Journal of Geophysics
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The normal modes of a uniform, compressible Maxwell half-space
Get PDFThe analytical solution for the load-induced deformation of a uniform, compressible, hydrostatically pre-stressed elastic half-space is derived. The solution is correct to first order in the quantity ε, which is inversely proportional to the wave number k of the deformation. Usually ε is very small compared with unity for Earth deformations on a scale amenable to the half-space approximation. Since pre-stress advection is included in the analysis, the correspondence principle allows us to solve the field equations governing the deformation of the associated Maxwell half-space. The viscoelastic solution shows that the relaxation of the Maxwell continuum is characterized by a fundamental mode and a rapidly decaying overtone of much smaller amplitude. In the incompressible limit the overtone is not excited. The significance of the results for the relaxation of the Earth's mantle is briefly discussed.
ARK: https://n2t.net/ark:/88439/y075549
Permalink: https://geophysicsjournal.com/article/207
 
Plasma-gas interactions in planetary atmospheres and their relevance for the terrestrial hydrogen budget
Get PDFThe relevance of charge exchange processes for the evaporation of neutral gases out of planetary gravitational fields has been known for quite a long time. For the planetary escape, however, collisions of the escaping constituent with both neutral and ionized atmospheric species operate in building up the escape flux. For the Earth, hydrogen collisions with O-atoms and O+- and H+- ions lead to about comparable contributions to the hydrogen escape at heights below 800 km, whereas at larger heights only charge exchange collisions need to be considered. In the present work, the lower region is therefore described by particle and energy flux conservation concepts, whereas in the upper region free-flight kinetic concepts are used. We give solutions for the height profiles of the hydrogen density, temperature and escape flux. The inclusion of charge exchange collisions increases the scale height, and consequently a smaller decrease of the hydrogen density is given. The hydrogen temperature, however, is reduced by including this effect. The charge exchange processes contribute about 70% to the total hydrogen escape which is of the order of a few 107 atoms/cm2 s at the plasma pause.
ARK: https://n2t.net/ark:/88439/y085188
Permalink: https://geophysicsjournal.com/article/14
 
The relaxation of spherical and flat Maxwell Earth models and effects due to the presence of the lithosphere
Get PDFThe radial surface deflection of a spherical Earth model appropriate to time scales characteristic of glacial loading is compared with the associated half-space response. For that purpose the analytical solution for a Maxwell sphere surrounded by a thick elastic shell is derived under the assumption of incompressibility. The half-space approximation is deduced as a special case directly from the spherical solution. Comparison of the response spectra, for different thicknesses of the elastic shell, reveals only minor differences. In the spatial domain, the half-space approximation may, nevertheless, diverge significantly from the spherical solution. For a disk load radius R = 800 km (Fennoscandia), the half-space approximation is adequate, whereas it is usually inappropriate if R = 1600 km (Laurentia). The sensitivity of the response to the thickness of the elastic shell is also investigated. For R = 800 km, the surface deflection in the central region below the load is fairly diagnostic of the shell thickness. If R = 1600 km, the peripheral region is more sensitive to this parameter.
ARK: https://n2t.net/ark:/88439/y005336
Permalink: https://geophysicsjournal.com/article/210
 
The electrical conductivity of the Andean crust in northern Chile and southern Bolivia as inferred from magnetotelluric measurements
Get PDFThe subducted Nasca plate produces anomalies of spectacular low electrical resistivity beneath the Andean knee. This is the result of two magnetotelluric field campaigns in 1982. The measurements were carried out on a profile from the Pacific coast to the Eastern Cordillera crossing northern Chile and southern Bolivia. Several anomalies may be distinguished. The strongest anomaly has been detected beneath the Pre- and West-Cordillera, with resistivities around 0.5 Ωm at depths greater than 5-10 km, strike direction roughly parallel to the main crest of the High-Cordillera. But also, the Altiplano is underlain – at a depth of 40-50 km – by very low-resistive material. In agreement with other geophysical and geological observations, we interpret these low resistivities as being due to the partially melted state. Further arguments identify the Pre-Cordillera anomaly, probably, with a plutonite which is still "alive", and the highly-conductive material 40 km beneath the Altiplano with the source of andesitic magma. A crucial point in this study is the distinction between the E- and B-polarization of the apparent resistivities. We distinguished both polarizations by the direction of the induction arrows and by the lateral continuity of the apparent resistivity.
ARK: https://n2t.net/ark:/88439/y029284
Permalink: https://geophysicsjournal.com/article/242
 
Long-term variations recorded by extensometers
Get PDFOn the basis of stress accumulation between two large earthquakes the maximum strain rate can be estimated. But the strain rates cannot be used to estimate directly the magnitude of the tectonic processes. The estimates of stress rates and horizontal crustal movement values are bigger than the real values in nature. Due to the significant differences among stations situated in the same region it seems that secular rates are modified or increased to a considerable extent by local conditions of different type, which are not connected to tectonic activity.
ARK: https://n2t.net/ark:/88439/y092577
Permalink: https://geophysicsjournal.com/article/15
 
The relationship between the magnetic anisotropy and the c-axis fabric in a massive hematite ore
Get PDFPreferred orientation of hematite ore from Minas Gerais, Brazil, was investigated by reflected-light microscopy, X-ray structural goniometry and magnetic anisotropy. A close relationship was found between c-axis fabrics determined by magnetic and non-magnetic methods; experiments confirmed the results of the theoretical treatment. For routine work it is advantageous to use both types of methods, profiting from rapidity of measurement of magnetic anisotropy and from detailed c-axis pole figures of pilot specimens provided by X-ray goniometry.
ARK: https://n2t.net/ark:/88439/y048352
Permalink: https://geophysicsjournal.com/article/244
 
Pc5 pulsations and their possible source mechanisms: a case study
Get PDFDuring the interval 0400-1600 UT on August 31, 1978 strong pc5 activity was observed in the morning and afternoon sector of the magnetosphere. Using data from a world-wide network of ground-magnetometer stations and from the geostationary satellites GEOS 2, GOES 2 and 3 as well as the satellite pair ISEE 1 and 2 a case study was performed with special respect to the question of possible source mechanisms responsible for the observed pulsations. Like earlier workers we came up with the result that the Kelvin-Helmholtz instability at the magnetopause or in the low latitude boundary layer is a likely candidate. In particular we found a change in the azimuthal phase propagation direction from westwards on the morning side and eastward on the afternoon side. Also the sense of polarization and the azimuth of the ground magnetic disturbance in the horizontal plane changed across the meridian of the stagnation point as predicted by a Kelvin-Helmholtz instability source mechanism. A more detailed analysis was carried out for an isolated, large amplitude pc5 event at 1040 UT. At the same time magnetic field observations from the ISEE satellite pair indicate a flux transfer event (FTE) like disturbance at the magnetopause. Also indications of magnetopause boundary oscillations were found, and we feel that this FTE-like event constitutes a possible source for the observed impulsive pc5 event recorded on the ground and on GEOS 2. Furthermore, as the observed pc5 pulsations exhibit a wave-packet structure, we studied whether these wave packets coincided with substorms or substorm intensifications observed simultaneously in the nighttime magnetosphere. However, only a partial one-to-one correlation was found.
ARK: https://n2t.net/ark:/88439/y054611
Permalink: https://geophysicsjournal.com/article/205
 
Magnetometer array studies in Finland - determination of single station transfer functions
Get PDFIn 1981 and 1982 four arrays of 30 or 31 magnetometers were operated on the Baltic Shield in central and south-eastern Finland to measure the natural magnetic field variations. These measurements were used to deduce some information about the lateral variation of the electrical conductivity within the Earth's crust. The stations were situated between latitudes 56° and 64° geomagnetic north. As substorms often extend over this area, most magnetic disturbance events have strong external spatial gradients and are not suitable for determining the electrical conductivity distribution inside the Earth. Some magnetic disturbance events with only smooth external spatial gradients could be selected and used for further analysis. For 11 of these events (2–6 h long), the horizontal spatial wavenumber k has been calculated. The product of the wavenumber k and the inductive scale length C was then used as an acceptance criterion and as a weighting function in the calculation of single station transfer functions. Most of the data were not acceptable for the criterion k · |C| < 0.3 for periods longer than 500 s. Because of the small number of acceptable data the statistical significance was not sufficient for all sites. Despite these problems induction vectors and conducted hypothetical vertical field maps could be used to locate conductivity anomalies. Intensive induction was found in three zones in the area under investigation.
ARK: https://n2t.net/ark:/88439/y036023
Permalink: https://geophysicsjournal.com/article/223
 
Static deformations and gravity changes at the Earth's surface due to atmospheric loading
Get PDF 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, however, 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.
ARK: https://n2t.net/ark:/88439/y011765
Permalink: https://geophysicsjournal.com/article/150
 
Magnetic expression of the continent-ocean boundary between the western margin of Australia and the eastern Indian Ocean
Get PDFA comprehensive review of the Early Cretaceous seafloor-spreading magnetic anomalies (M0 to M10) in the eastern Indian Ocean leads to the isolation of a distinctive magnetic anomaly at the continent-ocean boundary (COB). This anomaly is traceable 2000 km southward from the rifted margin of the magnetically smooth central Exmouth Plateau, through the transform-faulted and rifted margins of the Cuvier Abyssal Plain and Carnarvon Terrace and the set of narrow spreading segments south of the Zenith-Wallaby Fracture Zone to the area west of Perth. The anomaly corresponds to the COB as indicated by: 1. the lower part of the continental slope in a mean water depth of 3.75 km for the rifted margin and 4.5 km for the transform-faulted margin and 2. a change in seismic-reflection character from the faulted breakup unconformity on the continent to the smooth but hyperbolic oceanic layer 2. The COB anomaly at the rifted margin is modelled by modifying the magnetization of the oldest oceanic block of the seafloor-spreading sequence adjacent to the continental crust; in places, the COB anomaly is flanked by smaller anomalies modelled as rift-related dykes in the adjacent continental crust. The amplitude of the COB anomaly, commonly twice or more that of the adjacent oceanic magnetic anomalies, is due either to a thicker or a more intensely magnetized source. The COB anomaly at the transform-faulted margin is modelled by a thick vertical body that extends 10 km seaward of the COB. The Wallaby Plateau is probably underlain by oceanic crust, as shown by the continuity of the abandoned spreading ridge of the Sonne Ridge southwestward from the Cuvier Abyssal Plain; the shape of the Wallaby Plateau and the volcanic composition of dredge-hauls indicate that it is probably a volcanic upgrowth of the oceanic crust as exemplified by Iceland today. Furthermore, like lceland, the Wallaby Plateau is crossed by magnetic anomalies that are possibly degraded seafloor-spreading anomalies. The Zenith Plateau lacks magnetic lineations and its crustal structure, like that of the Naturaliste Plateau to the south remains unknown.
ARK: https://n2t.net/ark:/88439/y098447
Permalink: https://geophysicsjournal.com/article/239