The nature and cause of polarization anomalies of surface waves crossing northern and central Eurasia

Abstract

SUMMARY Results 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 Frequency-Time Polarization Analysis (FTPAN). All anomalies indicate deviations of wave-propagation paths to the north relative to the great circle paths. Polarization anomalies at OBN are uniformly small (<5°). 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 Gaussian-beam 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.