Journal of Geophysics
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Tidal triggering of earthquakes in the Swabian Jura?
Get PDFSeveral statistical tests were used to investigate the possibility that the earthquakes in the Swabian Jura are triggered by the tidal stress in the Earth. The results provide weak evidence that the earthquakes tend to occur when the tidal shear stress on the fault plane in the direction supporting the tectonic stress is greatest. A comparison of some of the statistical methods available for investigating tidal triggering effects is made.
ARK: https://n2t.net/ark:/88439/y090017
Permalink: https://geophysicsjournal.com/article/41
 
Full wave theory applied to a discontinuous velocity increase: the inner core boundary
Get PDFRecent developments in the computation of theoretical seismograms have shown the advantages of numerical integrations in the complex ray parameter plane. We here extend the approach, to calculate the amplitude of body waves interacting with a discontinuous velocity increase. The method incorporates a complex velocity profile to account for attenuation and the uniformly asymptotic method of Langer to account for frequency-dependence of reflection-transmission coefficients near grazing incidence. In the integrand of reflection-transmission coefficients, the Debye ray expansion is not made where it is poorly convergent near critical incidence. The method thereby correctly evaluates all the waves in the high velocity medium that are repeatedly refracted back up to, and reflected from, the low velocity medium. Calculations are equally efficient both for Earth models specified on discrete radii or as analytic functions of radius. The method is used to calculate theoretical seismograms for PKP waves in the PEM Earth model in the distance range 110°–152° and in Earth models 1066B and C2 in the distance range 112°–132°. Calculations of the amplitude near point D of the travel time curve for PKP (which is the critical distance of K waves incident from the fluid core upon the solid inner core) indicate that at finite frequencies the amplitude maximum associated with critical incidence is smaller than that that would be inferred from frequency independent reflection-transmission coefficients. At lower frequencies the maximum is displaced to a longer distance than D. Comparison of observed and calculated seismograms indicates that the PEM model having a P velocity jump of 0.83 km/s at the inner core can fit the observed amplitudes. Consequently the data do not require an anomalous P velocity gradient at the top of the inner core. A low frequency precursor to the DF branch of PKP at distances shorter than the B caustic at 146° agrees with Buchbinder's (1974) explanation of diffraction from the B caustic.
ARK: https://n2t.net/ark:/88439/y040552
Permalink: https://geophysicsjournal.com/article/164
 
Rockmagnetic studies of Upper Jurassic limestones from Southern Germany
Get PDFAcquisition of isothermal remanent magnetization (IRM) and the behaviour of IRM during high and low temperature treatment indicate that magnetite, goethite and minor quantities of hematite determine the magnetic properties of the Upper Jurassic limestones in Southern Germany. The direction of natural remanent magnetization (NRM) of magnetite is of detrital, synsedimentary origin, whereas the goethite, although its NRM is extremely stable against AF cleaning, has been magnetized only partly during the Jurassic. Part of the goethite NRM is interpreted to be of Tertiary (pre-Upper Miocene) age being formed during a period of Karstification in the Lower Tertiary. The apparent polarity sequence described earlier (Heller, 1977) is not of Jurassic age.
ARK: https://n2t.net/ark:/88439/y050321
Permalink: https://geophysicsjournal.com/article/165
 
Formal morphostructural zoning of mountain territories
Get PDFFormalization of morphostructural zoning of mountain regions, with special emphasis on determination of lineaments, is considered in this paper. The zoning is based on joint analysis of topography, geology, tectonics and geomorphology, represented on corresponding maps and aerial or space photos. Our goal is to make the zoning objective and reproducible. The importance of this goal follows from the fact that morphostructural zoning, especially the scheme of lineaments, is the starting point of our approach to prediction of strong earthquakes; it is important also to location of some mineral deposits. The definition of large elements of relief is formalized in the first place. On the basis of their characteristics a territory is divided into three types of areas: blocks, lineaments and knots. A precise and objective location of knot and lineament positions is the final aim of our formalized scheme. The application of the suggested algorithm to Eastern Tien Shan is described in the conclusion of the paper, as an illustration.
ARK: https://n2t.net/ark:/88439/y085028
Permalink: https://geophysicsjournal.com/article/158
 
Geoelectrical deep soundings in Southern Africa using the Cabora Bassa power line
Get PDFGeoelectrical deep soundings with electrode spacings between 30 and 1250 km have been carried out from 1973 to 1975 in southern Africa using the Cabora Bassa power line. At several sounding stations (Schlumberger and bipole-dipole arrays) the electric potential differences have been recorded in different directions. For short current-electrode spacings (< 200 km) the direction of the electric field is nearly perpendicular to the direction between the current electrodes whereas it becomes more or less parallel for large electrode spacings (> 400 km). A possible cause for this rotation of the field may be a dyke system observed in the granitic crust.
ARK: https://n2t.net/ark:/88439/y065100
Permalink: https://geophysicsjournal.com/article/216
 
Interpretation of wide-angle reflection travel-times in realistic crust-mantle structures
Get PDFA method for the interpretation of wide-angle reflection travel-times in laterally varying crust-mantle structures is formulated. A datum correction is first carried out by ray-tracing to remove the effects of refraction above the reflector. The resulting time-distance data may be then expressed in a form which permits, given multiple coverage of the sub-surface, the independent determination of velocity and reflector topography. The method has been tested on model data and found to be effective.
ARK: https://n2t.net/ark:/88439/y013185
Permalink: https://geophysicsjournal.com/article/58
 
The reflectivity method for a buried source
Get PDFThe reflectivity method for the computation of theoretical seismograms is extended for the case of a point source buried in a layered medium. Two sources are considered, an explosive source and a vertical single force. Appearing accuracy problems are solved. Poles of Rayleigh waves are shifted away from the real axis of the wavenumber plane by introducing attenuation, in order to allow numerical integration along the real axis. The results of several computations are discussed. This method allows the computation of complete seismograms including surface waves, leaking modes and all body wave phases, including depth phases like pP.
ARK: https://n2t.net/ark:/88439/y005596
Permalink: https://geophysicsjournal.com/article/75
 
On geophysical inverse problems and constraints
Get PDFMathematical methods for linearized geophysical inverse problems are reviewed, in cases with and in cases without constraints. The role of constraints receives particular attention, both in linear convex problems and in an exactly solvable non-linear example.
ARK: https://n2t.net/ark:/88439/y081128
Permalink: https://geophysicsjournal.com/article/178
 
Seismic energy transmission in an intensively scattering environment
Get PDFFor describing transmission of seismic energy through a medium in which seismic waves are intensively scattered, a statistical approach provides an attractive alternative to the conventional, deterministic approach. The energy transmission in such a medium with a given size distribution of scatterers is generally governed by a diffusion equation with a frequency-dependent diffusivity, rather than wave equations as in the conventional approach. By applying this theory, we can explain many unusual characteristics of lunar seismic signals, including those generated by surface impacts at near and far ranges and by continuous movement of the Lunar Rovers. The size distribution of scatterers can be estimated from the frequency dependence of diffusivity. Predominantly rectilinear particle motions observed indicate that the scattered energy is transmitted as body waves. When intensive scattering is limited to only a part of the transmitting medium, as in the case of far impacts on the Moon, a more general theory combining the two approaches is required. The theory is also useful for interpreting certain characteristics of terrestrial seismic signals because, while frequently ignored, appreciable scattering exists even for terrestrial cases.
ARK: https://n2t.net/ark:/88439/y052271
Permalink: https://geophysicsjournal.com/article/90
 
Characteristics of VRM in oceanic basalts
Get PDFLaboratory experiments, each lasting several weeks, have been conducted to establish the characteristics of viscous remanent magnetization (VRM) in oceanic basalts from many sites of the Deep Sea Drilling Program (DSDP). VRM is most pronounced in low-coercivity basalts whose natural remanences (NRM) have low median destructive fields, less than 100 Oe. A simple logarithmic acquisition law is rarely obeyed, but two or three distinct stages are instead observed, in each of which a logarithmic dependence of VRM intensity on acquisition time may be assumed. This observation leads to a simple interpretational model for the nature of VRM in DSDP basalts, but also implies that extrapolation of laboratory observations to geological times is not meaningful. Instead, the ratio of laboratory VRM (acquired in a 1 Oe field during 1000 h) to NRM is used as a minimum indicator of the potential seriousness of VRM. Experiments show that VRM acquired in the presence of NRM is more serious than VRM acquired in alternating field (AF) demagnetized samples. As most published VRM data in DSDP basalts were obtained after AF demagnetization, these are regarded also as minimum estimates of the significance of VRM acquired by oceanic basalts in situ. The consequences of the common occurrence of such an unstable component of magnetization in the oceanic basalt layer are considered in relation to the nature and distribution of oceanic magnetic quiet zones. The Cretaceous, and possibly the Jurassic, magnetic quiet zones are considered adequately explained by constant paleomagnetic field polarity. However, if VRM is a substantial and widespread magnetization component in the oceanic crust, it may not always be appropriate to interpret oceanic magnetic anomalies (or their absence) as an exact record of paleomagnetic field behavior. Remagnetization of the oceanic crust by VRM acquisition may be a viable alternative explanation of the origin of the marginal magnetic quiet zones.
ARK: https://n2t.net/ark:/88439/y064370
Permalink: https://geophysicsjournal.com/article/206