Secondary Microseism Sources in Macaronesia (North Atlantic)

• Main Authors: Silveira, G., Carvalho, Joana F., Schimmel, Martin, Stutzmann, E.
• Format: Still Image
• Language: unknown
• Published: 2018
• Subjects: North Atlantic
• Online Access: http://hdl.handle.net/10261/190359

When ocean-waves, coming from opposite directions meet, they generate pressure fluctuations that propagate in the ocean and are converted at ocean bottom into various seismic waves, which then travel within the Earth and are recorded everywhere on Earth. These signals, called secondary microseisms (SM), can be used to monitor the ocean activity. The islands of Macaronesia are of primary importance to improve our knowledge of the ocean wave activity in the North Atlantic Ocean at a local scale. Simultaneously, it allows improving our comprehension of the impact of islands geomorphology and the local bathymetry on the variability of the microseisms recorded in the Macaronesia stations. This study benefits from a vast seismic dataset of high-quality broadband seismic data in the Portuguese, Spanish and Cape Verde territories. We present here results obtained using a small subset of stations, located in the archipelagos of Cape Verde and Madeira. To analyze the three-component data records and characterize the secondary microseismic noise, we used the time-frequency polarization method (Schimmel et al., 2011) in the frequency band 0.10 - 0.33 Hz. This technique allows us to determine the back-azimuth (BAZ) of Rayleigh waves, as a function of time and frequency which we compare with theoretically determined sources obtained through the numerical ocean wave model (IFREMER model, Ardhuin et al., 2011). In all the Cape Verde stations, the SM is divided into short- and long-period bands, 3 - 5s and 5 - 10s, respectively. Contrary to the short-period SM, the long-period SM is consistent throughout the year, and BAZ does not reveal significant seasonal variations. The dominant BAZ points towards sources within the horse-shoe shaped archipelago or to nearby noise sources, showing that the most significant North Atlantic storms do not generate the recorded SM in Cape Verde. In Madeira, the SM is clearly visible between 3 and 10s and shows small seasonal variations, in both BAZ and frequency. The dominant BAZ points to ...