S U M M A R YResults of a pilot experiment preliminary to a systematic study of surface-wave polarization across Eurasia are presented. Long-period fundamental Rayleigh and Love waves recorded by the broad-band seismic stations KIV (Kislovodsk) and OBN (Obninsk) of the IRIS/IDA network deployed in the Former Soviet Union (FSU) were analysed from 14 events to the north-east, east and south-east of these stations in search of particle-motion anomalies using a technique called FrequencyTime Polarization Analysis (ITPAN). All anomalies indicate deviations of wavepropagation paths to the north relative to the great circle paths. Polarization anomalies at OBN are uniformly small (4"). Significant frequency-dependent polarization anomalies (5"-20") are found for Rayleigh and Love waves arriving at KIV from teleseismic events at a wide range of backazimuths (26"-103"). Polarization measurements are repeatable (both for a number of nearly degenerate events and for reciprocal paths) and vary smoothly and continuously as epicentral location is moved. Consequently, such measurements are robust and can provide useful structural information. The systematics of the frequency dependence of the polarization anomalies as epicentral position is moved from north to south display sensitivity to structures at a number of length-scales.Results from synthetic experiments using coupled normal-mode and Gaussianbeam synthetics reveal that: (1) recently constructed long-wavelength aspherical models produce polarization anomalies that are significantly smaller (<5" at KIV) but of the same sign (negative) as the observed anomalies, and (2) the frequency dependence of the observed anomalies must result from a combination of structures of differing wavelengths, with scale-lengths ranging from regional to global. Thus, polarization measurements provide new information about currently unmodelled structures. A model that fits the polarization anomalies observed at KIV includes a regional-scale low-velocity feature near to KIV in order to fit the large-magnitude, short-period polarization anomalies and a smaller magnitude, continent-scale increase in upper mantle velocities and/or the reduction of crustal thickness from south to north in central Eurasia in order to fit the broad-band, longer period anomalies. The small-scale, low-velocity feature is a model of the sedimentary basin of the sub-Caspian depression.We conclude that measurements of polarization anomalies can be obtained accurately, that they are reproducible, that they contain currently unmodelled information, and that they should prove to be useful in combination with velocity and amplitude information in future tomographic inversions, especially to help focus global-scale models to regional wavelengths.

lnhomogeneity in a real material may produce a seismic wavefield pat tern qualitatively indistinguishable from one caused by anisotropy. However, the quantitative description of such a medium as an apparently anisotropic elastic solid may lead t o geophysically invalid conclusions. Several numerjcal examples are presented for body and surface waves propagating through inhomogeneous isotropic elastic models.

The recent installation of six broadband digital IRIS/IDA seismic stations in the USSR has provided new opportunities for studying surface-wave propagation across Eurasia. Group velocities of fundamental Rayleigh and Love modes between epicenters and these stations were determined for 35 events that occurred since April 1989 to the middle of July 1990 near Eurasia. Differential phase velocities were found for the same arrivals along paths between several pairs of stations. Group and phase velocities were obtained in the period range from 15 to 300 sec. Frequency-time polarization analysis was used for studying polarization properties of surface waves. In some cases, significant anomalies in the particle motion for periods up to 100 sec were observed. They are attributed to surface-wave refraction and scattering due to lateral inhomogeneities at the boundaries and inside the Eurasia continent.

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