 and 5.2 Ma suggests an interlude of silica-rich intermediate water in the Cape Basin. The onset of heightened productivity under the Benguela Current is mirrored by similar increases reported between 6.9 and 6.7 Ma in the tropical eastern Pacific, the western and northern Pacific, and the Indian Ocean. The similarity between the patterns at Site 1085 and in the Pacific and Indian Oceans suggests that the dramatic productivity increase off southwest Africa is part of a global response to paleoceanographic changes.  相似文献   
73.
We consider four bodies in space with same masses forming two binaries, each one symmetric with respect to a fixed axis and moving under Newtonian gravitation in opposite directions about this axis. It is given a direct proof that all singularities of this model are due to collisions, and it is proved that the singularities due to simultaneous double collisions are regularizable. The set of equilibrium points on the total collision manifold is studied as well as the possible connections among them. We show that the set of initial conditions on a given energy surface going to quadruple collision is a union of twenty submanifolds: twelve of them have dimension 2 and the others have dimension 3. Similarly for ejection orbits from quadruple collision. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
74.
Abstract

This paper describes the tectono-metamorphic evolution of a segment of the Precambrian deep crust, in the southern Madagascar island. This crust corresponds to an Archaean basement reworked by a widespread, late panAfrican event (550–580 Ma) during the formation of the Mozambican belt. The finite geometry and associated metamorphism are depicted by satellite imaging, field mapping and P-T estimations using both conventional thermobarometric methods and TWEEQ software program with internally consistent thermodynamic data and uniform set of solution models. The structural pattern developed during high-grade metamorphism shows the juxtaposition of domains with complex fold geometries separated by a 15 km wide ductile shear zone. Within the folded domains, kilometre scale interference patterns associated with strongly dipping metamorphic stretching lineations can be described as superposed folding (F1 and F2 folds). The tight and upright F2 folds result from East-West horizontal shortening. The shear zone is defined by homogeneous orientations of steep foliations, sub-horizontal stretching lineations, and kilometre scale strain gradient. Within the shear zone, we observe dominant non-coaxial criteria at various scales that are consistent with a sinistral strike-slip system during D2 deformation stage. Nevertheless, we have also found in the shear zone, geometries typical of a horizontal shortening. Such a strain pattern is characteristic of transpression tectonics.

The synkinematic metamorphic conditions are estimated on mafic garnetiferous metabasites. Results show that regional transpression tectonics has developed under very high and constant thermal regime (about 800°C). A pressure gap, of about 3 kbar between the domains separated by the shear zone is identified. This implies tectonic coupling of two different structural levels during tranpressive tectonic.  相似文献   
75.
This study analyses some hydrological driving forces and their interrelation with surface‐flow initiation in a semiarid Caatinga basin (12 km2), Northeastern Brazil. During the analysis period (2005 – 2014), 118 events with precipitation higher than 10 mm were monitored, providing 45 events with runoff, 25 with negligible runoff and 49 without runoff. To verify the dominant processes, 179 on‐site measurements of saturated hydraulic conductivity (Ksat) were conducted. The results showed that annual runoff coefficient lay below 0.5% and discharge at the outlet has only occurred four days per annum on average, providing an insight to the surface‐water scarcity of the Caatinga biome. The most relevant variables to explain runoff initiation were total precipitation and maximum 60‐min rainfall intensity (I60). Runoff always occurred when rainfall surpassed 31 mm, but it never occurred for rainfall below 14 mm or for I60 below 12 mm h?1. The fact that the duration of the critical intensity is similar to the basin concentration time (65 min) and that the infiltration threshold value approaches the river‐bank saturated hydraulic conductivity support the assumption that Hortonian runoff prevails. However, none of the analysed variables (total or precedent precipitation, soil moisture content, rainfall intensities or rainfall duration) has been able to explain the runoff initiation in all monitored events: the best criteria, e.g. failed to explain 27% of the events. It is possible that surface‐flow initiation in the Caatinga biome is strongly influenced by the root‐system dynamics, which changes macro‐porosity status and, therefore, initial abstraction. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   
76.
An unidentified species of the genus Moina was found in a Pasveer sewage treatment plant at Glensiide, Wellington; it is described and illustrated. A survey of the literature shows that this is probably the first published record of the genus in New Zealand (M. lemnae, previously recorded, has since been tentatively referred to the genus Pseudomoina); sewage ponds in India and South Africa have yielded M. dubia.  相似文献   
77.
Wavefield decomposition forms an important ingredient of various geophysical methods. An example of wavefield decomposition is the decomposition into upgoing and downgoing wavefields and simultaneous decomposition into different wave/field types. The multi‐component field decomposition scheme makes use of the recordings of different field quantities (such as particle velocity and pressure). In practice, different recordings can be obscured by different sensor characteristics, requiring calibration with an unknown calibration factor. Not all field quantities required for multi‐component field decomposition might be available, or they can suffer from different noise levels. The multi‐depth‐level decomposition approach makes use of field quantities recorded at multiple depth levels, e.g., two horizontal boreholes closely separated from each other, a combination of a single receiver array combined with free‐surface boundary conditions, or acquisition geometries with a high‐density of vertical boreholes. We theoretically describe the multi‐depth‐level decomposition approach in a unified form, showing that it can be applied to different kinds of fields in dissipative, inhomogeneous, anisotropic media, e.g., acoustic, electromagnetic, elastodynamic, poroelastic, and seismoelectric fields. We express the one‐way fields at one depth level in terms of the observed fields at multiple depth levels, using extrapolation operators that are dependent on the medium parameters between the two depth levels. Lateral invariance at the depth level of decomposition allows us to carry out the multi‐depth‐level decomposition in the horizontal wavenumber–frequency domain. We illustrate the multi‐depth‐level decomposition scheme using two synthetic elastodynamic examples. The first example uses particle velocity recordings at two depth levels, whereas the second example combines recordings at one depth level with the Dirichlet free‐surface boundary condition of zero traction. Comparison with multi‐component decomposed fields shows a perfect match in both amplitude and phase for both cases. The multi‐depth‐level decomposition scheme is fully customizable to the desired acquisition geometry. The decomposition problem is in principle an inverse problem. Notches may occur at certain frequencies, causing the multi‐depth‐level composition matrix to become uninvertible, requiring additional notch filters. We can add multi‐depth‐level free‐surface boundary conditions as extra equations to the multi‐component composition matrix, thereby overdetermining this inverse problem. The combined multi‐component–multi‐depth‐level decomposition on a land data set clearly shows improvements in the decomposition results, compared with the performance of the multi‐component decomposition scheme.  相似文献   
78.
Exceptional ground motions have been observed in the near-source region in relatively small and moderate earthquakes such as the Lorca earthquake of 2011. In the Mw 5.2 mainshock, horizontal peak ground acceleration reached 0.37 g at the LOR accelerograph station in the north-central part of the town. Maximum ground acceleration and velocity in this station was strongly polarised in the fault’s strike-normal axis, a feature interpreted as a near-field effect of rupture directivity. This quake hit the Lorca area causing serious structural damage, unevenly distributed in the urban area as result of a heterogeneous spread of building vulnerability and seismic shaking severity. This study investigates the characteristics of the earthquake shaking at the LOR station (on rock) and other 11 zones of Lorca with different site conditions. We have computed the 1-D transfer functions of the shallow structure at these 11 points with available local Vs models obtained with SPAC technique. These transfer functions were used to convolve the LOR accelerogram and simulate the ground motion which occurred at each site. A set of key engineering ground-motion parameters obtained here, indicate different shake levels at each site and all these values are clearly above the LOR station ones. These parameter values are in the range of typical expected values for European earthquakes with local intensity VIII. Their large values, especially those of acceleration response spectra and energy input spectra, more clearly for periods between 0.4 and 0.6 s, point out that the seismic demand has been quite high and help to explain the heavy damage spread on the town’s buildings.  相似文献   
79.
This paper estimates fundamental translational period and damping ratio parameters and examines the changes in dynamic characteristics of a set of low-to-medium rise buildings in Lorca town (SE of Spain) affected by the May 11th, 2011 earthquake. These building parameters have been calculated analysing structural dynamic response from ambient vibration measurements recorded at top RC buildings pre- and post earthquake, using the Fast Fourier Transform and the Randomdec technique. The empirical expression relating period \((T)\) and number of floor \((N)\) here obtained analysing ambient noise recorded on 59 healthy buildings before the earthquake is \(T= (0.054\pm 0.002)\, N\) , very similar to others empirical period–height relationships obtained for RC structures in the European built environment but quite different from code provisions. Measurements performed in 34 damaged buildings show a period elongation after the quake according to \(T^{*} =(0.075\pm 0.002)\,N\) expression. Moreover, we found a rise of the fundamental period with the EMS’s grade of damage of buildings. In contrast to natural frequency, damping ratio \((\xi )\) do not shows a significant variation with earthquake damage degree and the product \(\xi \, T\) remains near constant.  相似文献   
80.
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