Journal of Geophysics
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Interactive high-resolution polarization analysis of broad-band seismograms
Get PDFIn the laterally heterogeneous and anisotropic Earth the particle motions of seismic waves are three-dimensionally polarized. This paper presents a method for determining a local wave coordinate system using the zeros of some component operators in rotated systems. It is applicable to broad-band composite waveforms and offers higher precision and resolution for the determination of azimuth and incidence angle than the usual least-squares techniques. The algorithms have been implemented in an interactive program for preprocessing three-component digital recordings. In addition, the program allows the use of particle motion diagrams and component products for the determination of wave types, onset times and pulse durations. Its performance is demonstrated on regional and teleseismic events recorded at the European broad-band stations GRF (Grafenberg, West Germany), KRC (Kasperske Hory, Czechoslovakia) and KSP (Ksiaz, Poland).
ARK: https://n2t.net/ark:/88439/y034913
Permalink: https://geophysicsjournal.com/article/203
 
Investigation of the velocity- and Q-structure of the lowermost mantle using PcP/P amplitude ratios from arrays at distances of 70°-84°
Get PDFInvestigations of the core reflection PcP at distances in the neighbourhood of the core shadow zone are especially appropriate for the study of isolated regions of D" because PcP for these distances has a large angle of incidence at the core-mantle boundary and is strongly influenced by the structure immediately above the core. A striking result of this investigation of PcP in the distance range 70°–84° is that PcP could be detected only in four cases (from a total of 16 earthquakes and 29 nuclear explosions) in the NORSAR- and GRF-array beams for PcP. This result is found although the P-wave-coda noise levels in the PcP beams are very low, mostly below the amplitude level of PcP predicted by standard Earth models. In the cases where PcP could not be identified, the PcP/P amplitude ratio, which is estimated from the beams with the aid of a cross-correlation procedure, must be regarded as an upper limit for the actual PcP/P ratio. The interpretation of the PcP/P amplitude ratios with one radially symmetric, elastic model proved to be impossible. The generally small PcP/P ratios can be explained by lateral variations in seismic wave absorption within D". Qα values for short-period P waves in D", between 800 (below Usbekistan) and 100 (below the northern Hudson Bay and Central Siberia), were found. Several recent seismological investigations of the P- and S-wave velocity of the lowermost mantle have suggested velocity models with first-order discontinuities (with velocity increases of the order of 1.5%–3.0%) 150–300 km above the core-mantle boundary. Through the calculation of synthetic short-period seismograms and comparison with the array data compiled for the main part of this study, it is shown that such models are very unlikely to be a global feature if the P velocity jump is of the order of 2.5%–3.0%. Even models with reduced P velocity jumps of 1.3% produce clear onsets in short-period seismograms. They are not observed for P waves with ray paths bottoming beneath Central Siberia.
ARK: https://n2t.net/ark:/88439/y055861
Permalink: https://geophysicsjournal.com/article/215
 
Spatial and temporal characteristics of impulsive structure of magnetospheric substorm
No full textAt least a dozen well-defined activations were recorded with high spatial resolution during the first 25 min of a substorm commencing at 1959 UT on 3 March 1976. The activations were determined by Pi1 and 2-type magnetic pulsations, magnetic variations and cosmic noise absorption. The activations exhibited differences in intensity, spatial extent and the accompanied auroral behaviour. In all cases but one an impulsive counterclockwise differential equivalent current vortex was observed superimposed on the continuously growing westward electrojet. The high-energy particle precipitation reached its maximum in these vortex regions. In cases where the activations observed on the ground occurred close to the footpoint of the IMP-J satellite, it registered simultaneously (within 2 min) burstlike enhancements of the high-energy particle fluxes. The satellite was at these times 37 RE away from the Earth in the plasma sheet boundary layer. Large, practically isotropic high-energy electron fluxes detected at the satellite during the substorm expansion suggested the existence of a large-scale magnetic loop structure in the far magnetotail. The results give evidence of a time-varying dissipation process operating in an impulsive manner in separate regions of the plasma sheet (within a few RE). According to the magnetic variations at mid-latitude, the intensity of the cross-tail current disruption (or the substorm current wedge) does not respond to these activations in the same impulsive manner, but seems to integrate their effects.
ARK: https://n2t.net/ark:/88439/y017215
Permalink: https://geophysicsjournal.com/article/221
 
The influence of atmospheric loading on VLBI-experiments
Get PDFAir pressure lows and highs with periods of some days and seasonal variations of mean air pressure can be regarded as loading functions on the Earth's surface. They result in quasi-periodic surface deformations. The influence of such displacements on the results and the accuracy of VLBI experiments (Very Long Baseline Interferometry) is investigated by simulation calculations for the transatlantic Effelsberg-Haystack baseline. Different models for the time dependence of air-pressure-induced station displacements are considered. It is shown that today's standard VLBI data analysis, including model parameters for clock- and atmospheric-refraction effects, is not able to correct the measurements for atmospheric loading effects. It leads to erroneous baseline vectors. Hence, for every geodetic VLBI experiment, the amount by which (1) the local air pressure at the station and (2) the mean air pressure in a surrounding area of 2,000 km radius has changed during the experiment should be tested. These two values give an estimate of the resulting vertical displacments by the use of a regression formula. The corresponding delay-time corrections have to be applied to the VLBI data. Most of the radiotelescopes participating in geodetic VLBI experiments are situated in regions with small seasonal variations of the station position. However, an increasing VLBI accuracy and an expanded and denser VLBI network will also require the consideration of seasonal displacements.
ARK: https://n2t.net/ark:/88439/y053411
Permalink: https://geophysicsjournal.com/article/8
 
Electric fields and currents at the Harang discontinuity: a case study
Get PDFOn 2 December 1977, 1600-1700 UT (around 19 MLT) a section of the Harang discontinuity moved westward over northern Scandinavia with a velocity of about 1 km/s, during a disturbed time interval. The westward movement was clearly identified in the IMS Scandinavian Magnetometer Array observations and is consistent with simultaneous ST ARE electric field measurements. The magnetic measurements showed a marked increase of the overhead current density during the westward movement while the electric field amplitudes stayed at the same level, thus indicating a temporal enhancement of the ionospheric conductivity. The westward movement gave the opportunity to combine subsequently observed two-dimensional distributions of equivalent current vectors (after correction for the magnetic field intensity increase) and electric field vectors into two corresponding patterns fairly extended in the east-west direction and covering the total extent of the most interesting part of the Harang discontinuity region. The electrically defined Harang discontinuity was located 200-300 km to the north of the magnetically defined discontinuity. The combined observational data were used to determine, as far as possible, the ionospheric distributions of height-integrated conductivities, of horizontal height-integrated current density vectors and of field-aligned currents. The results show a reduction of the Hall conductivities and upward Birkeland currents at and around the electrically defined Harang discontinuity. Some horizontal ionospheric current crosses the discontinuity, being diverged northward from the eastward electrojet and joining the westward electrojet, after further counterclockwise rotation.
ARK: https://n2t.net/ark:/88439/y096847
Permalink: https://geophysicsjournal.com/article/219
 
Rockmagnetism and palaeomagnetism of an Early Cretaceous/Late Jurassic dike swarm in Rio Grande do Norte, Brazil
Get PDFTen sites from a dike swarm of early Cretaceous/Late Jurassic age in northeast Brazil (5.7°S, 36.6°W) yielded a pole at 80.6°N, 95°E with A95 = 9.5°, K = 26.7 after AF cleaning. Rock-magnetic investigations and chemical analyses revealed titanomagnetites and maghemites, both with low titanium content showing ilmenite exsolution lamellae (oxidation class III). These are due to an internal high-temperature oxidation during cooling of the large dikes, follwed by low-temperature oxidation and/or hydrothermal alterations. There is evidence that with low-temperature oxidation the Ti-to-Fe ratio increases, a finding that is consistent with previous studies. Hysteresis parameters and susceptibility versus temperature curves can be interpreted in terms of pseudosingle-domain behavior with a trend toward multidomain behavior in accordance with moderate-to-weak stability of the remanence.
ARK: https://n2t.net/ark:/88439/y008126
Permalink: https://geophysicsjournal.com/article/126
 
Discrete frequency inequalities for magnetotelluric impedances of one-dimensional conductors
Get PDFFor the one-dimensional magnetotelluric inverse problem the ties between the impedances at neighbouring frequencies, reflecting the analytical properties of the transfer function, are expressed in terms of inequalities between the data. After the derivation of some elementary necessary constraints for data sets with two or three frequencies, a set of necessary and sufficient conditions warranting the existence of a one-dimensional conductivity model in the general M-frequency case is given. This set of constraints characterizes a 1-D data set by the signs of 2M determinants derived from the data.
ARK: https://n2t.net/ark:/88439/y087338
Permalink: https://geophysicsjournal.com/article/228
 
Ray theoretical strong motion synthesis
Get PDFWe present some results of the theory of high-frequency radiation by seismic sources. The emphasis will be placed on the kinematics of high-frequency waves, especially the stopping phases produced when the rupture encounters barriers of general shape. These results will be obtained from the representation theorem in which we replace the Green function by its asymptotic approximation at high frequencies, i.e. what is usually called the far-field approximation. This yields an expression akin to the Kirchhoff diffraction integral used in the modelling of reflection profiles and in seismic migration. The results obtained by this method are valid at distances from the fault which are longer than the dominant wavelength of the radiation. By a detailed analysis of the asymptotic method we find the wavefront discontinuities produced by rupture velocity jumps (barriers) or slip discontinuities (asperities) on the fault. Some examples of comparison between synthetics calculated with the new methods and those obtained by complete near-field synthesis will be presented. Among the examples we will consider is the circular fault, a model proposed by Bouchon for the Coyote Lake earthquake.
ARK: https://n2t.net/ark:/88439/y047732
Permalink: https://geophysicsjournal.com/article/234
 
Gaussian beam synthetic seismograms
Get PDFNumerical modelling of high-frequency seismic wave fields in complex, 2-D and 3-D, laterally varying, layered structures by the summation of elastodynamic Gaussian beams is discussed. The main attention is devoted to the expansion of the wave field into Gaussian beams, to the choice of initial parameters of Gaussian beams in these expansions and to the construction of synthetic seismograms. The Gaussian beam synthetic seismograms are regular even in regions where the ray method fails, such as the caustic region, critical region, etc. Due to the smoothing effects involved in the Gaussian beam procedure, the method is not too sensitive to the approximation of the medium and to minor details of the model. Moreover, the method does not require two-point (source-to-receiver) ray tracing. The eyaluation of Gaussian beam synthetic seismograms requires approximately the same amount of computer time as the evaluation of ray synthetic seismograms. The memory requirements are also approximately the same. Numerical examples of Gaussian beam synthetic seismograms for 2-D and 3-D structures are presented. Various possible applications of Gaussian beams to seismological problem of practical importance are outlined.
ARK: https://n2t.net/ark:/88439/y058811
Permalink: https://geophysicsjournal.com/article/144
 
Notes on wave propagation in laterally varying structure
Get PDFRecent models of Earth structure suggest large horizontal gradients, especially in shear velocities. Some changes in existing methods are required to construct synthetics for broadband signals in many situations, especially when energy can reach the receiver by up-going as well as down-going paths. This can be accomplished by allowing locally dipping structure and making some modifications to generalized ray theory. Local ray parameters are expressed in terms of a global reference which allows a de Hoop contour to be constructed for each generalized ray with the usual application of the Cagniard-de Hoop technique. Several useful approximations of ray expansions and WKBJ theory are presented. Comparisons of the synthetics produced by these two basic techniques alone, or in combination with known solutions, demonstrates their reliability and limitations.
ARK: https://n2t.net/ark:/88439/y018295
Permalink: https://geophysicsjournal.com/article/231