Rgobservations from four continents: inverse- and forward-modelling experiments

Abstract

SUMMARY Using short-period recordings of fundamental-mode Rayleigh waves (Rg) from seven arrays on four continents, we have explored structural properties in the respective siting areas through inversion of Rg-phase velocity-dispersion characteristics. The arrays studied were NORESS and ARCESS in Norway, GERESS in Germany, Eskdalemuir in Scotland, Yellowknife in Canada, Gauribidanur in India, and Alice Springs in Australia. The simplest model used consists of one layer over a half-space with shear velocities and layer thickness as unknowns. Densities were held fixed while P velocities were constrained by a constant Poisson ratio of 0.25. With one exception (Eskdalemuir), the estimated S velocities were remarkably consistent between the arrays with an average value of 2.9 ± 0.1 kms−1 in the upper layer and 3.55 ± 0.1 kms−1 in the half-space. However, estimated layer thicknesses varied considerably ranging from 0.12 km (Yellowknife) to 1.6 km (Alice Springs). Inversions were also performed with a simulated gradient layer near the surface. Model results and the ability to fit the observed velocity dispersion were very similar with the two types of models, thus revealing a resolution problem with the limited frequency range available and the lack of higher modes. To examine the effect of lateral inhomogeneities in terms of von Karman-velocity variations and topography on Rg propagation, 2-D finite difference synthetics were computed. In both cases, scattering was most pronounced for frequencies above 1 Hz and thus explain the lack of Rg-phase-velocity estimates for periods shorter than 0.6 s.