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Geoelectrical deep soundings with electrode spacings between 30 and 1250 km have been carried out from 1973 to 1975 in southern Africa using the Cabora Bassa power line. At several sounding stations (Schlumberger and bipole-dipole arrays) the electric potential differences have been recorded in different directions. For short current-electrode spacings (< 200 km) the direction of the electric field is nearly perpendicular to the direction between the current electrodes whereas it becomes more or less parallel for large electrode spacings (> 400 km). A possible cause for this rotation of the field may be a dyke system observed in the granitic crust.
ARK: https://n2t.net/ark:/88439/y065100
Permalink: https://geophysicsjournal.com/article/216
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Investigations of the core reflection PcP at distances in the neighbourhood of the core shadow zone are especially appropriate for the study of isolated regions of D" because PcP for these distances has a large angle of incidence at the core-mantle boundary and is strongly influenced by the structure immediately above the core. A striking result of this investigation of PcP in the distance range 70°–84° is that PcP could be detected only in four cases (from a total of 16 earthquakes and 29 nuclear explosions) in the NORSAR- and GRF-array beams for PcP. This result is found although the P-wave-coda noise levels in the PcP beams are very low, mostly below the amplitude level of PcP predicted by standard Earth models. In the cases where PcP could not be identified, the PcP/P amplitude ratio, which is estimated from the beams with the aid of a cross-correlation procedure, must be regarded as an upper limit for the actual PcP/P ratio. The interpretation of the PcP/P amplitude ratios with one radially symmetric, elastic model proved to be impossible. The generally small PcP/P ratios can be explained by lateral variations in seismic wave absorption within D". Qα values for short-period P waves in D", between 800 (below Usbekistan) and 100 (below the northern Hudson Bay and Central Siberia), were found. Several recent seismological investigations of the P- and S-wave velocity of the lowermost mantle have suggested velocity models with first-order discontinuities (with velocity increases of the order of 1.5%–3.0%) 150–300 km above the core-mantle boundary. Through the calculation of synthetic short-period seismograms and comparison with the array data compiled for the main part of this study, it is shown that such models are very unlikely to be a global feature if the P velocity jump is of the order of 2.5%–3.0%. Even models with reduced P velocity jumps of 1.3% produce clear onsets in short-period seismograms. They are not observed for P waves with ray paths bottoming beneath Central Siberia.
ARK: https://n2t.net/ark:/88439/y055861
Permalink: https://geophysicsjournal.com/article/215
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The relocation of a number of large shocks and the estimation of their fault-plane dip, along with seismotectonic evidence, allow some conclusions about the mechanisms of Friuli (NE Italy) earthquakes. The largest shocks of the 1976 sequence are located in an area with an ESE-WNW trend, extending on both sides of the Tagliamento River. The May earthquakes occurred over the whole area, while the September aftershocks were concentrated in the centre of this area. The 1977-1979 events are spread over a wider zone. These shocks can be ascribed mainly to Dinaric structures. The hypocentre distribution of the largest 1976 events shows that the stress involved deeper crustal sectors in the south of this area (about 8 km) than in the northern part (where it does not exceed 6 km). The motion occurred either in the south along gently dipping planes, which probably correspond to the detachment surface between the post-Hercynian cover and the Paleozoic basement, or in the north along more steeply dipping planes (thrusts) within the Mesozoic cover, along with triggering and rearrangement in the shallower Alpine and/or Dinaric structures.
ARK: https://n2t.net/ark:/88439/y045812
Permalink: https://geophysicsjournal.com/article/214
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Similar three-component microearthquake records have been observed in the Swabian Jura (SW Germany) seismic zone for different source-receiver geometries. This data set is used to study the resolution power of cross spectral analysis techniques for the estimation of relative differential times as well as the applicability to velocity monitoring. The differential times are estimated in the frequency domain by assuming a linear-phase cross spectrum with the slope indicating the individual time difference. All earthquakes have been relocated with respect to a master event, using the relative P and S delay times from the cross spectral analysis as a measure of source mislocation. The overall location error is strongly dependent on the inital distance between master and studied event. For earthquakes initially located farther apart than approximately 1.5 km, the relocalization result in terms of total location error was poorer, whereas for events initially located closer than 1 km the precision of the relocalization was improved. The remaining residuals are of the order of 10 ms, which is approximately 3 times the digitization interval. In order to test the applicability of cross spectral analysis to velocity monitoring, synthetic data were used to model the influences of noise and source time function differences. The effect of additive white noise seems to be acceptable in cases where the S/N ratio is sufficiently high. Small changes in the shape of the source time function, however, were found to be of great influence to the differential time estimates. Variation of rise, sustain and decay times, which were negligible in the coherence spectrum, spuriously introduced phase differences which, in terms of delay times, easily reach the magnitude of the digitization interval. Thus, velocity monitoring using cross spectral analysis techniques seems to depend strongly on the equality - in contrast to similarity - of the source time functions of the events which are compared. The coherence spectrum is not a sufficient measure to detect all the significant differences.
ARK: https://n2t.net/ark:/88439/y035103
Permalink: https://geophysicsjournal.com/article/213
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Domain structure observations and measurements of the temperature dependence of susceptibility on synthetic and natural titanomagnetites are reported. At room temperature Ti-rich titanomagnetite particles (x ≈ 0.6) of MD grain size normally develop a very complicated domain structure. The experimental results of our investigations, in addition to simple theoretical calculations, indicate that internal stress is the dominant cause of the observed anomalous domain patterns. Part of the results have already been published by Appel and Soffel (1984). The paper presented here, however, is a far more extended summary of the actual state of our research.
ARK: https://n2t.net/ark:/88439/y026104
Permalink: https://geophysicsjournal.com/article/212
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A pulsation event has been studied which showed Ps6-type characteristics with a period of about 20 min. Data from a number of magnetometer and riometer measurements over a large area have been combined with balloon measurements of the electric field to obtain a detailed description of the ionospheric current system during the event. The pulsations reported here occured over an interval longer than 3 h and showed large magnetic variations, up to 700 nT. A model of the three-dimensional current flow has been derived which agrees with the data. It is concluded that most of the magnetic disturbance measured on the ground is caused by Hall-currents.
ARK: https://n2t.net/ark:/88439/y016745
Permalink: https://geophysicsjournal.com/article/211
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The 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
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The compressional and shear wave anisotropy coefficients of 33 minerals of igneous and metamorphic rocks were calculated from published elastic constants and tabulated together with the orientation of velocity extremes in single crystals. The most abundant minerals of crustal crystalline rocks - alkali feldspars, plagioclases, quartz, micas and hornblende - have higher anisotropy coefficients than the upper mantle minerals - olivine, pyroxenes and garnets. Due to the orientation of mineral grains and their velocity extremes in a stress field, however, the olivine-rich ultramafites belong to the most anisotropic rocks and, in contrast, the crack-free anisotropy of crustal crystalline rocks is generally low, with the exception of metamorphic rocks rich in micas, hornblende and calcite.
ARK: https://n2t.net/ark:/88439/y095987
Permalink: https://geophysicsjournal.com/article/209
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The substorm on 2 March 1978 was selected for study as a relatively weak substorm, starting at about local magnetic midnight, that could be observed with instruments in Northern Scandinavia. The analysis is based on a comparative study of data from the IMS magnetometer network, all-sky cameras, pulsation magnetometers, and riometers in the Scandinavian area. In addition other data are used to support the results, e.g., a photograph from the DMSP-F2 satellite, showing the auroral situation over Scandinavia, and further west, immediately after the substorm onset. The substorm was preceded by a weak activation of aurora and magnetic disturbance about 3 min before the onset. After a fading that lasted for 20 s and could be observed only in optical aurora, the substorm onset led to a strong brightening of the aurora, an enhancement of the westward electrojet, a sudden rise in the ionospheric D-layer absorption, and Pi B type pulsations. Immediately after the onset, the ground magnetic data suggest the appearence of a pair of oppositely directed, localized, field-aligned currents (FACs). The main development of the signatures of the downward FAC was clearly delayed by about 3 min. There were significant correlations between the magnetic signatures of the two FACs and different features and spectra of the optical aurora, both in time and location. The observed Pi B type pulsations lasted as long as a growth in the local onset-connected FACs could be inferred. Within the first three minutes the localized three dimensional current system developed into a more sheet-like configuration. An expansion to the west, possibly accompanied by a westward travelling surge, was traced with riometers and magnetometers on Iceland and Greenland.
ARK: https://n2t.net/ark:/88439/y084828
Permalink: https://geophysicsjournal.com/article/208
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The 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
