Observations of high-frequency harmonic tremor in Fogo, Cape Verde Islands

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.