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  • 1
    Publication Date: 2007-12-12
    Description: The island of Crete represents a horst structure located in the central forearc of the retreating Hellenic subduction zone. The structure and dynamics of the plate boundary in the area of Crete are investigated by receiver function, surface wave and microseismicity using temporary seismic networks. Here the results are summarized and implications for geodynamic models are discussed. The oceanic Moho of the subducted African plate is situated at a depth of about 5060 km beneath Crete. The continental crust of the overriding Aegean lithosphere is about 35 km thick in eastern and central Crete, and typical crustal velocities are observed down to the upper surface of the downgoing slab beneath western Crete. A negative phase at about 4 s in receiver functions occurring in stripes parallel to the trend of the island points to low-velocity slices within the Aegean lithosphere. Interplate seismicity is spread out about 100 km updip from the southern coastline of Crete. To the south of western Crete, this seismically active zone corresponds to the inferred rupture plane of the magnitude 8 earthquake of AD 365. In contrast, interplate motion appears to be largely aseismic beneath the island. The coastline of Crete mimics the shape of a microseismically quiet realm in the Aegean lithosphere at 2040 km depth, suggesting a relation between active processes at this depth range and uplift. The peculiar properties of the lithosphere and the plate interface beneath Crete are tentatively attributed to extrusion of material from a subduction channel, driving differential uplift of the island by several kilometres since about 4 Ma.
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  • 2
    Publication Date: 2019-07-13
    Description: High resolution images of Europa taken by the Voyager 2 spacecraft were used to study a portion of Europa's dark lineations and the major white line feature Agenor Linea. Initial image processing of images 1195J2-001 (violet filter), 1198J2-001 (blue filter), 1201J2-001 (orange filter), and 1204J2-001 (ultraviolet filter) was performed at the U.S.G.S. Branch of Astrogeology in Flagstaff, Arizona. Processing was completed through the stages of image registration and color ratio image construction. Pixel printouts were used in a new technique of linear feature profiling to compensate for image misregistration through the mapping of features on the printouts. In all, 193 dark lineation segments were mapped and profiled. The more accurate multispectral data derived by this method was plotted using a new application of the ternary diagram, with orange, blue, and violet relative spectral reflectances serving as end members. Statistical techniques were then applied to the ternary diagram plots. The image products generated at LPI were used mainly to cross-check and verify the results of the ternary diagram analysis.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA-CR-174075 , NAS 1.26:174075 , LPI-CONTRIB-537 , Ann. Meteoritical Society Meeting; 30 Jul. - 2 Aug. 1984; Houston, TX; United States
    Format: application/pdf
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  • 3
    Publication Date: 2014-06-16
    Description: On a SW–NE profile from the Libyan coast towards central Turkey phase velocity curves of the fundamental Rayleigh mode were measured using a two-station method. The inversion of phase velocity curves yields 1-D models of shear wave velocity down to approximately 200 km depths that may be interpreted as estimates of average models between neighbouring stations on the profile. Strong lateral variations in the shear wave velocity structure are imaged along the profile. The subducted oceanic African mantle lithosphere is indicated in 1-D models for the region around Crete by significantly enlarged shear wave velocities. It is also imaged by an average model of the structure between stations on Crete and Santorini. On a path crossing the Libyan Sea south of Crete the resulting model is slower than a model expected for 110 Myr old oceanic lithosphere. The passive African margin is thus assumed to extend northwards beneath the Libyan Sea. Anomalous low shear wave velocities are found for the uppermost mantle beneath central Turkey down to a depth of approximately 130 km. Using two stations on Crete the average depth of the oceanic Moho within the subducting slab is estimated to be at approximately 50 km beneath Crete. For this arc-parallel path, an enlarged standard deviation of the measured phase velocities of approximately 0.2 km s−1 between 10 and 30 mHz is observed that is probably caused by strong lateral heterogeneity related to the subducting slab. In addition, in this frequency range an anomalous propagation of the fundamental Rayleigh mode is detected that is indicated by measured phase velocities that are approximately one standard deviation faster than phase velocities expected from a great-circle approximation. An average shear wave velocity of approximately 3.5 km s−1 is observed above the oceanic Moho. In order to explain the recent lithospheric structure of the Hellenic subduction zone a tectonic model is assumed for the NE–SW striking profile considered. It is based on the calculated 1-D models, tectonic reconstructions and on a model derived from the metamorphic history of rocks exposed on Crete. The suggested model summarizes the tectonic development at a lithospheric scale starting in the Late Cretaceous. Accretion of crustal material of two microcontinents to Eurasia is assumed, while continuous subduction of the oceanic lithosphere of different ocean basins and possibly of the mantle lithosphere of the microcontinents resulted in a single slab. The length of the oceanic lithosphere that was subducted south of Crete is estimated to be not greater than approximately 550 km.
    Keywords: ddc:550
    Language: English
    Type: http://purl.org/escidoc/metadata/ves/publication-types/article
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  • 4
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    In:  Geophysical Journal International ; Year: 2004 ; Volume: 158 ; Issue: 2 ; Pages: 592-608
    Publication Date: 2014-06-16
    Description: We present a case study of lithospheric structure in the forearc of a retreating subduction zone for the Hellenic Arc. Lateral structural variations along the arc beneath the island of Crete are jointly investigated by receiver functions and Rayleigh phase velocities. Data from temporary short-period networks amend previous results from broad-band stations by broadening the frequency range available for phase-velocity determination and increasing the spatial coverage of receiver function profiles. Both receiver functions and dispersion analysis reveal distinct structural differences between western and central Crete. Western Crete is characterized by nearly constant S-velocities of 3.72–3.75 km s−1 from 10 km depth down to a depth of 50 km and no distinct continental Moho signal. Meanwhile, central Crete shows lower S-velocities equal to 3.3 km s−1 in the crust between 10 and 20 km depth which are followed by the Aegean Moho in about 30 km depth and a mantle wedge with an S-velocity of 4.35 km s−1. Both methods lead to an average depth of 55 km for the subducted oceanic African Moho beneath Crete. This means that the slab is separated from the Aegean crust by a mantle wedge beneath central Crete, while beneath western Crete the corresponding depth region is characterized by crustal velocities. This thickened crust in the forearc might be formed by crustal material of the Aegean Plate dragged down with the subducting African lithosphere. Furthermore, rocks extruded from a melange circulating in a subduction channel might accumulate between a depth of 20 and 50 km and contain low-velocity material, e.g. in the form of serpentinized Aegean mantle. In addition, the lateral extent of a prominent negative phase observed around 4 s differential time in receiver functions from western Crete is mapped. This phase might point to low-velocity material around 30 km depth which could be extruded from a subduction channel. An important property of the forearc found in this study is its strong lateral heterogeneity.
    Keywords: ddc:550
    Language: English
    Type: http://purl.org/escidoc/metadata/ves/publication-types/article
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  • 5
    Publication Date: 2018-04-27
    Description: The increasingly dense coverage of Europe with broad-band seismic stations makes it possible to image its lithospheric structure in great detail, provided that structural information can be extracted effectively from the very large volumes of data. We develop an automated technique for the measurement of interstation phase velocities of (earthquake-excited) fundamental-mode surface waves in very broad period ranges. We then apply the technique to all available broad-band data from permanent and temporary networks across Europe. In a new implementation of the classical two-station method, Rayleigh and Love dispersion curves are determined by cross-correlation of seismograms from a pair of stations. An elaborate filtering and windowing scheme is employed to enhance the target signal and makes possible a significantly broader frequency band of the measurements, compared to previous implementations of the method. The selection of acceptable phase-velocity measurements for each event is performed in the frequency domain, based on a number of fine-tuned quality criteria including a smoothness requirement. Between 5 and 3000 single-event dispersion measurements are averaged per interstation path in order to obtain robust, broad-band dispersion curves with error estimates. In total, around 63,000 Rayleigh- and 27,500 Love-wave dispersion curves between 10 and 350 s have been determined, with standard deviations lower than 2 per cent and standard errors lower than 0.5 per cent. Comparisons of phase-velocity measurements using events at opposite backazimuths and the examination of the variance of the phase-velocity curves are parts of the quality control. With the automated procedure, large data sets can be consistently and repeatedly measured using varying selection parameters. Comparison of average interstation dispersion curves obtained with different degrees of smoothness shows that rough perturbations do not systematically bias the average dispersion measurement. They can, therefore, be treated as random but they do need to be removed in order to reduce random errors of the measurements. Using our large new data set, we construct phase-velocity maps for central and northern Europe. According to checkerboard tests, the lateral resolution in central Europe is ≤150 km. Comparison of regional surface-wave tomography with independent data on sediment thickness in North-German Basin and Polish Trough confirms the high-resolution potential of our phase-velocity measurements. At longer periods, the structure of the lithosphere and asthenosphere around the Trans-European Suture Zone (TESZ) is seen clearly. The region of the Tornquist-Teisseyre-Zone in the southeast is associated with a stronger lateral contrast in lithospheric thickness, across the TESZ compared to the region across the Sorgenfrei-Tornquist-Zone in the northwest.
    Keywords: ddc:550
    Language: English
    Type: http://purl.org/escidoc/metadata/ves/publication-types/article
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  • 6
    Publication Date: 2016-02-15
    Description: The time has come to exploit the full richness of broadband, three-component waveforms. The inversion of full seismograms leads to a tremendous improvement in imaging resolution due to the ability to map structures that are smaller than the seismic wavelength. In addition, it provides important constraints on density and attenuation. State-of-the-art software for full waveform tomography is available in scientific environments but is not yet accessible to the practitioner. The Toolbox for Applied Seismic Tomography (TOAST) will open a new window to seismic inversion. Due to advances in available computational resources and recent developments in high performance and parallelized computing, 3D inversion of full seismograms is within reach. By combining tested code collections for waveform modelling and the solution of large inverse problems, complemented by experience in the management of large software projects and by sound expertise in the inversion of elastic waves from the centimeter to the kilometer scale, the cooperation of the TOAST project partners will provide a unique knowledge base for implementing flexible and efficient tools for full waveform tomography and to transfer the knowledge to industrial practice. The TOAST project pursues the concept of modularization. It will provide modules that interact through standardized interfaces and thereby can be re-combined in application-specific and efficient ways. The Toolbox for Applied Seismic Tomography will prove its worth through application to surveys from commercial practice. Existing data from seismic experiments at different scales (e.g., monitoring of embankments, CO2 sequestration studies) and newly aquired shallow seismic and ultrasonic data will serve as case studies to validate the functionality of the toolbox.
    Keywords: ddc:550
    Language: English
    Type: http://purl.org/eprint/type/ConferencePaper
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  • 7
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    In:  Geophysical Research Letters ; Year: 2012 ; Volume: 39 ; Pages: n/a-n/a
    Publication Date: 2018-05-09
    Description: An instantaneous measure of the moment magnitude (Mw) of an ongoing earthquake is estimated from the moment rate function (MRF) determined in real-time from available seismic data using waveform inversion. Integration of the MRF gives the moment function from which an instantaneousMw is derived. By repeating the inversion procedure at regular intervals while seismic data are coming in we can monitor the evolution of seismic moment and Mw with time. The final size and duration of a strong earthquake can be obtained within 12 to 15 minutes after the origin time. We show examples of Mw monitoring for three large earthquakes at regional distances. The estimated Mw is only weakly sensitive to changes in the assumed source parameters. Depending on the availability of seismic stations close to the epicenter, a rapid estimation of the Mw as a prerequisite for the assessment of earthquake damage potential appears to be feasible.
    Language: English
    Type: http://purl.org/escidoc/metadata/ves/publication-types/article
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  • 8
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    In:  Journal of Petroleum Science and Engineering ; Year: 2015 ; Volume: 135 ; Pages: 702-722
    Publication Date: 2015-12-14
    Type: http://purl.org/escidoc/metadata/ves/publication-types/article
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  • 9
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    In:  Geophysical Research Letters ; Year: 2012 ; Volume: 39 ; Pages: n/a-n/a
    Publication Date: 2018-05-09
    Description: An instantaneous measure of the moment magnitude (Mw) of an ongoing earthquake is estimated from the moment rate function (MRF) determined in real-time from available seismic data using waveform inversion. Integration of the MRF gives the moment function from which an instantaneousMw is derived. By repeating the inversion procedure at regular intervals while seismic data are coming in we can monitor the evolution of seismic moment and Mw with time. The final size and duration of a strong earthquake can be obtained within 12 to 15 minutes after the origin time. We show examples of Mw monitoring for three large earthquakes at regional distances. The estimated Mw is only weakly sensitive to changes in the assumed source parameters. Depending on the availability of seismic stations close to the epicenter, a rapid estimation of the Mw as a prerequisite for the assessment of earthquake damage potential appears to be feasible.
    Keywords: ddc:550
    Language: English
    Type: http://purl.org/escidoc/metadata/ves/publication-types/article
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  • 10
    Publication Date: 2018-05-09
    Description: We present a new, S-velocity model of the European upper mantle, constrained by inversions of seismic waveforms from broad-band stations in Europe and surrounding regions. We collected seismograms for the years 1990–2007 from all permanent stations in Europe for which data were available. In addition, we incorporated data from temporary experiments. Automated multimode inversion of surface and S-wave forms was applied to extract structural information from the seismograms, in the form of linear equations with uncorrelated uncertainties. The equations were then solved for seismic velocity perturbations in the crust and mantle with respect to a 3-D reference model with a realistic crust. We present two versions of the model: one for the entire European upper mantle and another, with the highest resolution, focused on the upper 200 km of the mantle beneath western and central Europe and the circum Mediterranean. The mantle lithosphere and asthenosphere are well resolved by both models. Major features of the lithosphere–asthenosphere system in Europe and the Mediterranean are indentified. The highest velocities in the mantle lithosphere of the East European Craton (EEC) are found at about 150 km depth. There are no indications for a deep cratonic root below about 330 km depth. Lateral variations within the cratonic mantle lithosphere are resolved as well. The locations of kimberlites correlate with reduced S-wave velocities in the shallow cratonic mantle lithosphere. This anomaly is present in regions of both Proterozoic and Archean crust, pointing to an alteration of the mantle lithosphere after the formation of the craton. Strong lateral changes in S-wave velocity are found at the northwestern margin of the EEC and may indicate erosion of cratonic mantle lithosphere beneath the Scandes by hot asthenosphere. The mantle lithosphere beneath western Europe and between the Tornquist–Teisseyre Zone and the Elbe Line shows moderately high velocities and is of an intermediate character, between cratonic lithosphere and the thin lithosphere of central Europe. In central Europe, Caledonian and Variscian sutures are not associated with strong lateral changes in the lithosphere–asthenosphere system. Cenozoic anorogenic intraplate volcanism in central Europe and the circum Mediterranean is found in regions of shallow asthenosphere and close to changes in the depth of the lithosphere–asthenosphere boundary. Very low velocities at shallow upper-mantle depths are present from eastern Turkey towards the Dead Sea transform fault system and Sinai, beneath locations of recent volcanism. Low-velocity anomalies extending vertically from shallow upper mantle down to the transition zone are found beneath the Massif Central, Sinai and the Dead Sea, the Canary Islands and Iceland.
    Keywords: ddc:550
    Language: English
    Type: http://purl.org/escidoc/metadata/ves/publication-types/article
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