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The response at the surface of a layered transversely isotropic medium due to a buried dislocation source can be expressed by using propagator matrices and discrete wavenumber summation. These operations produce complete seismograms for earthquake or explosion sources which include all body- and surface-vave phases for this specialized anisotropic structure. In order to test the numerical procedures, synthetic seismograms at near distances for an isotropic model are compared with those generated by other methods. The agreement is found to be satisfactory in all cases. Comparisons of synthetic seismograms for anisotropic models having a small degree of anisotropy with similar but isotropic models, show that significant differences in travel times, amplitudes and wave forms can be caused by the anisotropy.
ARK: https://n2t.net/ark:/88439/y066890
Permalink: https://geophysicsjournal.com/article/226
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Clear P-to-S converted phases are observed in teleseismic seismograms recorded at the Grafenberg array. The seismic events used are nuclear explosions from east Kasakh and deep-focus earthquakes from the region of Japan and central Asia. The identification of the converted phases has been improved by the rotation of the coordinate system. Theoretical seismograms have been computed to obtain the response of various velocity structures of the lithosphere under the receivers, using the reflectivity method of Kind (1985) for different source and receiver structures. The interpretation of the P-to-S conversions leads to a model of the crust-mantle transition zone with a first-order discontinuity and a normal velocity contrast in the north, an increased velocity contrast in the centre, and a reduced sharpness of the discontinuity in the south of the array. The sedimentary layers underneath the stations also cause strong converted phases, and they also influence strongly the waveform of the incoming P wave.
ARK: https://n2t.net/ark:/88439/y056921
Permalink: https://geophysicsjournal.com/article/225
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The analysis of mislocation patterns and the three-dimensional inversion of travel-time residuals for P waves measured at the GRF array reveal the existence of strong lateral velocity variations beneath the array. The most expressive phenomenon consists of an increase of P-wave velocities in the upper mantle from north to south, in addition to a possible thickening of the lithosphere to the south; especially the Moldanubian part of the Bohemian massif located to the southeast of the GRF array is characterized by high P-wave velocities in the upper mantle. The systematic change of the magnitude of the residual variation across the array, depending on the incidence angle for P waves, leads to the conclusion that a low-velocity zone exists in the upper mantle to the northeast of subarray A. The appearance of low-velocity material in the vicinity of the border between the two tectonic units, namely the Saxothuringian zone to the north and the Moldanubian zone to the south, might be connected to the deep structure of the graben area which extends to the northeast into the Egergraben.
ARK: https://n2t.net/ark:/88439/y046952
Permalink: https://geophysicsjournal.com/article/224
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In 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
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Anisotropy and intensity variations of high-energy particles and magnetic variations detected by IMP-J at 37 Re in the central part of the magnetotail within 2 Re of the neutral sheet were studied during a few consecutive substorms on March 3, 1976, and related to a large body of ground observations. A close correlation is usually found between bursts observed in Pi pulsations on the ground (having a duration of about 1 min and usually being repeated in 1–3 min) and high-energy particle bursts observed in the far tail. The magnetic field response is examined relative to the onsets of high-energy particle bursts. A three-dimensional current system RIPD (Reconnection Induced Propagating Disturbance) is constructed which can, in principle, produce the magnetic field variations observed in the boundary part of the plasma sheet (PS). Previously reported properties of high-energy particle bursts (inverse energy dispersion, preferential acceleration of alpha particles, dawn-dusk asymmetry in the acceleration of electrons and protons and transient PS expansions) are found to be typical of these impulsive processes. Together with earlier results, these observations show that the expansion process (identified here as a transient reconnection or explosive tearing mode) clearly operates in an impulsive fashion. The superposition of impulse-induced propagating disturbances necessarily results in complex, variable patterns of magnetic field and thermal plasma behaviour, such as are frequently found in the PS during substorms.
ARK: https://n2t.net/ark:/88439/y027754
Permalink: https://geophysicsjournal.com/article/222
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At least a dozen well-defined activations were recorded with high spatial resolution during the first 25 min of a substorm commencing at 1959 UT on 3 March 1976. The activations were determined by Pi1 and 2-type magnetic pulsations, magnetic variations and cosmic noise absorption. The activations exhibited differences in intensity, spatial extent and the accompanied auroral behaviour. In all cases but one an impulsive counterclockwise differential equivalent current vortex was observed superimposed on the continuously growing westward electrojet. The high-energy particle precipitation reached its maximum in these vortex regions. In cases where the activations observed on the ground occurred close to the footpoint of the IMP-J satellite, it registered simultaneously (within 2 min) burstlike enhancements of the high-energy particle fluxes. The satellite was at these times 37 RE away from the Earth in the plasma sheet boundary layer. Large, practically isotropic high-energy electron fluxes detected at the satellite during the substorm expansion suggested the existence of a large-scale magnetic loop structure in the far magnetotail. The results give evidence of a time-varying dissipation process operating in an impulsive manner in separate regions of the plasma sheet (within a few RE). According to the magnetic variations at mid-latitude, the intensity of the cross-tail current disruption (or the substorm current wedge) does not respond to these activations in the same impulsive manner, but seems to integrate their effects.
ARK: https://n2t.net/ark:/88439/y017215
Permalink: https://geophysicsjournal.com/article/221
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The anisotropy of magnetic low-field susceptibility in the Triassic Plattensandstein formation (Upper Buntsandstein) from northern Bavaria has a typical sedimentary fabric. The anisotropy ellipsoids are strongly oblate with minimum susceptibility axes normal to sedimentary bedding. The directions of the maximum susceptibility axes are consistent with the NNE-NE-trending general sediment transport direction that is derived from geological field observations of cross-bedding structures in the sandstones. However, the very small intensity differences between maximum and intermediate susceptibility require extremely sensitive measurement techniques. Comparative measurements were made with a spinner magnetometer, a cryogenic magnetometer and a susceptibility bridge. Directionally, the most consistent results were obtained with the spinner magnetometer after it was stabilized by means of a low-pass active filter. The directional consistency of the anisotropy principal axes can be improved further by annealing the sandstones at 750° C in air. During this treatment a strongly magnetic, low-coercivity mineral phase - probably magnetite - is formed which enhances the degree of magnetic anisotropy as well as the bulk susceptibility. Low temperature measurements indicate that, in the natural unheated state, paramagnetic minerals contribute substantially to the low-field susceptibility of the sandstones at room temperature.
ARK: https://n2t.net/ark:/88439/y006806
Permalink: https://geophysicsjournal.com/article/220
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On 2 December 1977, 1600-1700 UT (around 19 MLT) a section of the Harang discontinuity moved westward over northern Scandinavia with a velocity of about 1 km/s, during a disturbed time interval. The westward movement was clearly identified in the IMS Scandinavian Magnetometer Array observations and is consistent with simultaneous ST ARE electric field measurements. The magnetic measurements showed a marked increase of the overhead current density during the westward movement while the electric field amplitudes stayed at the same level, thus indicating a temporal enhancement of the ionospheric conductivity. The westward movement gave the opportunity to combine subsequently observed two-dimensional distributions of equivalent current vectors (after correction for the magnetic field intensity increase) and electric field vectors into two corresponding patterns fairly extended in the east-west direction and covering the total extent of the most interesting part of the Harang discontinuity region. The electrically defined Harang discontinuity was located 200-300 km to the north of the magnetically defined discontinuity. The combined observational data were used to determine, as far as possible, the ionospheric distributions of height-integrated conductivities, of horizontal height-integrated current density vectors and of field-aligned currents. The results show a reduction of the Hall conductivities and upward Birkeland currents at and around the electrically defined Harang discontinuity. Some horizontal ionospheric current crosses the discontinuity, being diverged northward from the eastward electrojet and joining the westward electrojet, after further counterclockwise rotation.
ARK: https://n2t.net/ark:/88439/y096847
Permalink: https://geophysicsjournal.com/article/219
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This paper discusses cruxes of the method for inverting the P-time residual data introduced by Aki et al. (1976a) and summarizes the results obtained by the method on 3-dimensional seismic velocity anomalies in the lithosphere under several seismic arrays around the world. The velocity anomalies at shallow depths correlate well with geologic features in young, active areas such as California, Hawaii, and Yellowstone, but the correlation is not apparent in old, stable areas such as eastern Montana and Norway. Significant small scale (20~50 km) lateral inhomogeneity is observed everywhere to the depth of 100~150 km, with the minimum estimate of root mean square fluctuation about 3%. The lithosphere-asthenosphere boundary seems to manifest itself as change in the roughness of anomaly pattern or in the trend of anomaly.
ARK: https://n2t.net/ark:/88439/y086778
Permalink: https://geophysicsjournal.com/article/218
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Applications of the ray method to the construction of theoretical seismograms for laterally varying layered structures are discussed. Numerical examples are presented. It is shown that the refracted waves are very sensitive to the curvature of interfaces. Certain modifications and improvements of ray theoretical seismograms to increase their accuracy are suggested.
ARK: https://n2t.net/ark:/88439/y076019
Permalink: https://geophysicsjournal.com/article/217
