Showing papers on "Magnetic dip published in 1983"

Linear filtering of VLF dip-angle measurements

Abstract: The suggested interpretation technique is based on discrete linear filtering of VLF data. The output of the described filtering results is expressed in terms of an equivalent current density at a specified depth that would cause the measured magnetic field. The most practical six-point filter gives an accuracy of 8%. The filter is an extension of the commonly used Fraser filter to process VLF dip-angle data. Filtering the same data set for various depths gives an idea about the change of current density with depth. Areas with high current-density correspond to good conductors. The conductor dip can also be determined. The use of the technique is illustrated on theoretical and field examples. In all cases a good correlation with original models and other types of geophysical measurements was obtained. As shown in the last example, the filtering technique is also applicable in interpretation of other electromagnetic methods.

Spread F plasma bubble vertical rise velocities determined from spaced ionosonde observations

Abstract: Systematic time differences in the onsets of spread F events in the ionograms are observed between the magnetic equatorial station Fortaleza (4°S, 38°W, dip latitude 1.8°S) and the low-latitude station Cachoeira Paulista (23°S, 45°W, dip latitude 14°S), two stations in Brazil, located at close-by magnetic meridional planes (actually some 12° of magnetic longitude apart). On the assumption, justified from different experimental observations, that the spread F irregularities occur in strongly field-aligned plasma bubbles that extend several degrees on either side of the magnetic equator, and rise up in vertically elongated columns over the magnetic equator, we have related the observed time differences in the onsets of spread F events at the two stations to the plasma bubble vertical rise velocities over the magnetic equator. The vertical rise velocities of the plasma bubble so determined are found to be well within the values measured by VHF radar and satellite techniques, and further show, at times, good correlations with the amplitude of the prereversal peak in the vertical drift velocities and the heights of the evening equatorial F layer. Possible implications of these results are discussed.

Role of Shallow Phase Changes in the Subduction of Oceanic Crust

Abstract: Detailed studies of the seismicity of several subduction zones demonstrate that shallow-dipping thrust zones turn to steeper angles at depths of about 40 kilometers. An increased downward body force resulting from shallow phase changes in subducted oceanic crust may be the cause of this increased dip angle. In addition, the volume reduction associated with phase changes may produce sufficiently large stresses in neighboring rocks to cause the seismicity of the upper Benioff zone.

Double-planed structure of intermediatedepth seismic zone and thermal stress in the descending plate

Abstract: Recent seismic studies using a high-gain seismograph network have demonstrated the existence of a double-planed seismic zone in the descending plate beneath island arcs such as northeastern Japan, Kurile, and Central Aleutian. Several hypotheses in terms of plate unbending, phase changes, mechanical models have been proposed to explain the characteristic features of the double-planed structure. This paper presents a new hypothesis that thermal stress due to non-uniform temperature distribution in the descending plate is the main causative force for genesis of earthquakes in the double-planed seismic zone. Based on the estimated temperature distribution in the plate with a dip angle θ and a convergence velocity Vc, the thermal stress is calculated analytically under several assumptions. According to the results of these calculations, the upper and lower parts of the plate are characterized by compressional stress, and the central part by tensional stress. This stress pattern is well consistent with the focal mechanism solutions of earthquakes in the two planes of seismic zone. To verify out hypothesis quantitatively, a new parameter R, defined as the ratio of deviatric stress to the mean normal stress at a depth, is introduced as an index of the possibility of earthquake occurrence. In the case of the descending plate, for example, beneath northeastern Japan (θ=30°, Vc=8 cm/yr), two regions with R≥0.04 exist at the uppermost and central parts of the plate. These regions are parallel to each other with a distance of about 30km. The upper and central regions are characterized by compressional and tensional deviatric stress, respectively. These regions terminate at a depth of about 250km The above features explain the observed seismic activity under the northeastern Japan arc. This value of R=0.04 is not too different from the data of rock fracture experiments at high temperature and pressure. The value of R at the center of the plate is the largest in the case of θ=20-30° and Vc≥3cm/yr and decreases with increasing dip angle.

Magnetic exploration with reduction of magnetic data to the equator

Abstract: A method of reducing magnetic data recorded at latitudes close to the magnetic equator to the equator is disclosed. The result corrects magnetic field anomalies recorded with respect to geologic anomalies to more closely reflect the shape of the geologic anomalies. In the preferred embodiment, reduction-to-the-equator is carried out in the wave number domain using an operator ##EQU1## .