Separation of Source, Site, and Path Effects Based on Elastic Radiative Transfer Theory. Part 1: Mapping of Scattering and Absorption Properties and Application to Metropolitan France

We present the first 2D scattering and absorption maps of Metropolitan France in the 0.375–24 Hz frequency band. To obtain these maps, we use a hybrid linearized inversion/grid-search scheme to adjust the scattering and absorption quality factors, and the product of site amplification and source energy for each seismogram envelope separately. To compute seismogram envelopes and their partial derivatives, we perform Monte Carlo simulations of elastic radiative transfer, taking into account scattering anisotropy, coupling between P and S waves, as well as partial trapping of energy in the crust. Next, scattering and absorption parameters are mapped in a ray tomographic manner, using a least-squares algorithm with second-order Tikhonov regularization. We apply the mapping procedure to around 21,000 waveforms recorded by European Plate Observing System–France and Commissariat à l’Énergie Atomique et aux Énergies Alternatives stations at hypocentral distances less than 250 km, for earthquakes with local magnitudes ML ranging from 2.0 to 5.9. Attenuation maps show strong spatial and frequency variations. Spatial variations can reach a factor 3 for scattering and 2 for absorption compared with the average attenuation in France. Although scattering dominates absorption at low frequency (<1 Hz), the opposite occurs at higher frequency. Strong scattering anomalies can be associated with recent sedimentary basins (Quaternary and late Cenozoic) at low frequency, and a few Mesozoic basins or deep sedimentary basins at higher frequency. Absorption is strongest in the western Pyrenees and the French Alps, with a frequency-dependent pattern for the latter. Absorption is also strong at low frequency in the northern basins and Ardennes massif. By contrast, old Variscan units show low scattering attenuation especially at low frequency, and the Armorican massif is consistently the least absorbing region. Such spatial and frequency-dependent attenuation variations likely contribute to the strong variability of ground motions in France, making our model of attenuation of interest for future seismic hazard studies.