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- Characterization of microseismic noise in Cape VerdePublication . Carvalho, Joana; Silveira, Graça; Schimmel, Martin; Stutzmann, EleonoreThe interaction of ocean waves with either the seafloor or other ocean waves generates primary (PM) and secondary microseisms (SM) that propagate through the crust and mantle, predominantly as Rayleigh waves. The horseshoe geometry and surrounding bathymetry of the Cape Verde archipelago play a significant role in the ambient-noise generation in this region. We analyze the microseisms recorded in the region using two different temporary seismic networks, and we determine the number of signals polarized as Rayleigh waves and their back azimuth (BAZ) as a function of time and frequency. The relative number of polarized signals between PM and SM varies between the stations. At most of the stations, the SM can be divided into two frequency bands. At lower frequencies (0.1-0.2 Hz), the number of SM signals is stable throughout the year, whereas at higher frequencies (0.2-0.3 Hz) this number varies with the season, with more polarized signals during the northern hemisphere spring and summer. In both frequency ranges and at most stations, the BAZ does not vary significantly over the year and points toward sources within the archipelago and outside. We compute the source site effect and show that the local bathymetry around the Cape Verde Islands strongly amplifies local SM sources. Finally, we compare the measured BAZ with source areas derived from an ocean-wave model, which confirms that Cape Verde stations mostly record local sources.
- Evidence for high temperature in the upper mantle beneath Cape Verde archipelago from Rayleigh-wave phase-velocity measurementsPublication . Carvalho, Joana; Bonadio, Raffaele; Silveira, Graça; Lebedev, Sergei; Mata, João; Arroucau, Pierre; Meier, Thomas; Celli, Nicolas L.Cape Verde is an intraplate archipelago located in the Atlantic Ocean about 560 km west of Senegal, on an similar to 130 Ma old oceanic lithosphere. The upper-mantle structure beneath the islands was poorly known, until recently, in large part due to the lack of broadband seismic stations. In this study we used data from two temporary deployments across the archipelago, measuring the phase velocities of Rayleigh-waves fundamental-modes in a broad period range (8-250 s), by cross-correlating teleseismic earthquake data between pairs of stations. We derived a robust average, phase-velocity curve for the Cape Verde region, and inverted it for a shear-wave velocity profile. Our results show significantly low velocities of similar to 4.2 km/s in the asthenosphere, indicating the presence of anomalously high temperatures and, eventually, partial melting. The temperature anomaly is probably responsible for the thermal rejuvenation of the lithosphere to an effective age as young as about 30 Ma, which we infer from the comparison of seismic velocities beneath Cape Verde archipelago and those representative of different ages in the Central Atlantic. The anomalously high temperature in the asthenosphere, together with previously published evidence on low seismic velocities in the lower mantle and relatively He-unradiogenic isotopic ratios, suggests a hot plume, rooted deep in the lower mantle, as the origin of the Cape Verde hotspot.
- Mantle structure beneath the Macaronesian volcanic islands (Cape Verde, Canaries, Madeira and Azores): a review and future directionsPublication . Civiero, Chiara; Carvalho, Joana; Silveira, GraçaOcean island volcanism provides a unique window into the nature of mantle composition, dynamics and evolution. The four Macaronesian archipelagos-Cape Verde, the Canaries, Madeira and the Azores-are the main magmatic systems of the Central-East Atlantic Ocean with volcanic activity that in some islands poses significant risk for the population. The recent development of regional seismic networks in these settings has provided an important step forward in mapping the underlying mantle. However, difficulties in resolving the small-scale structure with geophysical techniques persist leading to discrepancies in the interpretation of the mechanisms responsible for volcanism. Here we review results from a number of studies on the seismic mantle structure beneath the Macaronesian archipelagos including seismic tomography, receiver functions, precursors and shear-wave splitting. Several regional models show low-velocity features in the asthenosphere below the islands, a relatively thinned transition zone and complex anisotropic patterns and attribute the volcanism to mantle plumes. This inference is supported by whole-mantle tomography models, which find broad low-velocity anomalies in the lower mantle below the Central-East Atlantic. Other models call for alternative mechanisms associated with shallower mantle upwellings and purely plate tectonism. Thus, there is still no generally accepted mechanism that explains volcanism in the Macaronesia region. Future research requires improvements in the resolving power of seismic techniques to better illuminate the velocity structure at a much higher resolution than the currently achieved and ultimately define the mechanisms controlling the ocean island volcanism.