-
The applicability of the finite-difference methods has been limited in most cases to simple geometric shapes. The problem of introducing boundary conditions into the scheme has usually restricted the models to structures in which the boundaries are parallel to the coordinates. Recently, several investigators have studied the effect of prominent topographic features on seismic signals. Most deal with SH waves. The behaviour of a P-SV pulse in media with prominent irregular surfaces is yet almost unknown. The difficulty of the last problem relative to the SH case lies in the vectorial form of the equation of motion and the more complicated boundary conditions. In the present work a technique is proposed for simulating the P-SV wave propagation in a two-dimensional half-space with an arbitrary polygonlike topography. This technique has been applied to compute seismograms due to a P-pulse on surfaces of ridges and canyons. The incident pulse is amplified at the crest of mountains and at the upper corners of canyons. The magnitude of amplification is a function of the steepness of the topographic structure and can increase by 50 % compared to a flat surface under the same conditions. The maximum attenuation computed at the bottom of a canyon was 25 %. It can be concluded that the influence of prominent topographic features on the incident P-pulse is similar to that on incident SH waves, which was computed in previous investigations.
ARK: https://n2t.net/ark:/88439/y066050
Permalink: https://geophysicsjournal.com/article/279
-
26 heat flow values were determined in the northern Italian lakes. The mean is calculated to be 1.59 μcal cm-2s-1 and after subtraction of the effect of uplift and denudation of the Alps 1.07 μcal cm-2s-1. With regard to the results of Austria and Hungary a rise in the mean values is observed from west to east. Heat flow maps have been constructed performing a trend analysis of all available European heat flow values. The Rhinegraben and lvrea anomaly having a "local" character are not recognizable on the maps. Simple transient models for the Ivrea anomaly and the two Mediterranean anomalies have been calculated to explain the present surface flow distribution. The existence of a low velocity layer in the crust west of the Ivrea body is not in disagreement with the measured heat flow values. This does not hold for the lvrea region itself if correction of the heat flow values for uplift and denudation is being made. However, it is perceivable that a heat wave caused by the possibly anomalous hot low velocity layer has not yet reached the Earth surface or can not be detected due to the inaccuracy of the measurements.
ARK: https://n2t.net/ark:/88439/y051571
Permalink: https://geophysicsjournal.com/article/278
-
Remanent magnetization of 92 samples (22 sites) collected from Oligocene basalts from the Ethiopian southern plateau (location 9.1°N, 41°E) yields a palaeomagnetic pole at 75.1°N, 170.3°E with A95 = 6.4° after AF-cleaning. This pole agrees with the Eocene-Oligocene pole for the western plateau. Palaeomagnetic data of 125 samples (26 sites) from Early Pliocene basaltic rocks from the Danakil block (12.7°N, 42.5°E) yield a pole at 79.5°N, 258.8°E with A95 = 2.6°. The deviation of this pole position from other Miocene Pliocene poles for Africa is consistent with the hypothesis of 10° counter-clockwise rotation of the Danakil block since the Early Pliocene. On theoretical grounds however this amount of deviation may also be caused by not having completely averaged out secular variation due to insufficient sampling.
ARK: https://n2t.net/ark:/88439/y042442
Permalink: https://geophysicsjournal.com/article/277
-
The methods of Magnetotellurics (MT) and Geomagnetic Deep Sounding (GOS) have been applied to study the electromagnetic response of the rift structure of the Rhine Graben. The measurements at 17 MT and 7 GOS stations were carried out along a profile running perpendicular across the Graben. Fourier analysis and numerical filters were tried for separation of the frequencies and a least squares technique was applied for data reduction. The thus gained transfer functions can be explained well by two-dimensional models under the following assumption: a well conducting layer at the depth between about 80 and 100 km exists at distances 50 km West and 100 km East of the Rhine Graben. Immediately below the Graben, however, a zone of similar good conductivity lies between about 25 and 45 km depth. The lateral extension of this zone is only some tens of km from the Graben's edges.
ARK: https://n2t.net/ark:/88439/y032853
Permalink: https://geophysicsjournal.com/article/276
-
The remanent magnetizations have been analysed in Cretaceous and Jurassic limestones, and Triassic dolomites, from Eastern and Central Sardinia. Ore microscopy and rock magnetic studies, including the acquisition of isothermal remanent magnetization (IRM) and the behavior of IRM during high temperature treatment, indicated that magnetite and goethite determine the paleomagnetic properties of these carbonate rocks. At all Cretaceous sites the remanences were either too weak or too unstable to be used for a paleomagnetic study. AF and thermal demagnetization were used to isolate the characteristic remanent magnetization (ChRM) in the Jurassic and Triassic samples. Rejection criteria were used to eliminate about one third of these samples because of directional instability, or because only very stable secondary magnetizations associated with goethite were present. For the remaining samples the ChRM direction associated with primary magnetite was extracted with the aid of vector diagrams or vector difference calculations. The ChRM directions are rotated by 70°–90° counterclockwise relative to European directions and by 35°–45° counterclockwise relative to African directions. Part of this results from the 30° microplate rotation of Sardinia in the Miocene. The remainder is due to an earlier phase of microplate rotation which took place in post-Late Jurassic time.
ARK: https://n2t.net/ark:/88439/y022994
Permalink: https://geophysicsjournal.com/article/274
-
Fuchs (1975) has described the theory and computation of synthetic seismograms for PS reflections from first order discontinuities and transition zones with various properties. A number of record sections were shown in which PS reflections were not discernable. In the course of a programme of explosion work which has been carried out around Scotland (SOSP, outlined by Jacob, 1974) a line of shots fired in the North Minch was observed by a station, WR, situated at Cape Wrath (Fig. 1). Though sub-critical PmP Moho reflections were seen, which indicated that PS reflections from the Moho might be expected, a section of vertical component seismograms (Fig. 2) gave only a weak indication of the phase. However, further processing of the data has shown a clear PS Moho reflection to be present.
ARK: https://n2t.net/ark:/88439/y012585
Permalink: https://geophysicsjournal.com/article/273
-
Theoretical investigations predict meridional currents in the dynamo region of the ionosphere a few degrees north and south of the magnetic equator as a consequence of the equatorial electrojet. The magnetic fields of these currents are perpendicular to the permanent magnetic field of the Earth and cause a height variation of the magnetic declination. It was the aim of six rocket launches performed in 1970 near noon at Natal, Brazil, to detect these currents. To reach this objective, identical payloads each consisting of two flux-gate magnetometers, a solar aspect sensor, an experiment to measure the angle between the magnetic field and the direction to the Sun, and an impedance probe to determine the electron density were launched under different magnetic conditions. The predicted variation of the declination has been observed but with considerably higher amplitude than was expected. On the other hand, the measured height integrated current density in the west-east direction was smaller than deduced from ground based magnetic H-variations. These discrepancies can be explained by currents flowing at 5 degrees off the magnetic equator, on both sides, with intensities of about 0.3 of the electrojet intensity at the same height but in reversed direction. Such reversed currents have recently been observed from ground based magnetic observations by others and have also been interpreted theoretically by ionospheric wind effects.
ARK: https://n2t.net/ark:/88439/y001006
Permalink: https://geophysicsjournal.com/article/272
-
Methods for the calculation of synthetic seismograms along vertical profiles through horizontally layered media are presented; the aim is to simulate numerically the technique of vertical seismic profiling (VSP), which is sometimes used for improved resolution of deep primary reflections. We present the theory for vertically travelling plane waves and for spherical waves, generated by a single force or an explosive point source. The solution is restricted to the acoustic case. The theoretical seismograms are complete, i.e., they contain all surface and internal multiples. Anelastic effects are taken into account with the method of complex velocities. In order to save computing time, seismogram segments are calculated, using the method of complex frequencies. These methods are applied to a layered model of the coal-bearing carboniferous in the Ruhr district of W. Germany. Different assumptions on overburden structure, recording geometry, and anelasticity are made, and the VSP seismograms are compared with seismograms calculated for horizontal profiles at the surface. It is suggested that VSP field surveys be preceeded by similar numerical studies for an estimate of the information that can be obtained by this expensive technique.
ARK: https://n2t.net/ark:/88439/y091727
Permalink: https://geophysicsjournal.com/article/271
-
In this paper an attempt is made to determine the frequency dependence of Q in the Earth's mantle in the frequency range 0.03–1.5 Hz from the spectral ratio of teleseismic S- and P-waves. Digital broad-band data of 17 earthquakes at 40° < ∆ < 90° recorded at the Central Seismological Observatory of the Federal Republic of Germany at Erlangen were analysed. The method implies the following assumptions: frequency independence of the crustal transfer function, proportionality of Qp (f) and Qs (f), and proportionality of P- and S-source spectra. This last and most critical assumption was carefully investigated by kinematic and dynamic source models. The calculated Q-spectra for the individual events vary considerably but all have in common a general increasing trend with frequency which can best be represented by a power law Q ≈ f α with 0.25 < α < 0.6. A further increase in slope near 1 Hz suggests an absorption band corner with an upper cut-off relaxation time τm = 0.33 ± 0.18 s. The significance of the Q-spectra and their variability is estimated by manipulating semi-synthetic seismograms with different error-producing processes such as length and shape of the time window, superposition of noise, digital filter process and source spectra. It is concluded that none of these processes is able to destroy or to imitate the observed increasing trend of Q with frequency. The results are compared with those from other seismological investigations and from laboratory experiments on mantle rocks at high temperature and in the seismic frequency band.
ARK: https://n2t.net/ark:/88439/y081538
Permalink: https://geophysicsjournal.com/article/270
-
Records of all known eruptions and damaging earthquakes in Iceland since 1550 were produced containing 82 and 44 events, respectively. Neither record exhibits a significant trend. Analysis of the second order properties of the records reveals no indication of clustering or periodicity in either record. There is a significant relationship between the two kinds of events. Some of this may be due to triggering effects, but there is also a more long term relationship with eruptions leading the earthquakes. This relationship was examined further by classification of the events geographically and by types of the eruptions. The relationship seems to embody the whole area rather than local effects.
ARK: https://n2t.net/ark:/88439/y071809
Permalink: https://geophysicsjournal.com/article/269
