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Ground magnetic perturbations and ionospheric electric fields have been observed by the Scandinavian Magnetometer Array and the STARE radars, respectively, during the passage of a westward travelling surge in the late evening sector. On the basis of these measurements, the three-dimensional current system in the vicinity of the westward travelling surge is modelled. The results support a current model recently proposed by Rostoker and co-workers on the basis of purely magnetic ground-based measurements. In particular, we can identify an upward field-aligned current of about 5 x 104 A in the head of the surge. Ahead of the surge, we find a south-eastward directed electric field. Additionally, we have some evidence for the existence of a southwestward directed electric field east of the surge. The latter may be explained by the generation of polarisation charges at the northern and southern boundaries of the higher conducting region east of the surge's head.
ARK: https://n2t.net/ark:/88439/y068400
Permalink: https://geophysicsjournal.com/article/156
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Study of earthquakes and moonquakes finds strong (M6+) quakes and plate tectonics are caused by externally excited resonance instead of mantle convection/inner heat.
ARK: https://n2t.net/ark:/88439/x042047
Permalink: https://geophysicsjournal.com/article/155
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We consider the periodic acceleration of the Earth's rotation rate caused by the oceanic M2-tide and the corresponding cumulative effect in universal time UT1. The necessary information stems from a hydrodynamical model of the world's oceans. The time variations of the contributions to the relative angular momentum balance and of the solid Earth's center-of-mass are provided. As for the theoretical aspects, two hypotheses are examined: the 'quasi-isolated-Earth' -hypothesis, which is basic for the derivation of the Δω- and ΔUT1-effects, is acceptable at least as a first-order approximation, whereas the idea of 'locked oceans' is not. The main result is a ΔUT1-contribution with a total range of 0.05 ms, thus nearly detectable by means of modern observational techniques: this effect essentially originates from the tidal currents.
ARK: https://n2t.net/ark:/88439/y059561
Permalink: https://geophysicsjournal.com/article/154
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In the period 1971 to 1974 the territory of Greece has been geophysically investigated by Greek and German Institutions. Gravity and magnetic stations have been distributed at a spacing of one station per 25 km2. Five deep seismic sounding cross-sections have been fired and recorded along the lines: Ionian Sea — Peloponnese, Amorgos — Mikonos — Evia, Crete: East-West, Cretan Sea: North-South, Cretan Sea: East-West. The results of the seismic programms revealed a pure continental structure of variable thickness. The most attenuated area is that of the Cretan Sea, with only 22 km depth to the Moho-Discontinuity at the Bouguer gravity maximum of +175 mgal. The very unevenly distributed sedimentary cover of the Cretan Sea is composed mainly of Neogene Sediments with thickness of 3–3,5 km in local basins (Jonsma et al., 1975). The Greek mainland along the Pindos Chains has minimum Bouguer anomalies of –120 to –140 mgal and Moho-Depths between 42–46 km. The Aegean Area builds a large dome and incorporates also a large part of the Taurides, Western Turkey (Makris, 1975).
ARK: https://n2t.net/ark:/88439/y049082
Permalink: https://geophysicsjournal.com/article/153
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A 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
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Shear-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
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Deformations and gravity changes at the Earth's surface due to regional and global air pressure variations are estimated for a radially stratified Earth. The results are as follows:- Vertical displacements of seasonal character have maximum amplitudes of ±0.5 cm. (Anti-)Cyclones can cause vertical displacements of up to ±2.5 cm.- Horizontal displacements have amplitudes less than ±2.5 mm.- Horizontal principal strains may have amplitudes up to 10-8. They reduce to about ±1.5✻10-9 for seasonal changes in the air pressure distribution.- The total gravity perturbation consisting of the Newtonian attraction of air masses and of self-gravitation due to the elastic deformation may go up to ±20 μgal in the case of (anti-)cyclones, and ±3 μgal in the case of seasonal air pressure changes.- The total tilt due to seasonal air pressure variations can be as high as ±1.5 mseca. For passing (anti-)cyclones this value may go up to ±10 mseca.
All the above values have to be modified in the direct vicinity of coastlines. The modification is only slight for the displacements and the secondary gravity effect, but it is important for the other components. There, the necessary modification may amount to several hundred percent depending on the type of deformation component and on the distance to the coastline. Precise air pressure corrections of radial displacements and gravity changes cannot be achieved by using a single regression coefficient. Either the characteristic wavelengths of the pressure distribution have to be taken into account or the following two-coefficient correction equations have to be used:
Radial displacement: u = -0.90 p̂ - 0.35 (p - p̂ ) Primary gravity: gp = 0.36 p̄ + 0.41 (p - p̄ ) Secondary gravity: gs = -0,17 p̂ - 0.08 (p - p̂ ) Total gravity: g = gp + gswith u = radial displacement in mm, gp, gs, g = primary, secondary and total gravity, respectively, in μgal, p = local pressure variation in mbar, p̄ = average of the pressure variation in a surrounding area of 2,000 km (in mbar) and p̂ the same average, except for setting the pressure values equal to zero over ocean areas. These corrections have been tested for seasonal air pressure variations and they have proved to be highly precise. The average errors are less than 0.5 mm, 0.1 μgal, 0.1 μgal and 0.2 μgal for the radial displacements, the primary, secondary and total gravity changes, respectively. The maximum errors are less than 1 mm in the case of the radial displacements, 0.3 μgal and 0.2 μgal for the primary and secondary gravity changes, respectively, and 0.4 μgal for the total gravity changes. Due to a small, spatially constant error term these values apply strictly only to spatial differences of the above deformation components. The differences, however, can be taken between any two points on the Earth's surface.
ARK: https://n2t.net/ark:/88439/y011765
Permalink: https://geophysicsjournal.com/article/150
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A method is presented for obtaining the direction and confidence oval for a paleomagnetic component at a site given a number of independently oriented samples, some of which give an estimate of the remanence direction, while others yield only remagnetization circles. Such mixed remagnetization circle-remanence direction data frequently characterise paleomagnetic sites carrying two remanence components where the component of interest is small and less dispersed compared to a more easily removed one. The method described maximises the amount of usable data per site and thus leads to an improved site direction estimate.
ARK: https://n2t.net/ark:/88439/y000446
Permalink: https://geophysicsjournal.com/article/149
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A set of 142 S and ScS absolute travel times and 62 ScS-S differential travel times from short-period recordings of the January 29, 1971, Sea of Okhotsk deep-focus earthquake has been analyzed to obtain estimates of the source anomaly as a function of position on the residual sphere. In the processing algorithm the station anomalies and travel times are treated as gaussian random variables with known variance matrices. A system of normal equations is found by minimizing a quadratic form that is the sum of three terms: a measure of the misfit to the absolute travel times, a measure of the misfit to the differential travel times, and a measure of the misfit to the estimated station anomalies. An approximate solution to the system of normal equations is derived by requiring that the source anomaly be a smooth function of position on the residual sphere. This fitting procedure yields a saddle-shaped source anomaly pattern. The pattern is compatible with the presence of a planar high-velocity zone beneath the source with a nearly vertical dip and a strike parallel to the Kuril-Kamchatka Arc. Ray tracing calculations have been used to model the anomaly. A high-velocity slab extending to a depth of 1000 km along the extrapolation of the Benioff Zone, with a velocity contrast of 5%, is consistent with the data. The anomaly is interpreted to be the expression of lithospheric material that has penetrated the lower mantle. This study and data from other subduction zones suggest that lithospheric slab penetration below the 650 km discontinuity is a general feature of mantle structure in regions of rapid plate convergence. It is concluded that the lower mantle participates in the thermal convection responsible for plate motions.
ARK: https://n2t.net/ark:/88439/y099157
Permalink: https://geophysicsjournal.com/article/148
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Asymptotic ray theory can be used to describe many seismic signals. Provided the wavefronts and amplitudes vary smoothly and the correct phase changes are included for caustics and reflection/transmission coefficients, it successfully describes direct and turning rays, on normal and reversed branches with multiple turning points, and partial and total reflections and transmissions. Nevertheless, many exceptions occur. Critical points, head waves, interference head waves, Airy caustics, Fresnel shadows, edge, point and interface diffractions and gradient coupling are examples discussed in this paper. Asymptotic ray theory can be simply extended to cover some of these problems. In this paper, the extension called the WKBJ or Maslov seismogram is discussed.
ARK: https://n2t.net/ark:/88439/y086318
Permalink: https://geophysicsjournal.com/article/147
