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  • 1
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A rigorous study of velocity dispersion of surface waves generated by teleseismic events propagating across the Iberian Peninsula and traversing main geological units, has been carried out from a set of selected analogue data, as digital records have only become available recently. Dispersed seismic signals have been obtained over a period of 16 years, between 1967 and 1982, at the five Iberian stations having long-period instruments. In our study, we have considered many earthquakes thus obtaining a fairly good path coverage of most of the peninsula for two-station Rayleigh wave velocity measurements. In all cases, the approach azimuths of the wavefronts were carefully checked. Several digital filtering techniques have been employed to remove the effects of multipathing and modal contamination, and to isolate the fundamental mode from Rayleigh wavetrains. Thus, we have obtained good estimates for both phase and group velocities. A time-variable filter has reduced the influence of noise and removed higher mode interference. Multiple filtering is then used to compute group velocity. Frequency-domain Wiener deconvolution is used to compute the interstation phase velocity. The determined average Rayleigh wave velocities reveal differences in the propagation conditions of the seismic energy across the peninsula. A mapping of velocities for various periods of reference, together with a mapping of errors in velocity, are the basis for obtaining the Rayleigh wave velocity distribution in the peninsula. Theoretical 2-D layered earth models are obtained by joint inversion of phase and group velocity dispersion curves using the stochastic inverse operator. In our inversion scheme, we use velocities corrected for anelastic effects. Finally, a 3-D mapping of S velocity is performed. This study shows important regional features of the deep structure of Iberia; we see small lateral inhomogeneities and also two low-velocity layers: one with shear velocities usually ranging from 4.23 to 4.31 km s-1 directly under the Moho, and another, the asthenosphere, with a negative velocity gradient for depths between 81 and 181 km, terminated at the bottom by a sharp discontinuity.
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  • 2
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Several filtering techniques have been used to remove the effects of multipathing and modal contamination, and to isolate the fundamental mode from Rayleigh wavetrains. Group velocity data are obtained by means of the multiple-filter technique. A time-variable filter has allowed the influence of noise as well as the interference produced by higher modes to be removed. Multiple filtering is then used again to compute group velocities at each station. the interstation group velocity for the fundamental mode Rayleigh wave is estimated according to the velocities at two stations. Frequency-domain Wiener deconvolution is used to compute the phase velocity between two stations. the well-known three-station method is applied to correct the distances travelled by the waves across the array and therefore to determine interstation phase and group velocities in a more accurate manner. On the other hand, lateral refraction at the Atlantic continental edge of the Peninsula is also studied. Phase velocities are corrected for the anelastic effect. Inversion of the interstation Rayleigh wave phase velocities is then made in accordance with generalized inversion theory to obtain theoretical 2-D layered earth models. In this paper, these methods are applied to Rayleigh waves generated by teleseismic events propagating across the Iberian Peninsula and recorded at WWSSN stations. As a consequence, new and principal features for the Iberian lithosphere-asthenosphere system are obtained. A very interesting feature of the the Iberian lithosphere was found-a low-velocity layer directly under the Moho, between 39 and 64 km depth, with shear velocities ranging from 4.12 to 4.37 km s-1. the Iberian asthenosphere, which lies between 100 and 180km depth, is not an homogeneous layer and shows a negative velocity gradient from top to bottom together with a sudden increase in shear velocity beneath the low-velocity zone.
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  • 3
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Coda wave analysis is used to obtain frequency-dependent coda-Q values for different seismic zones of the Iberian area. Seventeen source regions around the Geophysical Observatory of Toledo and some four seismic events per region have been considered in this study. We have used an iterative Fourier analysis technique to see the variation of the frequency along the coda, also taking into account the instrument response. We have applied a suitable criterion to select the predominant frequency every 5 s along the coda. The variation of the frequency with time for each region is averaged with a second-degree polynomial, which is compared to master curves obtained directly from the response of the seismograph system, in order to determine the elastic quality factor Q. It has been observed that the frequency-time curves thus obtained are better explained if Q is considered as an exponential function of the peak frequency. The main result of this work is a set of 1 Hz Q values with a clear indication that frequency dependence of Q exists, although the bandwidth from which our conclusions are reached is only 0.5–1 Hz. The coda-Q values obtained for the tectonically most stable areas (north Spain) appear somewhat higher, Q 〉 300, than those corresponding to the seismic active zones (south Spain), Q 〈 250. Thus, a clear relationship is established between Q values and the two major tectonic provinces in Iberia. These results may be helpful for seismic risk and earthquake engineering purposes.
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  • 4
    ISSN: 0031-9201
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
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  • 5
    Publication Date: 1992-06-01
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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  • 6
    Publication Date: 2013-03-01
    Description: The uppermost 50 km of the lithospheric structure of the northern part of the Iberian Peninsula is presented in this study, by means of a set of 2D images of shear-wave velocity for depths raging from 0 to 50 km. This goal will be attained by means of the inversion of the Rayleigh-wave dispersion. For this purpose, the 23 earthquakes occurred on the vicinity of the study area, from 2001 to 2003, will be considered. The dispersion curves of these earthquakes have been measured for periods from 2 to 45 s, by combination of two digital-filtering techniques: Multiple Filter Technique (MFT) and Time Variable Filtering (TVF). The resulting set of source-station averaged dispersion curves has been inverted according to the generalized inversion theory, to get S-wave velocity models for each source-station path. Finally, these models have been interpolated by kriging to obtain a 2D mapping of the S-wave velocity structure, for the northern part of Iberia. The results presented in this paper show that the crust of the study area consisted of three main layers of varying thickness with a clear S-velocity contrast, resulting in a Moho depth of 30 km that decreases to 25 km in the eastern border of the Iberian peninsula and to 20 km in the Valencia trough. The upper crust has a sedimentary cover of 2 km thick. This upper crust has a thickness of 8 km, showing a very small lateral variation. In the middle crust (from 10 to 20 km depth), the S-velocity increases from 3.4 km/s to 3.7 km/s for the whole study area except for the east and the Valencia trough, where the S-velocities reach values of 3.9 km/s. The lower crust (from 20 to 30 km depth), exhibits S-velocities of 3.8–3.9 km/s, which jump to 4.3–4.7 km/s in the upper mantle. ©2013 Akadémiai Kiadó, Budapest, Hungary
    Print ISSN: 2213-5812
    Electronic ISSN: 2213-5820
    Topics: Architecture, Civil Engineering, Surveying , Geosciences , Physics
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  • 7
    Publication Date: 2011-10-01
    Description: In this study, we present the lithospheric structure of the south-eastern part of the Iberian Peninsula by means of a set of 2D images of shear velocity, for depths ranging from 0 to 50 km. This goal will be attained by means of the inversion of the Rayleigh wave dispersion. For it, the traces of 25 earthquakes occurred on the neighbouring of the study area, from 2001 to 2003, will be considered. These earthquakes have been registered by 11 broadband stations located on Iberia. All seismic events have been grouped in source zones to get an average dispersion curve for each source-station path. The dispersion curves have been measured for periods between 2 and 45 s, by combination of two digital filtering techniques: Multiple Filter Technique and Time Variable Filtering. The resulting set of source-station averaged dispersion curves has been inverted according to the generalized inversion theory, to get S-wave velocity models for each source-station path. Later, these models have been interpolated using the method of kriging, to obtain a 2D mapping of the S-wave velocity structure for the south-eastern part of Iberia. The results presented in this paper show that the techniques used here are a powerful tool to investigate the crust and upper mantle structure, through the dispersion analysis and its inversion to obtain shear velocity distributions with depth. By means of this analysis, principal structural features of the south-eastern part of Iberia, such as the existence of lateral and vertical heterogeneity in the whole study area, or the location of the Moho discontinuity at 30 km of depth (with an average S-velocity of uppermost mantle of 4.7 km/s), have been revealed. Other important structural features revealed by this analysis have been that the uppermost of Iberian massif shows higher velocity values than the uppermost of the Alpine domain, indicating that the massif is old and tectonically stable. The average velocity of the crust in Betic cordillera is of 3.5 km/s, while in the Iberian massif is 3.7 km/s. All these features are in agreement with the geology and other previous geophysical studies. ©2010 Springer-Verlag
    Print ISSN: 1437-3254
    Electronic ISSN: 1437-3262
    Topics: Geosciences
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  • 8
    Publication Date: 2005-07-01
    Description: A detailed dispersion analysis of Rayleigh waves generated by local earthquakes and occasionally by blasts that occurred in southern Spain, was undertaken to obtain the shear-wave velocity structure of the region at shallow depth. Our database includes seismograms generated by 35 seismic events that were recorded by 15 single-component short-period stations from 1990 to 1995. All these events have focal depths less than 10 km and body-wave magnitudes between 3.0 and 4.0, and they were all recorded at distances between 40 and 300 km from the epicentre. We analysed a total of 90 source-station Rayleigh-wave paths. The collected data were processed by standard digital filtering techniques to obtain Rayleigh-wave group-velocity dispersion measurements. The path-averaged group velocities vary from 1.12 to 2.25 km/s within the 1.0-6.0 s period interval. Then, using a stochastic inversion approach we obtained 1-D shear-wave velocity–depth models across the study area, which were resolved to a depth of circa 5 km. The inverted shear-wave velocities range approximately between 1.0 and 3.8 km/s with a standard deviation range of 0.05–0.16 km/s, and show significant variations from region to region. These results were combined to produce 3-D images via volumetric modelling and data visualization. We present images that show different shear velocity patterns for the Betic Cordillera. Looking at the velocity distribution at various depths and at vertical sections, we discuss of the study area in terms of subsurface structure and S-wave velocity distribution (low velocity channels, basement depth, etc.) at very shallow depths (0–5 km). Our results characterize the region sufficiently and lead to a correlation of shear-wave velocity with the different geological units features. ©2005 Springer
    Print ISSN: 0169-3298
    Electronic ISSN: 1573-0956
    Topics: Geosciences , Physics
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  • 9
    Publication Date: 2013-04-01
    Description: The 3D S-velocity structure beneath Africa is shown by means of a 2D S-velocity mapping for depths raging from zero to 500 km, determined by the regionalization and inversion of Rayleigh-wave dispersion. The traces of 94 earthquakes, occurred from 1990 to 2009 in the study area, have been used to obtain the Rayleigh-wave dispersion. These earthquakes were registered by 61 seismic stations located on Africa and the surrounding area. The dispersion curves were obtained for periods between 5 and 300 s, by digital filtering with a combination of MFT and TVF filtering techniques. After that, all seismic events (and some stations) were grouped to obtain a dispersion curve for each source-station path. These dispersion curves were regionalized and after inverted according to generalized inversion theory, to obtain shear-wave velocity models for rectangular blocks with a size of 5° × 5°. The 3D S-velocity structure obtained through this procedure is shown in the 2D S-velocity maps plotted for several depths. These results agree well with the geology and other geophysical results previously obtained. The obtained S-velocity models suggest the existence of lateral and vertical heterogeneity. The zones with consolidated and old structures (as cratons) present greater S-velocity values than the other younger zones. Nevertheless, in the depth range from 20 to 40 km, the different Moho depths present in the study area generate the principal variation of S-velocity. A similar behaviour is found for the depth range from 60 to 230 km, in which the lithosphere–asthenosphere boundary generates the principal variations of S-velocity. Finally, it should be highlighted a new and interesting feature obtained in this study: the definition of the base of the asthenosphere, for depths ranging from 160 to 280 km, in the whole African continent. ©2012 Springer-Verlag Berlin Heidelberg
    Print ISSN: 1437-3254
    Electronic ISSN: 1437-3262
    Topics: Geosciences
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  • 10
    Publication Date: 2007-07-01
    Description: The lithospheric structure of the Sinai Peninsula is shown by means of nine shear velocity profiles for depths ranging from zero to 50 km, determined from the Rayleigh wave analysis. The traces of 30 earthquakes, which occurred from 1992 to 1999 in and around the study area, have been used to obtain Rayleigh wave dispersion. These earthquakes were registered by a broadband station located in Egypt (KEG station). The dispersion curves were obtained for periods between 3 and 40 s, by digital filtering with a combination of MFT and TVF filtering techniques. After that, all seismic events were grouped in source zones to obtain a dispersion curve for each source-station path. These dispersion curves were inverted according to generalized inversion theory, to obtain shear wave velocity models for each source-station path, which is the main goal of this study. The shear velocity structure obtained for the Sinai Peninsula is shown through the shear velocity distributions with depth. These results agree well with the geology and other geophysical results, previously obtained from seismic and gravity data. The obtained velocity models suggest the existence of lateral and vertical heterogeneity. The shear velocity increases generally with depth for all paths analyzed in the study area. Nevertheless, in some paths a small low velocity channel in the upper or lower crust occurs. Along these profiles, it is found that the crustal structure of the Sinai Peninsula consists of three principal layers: upper crust with a sedimentary layer and lower crust. The upper crust has a sedimentary cover of 2 km thick with an average S-velocity of 2.53 km/s. This upper crust has a variable thickness ranging from 12 to 18 km, with S-wave velocity ranging from 3.24 to 3.69 km/s. The Moho discontinuity is located at a depth of 30 km, which is reflected by a sharp increase in the S-velocity values that jump from 3.70–4.12 to 4.33–4.61 km/s. ©2007 Springer Science+Business Media B.V.
    Print ISSN: 0169-3298
    Electronic ISSN: 1573-0956
    Topics: Geosciences , Physics
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