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- 3D-ambient noise surface wave tomography of Fogo volcano, Cape VerdePublication . Carvalho, J.; Silveira, Graça; Dumont, Stéphanie; Ramalho, Ricardo Dos SantosFogo volcano belongs to the Cape Verde Archipelago, and it is one of the most active volcanoes in the Atlantic Ocean, which most recent eruption occurred from November 2014 to February 2015. We analyzed ambient seismic noise recordings of three different networks deployed in the island, totalizing 14 seismic stations, to derive a crustal 3D shear-wave crustal velocity model of the volcano. Through the phase cross-correlation technique followed by a time-domain phase weighted stack, we were able to measure Rayleigh wave group-velocity dispersion measurements in the period range from 1.0 to 10 s. These dispersion measurements were used to invert for 2D group velocity maps at selected periods, and then inverted to produce a 3D shear-wave velocity model of the island. The tomographic model shows three velocity domains. First, an asymmetric upper layer, above 5–6 km of depth, with lower velocities concentrated in the northeastern sector of the island and a clear higher-velocity horizontal body at 3–4 km of depth in the southwestern sector of the island; the spatial correlation between these two velocity zones and the Galinheiros normal fault suggests a genetic link between the high velocities and long-term surface deformation, which we related to sill intrusions between 3 and 4.5 km depth, beneath the southwestern sector of the island. Second, a marked higher-velocity horizontal layer in between 5 and 6 km and 8–9 km, interpreted as the seismic expression of pervasive sill and laccolith intrusions, now cooled, beneath the volcanic edifice and within the underlying oceanic crust. Third, a lower velocity layer below 8–9 km of depth, more pronounced beneath the northeastern sector, which could be explained by a hotter and possibly melt-rich zone beneath the volcano or a significantly altered/serpentinized crust. Finally, our study also confirms that Fogo lacks any sizable magma chambers (ancient or recent) within the volcanic edifice, in agreement with other geophysical and petrological studies. These observations demonstrate that 3D-ambient noise Rayleigh wave tomography is a powerful tool to image the crustal and upper mantle structure beneath volcanic islands, as shown here for Fogo volcano.
- Crustal and uppermost mantle structure of Cape Verde from ambient noise tomographyPublication . Carvalho, J.; Silveira, Graça; Kiselev, S; Custódio, Susana; Dos Santos Ramalho, Ricardo; Stutzmann, E; Schimmel, MWe present a seismic ambient noise tomography of the Cape Verde archipelago, located in the Atlantic Ocean, approximately 600 km west of Senegal. We used 38 seismic broad-band stations that continuously recorded for 10 months, in order to construct the first 3-D model of Sv-wave velocities for the crust and uppermost mantle beneath the Cape Verde region. We started by computing phase cross-correlations for vertical component recordings using all possible inter-island station pairs. Next, a time–frequency phase-weighted stack was applied to obtain robust Rayleigh-wave group-velocity dispersion curves in the period band between 10 and 24 s. Group-velocity maps at different periods are obtained by inverting the dispersion curves. We then inverted the group-velocity maps to obtain the 3-D shear wave velocity structure of the crust and uppermost mantle beneath Cape Verde. The final 3-D model extends from 8 km down to 23 km and has a lateral resolution of about 50 km. The crust in the southwestern sector, encompassing Fogo, presents lower S-wave velocities that may be caused by the presence of melt pockets and/or hydrothermal fluids circulation. The uppermost mantle beneath the northwestern sector is characterized by higher S-wave velocities in agreement with previous results obtained from Ps and Sp receiver functions. Those high-velocity anomalies can reflect non-altered crust or remnants of magma chambers or solidified basaltic intrusions, which fed the volcanism in these islands. Our maps revealed the presence of crustal underplating across the entire archipelago, yet stronger beneath the groups Santo Antão—São Vicente—São Nicolau and Fogo—Santiago—Maio.