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Over a year of seismic observations, ~5000 short duration icequakes were detected by a permanent broadband station installed at the Princess Elisabeth base, located ~180 km inland in eastern Dronning Maud Land, East-Antarctica. Icequake detection via seismic waveform pattern recognition indicates the presence of two dominating clusters of events, totalizing ~1500 icequakes. The corresponding icequake locations point towards two distinct zones of outcropping blue ice areas (BIAs) located respectively at 4 and 1 km from the seismic station, both on the leeward side of a nunatak protruding through the ice sheet. The temporal occurrence of these icequakes suggests a close genetic link with thermal contraction of ice caused by significant surface cooling controlled, in summer by variations in diurnal solar radiation and in winter by strong cooling during cold katabatic regimes. Further analysis demonstrates the dependence of these icequakes on the absolute surface temperature and on its temporal change. Besides providing information on the ice fracture mechanics and rheology, investigations of thermal icequakes may be regarded as a ground-based proxy for the monitoring of the thermal state of BIAs, and characterization of ice-sheet ablation zones. [ABSTRACT FROM AUTHOR]

Abstract: If upper mantle anisotropy beneath fast-moving oceanic plates is expected to align the fast azimuths close to the plate motion directions, the upper mantle flow pattern beneath slow-moving oceanic plates will reflect the relative motion between the moving plate and the underlying large-scale convecting mantle. In addition to the non-correlation of the fast azimuths with the plate motion direction, the flow and anisotropy pattern may be locally perturbed by other factors such as the upwelling and the sublithospheric spreading of mantle plumes. Investigating such plume–lithosphere interaction is strongly dependent on the available seismological data, which are generally sparse in oceanic environment. In this study, we take the opportunity of recent temporary deployments of 15 seismic stations and 5 permanent stations on the Piton de la Fournaise volcano, the active locus of La Réunion hotspot and of 6 permanent stations installed along or close to its fossil track of about 3700km in length, to analyze azimuthal anisotropy detected by SKS wave splitting and to decipher the various possible origins of anisotropy beneath the Western Indian Ocean. From about 150 good and fair splitting measurements and more than 1000 null splitting measurements, we attempt to distinguish between the influence of a local plume signature and large-scale mantle flow. The large-scale anisotropy pattern obtained at the SW-Indian Ocean island stations is well explained by plate motion relative to the deep mantle circulation. By contrast, stations on La Réunion Island show a complex signature characterized by numerous “nulls” and by fast split shear wave polarizations trending normal to the plate motion direction and obtained within a small backazimuthal window, that cannot be explained by either a single or two anisotropic layers. Despite the sparse spatial coverage which precludes a unique answer, we show that such pattern may be compatible with a simple model of sublithospheric spreading of La Réunion plume characterized by a conduit located at 100–200km north of La Réunion Island. Anisotropy beneath the new GEOSCOPE station in Rodrigues Island does not appear to be influenced by La Réunion plume-spreading signature but is fully compatible with either a model of large-scale deep mantle convection pattern and/or with a channeled asthenospheric flow beneath the Rodrigues ridge. [Copyright &y& Elsevier]

Abstract: Constraining mantle deformation beneath plate boundaries where plates interact with each other, such as beneath active or halted mountain belts, is a particularly important objective of “mantle tectonics” that may bring a depth extent to the Earth''s surface observation. Such mantle deformation can be mapped at scale lengths of several tens of kilometers through the analysis of seismological data and particularly by mapping seismic anisotropy from the splitting analysis of vertically-propagating SKS waves that largely reflect the strain-induced crystal preferred orientations of the rock-forming minerals within the upper mantle. In the present study, we analyse data from approximately 50 broadband seismic stations covering the Western Alps and we provide a coherent picture of upper mantle anisotropy beneath the belt. The large-scale anisotropy pattern is characterized by fast split directions that closely follow the trend of the belt. Moreover, the maximum anisotropy magnitude is not located beneath the internal zones of the belt but instead beneath external units. All suggests that the anisotropy is likely dominated by sublithospheric mantle deformation. We propose that the observed anisotropy pattern can be explained by recent or active mantle flow around the Eurasian slab presently plunging beneath the inner parts of the Alps. [Copyright &y& Elsevier]

Upper-mantle flow beneath the South Pacific is investigated by analysing shear wave splitting parameters at eight permanent long-period and broad-band seismic stations and 10 broad-band stations deployed in French Polynesia from 2001 to 2005 in the framework of the Polynesian Lithosphere and Upper Mantle Experiment (PLUME). Despite the small number of events and the rather poor backazimuthal coverage due to the geographical distribution of the natural seismicity, upper-mantle seismic anisotropy has been detected at all stations except at Tahiti where two permanent stations with 15 yr of data show an apparent isotropy. The median value of fast polarization azimuths (N67.5°W) is parallel to the present Pacific absolute plate motion direction in French Polynesia (APM: N67°W). This suggests that the observed SKS fast polarization directions result mainly from olivine crystal preferred orientations produced by deformation in the sublithospheric mantle due to viscous entrainment by the moving Pacific Plate and preserved in the lithosphere as the plate cools. However, analysis of individual measurements highlights variations of splitting parameters with event backazimuth that imply an actual upper-mantle structure more complex than a single anisotropic layer with horizontal fast axis. A forward approach shows that a two-layer structure of anisotropy beneath French Polynesia better explains the splitting observations than a single anisotropic layer. Second-order variations in the measurements may also indicate the presence of small-scale lateral heterogeneities. The influence of plumes or fracture zones within the studied area does not appear to dominate the large-scale anisotropy pattern but may explain these second-order splitting variations across the network. [ABSTRACT FROM AUTHOR]

Examines the upper mantle anisotropy beneath the African IRIS and Geoscope stations. Conduction of investigations of teleseismic shear wave splitting; Observation of anisotropy at continental stations; Difficulty in obtaining anisotropy measurements at the oceanic stations.

Abstract: We present new shear-wave splitting measurements performed at 16 stations on the East European Craton, and discuss their implications in terms of upper-mantle anisotropy for this geophysically poorly-known region. Previous investigations of mantle anisotropy in Central Europe have shown fast directions aligning smoothly with the craton''s margin and various suggestions have been proposed to explain their origin such as asthenospheric flow or lithospheric frozen-in deformation. Here, we aim at investigating the continuation of this shear-wave splitting pattern further to the East, into the East European Craton. For the craton, the interpretation appears to be less ambiguous than for central Europe since several arguments support lithospheric anisotropy in this region: 1) The large-scale coherence within either of the four constituting blocks and the significant variations between the blocks on a small-scale, 2) the weak correlation with absolute plate motion vectors, and 3) the good correlation between anisotropy and crustal features, for which we use magnetic field alignments as a proxy. Rather good correlation of these magnetic features with seismic fast orientations strongly supports the idea of vertically coherent deformation throughout upper mantle and crust. The observed splitting orientations thus reflect the last tectonic events of each block, frozen-in into the lithosphere for hundreds of millions of years. [Copyright &y& Elsevier]

Abstract: We compare a global compilation of shear-wave splitting measurements with azimuthal seismic anisotropy parameters inferred from surface-wave tomography. The currently available splitting dataset is taken from a novel comprehensive collection of available publications that is updated interactively online. The comparison between the two types of data is made by calculating predicted splitting parameters from the anisotropic tomography model. Comparing these predicted splitting parameters with the observed ones, we find a considerable correlation between the two datasets at global scale. This result is noteworthy, since such correlation did not seem to exist in previous studies. The spatial resolution associated with the two types of methods is rather different. While surface waves have good vertical resolution and poor lateral resolution of several hundreds of kilometers, SKS splitting measurements have good lateral, but poor vertical resolution. The correlation can be understood in light of recent propositions that anisotropy seen by SKS splitting constrains mostly the upper mantle, and therefore a similar depth region as surface waves. The correlation also confirms the generally good quality of the shear-wave measurements, as well as that of the anisotropic tomography model. [Copyright &y& Elsevier]

Abstract: We discuss mantle structure and dynamics under the Gibraltar arc. For that region, a large variety of geodynamic models has been proposed, and there was, up to recently, no consensus on which of these models is the best one to describe the geodynamic situation of the region. The key to distinguishing the different models probably lies in the upper mantle, and we propose two specially adapted seismological techniques that help to distinguish each of the two models that are more frequently invoked. These are, on one hand, the subduction-rollback (SR) model, and on the other, continental delamination or convective removal (CDCR). The first technique that we use is based on observations of dispersed P-wavetrains that are expected for certain ray directions. Observations made on the Gibraltar arc suggest the presence of subducted oceanic lithosphere under the Alboran Sea. The second technique is based on shear-wave splitting of SKS phases, and it allows us to look for the radially-oriented flow that must be present in the mantle beneath the Alboran Sea, for a CDCR mechanism. There are however no indications of any radial flow up to this point. Both approaches therefore suggest that subduction-rollback is the best model to explain the complex geodynamic situation under the region. [Copyright &y& Elsevier]

Abstract: We present a graphical user interface to facilitate the processing of teleseismic shear-wave splitting observations. In contrast to a fully automated technique, we present a manual, per-event approach that maintains user control during the sequence of processing. The SplitLab environment is intended to undertake the repetitive processing steps while enabling the user to focus on quality control and eventually the interpretation of the results. Pre-processing modules of SplitLab create a database of events and link the corresponding seismogram files. The seismogram viewer tool uses this database to perform the measurement interactively. Post-processing of the combined results of such a project includes a viewer and export option. Our emphasis lies in the application to teleseismic shear-wave splitting analysis, but our code can be extended easily for other purposes. SplitLab can be downloaded at http://www.gm.univ-montp2.fr/splitting/. [Copyright &y& Elsevier]