Journal of Geophysics
Not a member yet
262 research outputs found
Sort by
Ray theory and its extensions: WKBJ and Maslov seismograms
Get PDFAsymptotic ray theory can be used to describe many seismic signals. Provided the wavefronts and amplitudes vary smoothly and the correct phase changes are included for caustics and reflection/transmission coefficients, it successfully describes direct and turning rays, on normal and reversed branches with multiple turning points, and partial and total reflections and transmissions. Nevertheless, many exceptions occur. Critical points, head waves, interference head waves, Airy caustics, Fresnel shadows, edge, point and interface diffractions and gradient coupling are examples discussed in this paper. Asymptotic ray theory can be simply extended to cover some of these problems. In this paper, the extension called the WKBJ or Maslov seismogram is discussed.
ARK: https://n2t.net/ark:/88439/y086318
Permalink: https://geophysicsjournal.com/article/147
 
On the origin of the cusp field-aligned currents
Get PDFA complicated system of field-aligned currents is known to exist in the high-latitude region near noon, associated with the cusp. We suggest that the equatorward part of this system, referred to as the Region 1 field-aligned currents, is caused by the leakage of the field-aligned currents associated with the rotational discontinuities at the dayside magnetopause. The poleward part, referred to as the cusp field-aligned currents, is associated with the tail magnetopause. In this situation, it can be shown that the direction of the field-aligned currents at the magnteopause is controlled by the y-component of the interplanetary magnetic field (By). The poleward (equatorward) part of this field-aligned current system is found to flow out of (into) the northern polar ionosphere when By > 0 and into (out of) the northern ionsophere when By < 0. This current pattern reverses systematically in the southern polar ionosphere. Therefore, the suggested mechanism can explain qualitatively the observed changes of the cusp current systems. Further, the latitudinal width of the cusp field-aligned current system at the ionospheric altitude is estimated to be 100-400 km, consistent with observations.
ARK: https://n2t.net/ark:/88439/y057041
Permalink: https://geophysicsjournal.com/article/235
 
Wave propagation in multilayered media: the effect of waveguides in oceanic and continental Earth models
Get PDFWe investigate the effect of low-velocity waveguides on ground motion. The computation of eigenvalues and eigenfunctions of Rayleigh waves up to the frequency 10.0 Hz allows an analysis of the seismic response which is source independent. The main conclusions of this study are: (1) Extending the model to depths greater than that of the waveguides allows the exact computation of leaking modes. (2) A source in a layered structure generates P waves of higher frequency than S waves even if the structure is purely elastic. (3) At high frequencies (10.0 Hz) the modal components of P waves separate from those of SV waves. (4) Strong surface waves are generated by shallow sources in sedimentary basins, also at a source distance of a few tens of kilometres. These waves do not appear in structures without sediments. (5) The polarization of strong ground motion at the surface of low-velocity layers is mainly horizontal. (6) For oceanic models, the contribution of the sedimentary layers is separable from that of the water layer only at high frequencies.
ARK: https://n2t.net/ark:/88439/y014925
Permalink: https://geophysicsjournal.com/article/294
 
Focal parameters of some Friuli earthquakes (1976-1979) using complete theoretical seismograms
Get PDFThe 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
 
The normal modes of a layered, incompressible Maxwell half-space
Get PDFThe theory describing the relaxation of an incompressible, layered Maxwell half-space is developed. The approach is based on the analytic solution of the associated elastic model and the subsequent application of the correspondence principle. The viscoelastic theory follows normal-mode theory, which allows the independent and exact determination of the relaxation-time and amplitude spectra for each mode of relaxation. The solution is tested by calculating the response of several models in the wavenumber and spatial domains. The examples are selected with regard to postglacial adjustment in Fennoscandia and analyse effects caused by (a) varying lithospheric thickness, (b) adding an asthenosphere, (c) increasing lower-mantle viscosity, (d) permitting relaxation of the lower lithosphere or (e) introducing density contrasts at 400-km and 670-km depths.
ARK: https://n2t.net/ark:/88439/y058111
Permalink: https://geophysicsjournal.com/article/245
 
The formulae for the calculation of the Fresnel zones or volumes
Get PDFThe symmetrized invariant formulae for the calculation of Fresnel zones or volumes are derived. It is assumed that an inhomogeneous medium with curvilinear interfaces is located between the source and/or the receiver and along the central ray within the Fresnel zone or volume. In the vicinity of the zone centre, the medium is considered locally homogeneous. The formula for the leading term of the field of a wave scattered by a bent body immersed in the above-mentioned medium is obtained by the Kirchhoff approximation. With the help of this formula and the expressions for the Fresnel radii for a particular case, the formulae for the Fresnel zones in the general case considered are obtained on the basis of the reciprocity relation. The formulae for the Fresnel zones are used to obtain the expressions for the Fresnel volumes. The physical consequences of the derived formulae with respect to the validity of the ray formulae and the resolution of seismic methods etc. are discussed.
ARK: https://n2t.net/ark:/88439/y093067
Permalink: https://geophysicsjournal.com/article/292
 
Synthetic seismograms - the Rayleigh waves modal summation
No full textFrom the latest developments of algorithms for the computation of eigenvalues and eigenfunctions of Rayleigh waves for flat layered anelastic models of the Earth, it is possible to construct, with highly satisfactory efficiency and accuracy, "complete" synthetic seismograms also at high frequencies. Examples are given both for continental and oceanic structural models made up of 70 layers and more and extending to depths of about 1,100 km.
ARK: https://n2t.net/ark:/88439/y047312
Permalink: https://geophysicsjournal.com/article/67
 
The crustal structure of the southern Rhinegraben from re-interpretation of seismic refraction data
Get PDFThe Rhinegraben is one of the best studied rift structures in the world. In the past 20 years, it has been the site of many seismic refraction studies. However, most of the profiles have only been interpreted using 1-D methods. In this paper a combined 2-D and 1-D analysis of the only reversed profile within the graben proper is presented. The new interpretation has resulted in changes in the P-wave velocity model of the Rhinegraben. In the new model, the upper crust of the graben, representing the sedimentary graben-fill, is found to be 6-7 km thick with Vp less than 6.0 km/s. The mid-crust, which is probably of granitic/gneissic composition, shows a practically constant velocity of 6.25 km/s. This observation is supported by seismic reflection data. At the base of the crust is a velocity discontinuity followed by a 1.5 km thick transition zone into the upper mantle. From north to south along the graben, the crust thins by 3 km and the upper mantle velocity changes from about 8.4 km/s to 7.9 km/s. In comparison with the old 1-D model, the new model shows a simplification of the structure of the lower crust and a thinning of the crust/mantle transition zone from 5 km to 1.5 km. Furthermore, in the old model, the upper mantle velocity underneath the graben was found to be constant at 8.1 km/s, which is in contrast to the changing upper mantle velocity found in the new model.
ARK: https://n2t.net/ark:/88439/y019805
Permalink: https://geophysicsjournal.com/article/241
 
Seismicity and dynamics of the Upper Rhinegraben
Get PDFIn this paper we present the results of a 10-year period (1971-1980) of research on the seismicity of the Upper Rhinegraben. Our investigations are exclusively based on instrumentally recorded earthquakes with local magnitudes between 0.5 < ML < 5. The increase in the number of high-gain seismic stations during the past 2 decades improved the quality of the observations considerably, thus allowing detailed recognition of the spatial distribution of the earthquake loci in focal areas deduced from the analysis of historical events. No region, regarded up to now as aseismic, revealed itself as seismic, not even at the level of microearthquakes. Excluding the focal area of the Swabian Jura, the northernmost and southernmost parts of the Upper Rhinegraben show the highest degree of seismic activity. The middle part of the Rhinegraben, between Strasbourg and Karlsruhe, reveals only modest activity, somewhat in contrast to the historical record. The number of earthquakes increases towards the east of the river Rhine relative to the west. An even more pronounced asymmetry is shown in the southern graben by different maximum focal depths perpendicular to the strike of the Rhinegraben. In the Vosges mountains and in the graben proper, depths of 13 and 16 km, respectively, are not exceeded. Maximum depths down to about 20 km are found in the Black Forest. No earthquake was detected in the lower gabbroic crust or in the mantle. The maximum focal depth seems to be governed by variations in the temperature-depth distribution. Fault plane solutions of more than 30 earthquakes demonstrate that the seismic dislocations take place predominantly as strike slip mechanisms in the southern graben area whereas normal faulting prevails in the north. In the northern graben, most of the seismic dislocations occur on fault segments striking N30°W whereas in the south a strike ofN20°E or N60°W (the conjugate direction) is dominant. Furthermore, the fault plane solutions indicate a clockwise rotation of the principle stress directions from north to south by about 40°.
ARK: https://n2t.net/ark:/88439/y060010
Permalink: https://geophysicsjournal.com/article/81
 
Archaeomagnetic study of medieval fireplaces at Mannheim-Wallstadt and ovens from Herrenchiemsee (Southern Germany) and the problem of magnetic refraction
No full textTen fireplaces from Mannheim-Wallstadt (MW) and four ovens from Herrenchiemsee (HC) were studied with regard to their archaeomagnetic properties. The ages of the medieval structures were only poorly known and an age dating was intended with the help of the standard curves for the declination and inclination of the geomagnetic field of the past 2000 years, as set up by Thellier (1981) for France. The stability of the NRM was tested with Thellier's test. Of the ten MW fireplaces, only one passed the test, whereas of the four ovens of HC, all passed the test but one of them had to be excluded because of secondary displacements within the structure. The MW fireplace could be dated to 670-700 A.D., in agreement with other archaeological age determinations for the fireplaces. The ovens from HC yielded an age between 1100 and 1170 A.D. The ovens showed the effect of magnetic refraction. Model calculations carried out on circular ring structures require a susceptibility of the oven material of the order of 0.5 SI units to explain the observed effect. The presently observed mean susceptibility at room temperature is only around 5 x 10-3 SI units. However, heating of the material at 550° C for 2 h in a reducing environment was able to increase the susceptibility by a factor of 20. This effect is explained by the reduction of secondary iron oxides and hydroxides to magnetite. Another increase by a factor of about 1.5 is obtained from the Hopkinson effect (increase of susceptibility with temperature) at the blocking temperature of magnetite. Combining all effects, it is possible to attain values for the susceptibility at the blocking temperature of magnetite as high as 0.2 SI units, which is the order of magnitude required for the explanation of the observed effect of magnetic refraction. This effect demands that special considerations be made during archaeomagnetic sampling from archaeological ovens or kilns.
ARK: https://n2t.net/ark:/88439/y016135
Permalink: https://geophysicsjournal.com/article/117