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Abstract: We report on a two-year seismic deployment in the Cape Verde Islands, one goal of which was to study the upper mantle to determine its structure under a hot spot that is stationary in the hot spot reference frame. We find from analysis of P-to-S receiver functions estimated from broadband seismic recordings that, within uncertainty, the time separation between the 410 and 660km discontinuities is normal compared to radial earth models. Thus, to exist, even stationary hot spots do not require vertical thermal anomalies from deep melting sources anchored in the lower mantle or at the core–mantle boundary or their anomalies are narrower than ∼250km in the upper mantle. [Copyright &y& Elsevier]

Real-time monitoring of volcanic earthquakes forms a critical component of assessing volcanic hazard but requires a widely dispersed land-based instrument network and analysis infrastructure to be effective. Here we show how a sparse network of hydrophones can be used for volcano monitoring as well. Detection of acoustic events by the Mid-Atlantic Ridge hydrophone array includes many events in the vicinity of the Cape Verde Islands, upon which a volcano monitoring network operates. Correlating the hydroacoustic events with earthquake locations determined by the network, we find that earthquakes of magnitudes as low as ML 2.0 can be hydroacoustically located to ±25 km at ranges up to 2000 km. All correlatable events are volcano-tectonic in character. Acoustic propagation of small-magnitude earthquake signals is more efficient if their sources are perched in the island's volcanic edifice in the ocean sound transmission channel, which contributes to volcanic event detectability. The location accuracy at this low magnitude range, which includes magnitudes typical of seismic crises precursory to eruptions, provides an additional volcanic hazard monitoring capability. Seismicity and hydroacoustic activity in the northern Cape Verdes suggest that this monitoring element may be of use in assessing future risk there. [ABSTRACT FROM AUTHOR]

Abstract: We report observations of high-frequency (HF) volcanic tremor, with spectral peaks in the band 5–20 Hz, recorded in Fogo Island, Cape Verde archipelago. Several occurrences are characterized by regularly spaced spectral peaks, but the fundamental frequency is missing. The signal is strong enough to be weakly detected on the southern islands of the archipelago, at distances of 20, 80 and 120 km from Fogo. This widespread detection of the HF tremor suggests that the propagation of the energy is made as T-waves propagating in the ocean sound channel. After correcting for site effects, the spatial pattern of amplitudes in Fogo and Brava Islands clearly indicates an offshore source. Active seamounts recently discovered in the vicinity of Fogo and Brava Islands [Hansteen, T.H., Grevemeyer, I., Hanel, K., Kraus, G., Schneider, J., Masson, D.G., Le Bas, T., Faria, B., 2006. Seamounts at the Cape Verde Islands: the geosphere–hydrosphere–biosphere connection, Seamount Biogeosciences Network, Workshop volume, La Jolla, March 24–25, 2006, 27–29.] are proposed to be the source of the HF tremor. [Copyright &y& Elsevier]