Seismic attenuation beneath Europe and the North Atlantic: Implications for water in the mantle

It is well known that anelasticity has significant effects on the propagation of seismic waves, as manifested by physical dispersion and dissipation. Investigations of anelasticity provide complementary constraints on the physical properties of Earth materials, but - contrary to imaging with elastic waves - progress in mapping Earth's anelasticity has been relatively slow, and there is only limited agreement between different studies or methodologies. Here, within the framework of adjoint tomography, we use frequency-dependent phase and amplitude anomalies between observed and simulated seismograms to simultaneously constrain upper mantle wavespeeds and attenuation beneath the European continent and the North Atlantic Ocean. In the sea-floor spreading environment beneath the North Atlantic, we find enhanced attenuation in the asthenosphere and within the mantle transition zone (MTZ). In subduction zone settings, for example beneath the Hellenic arc, elevated attenuation is observed along the top of the subducting slab down to the MTZ. No prominent reductions in wavespeeds are correlated with these distinct attenuation features, suggesting that non-thermal effects may play an important role in these environments. A plausible explanation invokes the transport of water into the deep Earth by relatively cold subducting slabs, leading to a hydrated MTZ, as previously suggested by mineral physics and geodynamics studies.