The Experts below are selected from a list of 273 Experts worldwide ranked by ideXlab platform
Barbara Romanowicz - One of the best experts on this subject based on the ideXlab platform.
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effects of upper Mantle Structure beneath alaska on core sensitive seismic wave absolute and differential measurements implications for estimates of inner core anisotropy
Physics of the Earth and Planetary Interiors, 2021Co-Authors: Daniel A Frost, Barbara RomanowiczAbstract:Abstract Inner core anisotropy is often studied using differential travel times between the inner core phase, PKPdf, and one of two outer core phases, either PKPbc or PKPab, to eliminate contamination by crust and upper Mantle Structure. In particular, given the similarity of the two ray paths in the shallow Earth, the PKPbc-df differential travel time is assumed to robustly reflect the Structure of the inner core, and not be influenced by Mantle Structure. Measurements of PKPab-df and PKPbc-df differential times reveal that the inner core is anisotropic: PKPdf rays travel through the inner core ~3% faster along polar paths than along equatorial paths. Even so, measurements of both PKPbc-df and PKPab-df differential travel times on quasi-polar paths between sources in the South Sandwich Islands and stations in Alaska present greater scatter (6 s spread) than other equivalent quasi-polar paths from other parts of the globe (2 s spread). While the South Sandwich Islands to Alaska data help increase spatial sampling of the inner core, including these data in inner core models significantly increases estimates of average global inner core anisotropy strength, by more than 1%. Whether this reflects real spatial variability in the strength of inner core anisotropy or else results from complexity outside of the inner core is uncertain but is crucial for constraining the inner core composition and growth history. Using a regional tomographic model of the Alaskan upper Mantle to predict upper Mantle effects on PKP travel times, we show that the signature of the Alaska slab is present in trends of observed absolute PKPbc, ab, and df travel times, both as a function of distance and azimuth. Moreover, we demonstrate that the effect of the slab is not fully cancelled by differential measurements. This implies that past models of the inner core are biased towards too strong average anisotropy. In order to better constrain inner core anisotropy in future, differential measurements of core-phase travel times need to be more accurately corrected for upper Mantle three-dimensional Structure, which in turns requires the construction of higher resolution tomographic models.
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inferring upper Mantle Structure by full waveform tomography with the spectral element method
Geophysical Journal International, 2011Co-Authors: V Lekic, Barbara RomanowiczAbstract:SUMMARY Mapping the elastic and anelastic Structure of the Earth's Mantle is crucial for understanding the temperature, composition and dynamics of our planet. In the past quarter century, global tomography based on ray theory and first-order perturbation methods has imaged long-wavelength elastic velocity heterogeneities of the Earth's Mantle. However, the approximate techniques upon which global tomographers have traditionally relied become inadequate when dealing with crustal Structure, as well as short-wavelength or large amplitude Mantle heterogeneity. The spectral element method, on the other hand, permits accurate calculation of wave propagation through highly heterogeneous Structures, and is computationally economical when coupled with a normal mode solution and applied to a restricted region of the Earth such as the upper Mantle (SEM). Importantly, SEM allows a dramatic improvement in accounting for the effects of crustal Structure. Here, we develop and apply a new hybrid method of tomography, which allows us to leverage the accuracy of SEM to model fundamental and higher-mode long period (>60 s) waveforms. We then present the first global model of upper-Mantle velocity and radial anisotropy developed using SEM. Our model, SEMum, confirms that the long-wavelength Mantle Structure imaged using approximate semi-analytic techniques is robust and representative of the Earth's true Structure. Furthermore, it reveals Structures in the upper Mantle that were not clearly seen in previous global tomographic models. We show that SEMum favourably compares to and rivals the resolving power of continental-scale studies. This new hybrid approach to tomography can be applied to a larger and higher-frequency data set in order to gain new insights into the Structure of the lower Mantle and more robustly map seismic Structure at the regional and smaller scales.
Nilgün Sayil - One of the best experts on this subject based on the ideXlab platform.
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Determination of the Crust and Upper-Mantle Structure in Anatolia by Surface Wave Data
Natural Science, 2014Co-Authors: Nilgün SayilAbstract:In this study, the crust and upper Mantle Structure of Anatolia have been investigated by measuring the group velocity dispersion data of discriminated seismic surface waves. In the scope of the study, it has selected the profiles between six stations located in western Anatolia of Bogazici University Kandilli Observatory Earthquake Research Institute, national network of Turkey, and records of an earthquake (having about 10° epicentral distance) occurred in the eastern of Anatolia have been used. Firstly, surface wave discrimination filter based on the polarization properties has been applied tothree-component recordsand emphasized to surface waves. Then the group velocities have been calculated by multiple filter technique. A five-layered crustal model having total thickness of 38 - 40 km and Pn-wave velocity of 8.00 km/sec in the upper-Mantle has been determined through inversion of surface wave group velocity dispersion data in the period range of 10 sec to 60 sec.
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Investigation of Crust and Upper-Mantle Structure in the Black Sea with Group-Velocity Data
Bulletin of the Seismological Society of America, 2000Co-Authors: Nilgün Sayil, Ilhan OsmansahinAbstract:In this study, crust and upper-Mantle Structure in the Black Sea have been investigated by measuring the group-velocity dispersion data of seismic surface waves. Structural models for the profiles between Istanbul station (IST) and six earthquakes that occurred near the northeastern coast of the Black Sea have been constructed. These six profiles are approximately in the NE–SW direction and across the Black Sea. Seismograms of Rayleigh waves for four earthquakes and both Love and Rayleigh waves for two earthquakes have been analyzed. Group velocities have been computed with the multiple filter technique (MFT) from the vertical and/or horizontal component records of six selected earthquakes. Crust and upper-Mantle Structure for each profile has been estimated by trial and error fitting of these group-velocity values. A three-layered crustal model has been determined for all the profiles. Density values of the layers are identical for all models except for profile 1, of which one part lies in Anatolia. Thickness and density values of the layers in the crust and upper-Mantle Structure models obtained from the analyses of both Love and Rayleigh waves for the same profile (for profiles 2 and 6, individually) are identical. There are small changes in the velocity values. When the profiles are compared among themselves, it is shown that the crust is getting thinner from south to north, i.e., toward the center of the Black Sea basin.
Emeline Maufroy - One of the best experts on this subject based on the ideXlab platform.
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Mantle Structure under Gibraltar constrained by dispersion of body waves
Geophysical Research Letters, 2007Co-Authors: Götz Bokelmann, Emeline MaufroyAbstract:[1] WestudytheAfrica-Iberiaplateboundaryinthevicinity of Gibraltar. Numerous models have been proposed for that region throughout the last decades, proposing mechanisms that range widely from continental delamination, convective removal, to subduction of oceanic lithosphere. To better constrain upper-Mantle Structure under the region, we study waveforms of P-waves that traverse the Alboran Sea region between Spain and Morocco. These show dispersive behavior, which, together with early arrival times, confirms the presence of an anomalous upper Mantle Structure under the Alboran Sea. The dispersion is consistent with that expected from subducted lithosphere. Waveforms of body waves therefore provide a way to better constrain the elusive Mantle Structure and dynamics of the Alboran Sea region. Citation: Bokelmann, G., and E. Maufroy (2007), Mantle Structure under Gibraltar constrained by dispersion of body waves, Geophys. Res. Lett., 34, L22305, doi:10.1029/2007GL030964.
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Mantle Structure under Gibraltar constrained by seismic waveform complexity
Geophysical Research Letters, 2007Co-Authors: Götz Bokelmann, Emeline MaufroyAbstract:We study the Africa-Iberia plate boundary in the vicinity of Gibraltar. Numerous models have been proposed for that region throughout the last decades, proposing mechanisms that range widely from continental delamination, convective removal, to subduction of oceanic lithosphere. To better constrain upper-Mantle Structure under the region, we study waveforms of P-waves that traverse the Alboran Sea region between Spain and Morocco. These show dispersive behavior, which, together with early arrival times, confirms the presence of an anomalous upper Mantle Structure under the Alboran Sea. The dispersion is consistent with that expected from subducted lithosphere. Waveforms of body waves therefore provide a way to better constrain the elusive Mantle Structure and dynamics of the Alboran Sea region
Catherine Péquegnat - One of the best experts on this subject based on the ideXlab platform.
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High-resolution surface wave tomography beneath the Aegean-Anatolia region: constraints on upper-Mantle Structure
Geophysical Journal International, 2012Co-Authors: Gwénaëlle Salaun, Helle Pedersen, Denis Hatzfeld, Anne Paul, Véronique Farra, Hayrullah Karabulut, Costas Papazachos, Dean Childs, Catherine PéquegnatAbstract:This study provides new constraints on the upper-Mantle Structure from western Greece to central Anatolia using seismic data of permanent broad-band networks recently installed in Greece and Turkey and from a two-year temporary array (SIMBAAD experiment). We used ∼200 seismic events recorded at 146 broad-band stations with a typical interstation distance of 60–100 km across the study area. The high-resolution 3-D shear wave velocity model of the Mantle is obtained by inversion of fundamental-mode Rayleigh wave phase velocity maps for periods between 20 and 195 s. The tomography is based on ray tracing in heterogeneous media taking into account external propagation effects. The horizontal resolution is approximately 100 km, however small heterogeneities may suffer from some horizontal smearing and damp- ing. The vertical resolution is approximately 100 km. The vertical smoothing is necessary to avoid unresolved spurious shear wave velocity oscillations in the upper Mantle. The errors on shear wave velocities in our 3-D model (0.02–0.1km s−1) are significantly smaller than the amplitude of Vs variations (0.3–0.5 km s−1). In spite of the vertical and horizontal smoothing, our model shows details in the upper-Mantle Structure never reached at regional scale in the area. The overall Structure is characterized by a low-velocity zone (80–200km depth) re- flecting a slow and warm asthenosphere underlying a thin lithosphere. The southwesternmost termination of the low-velocity anomaly corresponds to the northward dipping Hellenic slab. The detailed shear velocity Structure of the upper Mantle beneath Anatolia appears to be far more geometrically complex than revealed in previous tomographic studies of the area. At depths larger than or equal to 160 km, velocities are overall high beneath Anatolia, partly due to the presence of dipping high-velocity anomalies which we tentatively interpret as remnant slabs. The southernmost high-velocity anomaly beneath Anatolia is separated from the eastern edge of the Hellenic slab by a major low-velocity anomaly which we interpret as the trace of asthenospheric Mantle material rising inside a vertical slab tear beneath southwestern Anatolia.
Gillian R. Foulger - One of the best experts on this subject based on the ideXlab platform.
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Surface wave waveform inversion for variation in upper Mantle Structure beneath Iceland
Geophysical Journal International, 2004Co-Authors: Gillian R. FoulgerAbstract:SUMMARY We study the Structure of the upper Mantle beneath Iceland using surface wave waveforms recorded at pairs of stations lying approximately on the same great circles as the sources used. We invert for local, path-average V s variations between the station pairs. The method used in this study is an extension of an algorithm proposed by Kushnir et al. (1989), which uses only the phase of the seismograms. In our waveform inversion not only the phases but also the amplitudes of the surface waves are used as structural constraints. We illustrate the resolution power of the new algorithm with synthetic examples. We apply the method to study upper Mantle Structure beneath Iceland using recordings of three events with northerly, southsouthwesterly and easterly orientated paths and 19 station pairs. Depending on the separation distance of the stations, we invert waveforms in the frequency range 0.0166‐0.08 Hz and 0.01‐ 0.08 Hz. Resolution is limited by the penetration depth of the surface wave fundamental mode, and is good down to ∼150 km for the narrower frequency band and ∼200 km for the wider band. Although the inversions of the differential waveforms only provide information on lateral V s variations between station pairs, the main structural features of the upper Mantle beneath Iceland are retrieved. We confirm that the strongest negative V s anomalies of up to ∼− 5 per cent underlie central Iceland, and extend down to the limit of our resolution at ∼200 km. The rift zones away from central Iceland are underlain by low velocities in the depth range ∼50‐100 km and high velocities below this, indicating that they are shallowly sourced. Such a Structure also underlies northwest Vatnajokull, where a Mantle plume is traditionally assumed to lie. Beneath intraplate areas, Mantle structural variations are small. Using our method, smaller-scale Mantle Structures are detectable than is possible with teleseismic tomography, which tends to smear anomalies throughout larger volumes.