-
Konigsberger ratios of DSDP basalts were rather low (mean value around 7) but were usually high enough to justify interpreting oceanic anomalies with a remanent magnetization model. Natural remanent magnetizations were considerably lower than in dredged basalts but were strong enough to account for amplitudes of oceanic magnetic anomalies. Both stable and unstable remanent types were encountered. The observed stable inclinations showed a large scatter when compared to expected inclinations, largely due to non-cancellation of secular variation. All the basalts were categorized in deuteric oxidation Class I by opaque petrology observations. Extensive maghemitization, inferred from thermomagnetic analyses, may explain the low NRM intensities. Unstable specimens easily acquired large viscous remanent magnetizations, in some cases as large as the NRM. The basalt magnetic properties were in general accord with the expectations of the Vine and Matthews hypothesis.
ARK: https://n2t.net/ark:/88439/y075179
Permalink: https://geophysicsjournal.com/article/103
-
The principle of conservation of energy, in the form of the equation of radiative transfer, is used to treat the case of strong scattering of elastic waves. If the medium is isotropic, if all the energy present has been scattered many times, and if the time and distance scales of the problem are long compared to the time and distance scales of the scattering process, then the average flow of energy is described by the diffusion equation with an additional term representing linear dissipation to heat. Model seismic experiments using holes drilled in aluminum plates as scatterers confirm the applicability of the formalism. The diffusion formalism qas been successfully applied to lunar seismograms and to some Earth data. The results of studies of lunar seismograms show that the zone of strong scattering on the Moon is confined to a near surface zone.
ARK: https://n2t.net/ark:/88439/y064280
Permalink: https://geophysicsjournal.com/article/102
-
Presented is a new application of Gephart-Forsyth inverse technique for predicting directions and senses of slip. In Vrancea region, the compression axis is thus found to act in a NE-SW direction and not in a SE-NW direction, as is commonly thought from focal mechanism results. In addition, evidence is provided that the stresses in the crust are decoupled from those in the subducted slab.
ARK: https://n2t.net/ark:/88439/y054021
Permalink: https://geophysicsjournal.com/article/101
-
In 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
-
Prior to the present activity of the Krafla volcano, which started in 1975, levelling and gravity surveys had been carried out in the area. The network has since been extended, and measurements are carried out about every second month. The measurements reveal a quasi-periodic behaviour of the tectonic activity, characterized by slow inflation of the caldera for several weeks or months at a rate of 6–10 mm/d, interrupted by sudden subsidence events lasting for one or a few days. Elevation changes within the caldera are described by a deflation-inflation swelling with its apex near the center of the caldera. Calculations. using a model of a spherical chamber with varying pressure at 3 km depth, show good agreement with the measured elevation values. Comparison of gravity and levelling data, using a Bouguer type relationship, suggests that inflation and deflation of the floor of the caldera is caused entirely by in- and outflow of magma. Some discrepancies between the levelling and gravity data immediately after subsidence events can be explained by additional mass flow of groundwater.
ARK: https://n2t.net/ark:/88439/y037843
Permalink: https://geophysicsjournal.com/article/99
-
Magnetotelluric results from 38 sites in North Iceland yield a detailed spatial distribution of conductivity versus depth. A high-conductivity layer, about 10 Ωm, marking the transition between crust and upper mantle is detected throughout the surveyed area, its depth increases from 10 km within the zones of present tectonic activity to 20–30 km at a 50–100 km distance from the rift axis. The high conductivity layer indicates some basaltic melt fraction at temperatures around 1,000°C. The calculated average crustal temperature gradients are 100–150°C/km at the rift axis and 50–70°C/km at about 100 km distance. Below this anomalous zone, down to 150 km depth, the conductivity decreases to 100 Ωm due to the ultrabasic chemical composition of the upper mantle and a possible decrease of melt content with increasing depth. The temperature gradient within the upper mantle cannot be more than a few °C/km. However, a highly conducting asthenosphere exists below 150 km depth. The following petrological model for Iceland is suggested: Diapiric updoming of the asthenosphere leads to partial fusion of parent ultra basic mantle material, resulting in segregation and upward migration of basaltic melt. Lower density, lower viscosity and higher mobility relative to the solid phase causes the melt to rise faster than the ultrabasic material. This melt fraction then forms the basaltic crust of Iceland which is thickening away from the ridge axis with increasing age.
ARK: https://n2t.net/ark:/88439/y023054
Permalink: https://geophysicsjournal.com/article/98
-
The case for dispersing charges fired at sea into a number of packages fired simultaneously at optimum depth is outlined and an experiment, carried out to check that linear addition of the signals from separate sources does occur, is described. The advantages of this system for lithospheric studies are demonstrated.
ARK: https://n2t.net/ark:/88439/y018705
Permalink: https://geophysicsjournal.com/article/97
-
The corrected frequency of mode 0S0.
ARK: https://n2t.net/ark:/88439/y000005
Notes: Corrected article
Permalink: https://geophysicsjournal.com/article/96
-
The remanent magnetization acquired by a slurry stirred in a magnetic field was measured as a function of the applied field, stirring rate and water content. The experimental results were fitted by a theoretical model in which the stirring process was approximated as a periodic randomization of the grains. The acquired remanence was proportional to the applied magnetic field and independent of the stirring rate only for weak fields ( <160 A/m) and slow stirring rates ( <10 rad/s). The remanent intensity decreased with decreasing water content. The implications for the laboratory modelling of post-depositional remanent magnetization are discussed.
ARK: https://n2t.net/ark:/88439/y004206
Permalink: https://geophysicsjournal.com/article/95
-
Two regional earthquake catalogues are analysed in order to investigate the existence of diurnal, annual and Earth-tide related periodicities of the occurrence time of seismic events. The first earthquake catalogue covers the total region of the Federal Republic of Germany for the time period 1021-1979 (1530 events) and is investigated with respect to the existence of diurnal and annual periodicities. With the help of the graphical representation for Schuster's test, a midnight and a midday maximum are separated for different intensity classes. A winter maximum present in the data prior to 1930 is absent in the more recent data. It is argued that both diurnal and annual maxima are artifacts due to data sampling problems. The second earthquake catalogue refers to the western part of Germany (Lower Rhine Graben, Rhenish Massif) and the time period 1979-1984 (1012 events of magnitude ML = 0.0–5.1). For the events of this catalogue, a tidal phase is computed according to a model for the triggering of the predominant dip-slip earthquakes. The distribution of phases supports the triggering hypothesis (at the 99% significance level if Schuster's test is applied).
ARK: https://n2t.net/ark:/88439/y094877
Permalink: https://geophysicsjournal.com/article/94
