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International audience; Seismic records from the RER very broad-band seismic station (La Réunion Island) belonging to the GEOSCOPE network are investigated to understand the eruptive succession (February to May) of Piton de la Fournaise and the caldera collapse episode of April 2007. Data first indicate that the short-lived, small volume, summit eruption of February 18 occurred during a phase of continuous inflation initiated in January 2007. Inflation decelerated around 2 weeks before a second short-lived small volume eruption on March 30-31 on the SE flank, almost simultaneous with a sudden, large deflation of the edifice. Deflation rate, which had stabilized at a relatively low level, increased anew on April 1 while no magma was emitted, followed on April 2 by a more distant and one of the most voluminous eruptions of the last two centuries at La Réunion Island. The RER station shows that very long period (VLP) and ultra long period (ULP) events developed during this period. Seven ULP events preceded the caldera collapse and 48 ones occurred during the caldera collapse over 9 days, most of which during the first 30 hours. A thorough examination of the seismic signals corrected for tide effects shows that each collapse event was coeval with VLP and ULP signals. Each individual collapse showed similar ULP and VLP signals characterized by periods of ∼ 500 s and ∼ 7 s, respectively. The back-azimuth of most ULP signals related to the caldera collapse points clearly toward the Dolomieu caldera. The strikingly constant duration of the VLP signals (around 20 s) related to the collapse events and their occurrence before the collapse initiation suggest a physical control of the volcanic edifice. Waveforms and spectrograms of the various caldera collapse events show very homogeneous patterns, suggesting a similar and repeating volcano-tectonic process for the formation of the VLP signals events. Although tilt may be responsible of part of the ULP signals observed during the collapse events, we show that it cannot explain most of the records. The ULP signals occurring during the collapse and also recorded by the OVPF GNSS (Global Navigation Satellite System) permanent network likely correspond to relaxation of the volcanic edifice. This analysis allows us to propose a scenario that may explain each successive collapse event as starting with a short-period event induced by the rock failure, followed by a VLP signal induced by dip-slip motion on the caldera ring fault, and ending with a ULP signal likely related to a relaxation process of the edifice.

International audience; Ocean wave activity excites seismic waves that propagate through the solid earth, known as microseismic noise. Here we use a network of 57 ocean bottom seismometers (OBS) deployed around La Réunion Island in the southwest Indian Ocean to investigate the noise generated in the secondary microseismic band as a tropical cyclone moved over the network. Spectral and polarization analyses show that microseisms strongly increase in the 0.1–0.35 Hz frequency band as the cyclone approaches and that this noise is composed of both compressional and surface waves, confirming theoretical predictions. We infer the location of maximum noise amplitude in space and time and show that it roughly coincides with the location of maximum ocean wave interactions. Although this analysis was retrospectively performed, microseisms recorded on the seafloor can be considered a novel source of information for future real-time tracking and monitoring of major storms, complementing atmospheric, oceanographic, and satellite observations.

International audience; Seismic anisotropy is a ubiquitous feature of the subcontinental mantle. This can be inferred both from direct seismic observations of shear wave splitting from teleseismic shear waves, as well as the petrofabric analyses of mantle nodules from kimberlite pipes. The anisotropy is principally due to the strain-induced lattice preferred orientation (LPO) of olivine. The combined use of these mantle samples, deformation experiments on olivine, and numerical modeling of LPO, provides a critical framework for making inferences about mantle deformation from observed seismic anisotropy. In most cases there is a close correspondence between mantle deformation derived from seismic observations of anisotropy, and crustal deformation, from the Archean to the present. This implies that the mantle plays a major, if not dominant role in continental deformation. No clear evidence is found for a continental asthenospheric decoupling zone, suggesting that continents are probably coupled to general mantle circulation.

International audience; The dynamics of mantle plumes and the origin of their associated swells remain some of the most controversial topics in geodynamics. Here we construct a numerical model of the mantle flow beneath the French Polynesia region. Our study is based on a new regional seismic tomography model, which high resolution allows obtaining information at the scale of plumes. We find excellent correlations between the observed and the modeled dynamic swells, between the modeled flow pattern and the active volcanism and between the buoyancy fluxes obtained from our numerical model and the ones deduced from the swells morphology. These outstanding fits reveal for the first time that a direct link exists between the surface observations and mantle flows.

International audience; The South Pacific region is characterized by a broadly elevated seafloor known as the South Pacific superswell. This region has a concentration of midplate volcanoes that experienced massive eruptions in the mid-Cretaceous period (90-120 Ma). These characteristics suggest the presence of a large-scale mantle plume beneath the South Pacific, called the South Pacific superplume. The geometry, origin depth, temperature, and composition of the superplume remain controversial, however, mainly due to the lack of seismological data that documents the mantle structure beneath the South Pacific. Seismic stations are sparse in the area due to its remote ocean environment. To obtain a better seismic image of the superplume, we deployed temporary broadband seismographs on oceanic islands and the seafloor in the South Pacific, which made possible the highest spatial resolution that has ever been achieved for the mantle structure beneath the region. The seismic image obtained from this new seismic data indicates that large-scale low-velocity anomalies (on the order of 1000 km in diameter), indicative of the superplume, are located from the bottom of the mantle to a depth of 1000 km, and small-scale low-velocity anomalies (on the order of 100 km in diameter) are present above it. A comparison of the seismic image with recent mantle convection studies based upon laboratory and numerical experiments suggests that the superplume may be a hot and chemically distinct mantle dome, and that the small-scale anomalies may be narrow plumes generated from the top of the dome. This model may explain various characteristics of hot spots in the South Pacific, such as the seafloor swell, short-lived hot spot chains, and the periodicity of massive eruptions.

International audience; The three-dimensional S-wave velocity structure beneath the South Pacific Superswell is obtained from joint broadband seismic experiments on the ocean floor and islands. We collected only approximately 800 relative times of long-period teleseismic SH-waves by using a waveform cross-correlation from 76 events occurring from January 2003 to May 2005. We conducted relative time tomography to obtain a 3D structure to depths of 1600 km. In the resultant image, we find a characteristic distribution of low- velocity regions. The most prominent features are a large doughnut-shaped low-velocity region at 800 km depth, and an elongated large low-velocity region beneath the Society to Pitcairn hotspots at 1200 km depth. Our model suggests that a large low shear velocity province rooted in the D00 extends upwards and culminates near the top of the lower mantle beneath the central part of the South Pacific Superswell although its perfect continuity is not still confirmed.

Azimuthal anisotropy is the dependence of local seismic properties on the azimuth of propagation. We present the azimuthally anisotropic component of a 3D SV velocity model for the Pacific Ocean, derived from the waveform modeling of over 56,000 multi-mode Rayleigh waves followed by a simultaneous inversion for isotropic and azimuthally anisotropic vsv structure. The isotropic vsv model is discussed in a previous paper (A. Maggi, E. Debayle, K. Priestley, G. Barruol, Multi-mode surface waveform tomography of the Pacific Ocean: a close look at the lithospheric cooling signature, Geophys. J. Int. 166 (3) (2006). doi:10.1111/ j.1365-246x.2006.03037.x). The azimuthal anisotropy we find is consistent with the lattice preferred orientation model (LPO): the hypothesis of anisotropy generation in the Earth's mantle by preferential alignment of anisotropic crystals in response to the shear strains induced by mantle flow. At lithospheric depths we find good agreement between fast azimuthal anisotropy orientations and ridge spreading directions recorded by sea-floor magnetic anomalies. At asthenospheric depths we find a strong correlation between fast azimuthal anisotropy orientations and the directions of current plate motions. We observe perturbations in the pattern of seismic anisotropy close to Pacific hot-spots that are consistent with the predictions of numerical models of LPO generation in plume-disturbed plate motion-driven mantle flow. These observations suggest that perturbations in the patterns of azimuthal anisotropy may provide indirect evidence for plume-like upwelling in the mantle.

Upper mantle flow beneath the French Massif Central is investigated using teleseismic shear wave splitting induced by seismic anisotropy. About 25 three component stations (short period, intermediate and broad-band) were installed during the period 1998-1999 in the southern Massif Central, from the Clermont Ferrand volcanic area to the Mediterranean sea. Teleseismic shear waves (SKS, SKKS and PKS) were used to determine the splitting parameters: the fast polarization direction and the delay time. Delay times ranging between 0.7 and 1.5 s have been observed at most of the sites. The azimuths of the fast split shear waves trend homogeneously NW-SE in the southern Massif Central suggesting a homogeneous mantle flow beneath this area. The observed NW-SE direction differs from the N100°E Pyrenean anisotropy further South. It does not appear to be correlated to Hercynian structures nor to the present-day motion of the plate but is well correlated to the Tertiary extension direction. We propose that the opening of the western Mediterranean induced by the rotation of the Corsica-Sardinia lithospheric bloc and the roll-back to the SE of the Tethys slab may have generated a large asthenospheric mantle flow beneath the southern Massif Central and a deflection of the up going plume centered beneath the northern Massif Central toward the SE.

International audience; Splitting of teleseismic shear waves has been measured in the Appalachians (eastern USA) and the Pyrenees (western Europe) using data recorded by permanent and portable stations. From a comparison of the results, it appears that an interpretation of the recorded seismic anisotropy in terms of geodynamics is not straightforward. Successive geodynamic events have generated structures that may have resulted in a similar pattern of mantle flow and that therefore may have contributed in the development of the recorded anisotropy. Combining geological and geophysical arguments, it appears that the mantle anisotropy measured across the Appalachians and the Pyrenees may not be systematically Appalachian or Pyrenean in age but may be mainly due to a lithospheric structure formed during earlier major tectonic events, i.e. the Grenvillian and the Hercynian orogenies, respectively. We suggest that during major episodes of continent assembly, a pervasive tectonic fabric is developed in the lithospheric mantle. In the subsequent evolution of the continent, this fabric may induce a significant mechanical anisotropy that will drastically influence the mechanical behaviour of the Iithosphere when submitted to new tectonic events.