Shear wave anisotropy in the upper mantle beneath the Aegean related to internal deformation (original) (raw)
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Mantle dynamics beneath Greece from SKS and PKS seismic anisotropy study
Acta Geophysica, 2018
SKS and PKS splitting parameters were determined in the broader Greek region using data from 45 stations of the Hellenic Unified Seismological Network and the Kandilli Observatory and Earthquake Research Institute, utilizing teleseismic events that occurred between 2010 and 2017. Data were processed for shear-wave splitting with the Minimum Energy Method that was considered the optimal. The results generally confirm the existence of anisotropic zonation in the Hellenic subduction system, with alternating trench-normal and trench-parallel directions. The zonation is attributed to the upper and lower olivine fabric layers that can, potentially, be present in the subduction zone. At the edges of this zone, two possible toroidal flow cases have been identified, implying the existence of tears that allow the inflow of asthenospheric material in the mantle wedge. The high number of null measurements in the KZN and XOR stations indicates a possible anisotropic transition zone between the fore-arc and back-arc areas. SKS and PKS splitting results are jointly interpreted, given that they yield similar values in most cases.
Shear wave anisotropy beneath the Aegean inferred from SKS splitting observations
Journal of Geophysical Research, 2011
1] SKS splitting parameters are measured in the Aegean region using events recorded at a dense temporary network in the south Aegean and the operating permanent networks, especially focusing in the back-arc and the near-trench areas of the Hellenic arc. In general, fast anisotropy directions are trench perpendicular in the back-arc area and trench parallel near the trench. Anisotropy measurements near the volcanic arc mark the transition between these two regions. In the back arc, a gradual increase is observed in delay times from south to north, with a prevailing NE-SW direction. In Cyclades, this pattern is correlated with GPS velocities and stretching lineations of metamorphic core complexes. Our preferred source of anisotropy in the back-arc region is the mantle wedge flow, induced by the retreating descending slab. The westernmost termination of the trench reveals directions parallel with the Kefalonia Transform Fault and perpendicular to the convergence boundary. Beneath Peloponnese, the trench-parallel flow is probably located beneath the shallow-dipping slab, although scattered measurements may also reflect fossil anisotropy from a past NW-SE strike of the trench. In western Crete, which may be entering a stage of continental collision, the anisotropy pattern changes to trench perpendicular, with a possible subslab source. Good nulls in central east Crete indicate a change in the anisotropy origin toward the east. At the easternmost side of the trench, fast directions are trench parallel. This reflects a similar subslab flow that may become toroidal around the slab edge beneath western Turkey. This may also produce a trench-parallel flow within the mantle wedge.
Elastic-Anelastic properties beneath the Aegean inferred from long period Rayleigh Waves
2007
The highest deformation rate along the Africa/Eurasia convergence zone is well documented in the Aegean area, being >4 cm/yr. However, it is still under question whether continental deformation is distributed along major faults which extend through the whole lithosphere or over large areas. Furthermore, our knowledge concerning the implication of lithosphere-asthenosphere coupling in lithospheric plates driving forces is poor. These questions can not easily be answered as most of the available information is mainly located at or close to the surface (geodesy, tectonics, seismicity). The high rates and type of surface continental deformation within the Aegean constitute this region particularly interesting in this perspective.
2017
To understand the dynamics of a subduction zone, seismic anisotropy is often used, despite the difficulty of constraining anisotropy in the sub-slab region. However, particularly due to constraints imposed by deformation patterns in the mantle surrounding subducting slabs, seismic anisotropy is usually chosen as a better tool. In this study, the dynamics and kinematics of the Hellenic subduction zone and its impact on mantle convection related deformation are investigated by using source-side seismic anisotropy. To this aim, shear wave splitting parameters of local and teleseismic S waves from intermediate and shallow earthquakes are measured. Then, they are combined with splitting parameters obtained from teleseismic SKS and SKKS waves in order to determine variations in seismic anisotropy with depth. Although in the study area strong deep earthquakes are not available, our preliminary splitting results are quite reliable. Also, they are fairly consistent with the presence of azimu...
ANELASTICITY BENEATH THE AEGEAN INFERRED FROM RAYLEIGH WAVE ATTENUATION
Anelasticity of the Earth crucially affects the propagation of seismic waves especially, in the long period range. However, even though the elastic properties of the Aegean deep lithosphere and upper mantle have been thoroughly investigated, their quantitative anelastic properties that influence the long period wavefield are still largely unknown. This work is towards contributing to the better knowledge of the deep structure of the Aegean by introducing experimental anelastic parameters via the study of long period Rayleigh waves attenuation. For this scope, fundamental mode attenuation coefficients (γ R ) have been obtained for different two-station great-circle paths across the Aegean. The data used were provided by a broadband array installed in the area for 6 months in 1997. More than 1100 seismograms were analyzed in the 10-100 s range to obtain 17 sets of path average γ R (T) functions. The attenuation coefficients are in the range 2.5×10 -3 -0.15×10 -3 km -1 and correlate sufficiently with both experimental measurements in active tectonic regions elsewhere and synthetics generated with the use of an attenuation reference model inferred from other sources. By applying a stochastic uncoupled causal inversion method an average joint Q β -1 and shear velocity model representative of the under study area was obtained. Furthermore, path average γ R (T) functions were combined in a continuous regionalization tomographic scheme to obtain local γ R (T) and tomograms were constructed in the range 10-60 s. The most prominent feature in the tomograms is a high attenuation region in the central and north Aegean. This region is located south of the North Anatolian Trough and correlates well with a low shear velocity zone inferred from surface wave phase velocities. Moreover, it is associated with observed intense extensional deformation rates, mantle olivine anisotropy, recent volcanism and high heat flow.
The highest deformation rate along the Africa/Eurasia convergence zone is well documented in the Aegean area, being >4 cm/yr. However, it is still under question whether continental deformation is distributed along major faults which extend through the whole lithosphere or over large areas. Furthermore, our knowledge concerning the implication of lithosphere-asthenosphere coupling in lithospheric plates driving forces is poor. These questions can not easily be answered as most of the available information is mainly located at or close to the surface (geodesy, tectonics, seismicity). The high rates and type of surface continental deformation within the Aegean constitute this region particularly interesting in this perspective.
Shear-wave anisotropy across the geothermal field of Milos, Aegean volcanic arc
Geophysical Journal International, 1991
The drilled geothermal reservoir of Milos island, Aegean sea (Greece) is densely sampled by seismic waves from local earthquakes occurring at depths of 4-6 km, recorded by numerous three-component seismographs. During the eight months of observation seismic activity on the island occurred in the form of a few swarms located in three different areas. The focal mechanisms of all earthquakes share an area of dilatational quadrants indicating a nearly vertical maximum compressive stress. This implies an extensional field consistent with the regional tectonics of the Aegean sea.
Earth and Planetary Science Letters, 2004
We study the 3-D strain evolution of the northern Aegean and the vertical coherency of deformation. We observe that finite strain orientations in the mantle inferred from published shear-wave (SKS) fast polarization orientations, mid-to upper-crustal stretching lineations in metamorphic core complexes of mainly Miocene age, and the gradient in regional crustal thickness variations, are subparallel to one another. This correlation suggests that the Miocene phase of extension is imprinted in the anisotropic fabric (i.e., lattice preferred orientation, LPO) of the lithospheric mantle, and that the orientation of finite strain due to extension is nearly constant with depth. The lateral variation of published seismic delay times shows a correlation with laterally varying finite strain in the crust inferred from topography and crustal thickness estimates. This correlation suggests that lateral variations in finite crustal and mantle strain are correlated and may point at a pure shear extension mechanism involving the entire lithosphere.