Towards a global seismic model of attenuation combining on- and offshore data

We present our new 2-D and 3-D global models of Rayleigh wave and shear attenuation (SGLOBE-Q2D and SGLOBE-Q3D, respectively), including uncertainties. We use a dataset of ~10 million fundamental and higher mode (up to 4th overtone) Rayleigh wave amplitude measurements with wave periods T~38-275 s. The amplitude measurements are corrected for source effects as well as for the influence of along-path and local receiver elastic structure. Extensive synthetic inversion tests are carried out to guide the model parameterisation used. This enables us to expand our fundamental mode Rayleigh wave attenuation maps up to spherical harmonic degree 20, which is higher than in recent global attenuation studies. We observe strong low attenuation beneath all major global cratons, including a clear separation between the Congo and Kalahari cratons in South Africa, and between the East European and Siberian cratons at T~40-100 s. We also observe low attenuation perturbations beneath the continents and high attenuation anomalies beneath the oceans. We compare the observed upper mantle variations in attenuation beneath different oceans as a function of plate age and speed, and contrast them with variations in shear wave speed and anisotropy. In addition, we carry out synthetic inversion tests combining land and offshore surface wave synthetic data considering the source-receiver paths recorded by the UPFLOW ocean bottom seismology experiment in the Atlantic to quantify the potential resolution improvements thanks to offshore data.