RG OBSERVATIONS FROM 4 CONTINENTS - INVERSE-MODELING AND FORWARD-MODELING EXPERIMENTS

Using short-period recordings of fundamental-mode Rayleigh waves (Rg) from seven arrays on four continents, we have explored structural prop erties in the respective siting areas through inversion of Rg-phase ve locity-dispersion characteristics. The arrays studied were NORESS and ARCESS in Norway, GERESS in Germany, Eskdalemuir in Scotland, Yellowkn ife in Canada, Gauribidanur in India, and Alice Springs in Australia. The simplest model used consists of one layer over a half-space with s hear velocities and layer thickness as unknowns. Densities were held f ixed while P velocities were constrained by a constant Poisson ratio o f 0.25. With one exception (Eskdalemuir), the estimated S velocities w ere 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 hal f-space. However, estimated layer thicknesses varied considerably rang ing from 0.12 km (Yellowknife) to 1.6 km (Alice Springs). Inversions w ere also performed with a simulated gradient layer near the surface. M odel results and the ability to fit the observed velocity dispersion w ere very similar with the two types of models, thus revealing a resolu tion problem with the limited frequency range available and the lack o f higher modes. To examine the effect of lateral inhomogeneities in te rms of von Karman-velocity variations and topography on Rg propagation , 2-D finite difference synthetics were computed. In both cases, scatt ering 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 .