-
Complete synthetic seismograms can be computed by the superposition of the fundamental and higher modes of Rayleigh waves. The usefulness of this approach is illustrated by the fact that it is possible to reproduce with sufficient detail experimental signals lasting several tens of seconds and having a high-frequency content (up to 1 Hz). The method has been proven to work even for higher frequencies, up to 10 Hz. To illustrate the source and structure modelling using this method, the whole experimental records from the Carder displacement meter at the station El Centro for the 1968 Borrego Mountain earthquake have been fitted. Seismic profiles (displacement, velocity and acceleration) have been synthesized and they clearly show the expected agreement between ray travel times and arrival times of different phases.
ARK: https://n2t.net/ark:/88439/y068630
Permalink: https://geophysicsjournal.com/article/246
-
The theory describing the relaxation of an incompressible, layered Maxwell half-space is developed. The approach is based on the analytic solution of the associated elastic model and the subsequent application of the correspondence principle. The viscoelastic theory follows normal-mode theory, which allows the independent and exact determination of the relaxation-time and amplitude spectra for each mode of relaxation. The solution is tested by calculating the response of several models in the wavenumber and spatial domains. The examples are selected with regard to postglacial adjustment in Fennoscandia and analyse effects caused by (a) varying lithospheric thickness, (b) adding an asthenosphere, (c) increasing lower-mantle viscosity, (d) permitting relaxation of the lower lithosphere or (e) introducing density contrasts at 400-km and 670-km depths.
ARK: https://n2t.net/ark:/88439/y058111
Permalink: https://geophysicsjournal.com/article/245
-
Preferred orientation of hematite ore from Minas Gerais, Brazil, was investigated by reflected-light microscopy, X-ray structural goniometry and magnetic anisotropy. A close relationship was found between c-axis fabrics determined by magnetic and non-magnetic methods; experiments confirmed the results of the theoretical treatment. For routine work it is advantageous to use both types of methods, profiting from rapidity of measurement of magnetic anisotropy and from detailed c-axis pole figures of pilot specimens provided by X-ray goniometry.
ARK: https://n2t.net/ark:/88439/y048352
Permalink: https://geophysicsjournal.com/article/244
-
The detection of hydromagnetic waves on spacecraft and at the Earth's surface indicate disturbances of the geomagnetosphere. These disturbances are diagnostic of processes and boundaries occurring within the plasma of the Earth's space environment. In order to delineate the processes and boundaries a dense network of measuring points is desirable. Parameters that have proved useful are the frequency and the relative amplitudes and phases of the waves at different positions in space. This paper summarises some of the characteristics of hydromagnetic waves in the magnetosphere and the modification of the signal brought about by the boundary conditions imposed by the ionosphere-neutral atmosphere-Earth system.
ARK: https://n2t.net/ark:/88439/y039423
Permalink: https://geophysicsjournal.com/article/243
-
The subducted Nasca plate produces anomalies of spectacular low electrical resistivity beneath the Andean knee. This is the result of two magnetotelluric field campaigns in 1982. The measurements were carried out on a profile from the Pacific coast to the Eastern Cordillera crossing northern Chile and southern Bolivia. Several anomalies may be distinguished. The strongest anomaly has been detected beneath the Pre- and West-Cordillera, with resistivities around 0.5 Ωm at depths greater than 5-10 km, strike direction roughly parallel to the main crest of the High-Cordillera. But also, the Altiplano is underlain – at a depth of 40-50 km – by very low-resistive material. In agreement with other geophysical and geological observations, we interpret these low resistivities as being due to the partially melted state. Further arguments identify the Pre-Cordillera anomaly, probably, with a plutonite which is still "alive", and the highly-conductive material 40 km beneath the Altiplano with the source of andesitic magma. A crucial point in this study is the distinction between the E- and B-polarization of the apparent resistivities. We distinguished both polarizations by the direction of the induction arrows and by the lateral continuity of the apparent resistivity.
ARK: https://n2t.net/ark:/88439/y029284
Permalink: https://geophysicsjournal.com/article/242
-
The Rhinegraben is one of the best studied rift structures in the world. In the past 20 years, it has been the site of many seismic refraction studies. However, most of the profiles have only been interpreted using 1-D methods. In this paper a combined 2-D and 1-D analysis of the only reversed profile within the graben proper is presented. The new interpretation has resulted in changes in the P-wave velocity model of the Rhinegraben. In the new model, the upper crust of the graben, representing the sedimentary graben-fill, is found to be 6-7 km thick with Vp less than 6.0 km/s. The mid-crust, which is probably of granitic/gneissic composition, shows a practically constant velocity of 6.25 km/s. This observation is supported by seismic reflection data. At the base of the crust is a velocity discontinuity followed by a 1.5 km thick transition zone into the upper mantle. From north to south along the graben, the crust thins by 3 km and the upper mantle velocity changes from about 8.4 km/s to 7.9 km/s. In comparison with the old 1-D model, the new model shows a simplification of the structure of the lower crust and a thinning of the crust/mantle transition zone from 5 km to 1.5 km. Furthermore, in the old model, the upper mantle velocity underneath the graben was found to be constant at 8.1 km/s, which is in contrast to the changing upper mantle velocity found in the new model.
ARK: https://n2t.net/ark:/88439/y019805
Permalink: https://geophysicsjournal.com/article/241
-
A new method for solving the full nonlinear problem of simultaneous inversion for 3D structure and hypocentres (SSH method) has been developed. In comparison with the simplified linear inversion procedure of Aki and Lee (1976), the method proposed here exhibits several extensions and improvements: a) The seismic forward problem is solved exactly by a newly developed 3D ray tracing method. This procedure has been conceived as a shooting method. The 3D heterogeneous medium is parametrized by rectangular blocks. b) The full nonlinear inverse problem is solved. The method works iteratively and was conceived in terms of the Levenberg - Marquardt (LM) algorithm. In each iteration step the seismic forward problem is solved with the ray tracing procedure and a linear inversion is performed. c) In order to improve the stability and uniqueness of the inverse solution, a priori information on the model space may be used in the inversion process. For example, a known crustal structure may be exploited for the retrieval of the deeper lithosphere. The SSH method has been tested on various theoretical 1D and 3D models and its capabilities and limitations are discussed. The results demonstrate the existence of a tradeoff between hypocentral depths, origin times and seismic velocity, particularly when the horizontal dimensions of the recording network are small relative to the depths of the earthquakes. The SSH method has been applied to real earthquake data of the Vrancea region, Romania. About 50 crustal and intermediate depth events were used in the inversion process. First, the crustal events were inverted to infer the crustal structure of the region. This crustal information was then used as an a priori constraint in the inversion of the intermediate depth events. The 3D-velocity model found for the Vrancea region reduces the RMS value of the travel-time residuals from 0.5 to 0.3 s. The most essential feature of the model is a high-velocity anomaly of about 4%-6% beneath the eastern Carpathian foredeeps, extending from about 80 to 160 km depth. This appears to be in agreement with results of teleseismic studies of other authors and may be understood in terms of the plate tectonic concept proposed for the Vrancea region. Due to lack of spatial resolution and high standard errors, the results obtained above should, nevertheless, be taken with some care and have to be substantiated by further investigations with higher quality data.
ARK: https://n2t.net/ark:/88439/y008846
Permalink: https://geophysicsjournal.com/article/240
-
A comprehensive review of the Early Cretaceous seafloor-spreading magnetic anomalies (M0 to M10) in the eastern Indian Ocean leads to the isolation of a distinctive magnetic anomaly at the continent-ocean boundary (COB). This anomaly is traceable 2000 km southward from the rifted margin of the magnetically smooth central Exmouth Plateau, through the transform-faulted and rifted margins of the Cuvier Abyssal Plain and Carnarvon Terrace and the set of narrow spreading segments south of the Zenith-Wallaby Fracture Zone to the area west of Perth. The anomaly corresponds to the COB as indicated by: 1. the lower part of the continental slope in a mean water depth of 3.75 km for the rifted margin and 4.5 km for the transform-faulted margin and 2. a change in seismic-reflection character from the faulted breakup unconformity on the continent to the smooth but hyperbolic oceanic layer 2. The COB anomaly at the rifted margin is modelled by modifying the magnetization of the oldest oceanic block of the seafloor-spreading sequence adjacent to the continental crust; in places, the COB anomaly is flanked by smaller anomalies modelled as rift-related dykes in the adjacent continental crust. The amplitude of the COB anomaly, commonly twice or more that of the adjacent oceanic magnetic anomalies, is due either to a thicker or a more intensely magnetized source. The COB anomaly at the transform-faulted margin is modelled by a thick vertical body that extends 10 km seaward of the COB. The Wallaby Plateau is probably underlain by oceanic crust, as shown by the continuity of the abandoned spreading ridge of the Sonne Ridge southwestward from the Cuvier Abyssal Plain; the shape of the Wallaby Plateau and the volcanic composition of dredge-hauls indicate that it is probably a volcanic upgrowth of the oceanic crust as exemplified by Iceland today. Furthermore, like lceland, the Wallaby Plateau is crossed by magnetic anomalies that are possibly degraded seafloor-spreading anomalies. The Zenith Plateau lacks magnetic lineations and its crustal structure, like that of the Naturaliste Plateau to the south remains unknown.
ARK: https://n2t.net/ark:/88439/y098447
Permalink: https://geophysicsjournal.com/article/239
-
The relative position of the VLF/ELF emission region with respect to the plasmapause is of essential importance in studying their generation and propagation mechanism. On occasions when whistler data are not available, providing extensively the experimental determination of the plasmapause, we are obliged to rely on the theoretical model calculation or, alternatively, on the empirical formulas. The present paper deals with the evaluation of the effectiveness of the use of a theoretical model calculation in estimating the plasmapause location with reference to its comparison with in-situ electron density measurements and empirical formulas, during a specific geomagnetic storm. It is concluded that the temporal evolution with the present theoretical calculation, under a more acceptable convection electric field model, would yield a sufficiently reliable value for the plasmapause configuration rather than the empirical formulas. It can be used in the study of wave-particle interactions when whistler data are not available and also in the study of the erosion of the plasmasphere itself.
ARK: https://n2t.net/ark:/88439/y088608
Permalink: https://geophysicsjournal.com/article/238
-
Whereas any finite set of impedance data does not constrain the electrical conductivity σ (z) at a fixed level z in a 1D-model, the conductance function S (z2) as the depth-integrated conductivity from the surface to the depth z2 will be constrained. Assuming only the non-negativity of σ (z), it is shown that for a given depth z2 the models generating the lower and upper bound of S (z2) consist of a sequence of thin sheets. The determination of the positions of the thin sheets and their conductances leads to a system of nonlinear equations. As a limitation the present approach requires the existence of a model, which exactly fits the data. The structure of the extremal models as a function of z2 is discussed in examples with a small number of frequencies. Moreover, it is shown that any set of complex 1D impedances for M frequencies can be represented by a partial fraction expansion involving not more than 2M (positive) constants. For exactly 2M constants there are two complementary representations related to the lower and upper bound of S (z2). For the simple one-frequency case, a more general extremal problem is briefly considered, where the admitted conductivities are constrained by a priori bounds σ – (z) and σ + (z) such that σ – (z) ≤ σ (z) ≤ σ + (z). In this case, the extremal models for S (z2) consist of a sequence of sections with alternating conductivities σ – (z) and σ + (z). The sharpening of conductance bounds by incorporating a priori information is illustrated by an example.
ARK: https://n2t.net/ark:/88439/y078099
Permalink: https://geophysicsjournal.com/article/237
