Journal of Geophysics
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Shear-wave singularities of wave propagation in anisotropic media
Get PDFShear-wave singularities in systems of anisotropic symmetry are comparatively well known, but it has not been generally realised that they may cause anomalies in shear-wave propagation for neighbouring directions due to the behaviour of the polarizations. Singularities are places where the two shear-wave slowness surfaces are continuous with each other through common points. The most frequent type, a point singularity, is a place where the two surfaces are continuous with each other through the vertices of cone-shaped projections from the surfaces. For directions of propagation in a plane, which cuts the slowness surfaces near a singularity, the velocities of the two shear-waves approach each other in a pinch and at the pinch exchange polarizations and velocity gradients. These singularities do not cause anomalies in plane waves propagating in a uniform medium, but may cause mode conversion and pulse-shape modification to waves with spherical wave-fronts, and to rays of shear-waves, in varying anisotropic media.
ARK: https://n2t.net/ark:/88439/y021254
Permalink: https://geophysicsjournal.com/article/151
 
Joint two-dimensional observations of ground magnetic and ionospheric electric fields associated with auroral currents. 5. Current system associated with eastward drifting omega bands
Get PDFMagnetograms from the Scandinavian Magnetometer Array (SMA) and ionospheric electric field measurements from the Scandinavian Twin Auroral Radar Experiment (STARE) have been evaluated during the passage of several omega bands (or eastward travelling surges) over Northern Scandinavia around 0400 MLT on 16 February 1977. The eastward motion of the omega bands was a pure E × B drift and was associated with Ps6 disturbances of the east-west component particularly of magnetic and electric fields. The two-dimensional distributions of electric and magnetic disturbance fields strongly support the hypothesis of a three-dimensional current system which is embedded in a rather homogeneous westward Hall current and where north-south aligned localized regions of field-aligned currents, flowing upward close to the omega band's wave-crest and downward east and west of it, are travelling eastward with the omega band. These field-aligned currents are associated with eastward and westward electric field disturbances and southward and northward Hall currents between them. It remains unclear whether these Hall currents close within the ionosphere, i.e. forming counterclockwise and clockwise ellipsoidal Hall current vortices, or if they diverge from the ionosphere as field-aligned currents at the southern and northern boundary of the region that is disturbed by the omega band.
ARK: https://n2t.net/ark:/88439/y015335
Permalink: https://geophysicsjournal.com/article/201
 
Steady state creep of fine grain granite at partial melting
Get PDFSteady state creep under constant stress has been measured in a fine grain granite (aplite) from Schauinsland, Germany for the temperature range of partial melting 860°–1060°C, under a hydrostatic pressure of 4.2 kbar, and at low shear stresses of 5–50 bar. The apparatus used is described briefly. Rheological measurements are complemented by microscopic investigations. With a melt fraction of up to about 20%, creep can be described by a power law with a stress exponent of 3-4 and an activation energy of 80 kcal/mole, typical for creep in solids. Above 1010°C or 20% melt, the activation energy increases rapidly to a value of 200 kcal/mole simultaneously with a rapid increase of the melt fraction and a decrease of feldspar content. From the grain structure and from etching tests it is concluded that quartz contributes little to the plastic deformation which is governed mainly by the stress and temperature induced recrystallization of feldspar. The large temperature dependence of the creep rate above 1010° C may be caused by the decreasing area of grain contacts and consequent rise in local stress. These results support those of Arzi (1974) and Roscoe (1952).
ARK: https://n2t.net/ark:/88439/y060550
Permalink: https://geophysicsjournal.com/article/258
 
Grain size effect on the low-temperature oxidation of titanomagnetite
Get PDFThe low-temperature oxidation process in titanomagnetite has been investigated. Using samples with controlled grain sizes, it has been shown that there is a critical grain size in the oxidation behaviour of titanomagnetites; smaller grains undergo low-temperature oxidation, while larger ones separate to Fe-rich titanomagnetite and Ti-rich hemo-ilmenite (high-temperature oxidation). The difference between the results of similar experiments by Ozima and Sakamoto (1971) and by Readman and O'Reilly (1972) can be explained in terms of differences in partide sizes of titanomagnetites. Comparable results were obtained from natural subaerial and submarine basalts; when they are heated to temperatures between 150° C and 375° C, oxidation proceeds in submarine basalts, but it does not in subaerially erupted basalts and a high-temperature oxidation process occurs. This fact indicates that the grain size of the magnetic minerals is one of the most important controlling factors in low-temperature oxidation of titanomagnetites.
ARK: https://n2t.net/ark:/88439/y041502
Permalink: https://geophysicsjournal.com/article/266
 
Monte Carlo simulation of collisionless shocks showing preferential acceleration of high A/Z particles
Get PDFA collisionless, quasi-parallel shock is simulated using Monte Carlo techniques. In this kinetic approach, scattering of all velocity particles (from thermal to high energy) is assumed to occur such that the mean free path is directly proportional to velocity times the mass to charge ratio and inversely proportional to the plasma density. Within the constraints of this assumption, the shock profile and velocity spectra are obtained showing preferential acceleration of high A/Z particles relative to protons.
ARK: https://n2t.net/ark:/88439/y033533
Permalink: https://geophysicsjournal.com/article/152
 
On modelling the lithosphere in mantle convection with non-linear rheology
Get PDFNumerical convection experiments were carried out with the aim of simulating the lithosphere as a strong mechanical boundary layer participating in the circulation, and to study its dynamical role and the governing parameters. The rheological model parameters were successively refined, effective viscosity depending on (1) depth, (2) temperature and pressure, and (3) temperature, pressure, and stress. In all cases a high-viscosity plate rested on a low-viscosity asthenosphere; in the two latter cases it could in principle subduct, but did so only if zones of weakness were built into it. It was possible to model active or inactive plates (moving faster or slower than the asthenosphere below). Because of a lack of numerical resolution it was however, not possible to simulate a narrow sinking slab; rather a broad zone of cooled and highly viscous material developed, often limiting the rate of descent and leading to non-steady convection. The circulation, including subduction, was stabilized by introduction of stress-dependence of viscosity (non-linearity), dissipation, and adiabatic heating. The parameter chiefly responsible for deciding the (active or passive) role of the plate is its decoupling from its neighbours, achieved in the models by assuming weakness zones. Another important result seems to be that the assumption of plausible mantle rheologies and heat input leads to equally plausible effective viscosities, plate velocities, and to upper-mantle temperatures which are relatively low by current ideas, but conforming to earlier estimates based on convection theory. Viscosity distribution and flow pattern are also in reasonable agreement with more detailed boundary layer computations. The main obstacles to our modelling are the numerical limitations, forcing upon us such artificialities as two-dimensionality, rectangular model boxes, coarse grids, and generalized weakness zones.
ARK: https://n2t.net/ark:/88439/y061130
Permalink: https://geophysicsjournal.com/article/176
 
Reflection and transmission of Love channel waves at coal seam discontinuities computed with a finite difference method
Get PDFChannel waves serve as a tool for the detection of discontinuities in coal seams caused, for example, by tectonic faulting. In this paper we study Love waves propagating along two-dimensional discontinuous coal seams. Synthetic seismograms, computed with an explicit finite difference method, are presented for three types of discontinuities: the seam end, the horizontal and the vertical offset. In all cases the discontinuity reflects mainly those waves with short wavelength and transmits those waves with large wavelength. An additional damping term is introduced into the finite difference formulation in order to prevent reflections from the edges of the computational grid. The boundary conditions at interfaces are all approximated with a truncation error of second order.
ARK: https://n2t.net/ark:/88439/y000566
Permalink: https://geophysicsjournal.com/article/170
 
Impulse in global geomagnetic "secular variation", 1977-1979
Get PDFThe global geomagnetic secular variation in the l960s and l970s was characterized by a rather rapid intensification which was shown to be of internal origin (Nevanlinna, 1980). An external 11-year wave could also be identified (Nevanlinna, 1980; Yukutake and Cain, 1979). In studying the global secular variation for the last few years, an even more rapid global change can be identified, namely a sudden enhancement followed by a deficit, lasting roughly two years from the end of 1977 to 1979. Here we demonstrate that the pulse is of external origin and that it resembles an intensification of the magnetospheric ring-current system lasting for about two years.
ARK: https://n2t.net/ark:/88439/y074919
Permalink: https://geophysicsjournal.com/article/197
 
Paleomagnetic evidence from Mesozoic carbonate rocks for the rotation of Sardinia
Get PDFThe remanent magnetizations have been analysed in Cretaceous and Jurassic limestones, and Triassic dolomites, from Eastern and Central Sardinia. Ore microscopy and rock magnetic studies, including the acquisition of isothermal remanent magnetization (IRM) and the behavior of IRM during high temperature treatment, indicated that magnetite and goethite determine the paleomagnetic properties of these carbonate rocks. At all Cretaceous sites the remanences were either too weak or too unstable to be used for a paleomagnetic study. AF and thermal demagnetization were used to isolate the characteristic remanent magnetization (ChRM) in the Jurassic and Triassic samples. Rejection criteria were used to eliminate about one third of these samples because of directional instability, or because only very stable secondary magnetizations associated with goethite were present. For the remaining samples the ChRM direction associated with primary magnetite was extracted with the aid of vector diagrams or vector difference calculations. The ChRM directions are rotated by 70°–90° counterclockwise relative to European directions and by 35°–45° counterclockwise relative to African directions. Part of this results from the 30° microplate rotation of Sardinia in the Miocene. The remainder is due to an earlier phase of microplate rotation which took place in post-Late Jurassic time.
ARK: https://n2t.net/ark:/88439/y022994
Permalink: https://geophysicsjournal.com/article/274
 
Palaeomagnetism of Upper Cretaceous volcanics and Nubian sandstones of Wadi Natash, SE Egypt and implications for the polar wander path for Africa in the Mesozoic
Get PDFEighteen sites (342 samples) from Upper Cretaceous Wadi Natash volcanics (24.5° N, 34.5° E) yield a mean direction of magnetization D = 345.4°, I = 16.7° with α95 = 8.5°, k = 21.4, N = 15 after AF cleaning resulting in a pole at 69.3° N, 258° E with A95 = 5.8° All sites have normal polarity consistent with their age and the magnetic stratigraphy in the Cretaceous. From 5 sites (85 samples) from Upper Cretaceous Nubian sandstone at Wadi Natasha mean direction D = 358.1°, I = 32.0° with α95 = 8.7°, k = 143 (mixed polarity) was obtained after thermal demagnetization. Combined with previous investigations on Nubian sandstone at other locations in Southern Egypt (Schult et al. 1978) this yields a pole at 81.8° N, 223° E with A95 = 3.3°, N = 23. 9 sites from Eocene Baharia iron ores (27.5° N, 29.0° E) yield a mean direction D = 188.0°, I = –43.6° and α95 = 6.4°, k = 65 with a pole at 83.5° N, 139° E and A95 = 7° In addition the palaeomagnetism of some Tertiary basaltic rocks in Northern Egypt was studied. The polar wander path for Africa in Mesozoic time is presented showing more mobility than in earlier papers. For appropriate reconstructions of South America with respect to Africa the polar wander paths of both continents are substantially in agreement.
ARK: https://n2t.net/ark:/88439/y010555
Permalink: https://geophysicsjournal.com/article/252