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  • 1
    ISSN: 1365-2478
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences , Physics
    Notes: The use of parallel computers makes simulation of elastic waves feasible throughout large structures by means of recent advances in domain decomposition methods. We introduce a competitive parallel algorithm for the propagation of elastic waves in complex heterogeneous media using finite-element discretization. This parallel method, called the multiblock method, performs more efficiently than classical domain decomposition techniques based on substructuration, such as the Schur complement technique. It reduces considerably the amount of communication amongst processors because the interface problem between subdomains is solved by taking advantage of Huygens' principle for wave propagation. We provide some numerical examples and detailed studies on the efficiency and performance of the algorithm, proving that it is competitive and less costly, from the computational viewpoint, than algorithms based on the Schur technique.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: bA method based on the coda attenuation law: Q=Q0(f/f0)v leads to the determination of the lateral variation of coda-Q in the southern part of the Iberian Peninsula using seismograms belonging to the seismological network of the Cartuja Observatory, located in Granada. The lateral variation of Q0 (Q value corresponding to a reference frequency f0 of 1 Hz) and its frequency dependence for the 1 to 5 Hz frequency range are, in general, in agreement with coda-Q values for frequencies less than about 1 Hz, previously determined in the region under study.To determine the coda-Q values analytical functions have been used to fit the magnification curves of the vertical component short-period seismographs belonging to the Cartuja network. The problem is solved by using least-squares techniques and non-linear inversion. The determined coda-Q0 values and its frequency dependence correlate well with several known geophysical parameters in the southern part of the Iberian Peninsula.
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  • 3
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A detailed dispersion analysis of Rayleigh waves propagating across the Iberian Peninsula is carried out. The starting data are high-quality long-period data recorded at the broad-band NARS stations installed in (he Iberian Peninsula during the ILIHA project. We apply methods to obtain a correct selection of data and subsequent two-station surface-wave velocity measurements. A total of 64 teleseismic events recorded by the NARS array and 143 seismic paths have been studied. Several techniques which provide a significant improvement in the signal-to-noise ratio are employed to remove higher-mode interference efficiently and improve the isolation of the fundamental-mode Rayleigh wave from the seismograms. Thereafter, the interstation Rayleigh wave phase and group velocities are determined. We perform simultaneous inversion of phase-and group-velocity dispersion data by means of the stochastic inverse operator, and lest the reliability of the results by computing resolving kernels and also by forward modelling. A regionalization procedure based on the Backus-Gilbert approach for linear inversion of traveltime data is applied.Both the inversion results and the contoured shear-wave velocity panoramas display the main features of the deep structure of Iberia. We find a subcrustal low-velocity channel which extends over practically the whole peninsular area and spans a depth interval of approximately 40–50 km; it exhibits velocities of between 4.30 and 4.50 km s-1. At depths of 66–81 km, we find the highest velocities in the lithosphere, which reach values of 4.85 km s-1 in many cases. The low-velocity channel of the asthenosphere spans a large depth interval of approximately 80-180 km; it shows the lowest velocity values computed by us. We find velocities decreasing with depth, which are of the order of 4.25–4.36 km s-1 for the first 40 km and of the order of 4.00–4.25 km s-1 for the rest. The upper mantle under the asthenosphere exhibits high velocities, which range between 4.62 and 4.82 km s-1 in most cases.The shear-wave velocity structure of the Iberian subcrustal lithosphere and asthenosphere is mapped at 11 depth intervals from 24 to 201 km. At the top of the mantle, relatively low velocities span the Ebro Valley and also the southern third of the peninsula. Low velocities appear in the south-southwest quadrant, and high velocities occur over the Hercynian basement. At greater lithospheric depths, very low velocities extending over the whole peninsula suggest a low-velocity channel of non-uniform lateral structure, where a reduced zone to the west of the Iberian plateau shows relatively high velocities. At the greatest lithospheric depths, the whole Iberian block is fairly homogeneous laterally. The asthenosphere shows a notable lateral heterogeneity as well. We distinguish two parts: the upper asthenosphere, a 40 km thick layer with predominant velocity values of 4.25 km s or more; and the lower asthenosphere, a 60 km thick layer with velocity values generally below 4.25 km s-1. The upper asthenosphere seems to be less laterally heterogeneous than the lower asthenosphere. The lower asthenosphere exhibits a more pronounced negative velocity gradient than the upper asthenosphere.
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  • 4
    ISSN: 1573-0840
    Keywords: early instrumental earthquakes ; magnitude ; intensity–magnitude relationships ; Iberian region
    Source: Springer Online Journal Archives 1860-2000
    Topics: Energy, Environment Protection, Nuclear Power Engineering , Geography , Geosciences
    Notes: Abstract The magnitudes of early instrumentally recorded earthquakes in the Iberian region (1912–1962) have been studied through processing of digitized seismograms of Wiechert seismograph and analysis of macroseismic information. A magnitude system based on instrumental registrations and macroseismic observations has been proposed. It consists of two compatible magnitude formulae depending on the total duration of seismic oscillations and on the maximum ground amplitude/period ratio of surface waves and includes correspondent intensity–magnitude relationships.
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  • 5
    Publication Date: 1997-08-01
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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  • 6
    Publication Date: 2010-09-13
    Description: Surface wave data were initially collected from events of magnitude Ms ≥ 5.0 and shallow or moderate focal depth occurred between 1980 and 2002: 713 of them generated Rayleigh waves and 660 Love waves, which were recorded by 13 broadband digital stations in Eurasia and India. Up to 1,525 source-station Rayleigh waveforms and 1,464 Love wave trains have been processed by frequency-time analysis to obtain group velocities. After inverting the path-averaged group times by means of a damped least-squares approach, we have retrieved location-dependent group velocities on a 2° × 2°-sized grid and constructed Rayleigh- and Love-wave group velocity maps at periods 10.4–105.0 s. Resolution and covariance matrices and the rms group velocity misfit have been computed in order to check the quality of the results. Afterwards, depth-dependent SV- and SH-wave velocity models of the crust and upper mantle are obtained by inversion of local Rayleigh- and Love-wave group velocities using a differential damped least-squares method. The results provide: (a) Rayleigh- and Love-wave group velocities at various periods; (b) SV- and SH-wave differential velocity maps at different depths; (c) sharp images of the subducted lithosphere by velocity cross sections along prefixed profiles; (d) regionalized dispersion curves and velocity-depth models related to the main geological formations. The lithospheric root presents a depth that can be substantiated at ~140 km (Qiangtang Block) and exceptionally at ~180 km in some places (Lhasa Block), and which exhibits laterally varying fast velocity very close to that of some shields that even reaches ~4.8 km/s under the northern Lhasa Block and the Qiangtang Block. Slow-velocity anomalies of 7–10% or more beneath southern Tibet and the eastern edge of the Plateau support the idea of a mechanically weak middle-to-lower crust and the existence of crustal flow in Tibet. ©2009 Birkhäuser Verlag Basel/Switzerland
    Print ISSN: 0033-4553
    Electronic ISSN: 1420-9136
    Topics: Geosciences , Physics
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  • 7
    Publication Date: 2003-02-01
    Description: The seismic velocity structure of the Granada Basin (southern Spain) at depths of 1-4 km is investigated by dispersion analysis of 0.5- to 5.9-sec Rayleigh waves. We have used records from 12 quarry blasts detonated between 1990 and 2000 and 16 local earthquakes with magnitudes between 2.6 and 3.8 that occurred between 1990 and 1997. All events were recorded on the Regional Seismic Network of Andalucia, Spain, and on the National Seismic Network. P-, S- and Rg-phases were recorded at 13 stations at distances between 10 and 72 km from the source. The analysis and inversion of Rg waveforms is performed using multiple filtering techniques and a generalized inversion approach. We measured ray-path group velocities from 0.83 to 2.15 km/sec in the 0.6- to 5.8-sec period interval, and obtained shear-wave velocity profiles for 29 source-station paths. The shear velocities lie between 0.8 and 3.34 km/sec within a standard deviation band of 0.05-0.14 km/sec. From these models, it is possible to infer the most conspicuous features in the upper 4 km of the region and the existence of some degree of lateral variation in velocity. After applying an imaging method aimed at volumetric modeling and data visualization, we present the first velocity images obtained for the Granada Basin. Our results characterize the basin and are correlated with the surface geology.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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  • 8
    Publication Date: 2000-10-01
    Description: A collection of 18 Iberian earthquakes, felt with epicentral intensity I (sub 0) 〉 or =VI (MSK) in the Iberian Peninsula during the period 1923-1961, just before the introduction of the WWSSN stations in 1962, have been investigated through digital reconstruction of old seismograms recorded in four Spanish seismological observatories: Alicante, Almeria, Ebro, and Toledo. Spectral analysis of the seismic waves recorded at the different stations has been performed and the seismic moment of the early instrumentally recorded earthquakes considered has been calculated according to the model of Brune (1970) and applying corrections by instrumental response, anelastic attenuation, free surface, and radiation pattern. A seismic moment-magnitude relation has been obtained, and the result has been compared with other regressions obtained for different seismo-active regions in Spain and in the world. The moment magnitudes of the earthquakes considered have been calculated according to the scale of Kanamori (1977) and Hanks and Kanamori (1979). Finally, a magnitude correction for the conversion of surface wave magnitude to moment magnitude has been proposed: M = 0.96 M(A/T)-0.36. This conversion should be considered valid for the 4.0-6.0 surface-wave magnitude interval, which spans well the characteristic range for the moderate seismic activity in the Iberian Peninsula.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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  • 9
    Publication Date: 2002-08-01
    Description: The human losses after strong earthquakes that occurred in the world during the twentieth century have been analyzed, and a quantitative model for a preliminary assessment of casualties is proposed. It consists of a correlation between the number of casualties and the earthquake magnitude as a function of population density. We tackle the distribution of the total number of casualties within areas of different macroseismic intensity. Prognostic estimations of the expected number of killed or injured people caused by a supposed strong earthquake in Andalucia (Spain), using the model based on worldwide data, are suggested. Prognostic estimations based on specific data about the Kanto-Tokai (Japan) region are likewise given and compared with the number of casualties due to the 1995 Kobe (Japan) earthquake. In relation to the expected number of victims in areas affected by strong seismic impacts, we compute the casualty rate as the number of people killed divided into the inhabitants of a region and show its variation for different population density groups in the case of two extreme earthquake magnitudes.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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  • 10
    Publication Date: 2005-03-01
    Description: Prognostic estimations of the expected number of killed or injured people and about the approximate cost associated with the damages caused by earthquakes are made following a suitable methodology of wide-ranging application. For the preliminary assessment of human life losses due to the occurrence of a relatively strong earthquake we use a quantitative model consisting of a correlation between the number of casualties and the earthquake magnitude as a function of population density. The macroseismic intensity field is determined in accordance with an updated anelastic attenuation law, and the number of casualties within areas of different intensity is computed using an application developed in a geographic information system (GIS) environment, taking advantage of the possibilities of such a system for the treatment of space-distributed data. The casualty rate, defined as the number of killed people divided by the number of inhabitants of the affected region, is also computed and we show its variation for some urban concentrations with different population density. For a rough preliminary evaluation of the direct economic cost derived from the damages, equally through a GIS-based tool, we take into account the local social wealth as a function of the gross domestic product of the country. This last step is performed on the basis of the relationship of the macroseismic intensity to the earthquake economic loss in percentage of the wealth. Such an approach to the human casualty and damage levels is carried out for sites near important cities located in a seismically active zone of Spain, thus contributing to an easier taking of decisions in emergency preparedness planning, contemporary earthquake engineering and seismic risk prevention. ©2005 Springer
    Print ISSN: 0921-030X
    Electronic ISSN: 1573-0840
    Topics: Energy, Environment Protection, Nuclear Power Engineering , Geography , Geosciences
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