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  • 1
    Electronic Resource
    Electronic Resource
    Wave propagation in a multiple-scattering upper mantle—observations and modelling (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 127 (1996), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A striking feature of Russian long-range seismic refraction data from Peaceful Nuclear Explosions is the observation of a high-frequency teleseismic Pn, phase, which travels with a group velocity of 8.0 km s-1 out to distances of several thousands of kilometres. Modelling using the reflectivity method shows that this phase can be understood as the response of an upper mantle that contains random RMS velocity fluctuations of about ±4 per cent superimposed on a positive velocity gradient. This class of model explains the existence of the teleseismic Pn, its high-frequency content and its coda length. A teleseismic Pn can only be generated if velocity flucluations are strong enough to cause multiple scattering and occur on a subwavelength scale. Cross-correlation properties of P- and S-wave velocity fluctuations exert a substantial influence on the wavefield. A completely unexpected phase can be observed if the fluctuations are imposed on a negative gradient
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1996.tb04735.x
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    Paper (German National Licenses)
    Fulltext
  • 2
    Electronic Resource
    Electronic Resource
    A new approach to describe the seismic wavefield using higher order Gaussian beam modes (1991)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 105 (1991), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: In the present paper a method is developed to calculate synthetic body wave seismograms using higher order Gaussian beams as a solution of the equation of motion. We obtain an approximated, analytic solution of the equation of motion which is valid in a cone-shaped sector of space. The advantage of this approach is that it is not necessary to perform the beam summation as in the Gaussian beam method. This approach is applied to homogeneous elastic horizontally layered models for the case of near vertical incidence. The computing algorithm is expressed in matrix product form which is convenient for wave modelling. Numerical examples for homogeneous elastic horizontally layered models are given and compared with results obtained by the reflectivity method and a standard ray method.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1991.tb00800.x
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    P-wave mantle velocity structure beneath northern Eurasia from long-range recordings along the profile Quartz
    Amsterdam : Elsevier
    Physics of the Earth and Planetary Interiors 79 (1993), S. 269-286 
    Details
    ISSN: 0031-9201
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Geosciences , Physics
    Type of Medium: Electronic Resource
    URL: http://linkinghub.elsevier.com/retrieve/pii/0031-9201(93)90151-X
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    Paper (German National Licenses)
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  • 4
    Electronic Resource
    Electronic Resource
    Scales of Heterogeneities in the Continental Crust and Upper Mantle (1999)
    Springer
    Pure and applied geophysics 156 (1999), S. 29-52 
    Details
    ISSN: 1420-9136
    Keywords: Key words: Inhomogeneous media, scattering, lower crust, upper mantle, transition zone.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract —A seismological characterization of crust and upper mantle can refer to large-scale averages of seismic velocities or to fluctuations of elastic parameters. Large is understood here relative to the wavelength used to probe the earth.¶In this paper we try to characterize crust and upper mantle by the fluctuations in media properties rather than by their average velocities. As such it becomes evident that different scales of heterogeneities prevail in different layers of crust and mantle. Although we cannot provide final models and an explanation of why these different scales exist, we believe that scales of inhomogeneities carry significant information regarding the tectonic processes that have affected the lower crust, the lithospheric and the sublithospheric upper mantle.¶We focus on four different types of small-scale inhomogeneities (1) the characteristics of the lower crust, (2) velocity fluctuations in the uppermost mantle, (3) scattering in the lowermost lithosphere and on (4) heterogeneities in the mantle transition zone.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s000240050288
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  • 5
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    The onset of Walvis Ridge: Plume influence at the continental margin (2017)
    Elsevier
    In:  Tectonophysics, 716 . pp. 90-107.
    Details
    Publication Date: 2020-02-06
    Description: Highlights • Crustal structure of Walvis Ridge reveals high seismic velocities in the lower crust intruding the African continent. • This modified crust is localized to approx. 100 × 100 km within the continent. • No indication for a large plume head observed The opening of the South Atlantic is a classical example for a plume related continental breakup. Flood basalts are present on both conjugate margins as well as aseismic ridges connecting them with the current plume location at Tristan da Cunha. To determine the effect of the proposed plume head on the continental crust, we acquired wide-angle seismic data at the junction of the Walvis Ridge with the African continent and modelled the P-wave velocity structure in a forward approach. The profile extends 430. km along the ridge and continues onshore to a length of 720. km. Crustal velocities beneath the Walvis Ridge vary between 5.5. km/s and 7.0. km/s, a typical range for oceanic crust. The crustal thickness of 22. km, however, is approximately three times larger than of normal oceanic crust. The continent-ocean transition is characterized by 30. km thick crust with strong lateral velocity variations in the upper crust and a high-velocity lower crust (HVLC), where velocities reach up to 7.5. km/s. The HVLC is 100 to 130. km wider at the Walvis Ridge than it is farther south, and impinges onto the continental crust of the Kaoko fold belt. Such high seismic velocities indicate Mg-rich igneous material intruded into the continental crust during the initial rifting stage. However, the remaining continental crust seems unaffected by intrusions and the root of the 40. km-thick crust of the Kaoko belt is not thermally abraded. We conclude that the plume head did not modify the continental crust on a large scale, but caused rather local effects. Thus, it seems unlikely that a plume drove or initiated the breakup process. We further propose that the plume already existed underneath the continent prior to the breakup, and ponded melt erupted at emerging rift structures providing the magma for continental flood basalts.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.tecto.2017.03.011
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    South Atlantic opening: A plume-induced breakup? (2015)
    GSA, Geological Society of America
    In:  Geology, 43 (10). pp. 931-934.
    Details
    Publication Date: 2017-05-17
    Description: Upwelling hot mantle plumes are thought to disintegrate continental lithosphere and are considered to be drivers of active continental breakup. The formation of the Walvis Ridge during the opening of the South Atlantic is related to a putative plume-induced breakup. We investigated the crustal structure of the Walvis Ridge (southeast Atlantic Ocean) at its intersection with the continental margin and searched for anomalies related to the possible plume head. The overall structure we identify suggests that no broad plume head existed during opening of the South Atlantic and anomalous mantle melting occurred only locally. We therefore question the importance of a plume head as a driver of continental breakup and further speculate that the hotspot was present before the rifting, leaving a track of kimberlites in the African craton.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1130/G36936.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    The Fine Structure of the Subducted Investigator Fracture Zone in Western Sumatra as Seen by Local Seismicity (2010)
    Elsevier
    In:  Earth and Planetary Science Letters, 298 (1-2). pp. 47-56.
    Details
    Publication Date: 2016-06-22
    Description: The Sumatran margin suffered three great earthquakes in recent years (Aceh-Andaman 26 December 2004 Mw = 9.1, Nias 28 March 2005 Mw = 8.7, Bengkulu 12 September 2007 Mw = 8.5). Here we present local earthquake data from a dense, amphibious local seismic network covering a segment of the Sumatran margin that last ruptured in 1797. The occurrence of forearc islands along this part of the Sumatran margin allows the deployment of seismic land-stations above the shallow part of the thrust fault. In combination with ocean bottom seismometers this station geometry provides high quality hypocentre location for the updip end of the seismogenic zone in an area where geodetic data are also available. In this region, the Investigator Fracture Zone (IFZ), which consists of 4 sub-ridges, is subducted below the Sunda plate. This topography appears to influence seismicity at all depth intervals. A well-defined linear streak of seismicity extending from 80 to 200 km depth lies along the prolongation of closely spaced IFZ sub-ridges. More intermediate depth seismicity is located to the southeast of this string of seismicity and is related to subducted rough oceanic seafloor. The plate interface beneath Siberut Island which ruptured last in 1797 is characterised by almost complete absence of seismicity.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.epsl.2010.07.020
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    Crustal Structure Around the Tristan da Cunha Hotspot Derived from Ambient Noise (2015)
    In:  [Talk] In: AGU 2015, 14. – 18.12.15, San Francisco, USA .
    Details
    Publication Date: 2016-02-04
    Description: According to classical plume theory, the Tristan da Cunha hotspot is thought to have played a major role in the rifting of the South Atlantic margins and the creation of the aseismic Walvis Ridge during and after the breakup of the South Atlantic. Between February 2012 and January 2013 a network of 24 broadband ocean-bottom seismometers was in operation around the volcanic archipelago of Tristan da Cunha. Ambient noise data from the OBS and a seismic station on Nightingale Island were used to constrain the crustal and uppermost structure around the island. From the vertical and hydrophone recordings of more than 300 days we could reconstruct the ambient noise Green's functions by cross-correlation. The dispersion curves of Rayleigh/Scholte waves could be derived from the cross-correlations in the period range from 2 to 32 seconds. Group velocity maps were determined for each individual period using travel time tomography. These group velocity maps were converted to depth by dispersion curve inversion to construct a 3D S-wave velocity model of the crust and uppermost mantle in the region. This model shows a strong velocity anomaly beneath the Tristan da Cunha archipelago. The influence of the water depth on the inversion is discussed.
    Type: Conference or Workshop Item , NonPeerReviewed
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    OceanRep: Talk
  • 9
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    Insight into the shallow lithosphere structure and magmatic plumbing system beneath the Tristan da Cunha hot spot from geophysical and petrological studies (2015)
    In:  [Talk] In: 2. European Mantle Workshop, 25.-28.08.2015, Wroclaw, Poland .
    Details
    Publication Date: 2016-03-10
    Type: Conference or Workshop Item , NonPeerReviewed
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    OceanRep: Talk
  • 10
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    Crustal Structure Around the Tristan da Cunha Hotspot Derived from Ambient Noise (2016)
    In:  [Talk] In: EGU General Assembly 2016, 17.-22.04.2016, Vienna, Austria .
    Details
    Publication Date: 2016-02-04
    Description: According to classical plume theory, the Tristan da Cunha hotspot is thought to have played a major role in the rifting of the South Atlantic margins and the creation of the aseismic Walvis Ridge during and after the breakup of the South Atlantic. Between February 2012 and January 2013 a network of 24 broadband ocean-bottom seismometers was in operation around the volcanic archipelago of Tristan da Cunha. Ambient noise data from the OBS and a seismic station on Nightingale Island were used to constrain the crustal and uppermost structure around the island. From the vertical and hydrophone recordings of more than 300 days we could reconstruct the ambient noise Green's functions by cross-correlation. The dispersion curves of Rayleigh/Scholte waves could be derived from the cross-correlations in the period range from 2 to 32 seconds. Group velocity maps were determined for each individual period using travel time tomography. These group velocity maps were converted to depth by dispersion curve inversion to construct a 3D S-wave velocity model of the crust and uppermost mantle in the region. This model shows a strong velocity anomaly beneath the Tristan da Cunha archipelago. The influence of the water depth on the inversion is discussed.
    Type: Conference or Workshop Item , NonPeerReviewed
    Format: text
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    OceanRep: Talk
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