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  • 1
    ISSN: 1573-157X
    Keywords: Roermond ; source inversion ; regionalstructure
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The Mw = 5.4 Roermond earthquake of April 13, 1992, is used as a 'test' earthquake for the development of source inversion methods at a regional scale in Europe. We combine structural modelling of the European continent (Du et al., 1997) with two source inversion methods derived from Sileny et al. (1992), and Mao et al. (1994). We show that following this strategy, it is possible to fully analyze the inverse problem of the hypocentral relocation, source mechanism and rupture history. We define and discuss our methodology on the basis of the inverse problem and of the associated tools. The results of our application to the Roermond earthquake are discussed in the light of other previously published solutions. Such an approach appears to offer a promising tool for the global description of seismic sources in regions well studied from the structural point of view, through waveform inversion of a few regional records.
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  • 2
    ISSN: 1573-1626
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Geosciences , Physics
    Description / Table of Contents: Summary The technique of spherical tensors is applied to the problem of stress in the mantle. An efficient method for stress computation is developed for the Newtonian mantle with spherical boundaries assuming that the density and gravitational potential in the form of harmonic expansions are known. The ways of including more complicated rheological models as well as the non-spherical shape of boundaries are outlined.
    Notes: Резюме Пре¶rt;ложен меmо¶rt; рaсчеma нanряженuŭ в мaнmuu, коmорыŭ основaн нa nрuмененuu annaрama сферuческuхгaрмонuческuх mензоров. Пре¶rt;nолaгaеmся, чmо рaзложенuе nлоmносmu uгрaвumaцuонного nоmенцuaлa nо сферuческuм функцuям uзвесmно u чmо мaнmuя nре¶rt;сmaвляеm собоŭ вязкую жu¶rt;косmь со сферuческuмuгрaнuцaмu. Обсуж¶rt;ены возможносmu обобщенuя меmо¶rt;a нa случaŭ более сложноŭ реологuu сре¶rt;ы u несферuческоŭ формыгрaнuцы.
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  • 3
    ISSN: 1573-1626
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Geosciences , Physics
    Notes: Summary The results of laboratory measurements on square perspex models with a stress concentrator under uniaxial compression are presented. An attempt was made to interpret these results in order to determine the mutual relations between the mechanical parameters characterizing the treated models, including their focal zones, parameters of model loading, parameters of seismogenic displacement and parameters of radiated elastic pulses. Particular attention was given to the study of slip displacement, slip velocity, rupture velocity, nucleation points of rupture propagation, nucleation points of elastic radiation and to the frequency analysis of radiated pulses. Measurements of the displacement in the focal zone enabled us to determine the source function, to construct theoretical seismograms in a far field and to compare them with the real pulses from ultrasonic transducers located there. This allowed testing theoretical and experimental approaches to the study of how slip displacement, slip velocity and rupture velocity are related to the parameters of radiated pulses.
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  • 4
    ISSN: 1573-1626
    Keywords: source mechanism ; Vrancea region ; moment tensor ; waveform inversion ; modal summation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Geosciences , Physics
    Notes: Summary The inversion of high-frequency seismograms is performed to retrieve source mechanisms, hypocentral depths and source time functions of two weak Vrancea earthquakes (ML=3.3), one that occurred in the crust, the other in the lithospheric part of the mantle. The digital waveforms recorded by the local Romanian network are used. Synthetic seismograms are computed by the modal summation method, using the point source approximation, for horizontally layered anelastic media. For each source-to-station path a different structural model is adopted which represents the best 1-D approximation of the medium in this azimuth. Thus, lateral inhomogeneities are taken into account in a simplified way. The source is described by the full moment tensor, allowing both deviatoric and volumetric components to be resolved. Although the structural models are simplified for the range of epicentral distances (15〈Δ〈〈170 km) considered, we find that the fit between the synthetic and observed seismograms is satisfactory for frequencies less than a few Hz. The few P-wave polarities available are not sufficient to determine a reliable source mechanism by standard methods, while the waveform inversion allows us to retrieve source mechanisms that are stable with respect to different boundary conditions and in good agreement with the observed polarities. The source time function is the less stable inverted parameter, being the most influenced by the simplification of the structural models.
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  • 5
    ISSN: 1573-1626
    Keywords: West Bohemia/Vogtland region ; earthquake swarm ; multiplet events ; seismic source mechanisns ; seismic moment tensor ; non-double-couple components
    Source: Springer Online Journal Archives 1860-2000
    Topics: Architecture, Civil Engineering, Surveying , Geosciences , Physics
    Notes: Abstract In order to learn more about the nature of the dynamic processes taking place in the West Bohemia/Vogtland earthquake swarm region, we investigated the temporal and spatial variations of the source mechanisms of the January 1997 swarm beneath Nový Kostel (NKC). Visual analyses of WEBNET seismograms of over 800 events revealed that a specific feature of this swarm was the occurrence of eight classes of multiplet events. The result of single-source, absolute moment tensor inversion of the P and SH peak amplitudes of a subset of 70 events representing all multiplet classes indicated that eight statistically significant types of mechanisms occurred during the swarm. Two of them, types A and B in our denotation, comprised all M L ≥ 1.3 events and predominated in the swarm. Type A were pure strike-slip mechanisms or strike-slip mechanisms containing a small normal component, with a nearly pure double-couple source. For class B events, oblique-thrust faulting and non-double-couple components significant at a fairly high confidence level were typical. Type A events predominated in the southern subcluster of the swarm, whereas most of type B events occurred in the subcluster northwards from NKC. This indicates that two major seismogenic planes were active during the swarm. The swarm essentially developed in four phases: in the first, type A events prevailed and the southern plane was active; during the second, characterised by the occurrence of both type A and B events (the former in the southern, the latter predominantly in the northern subcluster), the activity of the swarm culminated; in the third and fourth, the occurrence of type B events in the northern plane predominated, and only weak single events occurred southwards from NKC. Mechanisms of types AB , C , D , E , F and G , which were typical for M L ≤1.2 events, occurred randomly throughout the swarm. Type AB events were identified in both the southern and northern clusters, type C , E , F and G mechanisms only southwards from NKC. Type D events exhibited a large scatter of hypocentres which fell in neither the southern nor the northern cluster. Focal mechanisms like those reported in this study and with analogous temporal and spatial variations were observed by other authors already fifteen years ago in the 1985/86 earthquake swarm and may, therefore, be typical for the region under study.
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  • 6
    ISSN: 0031-9201
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Geosciences , Physics
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  • 7
    ISSN: 1420-9136
    Keywords: Rockburst mechanism ; implosion source component ; radiation pattern ; inversion of first motion amplitudes
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract One of the important questions of rockburst prevention is the understanding of the mechanism of rockburst source. This question can be effectively studied by direct seismic observations in the rockburst regions. For this purpose, the distribution ofP-wave onset signs and the inversion of first motion amplitudes were utilized as the basic method. In such a way the coal mine regions in Poland (Upper Silesia) and in Czechoslovakia (Kladno coal mine district) were studied as part of the Polish-Czechoslovak rockburst investigation project. More than 250 rockburst events were recorded here in the decade 1977–1983, and analyzed. The results of the statistical analysis of these data allow us to formulate and introduce a model of the rockburst source with an implosion component. The suitability of this conception was verified by laboratory simulation conditions; it was confirmed that the seismoactive displacements with a clear implosive component were recorded in the neighbourhood of a stress concentrator weakened by holes. The results of both the field observations and laboratory tests were in good agreement with the theoretically derived radiation patterns for a combined shear-implosive source and also with the theoretical conception of such a source based on real geometrical configurations of mine excavations and tectonic dislocations. The rockbursts treated exhibit a dominant shear component, the magnitude of the additional implosive component not exceeding 10 percent of the shear component.
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  • 8
    ISSN: 1420-9136
    Keywords: Physical modelling ; tensile fracturing ; shallow earthquakes ; tensile source component
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract The laboratory tests carried out in studying shear and tensile seismogenic displacements occurring in compressed samples, led us to search for earthquakes with a tensile source component. To determine this component in the seismic focus, a special procedure based on the construction of radiation patterns of the combined shear-tensile type is introduced. The criteria for selecting the events produced by the combined source mechanism are listed, and their limitations are mentioned. From the seismic zones with good azimuthal distribution of stations in the world seismic network nine earthquakes which occurred in the 6-year period 1976–1981 were analyzed; for these events better agreement of the observed and theoretical patterns was found for the combined shear-tensile source mechanism than for the pure double-couple mechanism. However, the share of the tensile component was always, found to be relatively small, ranging from 1 to 13 percent of the shear component. The comparison of the two solutions (double-couple vs. combined shear/tensile) is based on the first onset signs statistics. The results obtained indicate that tensile fracturing does not play a substantial role in the total amount of released seismic energy; on the orther hand, it is expected to be more important in the creation and development of focal zone morphology from both the instantaneous and long-term point of view.
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  • 9
    ISSN: 1420-9136
    Keywords: Key words: Upper mantle anisotropy, inversion of shear-wave splitting parameters, P-residual spheres, joint interpretation of body wave observations.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract —We report on results of a passive seismic experiment undertaken to study the 3-D velocity structure and anisotropy of the upper mantle around the contact zone of the Saxothuringicum and Moldanubicum in the western margin of the Bohemian Massif in central Europe. Spatial variations of P-wave velocities and lateral variations of the particle motion of split shear waves over the region monitor changes of structure and anisotropy within the deep lithosphere and the asthenosphere. A joint interpretation of P-residual spheres and shear-wave splitting results in an anisotropic model of the lithosphere with high velocities plunging divergently from the contact of both tectonic units. Lateral variations of the mean residuals are related to a southward thickening of the lithosphere beneath the Moldanubicum.
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  • 10
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Seismic sources with non-double-couple (non-DC) components are frequently observed in volcanic and geothermal areas. This suggests that the non-DC components may be generated by phenomena such as movements of magmatic fluids under high pressure and intrusions of dykes, which characterize volcanic areas and are believed to be responsible for volcanic tremors.Several volcanic tremors from the Phlegraean Fields, Southern Italy, have been processed with the waveform inversion method, which allows us to estimate the error on the source mechanism and on the source time function due to the noise present in the seismograms, the hypocentre mislocation and, in a simplistic way, an improper structure modelling.The events treated are examples of the seismicity accompanying the upward bradyseism of the area in 1984, and the following downward bradyseism in 1986. Significant non-DC components, i.e. a volumetric part and a compensated linear vector dipole (CLVD), are found in the mechanism, which indicates movement of magmatic fluids in the area.
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