THE NATURE AND CAUSE OF POLARIZATION ANOMALIES OF SURFACE-WAVES CROSSING NORTHERN AND CENTRAL EURASIA

Results of a pilot experiment preliminary to a systematic study of sur face-wave polarization across Eurasia are presented. Long-period funda mental Rayleigh and Love waves recorded by the broad-band seismic stat ions KIV (Kislovodsk) and OBN (Obninsk) of the IRIS/IDA network deploy ed in the Former Soviet Union (FSU) were analysed from 14 events to th e north-east, east and south-east of these stations in search of parti cle-motion anomalies using a technique called Frequency-Time Polarizat ion Analysis (FTPAN). All anomalies indicate deviations of wave-propag ation paths to the north relative to the great circle paths. Polarizat ion anomalies at OBN are uniformly small (<5-degrees). Significant fre quency-dependent polarization anomalies (5-degrees-20-degrees) are fou nd for Rayleigh and Love waves arriving at KIV from teleseismic events at a wide range of backazimuths (26-degrees-103-degrees). Polarizatio n 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 rob ust and can provide useful structural information. The systematics of the frequency dependence of the polarization anomalies as epicentral p osition is moved from north to south display sensitivity to structures at a number of length-scales. Results from synthetic experiments usin g coupled normal-mode and Gaussian-beam synthetics reveal that: (1) re cently constructed long-wavelength aspherical models produce polarizat ion anomalies that are significantly smaller (<5-degrees at KIV) but o f the same sign (negative) as the observed anomalies, and (2) the freq uency dependence of the observed anomalies must result from a combinat ion 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 t he polarization anomalies observed at KIV includes a regional-scale lo w-velocity feature near to KIV in order to fit the large-magnitude, sh ort-period polarization anomalies and a smaller magnitude, continent-s cale increase in upper mantle velocities and/or the reduction of crust al thickness from south to north in central Eurasia in order to fit th e broad-band, longer period anomalies. The small-scale, low-velocity f eature is a model of the sedimentary basin of the sub-Caspian depressi on.We conclude that measurements of polarization anomalies can be obta ined accurately, that they are reproducible, that they contain current ly unmodelled information, and that they should prove to be useful in combination with velocity and amplitude information in future tomograp hic inversions, especially to help focus global-scale models to region al wavelengths.