Diffraction of P, S and Rayleigh waves by three-dimensional topographies

The diffraction of P, S and Rayleigh waves by 3-D topographies in an elastic half-space is studied using a simplified indirect boundary element method (IBEM). This technique is based on the integral representation of the diffracted elastic fields in terms of single-layer boundary sources. It can be seen as a numerical realization of Huygens principle because diffracted waves are constructed at the boundaries from where they are radiated by means of boundary sources. A Fredholm integral equation of the second kind for such sources is obtained from the stress-free boundary conditions. A simplified discretization scheme for the numerical and analytical integration of the exact Green's functions, which employs circles of various sizes to cover most of the boundary surface, is used.
The incidence of elastic waves on 3-D topographical profiles is studied. We analyse the displacement amplitudes in the frequency, space and time domains. The results show that the vertical walls of a cylindrical cavity are strong diffractors producing emission of energy in all directions. In the case of a mountain and incident P, SV and SH waves the results show a great variability of the surface ground motion. These spatial variations are due to the interference between locally generated diffracted waves. A polarization analysis of the surface displacement at different locations shows that the diffracted waves are mostly surface and creeping waves.     

Horizontal layered structure with heterogeneity beneath continents and island arcs from particle orbits of long-period P waves

We investigate the particle orbits of long-period (about 20 s) P waves observed with the global seismic network. By analysing 84 three-component seismograms recorded at 25 stations from 60 earthquakes occurring beneath 300 km, we quantitatively evaluate the orbits by three sets of eigenvalues and eigenvectors, using a covariance matrix method. The eigenvalues for P waves recorded at stations located on continents are explained by the standard horizontal layered structure model (iasp91). On the other hand, the orbits observed at stations close to island arcs are affected not only by the horizontal layered structure but also by heterogeneity due to subducting plates, mantle diapirs and so on. On the basis of a single-scattering model for a plane P wave, we quantify the heterogeneities by an isotropic scattering coefficient g0. Fitting the theoretical eigenvalues to the observed ones, we estimate g0 for the crust and upper mantle beneath continents to be less than 0.0005 km^-1, and the mean g₀ for the structure beneath island arcs to be about 0.0015 to 0.003 km^-1.     

Wave propagation in transversely isotropic and periodically layered isotropic media

Summary. A set of stable algorithms for computing synthetic seismograms in attenuating transversely isotropic media is presented. The structures of these algorithms for anisotropic media are formally equivalent to their counterparts for isotropic media. The seismic responses of a periodically layered isotropic medium are compared with those of its long-wave equivalent transversely isotropic medium. The synthetics for the two media show observable differences in the range of frequencies considered. The differences are small in the P -waves, but partly large in later arrivals.     

Length scales, patterns and origin of azimuthal seismic anisotropy in the upper mantle as mapped by Rayleigh waves

We measure the degree of consistency between published models of azimuthal seismic anisotropy from surface waves, focusing on Rayleigh wave phase-velocity models. Some models agree up to wavelengths of ∼2000 km, albeit at small values of linear correlation coefficients. Others are, however, not well correlated at all, also with regard to isotropic structure. This points to differences in the underlying data sets and inversion strategies, particularly the relative 'damping' of mapped isotropic versus anisotropic anomalies. Yet, there is more agreement between published models than commonly held, encouraging further analysis. Employing a generalized spherical harmonic representation, we analyse power spectra of orientational (2Ψ) anisotropic heterogeneity from seismology. We find that the anisotropic component of some models is characterized by stronger short-wavelength power than the associated isotropic structure. This spectral signal is consistent with predictions from new geodynamic models, based on olivine texturing in mantle flow. The flow models are also successful in predicting some of the seismologically mapped patterns. We substantiate earlier findings that flow computations significantly outperform models of fast azimuths based on absolute plate velocities. Moreover, further evidence for the importance of active upwellings and downwellings as inferred from seismic tomography is presented. Deterministic estimates of expected anisotropic structure based on mantle flow computations such as ours can help guide future seismologic inversions, particularly in oceanic plate regions. We propose to consider such a priori information when addressing open questions about the averaging properties and resolution of surface and body wave based estimates of anisotropy.