Journal of Geophysics
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Initial results from the Helios-1 Search-coil Magnetometer experiment
Get PDFThe Helios search-coil magnetometer experiments provide a unique potential for studies of electromagnetic plasma wave modes in the solar wind between 0.3–1 A.U. because of excellent background noise levels, high time-resolution and high accuracy. Daily average spectral densities (γ √Hz) in the frequency range from 4.7–220 Hz show a pronounced increase as the sun is approached with complex superposed variations. Their values have a range of more than an order of magnitude. The shock of January 6, 1975 provides an interesting example for the use of high time-resolution spectral data. The fine structure and the long term variations in wave fields after the perpendicular shock are discussed. Also two interesting examples of "magnetic holes" are presented.
ARK: https://n2t.net/ark:/88439/y002056
Permalink: https://geophysicsjournal.com/article/116
 
Tunneling of low-frequency waves through the subcrustal lithosphere
Get PDFTunnel waves are long period waves which on overcritical incidence have penetrated through thin high-velocity layers. The generation and propagation of these waves are studied by numerical experiments using synthetic seismograms. Tunnel waves may occur as secondary and primary arrivals, on retrograde and prograde branches. Observations from explosion and teleseismic data with low-frequency tunnel waves are presented. A model of the lower lithosphere with thin high-velocity layers explains not only the occurrence of tunnel waves but also the high-frequency transmission of Pn and Sn waves to teleseismic distances. This model is in accordance with the observation of seismic anisotropy in the upper mantle and suggests refinements of existing petrological models of the lower lithosphere.
ARK: https://n2t.net/ark:/88439/y017705
Permalink: https://geophysicsjournal.com/article/86
 
Rayleigh channel waves for the in-seam seismic detection of discontinuities
Get PDFTwo-dimensional seismic models are used to reproduce the geological sequence cheek-seam-cheek in deep coal mines. The known reflection method based on Rayleigh channel waves is investigated with respect to discontinuities being composed of a termination of the seam and a fractured zone outside the seam. By means of fractured zones having different reflection coefficients discontinuities are designed representing clay hogs or microtectonic faults. Results obtained by investigating amplitudes, frequencies and phase velocities show a strong increase of the rate of amplitude decay with decreasing phase velocity. Furthermore it is demonstrated that the part of the Rayleigh channel wave propagating outside the seam is responsible for the detectability of a discontinuity. Expressing the detectability numerically the ratio ER of the channel wave's energy inside the seam to its total energy has been used. The value 0.5 for ER is derived to be a standard value for distinguishing the phase velocity range for which reliable reflection surveys are possible (ER > 0.5) or not (ER < 0.5).
ARK: https://n2t.net/ark:/88439/y015755
Permalink: https://geophysicsjournal.com/article/107
 
Heat flow map of the Bohemian Massif
Get PDFThe heat flow pattern of the Bohemian Massif is presented by the compilation of 47 heat flow values. The isolines of the geothermal activity were constructed using also data in the neighbouring countries. A clear relationship between heat flow and the tectonic structure can be observed, higher geothermal activity corresponds to the zones of the weakened Earth's crust coinciding with two major deep faulted zones bordering the most rigid central part of the whole massif.
ARK: https://n2t.net/ark:/88439/y053881
Permalink: https://geophysicsjournal.com/article/195
 
Palaeomagnetism of Upper Jurassic limestones from Southern Germany
Get PDFFlat-bedded, shallow water, marine limestones interlayered with marls are well exposed in Franconia and Swabia, Southern Germany. The Upper Jurassic sediments sampled have been dated in detail palaeontologically, and cover the interval from middle Lower Oxfordian to middle Lower Kimmeridgian. Bed by bed sampling of sections at 11 localities throughout a thickness of more than 130 m yielded about 400 samples (1100 specimens) for palaeomagnetic analysis. The NRM intensities appear to correlate with the clay content and averaged 2 x 10–7 Gauss. Progressive alternating field demagnetization of each specimen was used to isolate the direction of the characteristic remanence, whose mean intensity averaged 5 x 10–8 Gauss. Only normally magnetized rocks are found in the lower part of the section, but distinct zones of normal and reversed polarity are found in the upper part (middle Lower Kimmeridgian) in both regions of investigation. The polarity sequence is tentatively correlated with the ocean floor Mesozoic magnetic anomaly sequence. The data yield a reliable estimate of a Late Jurassic palaeomagnetic pole position for stable Europe.
ARK: https://n2t.net/ark:/88439/y044102
Permalink: https://geophysicsjournal.com/article/204
 
Computation of reflection coefficients for layered media
Get PDFA fast computer program of the Thomson-Haskell matrix formalism is presented for the computation of the P—SV reflection coefficients Rpp, Rps, Rss and Rsp for layered solid media. A matrix formalism and a computer program are also derived for the computation of P reflection coefficients for layered liquid media and of SH reflection coefficients for layered solid media.
ARK: https://n2t.net/ark:/88439/y003056
Permalink: https://geophysicsjournal.com/article/85
 
The Iranian long period array (ILPA)
Get PDFThe extended range 7 elements (ER7) seismic array, ILPA, has been installed in Iran. Construction was started in the winter of 1973 as a joint undertaking of the governments of Iran and the United States of America. This array began its operation in January 1976.
ARK: https://n2t.net/ark:/88439/y083518
Permalink: https://geophysicsjournal.com/article/198
 
Fault-plane solution of the earthquake in Northern Italy, 6 May 1976, and implications for the tectonics of the Eastern Alps
No full textThe fault-plane solution of the North Italian earthquake of May 6, 1976, has been constructed from long-period records of 37 stations (mainly WWNSS stations), assuming a focus in the crust. It is essentially a thrust type solution, the compressional axis having an azimuth of about 160°, perpendicular to the strike direction of the Eastern Alps. One nodal plane is very well defined from first P-wave motions: it is steeply dipping approximately SSE with a dip angle of 77°. The second nodal plane has been determined from S-wave polarisations at stations in the United States and Japan: it dips roughly NW with a dip angle of about 16°. The dip direction of this nodal plane is less well defined. From the two possible interpretations of the fault-plane solution, either a shallow thrust of Italy under the Alps or a steep thrust of the Alps under Italy, only the first one is geologically plausible. The slip direction of the underthrusting block (or plate?) has an azimuth of 348° and a dip angle of 13°.
ARK: https://n2t.net/ark:/88439/y072059
Permalink: https://geophysicsjournal.com/article/280
 
Crustal structure of the Central Aegean Sea and the islands of Evia and Crete, Greece, obtained by refractional seismic experiments
Get PDFIn 1973 and 1974 deep seismic sounding experiments were performed in the Aegean area by German and Greek geophysicists, from which two crustal sections were established. The one strikes along the islands of Amorgos-Mikonos-Andros and Evia and the other along Crete, in the E-W direction. The main results obtained are: Along the Amorgos-Mikonos-Evia section the crust is updipping from 32 km below Evia to 26 km below Amorgos in the southern Aegean Sea. The crust is of the continental type with Vpg = 6.0 km/s for the crystalline basement and 7.7 km/s for the upper mantle. The average velocity of the crust computed from Pm P-reflections has a value of 6.21 km/s. The sedimentary cover is very unevenly distributed with maximum thickness at North Evia. The crystalline basement outcrops at the southern part of the island and the Cyclades. Along Crete the crust is somewhat thicker, than that below South-Evia with 34 km at the western part and about 30 km at the eastern part of the island. At the western part of the island the nappes have their greatest thickness, the Messara Basin in the east containing the largest neogene sequences on Crete. The crustal structure of Crete is also of the average continental type.
ARK: https://n2t.net/ark:/88439/y044462
Permalink: https://geophysicsjournal.com/article/100
 
Earth-flattening approximation for body waves derived from geometric ray theory - improvements, corrections and range of applicability
Get PDFA new derivation of the Earth-flattening approximation (EFA) for body waves from geometric ray theory is given which results in an improved version of the EFA. This version agrees with the EFA, derived by Chapman (1973) from wave theory. Moreover, it allows absolute, not only relative, body-wave amplitude calculations for given source time functions. The choice of the density transformation of the EFA is shown, by numerical calculations, to be uncritical for body-wave amplitudes in the period range up to 30 s. An error in an earlier derivation of the EFA (Muller, 1973a) is corrected. This error requires a new investigation of the range of applicability of the EFA, which is performed for the P-wave propagation through a homogeneous sphere. The results are similar to those of the earlier paper: long-period P waves with dominant periods up to about 20s can be treated practically exactly, as long as they do not pass closer than about 800 km to the Earth's center.
ARK: https://n2t.net/ark:/88439/y046102
Permalink: https://geophysicsjournal.com/article/89