Open AccessScattering of S waves diffracted at the core–mantle boundary: forward modelling
Author(s) -
Emery Valérie,
Maupin Valérie,
Nataf HenriClaude
Publication year - 1999
Publication title -
geophysical journal international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0956-540X
DOI - 10.1046/j.1365-246x.1999.00941.x
Subject(s) - mantle (geology) , diffraction , geology , geophysics , core–mantle boundary , amplitude , scattering , seismic wave , seismology , physics , optics
Summary The lowermost 200–300 km of the Earth’s mantle, known as the D″ layer, is an extremely complex and heterogeneous region where transfer processes between the core and the mantle take place. Diffracted S waves propagate over large distances and are very sensitive to the velocity structure of this region. Strong variations of ampli‐tudes and waveforms are observed on recordings from networks of broad‐band seismic stations. We perform forward modelling of diffracted S waves in laterally heterogeneous structures in order to analyse whether or not these observations can be related to lateral inhomogeneities in D″. We combine the diffraction due to the core and the scattering due to small‐scale volumetric heterogeneities (10–100 km) by coupling single scattering (Born approximation) with the Langer approximation, which describes S diff wave propagation. The influence on the direct as well as on the scattered wavefields of the CMB as well as of possible tunnelling in the core or in D″ is fully accounted for. The SH and the SV components of the diffracted waves are analysed, as well as their coupling. The modelling is applied in heterogeneous models with different geometries: isolated heterogeneities, vertical cylinders, horizontal inhomogeneities and random media. Amplitudes of scattered waves are weak and only velocity perturbations of the order of 10 per cent over a volume of 240 × 240 × 300 km 3 produce visible effects on seismograms. The two polarizations of S diff have different radial sensitivities, the SH components being more sensitive to heterogeneities closer to the CMB. However, we do not observe significant time‐shifts between the two components similar to those produced by anisotropy. The long‐period S diff have a poor lateral resolution and average the velocity perturbations in their Fresnel zone. Random small‐scale heterogeneities with ± 10 per cent velocity contrast in the layer therefore have little effect on S diff , in contrast to their effect on PKIKP .