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Contents1. Introduction2. Low-latitude Pc 3 magnetic pulsations 2.1 Introduction 2.2 Upstream waves as a source of low-latitude Pc 3 2.3 Transmission of upstream waves through bow shock and magnetopause 2.4 Propagation mechanism of Pc 3 source waves into deep magnetosphere 2.5 Transmitted Pc 3 source waves in the magnetosphere 2.6 Characteristic frequencies of various Pc 3 oscillations in magnetosphere 2.7 Wave characteristics of low-latitude Pc 3 2.8 Pc 2–3 at very low latitudes (Φ< 22°)3. Low-latitude Pi 2 magnetic pulsations 3.1 Introduction 3.2 Unresolved characteristics of Pi 2 pulsations 3.3 Wave and current fluctuation models for Pi 2 generation 3.4 Model of instantaneous transmission from polar electric field to equator 3.5 A possible model for daytime Pi 24. Summary and conclusions.
ARK: https://n2t.net/ark:/88439/y071009
Permalink: https://geophysicsjournal.com/article/146
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Measurements of compressional and shear wave velocities, VP and VS, were made in a cubic anvil apparatus up to 700° C at 6 kbar in igneous and metamorphic rocks typical of the crust and mantle. Samples range in composition from acidic to ultramafic, with bulk density of 2.67–3.46 g/cm3 at 6 kbar. Mean atomic weights of the rocks vary between 20.37 and 23.03. A rough dependence of wave velocity on bulk density is apparent. However, there is considerable scatter, and the velocities do not generally follow lines of constant mean atomic weight. Both VP and VS increase with increasing amphibole, garnet, pyroxene and olivine content. High quartz content produces relatively low P-wave and high S-wave velocities, giving significantly low Poisson's ratios. In contrast, a high feldspar content is related to relatively high P-wave and low S-wave velocities and consequent high Poisson's ratios. The calculated temperature derivatives of VP and VS for the range 20-500° C at 6 kbar confining pressure, using best-fit solutions, cover the range –1.60 to –4.94 x 10–4 km/s° C and –1.39 to –3.93 x 10–4 km/s° C, respectively. The results compare fairly well with the few data published so far. For the dunite and peridotite rocks the (dVP/dT)p-values are in excellent agreement with published Voigt-Reuss-Hill values calculated from single crystal data. Using the experimental results, P-velocity profiles were calculated along a geotherm of a cold Precambrian shield crust and a warm continental crust. The temperature gradients for the latter cause velocity inversion in numerous rocks, rich in olivine or quartz.
ARK: https://n2t.net/ark:/88439/y069600
Permalink: https://geophysicsjournal.com/article/145
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Numerical modelling of high-frequency seismic wave fields in complex, 2-D and 3-D, laterally varying, layered structures by the summation of elastodynamic Gaussian beams is discussed. The main attention is devoted to the expansion of the wave field into Gaussian beams, to the choice of initial parameters of Gaussian beams in these expansions and to the construction of synthetic seismograms. The Gaussian beam synthetic seismograms are regular even in regions where the ray method fails, such as the caustic region, critical region, etc. Due to the smoothing effects involved in the Gaussian beam procedure, the method is not too sensitive to the approximation of the medium and to minor details of the model. Moreover, the method does not require two-point (source-to-receiver) ray tracing. The eyaluation of Gaussian beam synthetic seismograms requires approximately the same amount of computer time as the evaluation of ray synthetic seismograms. The memory requirements are also approximately the same. Numerical examples of Gaussian beam synthetic seismograms for 2-D and 3-D structures are presented. Various possible applications of Gaussian beams to seismological problem of practical importance are outlined.
ARK: https://n2t.net/ark:/88439/y058811
Permalink: https://geophysicsjournal.com/article/144
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Clear observations of small but abrupt changes in gravity during the passage of cold fronts are reported. Instrumental effects can be ruled out by special experiments. Simple models for the atmosphere show that the direction and the order of magnitude of the observed effects can be explained by changes in gravitational attraction of the sensor mass by the atmosphere or by downward acceleration of the ground due to the increasing air pressure. This is additional evidence for the high importance of meteorological causes for long-period seismic noise.
ARK: https://n2t.net/ark:/88439/y040712
Permalink: https://geophysicsjournal.com/article/143
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The elastodynamic Gaussian beams in 3D elastic inhomogeneous media are derived as asymptotic high-frequency one-way solutions of the elastodynamic equation concentrated close to rays of P and S waves. In this case, the elastodynamic equation is reduced to a parabolic (Schroedinger) equation which further leads to a matrix Riccati equation and the transport equation. Both these equations can be simply solved along the ray, the first numerically and the other analytically. The amplitude profile of the principal displacement component of the elastodynamic Gaussian beams is Gaussian in the plane perpendicular to the ray, with its maximum at the ray. The Gaussian beams are regular along the whole ray, even at caustics. As a limiting case of infinitely broad Gaussian beams, the paraxial ray approximation is obtained. The properties and possible applications of Gaussian beams and paraxial ray approximations in the numerical modelling of seismic wave fields in 3D inhomogeneous media are discussed.
ARK: https://n2t.net/ark:/88439/y030593
Permalink: https://geophysicsjournal.com/article/142
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In the 1D interpretation of magnetotelluric data, it is often useful to discover a reasonable first-approximation to the true conductivity-depth distribution beneath the recording location. This may be undertaken in the field, in order to ascertain if the station spacing is satisfactory or whether a greater station density of coverage is required, or at the base laboratory, as a prelude to a more sophisticated 1D inversion of the data.
ARK: https://n2t.net/ark:/88439/y020744
Permalink: https://geophysicsjournal.com/article/141
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In order to constrain the motions of the allochthonous Ardenne during the Paleozoic, standard paleomagnetic techniques have been applied on acidic and basic sills of the Cambrian Rocroi massif and on Cambrian-to-Visean sedimentary series sampled along the Meuse valley. Ordovician–Silurian volcanics and Devonian–Visean limestones have also been collected in the autochtonous Brabant and Namur basin, in order to get paleomagnetic reference poles. In the Ardenne, two groups of in situ paleomagnetic directions arise. The A components (mean: D = 212°, I = -11°, α95 = 6°, for 10 sites), that are characterized by unblocking temperatures around 330° C, represent Late Variscan (Stephanian–Permian) overprints. The B components (mean: D = 236°, I = 2°, α95 = 11°, for 13 sites) display higher unblocking temperatures, in the range 400°-580° C. They show a large scatter in inclination (–25° < I < 35°) that is not reduced by tectonic correction. The B components are interpreted as post- and partly synfolding overprints of Middle-Late Carboniferous age. In the Brabant, all series display Late Variscan remagnetizations (mean: D = 204°, I = -7°, α95 = 9°, for 5 sites), consistent with the Stephanian- Permian pole of Europe. The existence of Middle-to-Late Variscan B directions in the Armorican Massif, Central Massif, Vosges and Black Forest indicates that in Namurian–Westphalian times the whole investigated Variscan belt, including the Ardenne, was trending N-S. A 45° clockwise rotation relative to the paleomeridian, during the latest Westphalian–Stephanian, has lead the massif to its Permian position.
ARK: https://n2t.net/ark:/88439/y010025
Permalink: https://geophysicsjournal.com/article/140
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A simple convective model that can maintain observed plate motions consists of a viscous upper mantle of uniform density overlain by denser rigid plates. In the absence of density differences within the upper mantle the viscous stresses exerted by the flow are easily obtained and demonstrate that the buoyancy forces associated with plate creation and destruction can maintain plate motions. A model having a uniform viscosity upper mantle is, however, unsatisfactory because it predicts gravity and residual depth anomalies two orders of magnitude larger than those observed. This problem can be overcome by introducing a thin low viscosity layer beneath the plates. The resulting model is then similar to that proposed by Forsyth and Uyeda and by Chapple and Tullis despite a very different approach. This agreement suggests that the energetics of plate motion are now understood in outline. The model cannot, however, account for the existence of the longwavelength gravity anomalies which are not associated with plate motions.
ARK: https://n2t.net/ark:/88439/y001916
Permalink: https://geophysicsjournal.com/article/139
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Eighty-five sites of West Eifel volcanoes were investigated paleomagnetically, giving 64 independent virtual geomagnetic poles (VGP). The VGP distribution is strongly asymmetric: about 30% of the VGPs have latitudes below 60° N and are confined to the longitude sector between 30° E and 120° E. This leads to a mean VGP situated at 74.0° N/63.9° E, deviating significantly from the north geographical pole. The VGP distribution is non-Fisherian, but the radial component is rather similar to that observed for Tertiary to Quaternary Icelandic lavas. Tectonic, petrographic, rock magnetic properties and secular variation do not seem to be responsible for the anomalous VGP positions. We propose that the volcanoes with anomalous VGPs erupted in a short period during an excursion or event of the Earth's magnetic field.
ARK: https://n2t.net/ark:/88439/y098537
Permalink: https://geophysicsjournal.com/article/138
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A method is described for calculating the seismic response of an arbitrarily shaped interface below a homogeneous medium by the Kirchhoff integral in the time domain. It is shown, by comparison with other numerical techniques, that this method yields accurate results for reflections. The errors in calculating diffractions are tolerable if the distance of the receiver from the shadow boundary of reflection is not too large; this is usually the case in horizontal seismic profiling. The method has been applied to model qualitatively some typical features in record sections of the deep seismic reflection profile DEKORP2-S. This profile is characterized by numerous strongly curved events that are concentrated mainly in two areas of the profile. These signals can be addressed as diffractions from an interpretation of the travel times. Dynamic calculations, however, show that the surprisingly high amplitudes cannot be explained by diffracting elements like fault edges or small-scale inhomogeneities; instead, one has to assume cylindrically or spherically curved reflectors with a radius of at least 4 km. Some possible geological explanations for these structures, like diapiric intrusions or antiformal stacks, are discussed in view of the tectonic evolution of the Central European Variscides.
ARK: https://n2t.net/ark:/88439/y087788
Permalink: https://geophysicsjournal.com/article/137
