Journal of Geophysics
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Non-linear rheology and return flow in the mantle
Get PDFA simple model of mantle return flow in response to lithospheric plate motions is developed. Such a model is realistic if the buoyancy forces are concentrated in the plates. One-dimensionality is chosen as a simplification to study effects of mantle rheology in as much isolation as possible. Rheology is modelled as a combination of dislocation creep, diffusion creep and fluid phase transport; parameters are those appropriate for olivine. We have varied temperature, grain size, influence of partial melt, diffusivity and activation energy, grain deformation versus grain boundary sliding dominated creep, and surface plate velocity. A peculiar feature of non-linear rheology is the existence of low-stress high-viscosity regions, which, however, are of little dynamic importance because deformation there is very small. The main results are (1) that the model does not predict an excessive pressure gradient to be required by the return flow (which would be evident in a rise of the sea floor and strong increase in free air gravity anomalies toward the trenches); (2) that no excessive shear stresses at the plate bottom are predicted (which might result in observable heat flow effects and intra-plate seismicity and would require implausibly great driving forces at the plate ends); (3) that the model predicts the return flow to extend into the deeper mantle; this follows, however, from the simplifying assumption of olivine rheology below 400 km depth and would then argue for rather high temperatures, small grain sizes, possibly important fluid phase transport, and small activation volume. Recent work on the variation of activation volume with pressure and phase changes suggests a rather 'soft' lower mantle and thus supports the notion of 'deep' return flow. In interpreting the results one must, of course, keep in mind that the model is a purely mechanical one with a predetermined temperature profile (varied within plausible limits) and that the physics of the thermodynamic aspects of the flow problem is ignored.
ARK: https://n2t.net/ark:/88439/y009016
Permalink: https://geophysicsjournal.com/article/43
 
Inversion of satellite magnetic anomaly data
Get PDFA method of finding a first approximation to a crustal magnetization distribution from inversion of satellite magnetic anomaly data is described. Magnetization is expressed as a Fourier series in a segment of spherical shell. Input to this procedure is an equivalent source representation of the observed anomaly field. Instability of the inversion occurs when high frequency noise is present in the input data, or when the series is carried to an excessively high wave number. Preliminary results are given for the United States and adjacent areas.
ARK: https://n2t.net/ark:/88439/y090977
Permalink: https://geophysicsjournal.com/article/159
 
Magnetotelluric investigation of the lower crust and upper mantle beneath Iceland
Get PDFMagnetotelluric measurements were made at 12 sites on a 210 km long profile in northern and eastern Iceland. The profile crosses areas of different geological age ranging from Tertiary flood basalt to the presently active zone of rifting and volcanism. Beneath the investigated area, at a depth of a few km down to about 20 km there exists a 5 km thick layer with low resistivity amounting to 15 Ωm which is imbedded between layers of higher resistivities. The depth to the low resistivity layer increases with increasing distance from the spreading axis. The low resistivity layer is presumably caused by partial melting at the base of the crust and at the top of the anomalous mantle beneath Iceland. Comparison with laboratory measurements confirms a basaltic composition and a temperature of 1,000-1,100° C of the good conductor, and probably partially molten peridotite in the upper mantle beneath. Mean temperature gradients in the crust calculated from the magnetotelluric data are in good agreement to surface temperature gradients measured in drill holes.
ARK: https://n2t.net/ark:/88439/y080108
Permalink: https://geophysicsjournal.com/article/168
 
Palaeomagnetism of Permo-Triassic red sandstones from the Northern Calcareous Alps
Get PDFIt is generally agreed (Gwinner, 1971) that the Northern Calcareous Alps (together with the Northern Grauwackenzone) rest allochthonously on top of a Penninic basement and have been transported into their present position from the South after Upper Eocene. Their complicated structure is characterized by a series of nappes of various lateral extensions and thicknesses. A sketch map of the Western part of the Northern Calcareous Alps (Fig. 1) shows the main nappes (after Gwinner, 1971).
ARK: https://n2t.net/ark:/88439/y004586
Permalink: https://geophysicsjournal.com/article/200
 
Ionospheric observation of gravity-waves associated with hurricane Eloise
Get PDFAn experiment conducted by using a continuous wave-spectrum high frequency radio wave Doppler sounder array with three sites and nine transmitters (each site with three transmitters) was carried out to observe the coupling of energy between the troposphere and the ionosphere during the period of Hurricane Eloise. The analysis of the Doppler sounder records indicated that gravity waves were detected when the eye wall of Hurricane Eloise was located at the Gulf of Mexico. A group ray tracing has been used in an attempt to locate the sources of these waves. Wave sources are located along the storm track and near the storm center. The wave excitation mechanism is discussed.
ARK: https://n2t.net/ark:/88439/y031553
Permalink: https://geophysicsjournal.com/article/173
 
Extensions of the reflectivity method
Get PDFThe reflectivity method for the computation of theoretical seismograms is extended to include a dislocation point source of arbitrary orientation buried in a layered medium. The second extension is a change in the numerical integration over the circular frequency in order to avoid time aliasing effects.
ARK: https://n2t.net/ark:/88439/y041182
Permalink: https://geophysicsjournal.com/article/174
 
Crustal and upper mantle structure beneath the apennines region as inferred from the study of Rayleigh waves
Get PDFRayleigh wave phase velocities were obtained in the period range 12.5–83.3 s using the almost linear array of long period seismic stations installed at Bari (BAI), Grosseto (GSO), Bologna (BOL), and Torino (TNO). The Hedgehog inversion gives a crustal thickness in the range 25–37 km. The presence of a low-velocity layer in the crust is allowed, while low velocity material within a few kilometers of the Moho is required. The shear-wave velocities below 60 km are rather higher than usual channel values. If crustal thicknesses of the order of 37 km are rejected as suggested by other geophysical data then the low-velocity layer in the crust is required in order to satisfy the observed dispersion relation.
ARK: https://n2t.net/ark:/88439/y089438
Permalink: https://geophysicsjournal.com/article/114
 
Observations of sPn from Swabian Alb earthquakes at the GRF array
Get PDFA number of the largest earthquakes from the Swabian Alb recorded at the Grafenberg array have been examined (the geographic coordinates of the main station GRF are 49.69°N and 11.22°E; see Harjes and Seidl (1978) for a description of the array). It has been shown that the GRF records of some of the largest Swabian Alb earthquakes have a clear additional arrival between Pn and Pg.
ARK: https://n2t.net/ark:/88439/y003616
Permalink: https://geophysicsjournal.com/article/190
 
Reinterpretation of palaeomagnetism of the Colli Euganei and Monti Lessini (Italy)
Get PDFNew information on the age of the volcanics from the Colli Euganei and Monti Lessini (Northern Italy) collected by Channell et al. (1978) show that the divisions made by Soffel (1972, 1974, 1975a) of palaeomagnetic directions from the two areas into Eocene and Oligocene groups had to be modified. From the data of both sampling areas alone an anticlockwise rotation of the area between Eocene and Oligocene can no longer be clearly demonstrated. However, a combination of the predominantly Oligocene data from the Colli Euganei with the predominently Eocene data from the Monti Lessini as well as their comparison with corresponding palaeomagnetic data from Stable Europe brings enough statistical evidence for an anticlockwise rotation of the area of at least 20-30° between Upper Eocene and Lower to Middle Oligocene.
ARK: https://n2t.net/ark:/88439/y094107
Permalink: https://geophysicsjournal.com/article/199
 
Analysis of an eastward electrojet by means of upward continuation of ground-based magnetometer data
Get PDFOn October 26, 1975, data from two parallel meridian chains of densely spaced magnetometers in Northern Scandinavia indicated the presence of an evening sector eastward electrojet which between 1606 and 1710 UT was two-dimensional. By separating the relevant components of the magnetic disturbance fields into internal and external parts, and by subsequent upward continuation (towards the source) of the external horizontal component, equivalent height-integrated ionospheric current densities were derived as a function of latitude and time. Whereas the demarcation line between the eastward and westward ionospheric electrojet currents remained stationary, the sharp southern border of the eastward electrojet moved towards the south with a speed of about 50 ms-1 , possibly indicating an earthward movement of the inner edge of the magnetospheric plasma sheet. The maximum eastward height-integrated current density was of the order of 0.5 Am-1 , the corresponding large-scale field-aligned current density has been estimated to have been of the order of 1.0 μAm-2 . After 1630 UT there was evidence for a superposed stationary small-scale structure (wavelength of the order of 250 km) in the eastward flow, which would imply a pair of local field-aligned current sheets possibly related to an auroral arc.
ARK: https://n2t.net/ark:/88439/y092617
Permalink: https://geophysicsjournal.com/article/179