FREQUENCY-DEPENDENT POLARIZATION MEASUREMENTS OF LONG-PERIOD SURFACE-WAVES AND THEIR IMPLICATIONS FOR GLOBAL PHASE-VELOCITY MAPS

Surface-wave dispersion maps provide important constraints on global models of shear-wave velocity structure. Current surface-wave dispersion maps show significant differences from researcher to researcher, and it is clear that further work is required. In addition to dispersion data, polarization measurements obtained from long-period (100 s or more) three-component recordings from the various global networks can also be used to constrain dispersion maps. The off great circle propagation of the surface-wave packets is relatively easy to interpret within a ray-theoretic framework, and provides sensitivity to higher-order structure. The polarization angles as a function of frequency are readily measured using a multi-taper technique, which also has the benefit of providing an error estimate for the measurements. Application of the technique to three-component seismograms from the global GEOSCOPE array reveals large deviations from great circle propagation (up to 15-degrees- for low-orbit Love waves and 10-degrees for Rayleigh waves in the frequency band 5-12.5 mHz). On a more regional scale, an analysis of seismograms from the German Regional Seismic Network (GRSN) reveals even larger, strongly frequency-dependent deviations from great circle propagation in the frequency range 10-50 mHz.