Journal of Geophysics
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Simple plate models of mantle convection
Get PDFA 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
 
Lithospheric slab penetration into the lower mantle beneath the Sea of Okhotsk
Get PDFA 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
 
Palaeomagnetic data from the central part of the Northern Calcareous Alps, Austria
Get PDFPalaeomagnetic data from the Osterhorngruppe in the Northern Calcareous Alps (NCA) southeast of Salzburg are presented. The investigations were concentrated on the red nodular Adneter Kalk (limestone) of Liassic age which carries a stable natural remanent magnetisation. Rockmagnetic investigations revealed magnetite as the carrier of the remanent magnetization. The obtained palaeomagnetic results do not contradict a northward shift of the NCA of the order of several hundred kilometres and indicate a clockwise rotation of about 45° of the Northern Calcareous Alps with respect to Eurasia, since the Jurassic. Time of possible shift and rotation are briefly discussed.
ARK: https://n2t.net/ark:/88439/y040372
Permalink: https://geophysicsjournal.com/article/256
 
Observation of PS reflections from the Moho
Get PDFFuchs (1975) has described the theory and computation of synthetic seismograms for PS reflections from first order discontinuities and transition zones with various properties. A number of record sections were shown in which PS reflections were not discernable. In the course of a programme of explosion work which has been carried out around Scotland (SOSP, outlined by Jacob, 1974) a line of shots fired in the North Minch was observed by a station, WR, situated at Cape Wrath (Fig. 1). Though sub-critical PmP Moho reflections were seen, which indicated that PS reflections from the Moho might be expected, a section of vertical component seismograms (Fig. 2) gave only a weak indication of the phase. However, further processing of the data has shown a clear PS Moho reflection to be present.
ARK: https://n2t.net/ark:/88439/y012585
Permalink: https://geophysicsjournal.com/article/273
 
Elastic wave propagation in a highly scattering medium - a diffusion approach
Get PDFThe principle of conservation of energy, in the form of the equation of radiative transfer, is used to treat the case of strong scattering of elastic waves. If the medium is isotropic, if all the energy present has been scattered many times, and if the time and distance scales of the problem are long compared to the time and distance scales of the scattering process, then the average flow of energy is described by the diffusion equation with an additional term representing linear dissipation to heat. Model seismic experiments using holes drilled in aluminum plates as scatterers confirm the applicability of the formalism. The diffusion formalism qas been successfully applied to lunar seismograms and to some Earth data. The results of studies of lunar seismograms show that the zone of strong scattering on the Moon is confined to a near surface zone.
ARK: https://n2t.net/ark:/88439/y064280
Permalink: https://geophysicsjournal.com/article/102
 
Digital recording and analysis of broad-band seismic data at the Graefenberg (GRF) array
Get PDFAn array of 19 broad-band seismometers with flat velocity response between 0.05 Hz and 5 Hz combined with a binary gain-ranging data acquisition system of 138 db dynamic range is being installed in Southern Germany. The array has a maximum extension of 80 km, it consists mainly of three triangular subarrays, the dimensions of which are chosen so as to reduce the microseisms by simple summation. The dynamic range is accomplished by digitizing the output at the seismometer sites and transmitting the digital information on telephone lines to a central control station. Single station data are available since 1975 and continuous recordings of the first subarray exist since 1976. They provide extended information of the fine structure of the seismic wave field in the medium frequency range between 0.5 Hz and 0.05 Hz and allow the quantitative investigation of frequency-dependent seismic phenomena over a wide range of amplitude and frequency. An elementary problem is the numerical analysis of broad-band seismograph systems especially seismometer-galvanometer systems and the determination of the true ground displacement or velocity. This simulation and restitution problem can be solved by digital recursive filters applying the z-form method. The simulation may include high sensitive short-period detection instruments.
ARK: https://n2t.net/ark:/88439/y012005
Permalink: https://geophysicsjournal.com/article/127
 
Detection of meridional currents in the equatorial ionosphere
Get PDFTheoretical investigations predict meridional currents in the dynamo region of the ionosphere a few degrees north and south of the magnetic equator as a consequence of the equatorial electrojet. The magnetic fields of these currents are perpendicular to the permanent magnetic field of the Earth and cause a height variation of the magnetic declination. It was the aim of six rocket launches performed in 1970 near noon at Natal, Brazil, to detect these currents. To reach this objective, identical payloads each consisting of two flux-gate magnetometers, a solar aspect sensor, an experiment to measure the angle between the magnetic field and the direction to the Sun, and an impedance probe to determine the electron density were launched under different magnetic conditions. The predicted variation of the declination has been observed but with considerably higher amplitude than was expected. On the other hand, the measured height integrated current density in the west-east direction was smaller than deduced from ground based magnetic H-variations. These discrepancies can be explained by currents flowing at 5 degrees off the magnetic equator, on both sides, with intensities of about 0.3 of the electrojet intensity at the same height but in reversed direction. Such reversed currents have recently been observed from ground based magnetic observations by others and have also been interpreted theoretically by ionospheric wind effects.
ARK: https://n2t.net/ark:/88439/y001006
Permalink: https://geophysicsjournal.com/article/272
 
Pseudo-single-domain effects and single-domain multidomain transition in natural pyrrhotite deduced from domain structure observations
Get PDFThe domain configuration of primary pyrrhotite in a Devonian diabase was studied using the Bitter pattern technique. Due to the uniaxial symmetry the multidomain grains have a rather simple domain structure. The single-domain — multidomain transition occurs at an average particle diameter of 1.6 μm. In the multidomain grains clusters of inclusions seem to produce pseudo-single-domain effects with complicated domain configurations. Such pseudo-single-domain effects are necessary for the interpretation of the magnetically hard component of remanence which cannot be explained by the observed abundance of true single-domain particles alone.
ARK: https://n2t.net/ark:/88439/y061450
Permalink: https://geophysicsjournal.com/article/91
 
Radial variation of the interplanetary magnetic field between 0.3 AU and 1.0 AU - observations by the Helios-1 spacecraft
Get PDFWe have investigated the radial dependence of the radial and azimuthal components and the magnitude of the interplanetary magnetic field obtained by the Technical University of Braunschweig magnetometer experiment on-board of Helios-1 from December 10, 1974 to first perihelion on March 15, 1975. Absolute values of daily averages of each quantity have been employed. The regression analysis based on power laws leads to 2.55 γ x r-2.0 , 2.26 γ x r-1.0 and F = 5.53 γ x r-1.6 with standard deviations of 2.5 γ, 2.0 γ and 3.2 γ for the radial and azimuthal components and magnitude, respectively. Here r is the radial distance from the Sun in astronomical units. The results are compared with results obtained for Mariners 4, 5 and 10 and Pioneers 6 and 10. The differences are probably due to different epochs in the solar cycle and the different statistical techniques used.
ARK: https://n2t.net/ark:/88439/y077849
Permalink: https://geophysicsjournal.com/article/113
 
Cosmic ray measurements on board Helios 1 from December 1974 to September 1975: Quiet time spectra, radial gradients, and solar events
Get PDFThe University of Kiel cosmic ray experiment on board Helios 1 measures nucleons above 1. 7 MeV/nucleon and electrons above 0.3 MeV in the inner solar system between 1.0 and 0.3 AU from the Sun. A first survey is given on quiet time proton and Helium spectra which are compared near Earth and close to perihelion. The anomalous Helium component is also present at radial distances within 0.4 AU. Quiet time Helium spectra from 3.8 to 48 MeV/nucleon gradually increase between December 1974 and June 1975. For the integral radial gradient (protons above 51 MeV) we estimate a value of (11±2.5) %/AU during a period of slowly increasing cosmic ray intensity. We discuss solar particle events on January 5 (at 0.93 AU), March 7 (at 0.41 AU), and March 19, 1975 (at 0.32 AU). The March 19 event was measured closer to the Sun than any other event before. It exhibits sharp temporal structures, differences in the time profiles of various particle species, and a large abundance of Helium 3, with a 3He/4He ratio of 2 to 3 in the range 5 to 7 MeV/nucleon. This event occurred close to the peak of a high speed solar wind stream.
ARK: https://n2t.net/ark:/88439/y032443
Permalink: https://geophysicsjournal.com/article/183