Seismic scattering and absorption of oceanic lithospheric S waves in the Eastern North Atlantic

Seismic scattering and absorption parameters provide valuable information about the propagation of seismic waves within the lithosphere. For the analysis of the seismic scattering and absorption parameters in the oceanic lithosphere of the Eastern North Atlantic, we use a seismological array which was installed in 5000 m water depth about 100 km North of the Gloria fault, defining the plate boundary between the Eurasian and African plate at this location. During our seismological experiment, more than 350 local and regional earthquakes were identified within 10 month for epicentral distances up to 900 km. The acquired regional earthquake recordings show up to 30 Hz P and S wave arrivals with long codas lasting tens to hundreds of seconds. Modelling results suggest that these long codas originate from scattering in the oceanic lithosphere. The waves travel with upper mantle apparent velocities and are therefore referred to as oceanic Pn (Po) and Sn (So) waves. We use direct So waves and their coda of pre-selected earthquakes to estimate frequency-dependent seismic scattering and intrinsic attenuation parameters. The results for the analysed events show that intrinsic attenuation is stronger than scattering attenuation and the estimated transport mean free path lengths between 30-800 km indicate that the So wave coda is weakly influenced by the oceanic crust. Furthermore, the calculated parameters show higher attenuation for western to northern event azimuths than for eastern to southern ones which might be related to differences in lithospheric ages for example of the Eurasian and African plates, or the influence of the Gloria fault itself.