Journal of Geophysics
Not a member yet
262 research outputs found
Sort by
Lateral variations of the structure of the crust-mantle boundary from conversions of teleseismic P waves
Get PDFClear P-to-S converted phases are observed in teleseismic seismograms recorded at the Grafenberg array. The seismic events used are nuclear explosions from east Kasakh and deep-focus earthquakes from the region of Japan and central Asia. The identification of the converted phases has been improved by the rotation of the coordinate system. Theoretical seismograms have been computed to obtain the response of various velocity structures of the lithosphere under the receivers, using the reflectivity method of Kind (1985) for different source and receiver structures. The interpretation of the P-to-S conversions leads to a model of the crust-mantle transition zone with a first-order discontinuity and a normal velocity contrast in the north, an increased velocity contrast in the centre, and a reduced sharpness of the discontinuity in the south of the array. The sedimentary layers underneath the stations also cause strong converted phases, and they also influence strongly the waveform of the incoming P wave.
ARK: https://n2t.net/ark:/88439/y056921
Permalink: https://geophysicsjournal.com/article/225
 
Complete seismogram synthesis for transversely isotropic media
Get PDFThe response at the surface of a layered transversely isotropic medium due to a buried dislocation source can be expressed by using propagator matrices and discrete wavenumber summation. These operations produce complete seismograms for earthquake or explosion sources which include all body- and surface-vave phases for this specialized anisotropic structure. In order to test the numerical procedures, synthetic seismograms at near distances for an isotropic model are compared with those generated by other methods. The agreement is found to be satisfactory in all cases. Comparisons of synthetic seismograms for anisotropic models having a small degree of anisotropy with similar but isotropic models, show that significant differences in travel times, amplitudes and wave forms can be caused by the anisotropy.
ARK: https://n2t.net/ark:/88439/y066890
Permalink: https://geophysicsjournal.com/article/226
 
Impulsive processes in the magnetotail during substorm expansion
Get PDFAnisotropy and intensity variations of high-energy particles and magnetic variations detected by IMP-J at 37 Re in the central part of the magnetotail within 2 Re of the neutral sheet were studied during a few consecutive substorms on March 3, 1976, and related to a large body of ground observations. A close correlation is usually found between bursts observed in Pi pulsations on the ground (having a duration of about 1 min and usually being repeated in 1–3 min) and high-energy particle bursts observed in the far tail. The magnetic field response is examined relative to the onsets of high-energy particle bursts. A three-dimensional current system RIPD (Reconnection Induced Propagating Disturbance) is constructed which can, in principle, produce the magnetic field variations observed in the boundary part of the plasma sheet (PS). Previously reported properties of high-energy particle bursts (inverse energy dispersion, preferential acceleration of alpha particles, dawn-dusk asymmetry in the acceleration of electrons and protons and transient PS expansions) are found to be typical of these impulsive processes. Together with earlier results, these observations show that the expansion process (identified here as a transient reconnection or explosive tearing mode) clearly operates in an impulsive fashion. The superposition of impulse-induced propagating disturbances necessarily results in complex, variable patterns of magnetic field and thermal plasma behaviour, such as are frequently found in the PS during substorms.
ARK: https://n2t.net/ark:/88439/y027754
Permalink: https://geophysicsjournal.com/article/222
 
A comparison of upper mantle subcontinental electrical conductivity for North America, Europe, and Asia
Get PDFSpherical harmonic analysis coefficients of the external and internal parts of the quiet-day geomagnetic field variations (Sq), separated for the North American, European, Central Asian and East Asian regions, were used to determine conductivity profiles to depths of about 600 km by the Schmucker equivalent-substitute conductor method. All three regions showed a roughly exponential increase of conductivity with depth. Distinct discontinuities seemed to be evident near 255-300 km and near 450-600 km. Regional differences in the conductivity profiles were shown by the functional fittings to the data. For depths less than about 275 km, the North American conductivities seemed to be significantly higher than the other regions. For depths greater than about 300 km, the East Asian conductivities were largest.
ARK: https://n2t.net/ark:/88439/y028414
Permalink: https://geophysicsjournal.com/article/232
 
Magnetovariational and magnetotelluric studies of the Oulu anomaly on the Baltic Shield in Finland
Get PDFThe electrical conductivity structure of the Baltic Shield in Finland has been studied by magnetovariational (MV) and magnetotelluric (MT) work. First magnetometer arrays revealed the crustal Oulu conductivity anomaly which consists of a crustal conductive zone and a conductivity boundary. Since 1983 the magnetotelluric technique has been used to study the Oulu anomaly in more detail. The information obtained from the induction vectors of the MV and MT data are compared. 1 D and 2D models of the Oulu anomaly were constructed from MT data using induction vectors as additional information. In the centre of the research area the anomalous body (with a resistivity of 0.5 ohm m) lies below a depth of 4-7 km. Its width is about 25 km and its length is more than 100 km. To the south-west of this anomaly a low-resistive crustal layer exists at a depth of 14 km, whereas to the north-east no crustal layer was identified in the very resistive Karelidic realm.
ARK: https://n2t.net/ark:/88439/y097907
Permalink: https://geophysicsjournal.com/article/229
 
Paleomagnetism of the tertiary intrusives from Chalkidiki (Northern Greece)
Get PDFPaleomagnetic measurements made on Eocene-Oligocene intrusives of Chalkidiki (northern Greece) have given normal and reversed directions. The mean direction is D = 37°, I = 30°, α95 = 9° and the pole is 50°N, 140°E. This direction indicates a clockwise rotation of about 25° for Chalkidiki, similar to the rotations observed in western Greece but different from Bulgarian results.
ARK: https://n2t.net/ark:/88439/y081908
Permalink: https://geophysicsjournal.com/article/104
 
Cross spectral analysis of Swabian Jura (SW Germany) three-component microearthquake recordings
Get PDFSimilar three-component microearthquake records have been observed in the Swabian Jura (SW Germany) seismic zone for different source-receiver geometries. This data set is used to study the resolution power of cross spectral analysis techniques for the estimation of relative differential times as well as the applicability to velocity monitoring. The differential times are estimated in the frequency domain by assuming a linear-phase cross spectrum with the slope indicating the individual time difference. All earthquakes have been relocated with respect to a master event, using the relative P and S delay times from the cross spectral analysis as a measure of source mislocation. The overall location error is strongly dependent on the inital distance between master and studied event. For earthquakes initially located farther apart than approximately 1.5 km, the relocalization result in terms of total location error was poorer, whereas for events initially located closer than 1 km the precision of the relocalization was improved. The remaining residuals are of the order of 10 ms, which is approximately 3 times the digitization interval. In order to test the applicability of cross spectral analysis to velocity monitoring, synthetic data were used to model the influences of noise and source time function differences. The effect of additive white noise seems to be acceptable in cases where the S/N ratio is sufficiently high. Small changes in the shape of the source time function, however, were found to be of great influence to the differential time estimates. Variation of rise, sustain and decay times, which were negligible in the coherence spectrum, spuriously introduced phase differences which, in terms of delay times, easily reach the magnitude of the digitization interval. Thus, velocity monitoring using cross spectral analysis techniques seems to depend strongly on the equality - in contrast to similarity - of the source time functions of the events which are compared. The coherence spectrum is not a sufficient measure to detect all the significant differences.
ARK: https://n2t.net/ark:/88439/y035103
Permalink: https://geophysicsjournal.com/article/213
 
Absolute intensity of daytime whistlers at low and middle latitudes and its latitudinal variation
Get PDFThe statistical study on the field intensity of daytime whistlers at low (geomag. lat. 25°) and middle (35°) latitudes has been made, based on a lot of data obtained by the field-analysis direction finding system based on the simultaneous measurement of two horizontal magnetic field components and one vertical electric field component. At low latitude, the maximum absolute intensity is estimated to be 250 μV/m, while the corresponding maximum intensity at middle latitude amounts to 600 μV/m, being about 2.4 times that at low latitude. This latitudinal variation of daytime intensity is interpreted in terms of the joint influence of (a) source activity, (b) magnetospheric propagation effect and (c) ionospheric transmission loss. As the result, it is found that whistlers at each station are attributed to ducted propagation in the magnetosphere and have exited the ionosphere close to each observing station, as determined by the direction finding results. Furthermore, the cloud distributions observed by meteorological satellites have yielded that the duct entrance point of whistlers at each station fall within the active thunderstorm region. Hence, we can conclude that daytime whistlers are originated in the active thunderstorms in each conjugate region, are trapped in field-aligned ducts and followed by nearly the vertical exit from the ionosphere at each station. Finally the latitudinal difference of the intensity is satisfactorily interpreted in terms of the difference in the ionospheric transmission loss on the assumption of the same source intensity at each conjugate point and of no amplification in the magnetosphere.
ARK: https://n2t.net/ark:/88439/y007596
Permalink: https://geophysicsjournal.com/article/230
 
Nonlinear inversion of local seismic travel times for the simultaneous determination of the 3D-velocity structure and hypocentres - application to the seismic zone Vrancea
Get PDFA 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 conceptual model of core dynamics and the Earth's magnetic field
Get PDFA conceptual model of core dynamics and the Earth's magnetic field is presented. It differs from previous investigations in the use of an estimated core viscosity of 2 x 107 cm2 s- 1. The simplified derivations predict the correct order of magnitude for the external magnetic field and for the westward drift of the nondipole field.
ARK: https://n2t.net/ark:/88439/y077129
Permalink: https://geophysicsjournal.com/article/227