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  • 1
    Monograph available for loan
    Monograph available for loan
    Bochum : Institut für Geophysik der Ruhr-Universität Bochum
    Call number: M 10.0118
    Type of Medium: Monograph available for loan
    Pages: 100 S. , Ill., graph. Darst.
    Location: Upper compact magazine
    Branch Library: GFZ Library
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  • 2
    Call number: S 90.0907(51)
    In: Berichte des Institutes für Geophysik der Ruhr-Universität Bochum
    Type of Medium: Series available for loan
    Pages: 168 S.
    Series Statement: Berichte des Instituts für Geophysik der Ruhr-Universität Bochum : Reihe A 51
    Language: English
    Location: Lower compact magazine
    Branch Library: GFZ Library
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  • 3
    Publication Date: 2019-07-05
    Description: We present a new 3-D shear-velocity model for the top 30 km of the crust in the wider Vienna Basin region based on surface waves extracted from ambient-noise cross-correlations. We use continuous seismic records of 63 broad-band stations of the AlpArray project to retrieve interstation Green’s functions from ambient-noise cross-correlations in the period range from 5 to 25 s. From these Green’s functions, we measure Rayleigh group traveltimes, utilizing all four components of the cross-correlation tensor, which are associated with Rayleigh waves (ZZ, RR, RZ and ZR), to exploit multiple measurements per station pair. A set of selection criteria is applied to ensure that we use high-quality recordings of fundamental Rayleigh modes. We regionalize the interstation group velocities in a 5 km × 5 km grid with an average path density of ∼20 paths per cell. From the resulting group-velocity maps, we extract local 1-D dispersion curves for each cell and invert all cells independently to retrieve the crustal shear-velocity structure of the study area. The resulting model provides a previously unachieved lateral resolution of seismic velocities in the region of ∼15 km. As major features, we image the Vienna Basin and Little Hungarian Plain as low-velocity anomalies, and the Bohemian Massif with high velocities. The edges of these features are marked with prominent velocity contrasts correlated with faults, such as the Alpine Front and Vienna Basin transfer fault system. The observed structures correlate well with surface geology, gravitational anomalies and the few known crystalline basement depths from boreholes. For depths larger than those reached by boreholes, the new model allows new insight into the complex structure of the Vienna Basin and surrounding areas, including deep low-velocity zones, which we image with previously unachieved detail. This model may be used in the future to interpret the deeper structures and tectonic evolution of the wider Vienna Basin region, evaluate natural resources, model wave propagation and improve earthquake locations, among others.
    Type: Article , PeerReviewed
    Format: text
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  • 4
    Publication Date: 2019-09-23
    Description: The dense AlpArray network allows studying seismic wave propagation with high spatial resolution. Here we introduce an array approach to measure arrival angles of teleseismic Rayleigh waves. The approach combines the advantages of phase correlation as in the two-station method with array beamforming to obtain the phase-velocity vector. 20 earthquakes from the first two years of the AlpArray project are selected, and spatial patterns of arrival-angle deviations across the AlpArray are shown in maps, depending on period and earthquake location. The cause of these intriguing spatial patterns is discussed. A simple wave-propagation modelling example using an isolated anomaly and a Gaussian beam solution suggests that much of the complexity can be explained as a result of wave interference after passing a structural anomaly along the wave paths. This indicates that arrival-angle information constitutes useful additional information on the Earth structure, beyond what is currently used in inversions.
    Type: Article , PeerReviewed
    Format: text
    Format: text
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  • 5
    Publication Date: 2019-02-01
    Description: The AlpArray programme is a multinational, European consortium to advance our understanding of orogenesis and its relationship to mantle dynamics, plate reorganizations, surface processes and seismic hazard in the Alps–Apennines–Carpathians–Dinarides orogenic system. The AlpArray Seismic Network has been deployed with contributions from 36 institutions from 11 countries to map physical properties of the lithosphere and asthenosphere in 3D and thus to obtain new, high-resolution geophysical images of structures from the surface down to the base of the mantle transition zone. With over 600 broadband stations operated for 2 years, this seismic experiment is one of the largest simultaneously operated seismological networks in the academic domain, employing hexagonal coverage with station spacing at less than 52 km. This dense and regularly spaced experiment is made possible by the coordinated coeval deployment of temporary stations from numerous national pools, including ocean-bottom seismometers, which were funded by different national agencies. They combine with permanent networks, which also required the cooperation of many different operators. Together these stations ultimately fill coverage gaps. Following a short overview of previous large-scale seismological experiments in the Alpine region, we here present the goals, construction, deployment, characteristics and data management of the AlpArray Seismic Network, which will provide data that is expected to be unprecedented in quality to image the complex Alpine mountains at depth.
    Type: Article , PeerReviewed
    Format: text
    Format: video
    Format: other
    Format: other
    Format: other
    Format: text
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  • 6
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: P-wave polarization constrains local anisotropy in the vicinity of the receivers. Using three-component and array data from the regional GERESS array in south-eastern Germany, we measure polarization a and propagation vectors s for P phases of 120 events. Angular deviations a-s between these normalized vectors often approach 10°, rendering them easily measurable. The effect of anisotropy can be distinguished from remote effects, since all remote effects, such as source mislocation, distant lateral heterogeneity or distant anisotropy, affect polarization and propagation vectors simultaneously. Averaging removes sensitivity to near-receiver heterogeneity, and local anisotropy is left as the sole cause of the effects in a-s. This method hence gives local effective anisotropy in the near-receiver crust averaging over a depth interval of a wavelength (∼6 km). We resolve strike and dip of the symmetry plane and also two dimensionless numbers η and τ which give constraints on four of the elastic parameters. The optimum model (variance reduction 44 per cent) has symmetry plane orientation of strike 113° and dip 49° to the north, which corresponds closely to the consistently observed gneiss foliation direction in the area (120°, 50°-60°). Comparing η and τ with predictions from different physical models we find that the data are fit by a gneiss model assuming that the anisotropy is dominated by the mica, if 3-8 per cent of the mica are well aligned. This suggests that anisotropy in the region studied is dominated by the effect of local foliation rather than the regional stress field.
    Type of Medium: Electronic Resource
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  • 7
    Publication Date: 1999-06-01
    Description: We develop and apply a new technique to determine array-averaged particle motions from three-component seismic array data. The method is based on multi-wavelets, which are an extension of multi-taper spectral methods, and is a hybrid of Fourier and time-domain methods of array processing. Particle motions are determined by a time-domain principal-component method. A complex singular value decomposition is used on wavelet transformed signals assembled into multiple matrices (one for each wavelet). The eigenvector of the largest singular value of each matrix is used to estimate the phase between individual signals. We determine the relative phase between components to estimate an average particle motion ellipse for the array. The estimation procedure is made more stable by the redundancy inherent in the multi-wavelets and by M-estimators applied to individual phase factors in the complex plane. The method is applied to data from three-component array experiments conducted at Pinon Flats, California, in 1990 and 1991. We find remarkable departures of P-wave particle motions from the pure longitudinal motion expected for an isotropic media. Anomalies as large as 40 degrees are measured from some azimuths. The azimuthally varying particle-motion anomalies are frequency dependent, generally increasing in magnitude as frequency increases. Borehole measurements from sensors at 153 and 274 m depth below the array show a pattern indistinguishable from the surface sensors. The data are fit with a dipping, transversely isotropic medium with a symmetry plane having a strike of 70 degrees and a dip of 30 degrees to the northwest. We attribute our results to three superimposed effects: (1) an anisotropy of the near surface induced by preferential weathering of the granodiorite bedrock along joints, (2) a larger scale anisotropy induced by structural and intrinsic anisotropy related to the Santa Rosa mylonite, and (3) near-surface scattering.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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  • 8
    Publication Date: 1999-02-01
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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  • 9
    Publication Date: 2004-04-01
    Description: Earthquake location using relative arrival time measurements can lead to dramatically reduced location errors and a view of fault-zone processes with unprecedented detail. There are two principal reasons why this approach reduces location errors. The first is that the use of differenced arrival times to solve for the vector separation of earthquakes removes from the earthquake location problem much of the error due to unmodeled velocity structure. The second reason, on which we focus in this article, is that waveform cross correlation can substantially reduce measurement error. While cross correlation has long been used to determine relative arrival times with subsample precision, we extend correlation measurements to less similar waveforms, and we introduce a general quantitative means to assess when correlation data provide an improvement over catalog phase picks. We apply the technique to local earthquake data from the Calaveras Fault in northern California. Tests for an example streak of 243 earthquakes demonstrate that relative arrival times with normalized cross correlation coefficients as low as approximately 70%, interevent separation distances as large as to 2 km, and magnitudes up to 3.5 as recorded on the Northern California Seismic Network are more precise than relative arrival times determined from catalog phase data. Also discussed are improvements made to the correlation technique itself. We find that for large time offsets, our implementation of timedomain cross correlation is often more robust and that it recovers more observations than the cross spectral approach. Longer time windows give better results than shorter ones. Finally, we explain how thresholds and empirical weighting functions may be derived to optimize the location procedure for any given region of interest, taking advantage of the respective strengths of diverse correlation and catalog phase data on different length scales.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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  • 10
    Publication Date: 1995-10-01
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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