Polarization of the Earth's teleseismic wavefield

Abstract

The average polarization of the Earth's teleseismic wavefield is imaged by applying a vector stacking technique to ~ 32 000 three-component broad-band seismograms. Polarization record sections capture the average radial-vertical particle motion in two narrow frequency bands: one above the microseism peak (0.5-2.0 Hz) and one below (0.03-0.07 Hz). Within these records sections, colour indicates polarization angle and brightness denotes particle motion linearity. These parameters are estimated using the time-domain method of Jurkevics (1988), enhanced to allow the simultaneous analysis of multiple sources. The low- and high-frequency record sections highlight different features in the teleseismic wavefield. The low-frequency stack shows P, S and their surface multiples as well as more exotic phases originating from combinations of reflections, refractions and conversions. The high-frequency stack detects fewer arrivals. However, the attenuation of the multiple S waves allows the detection of later-arriving core phases. Both the high- and low-frequency stacks illuminate regions containing consistent waverforms and warm of contaminating arrivals. The coherence of lower-mantle-turning P waves is indicated by the agreement between the observed and predicted polarization angles. In contrast, a large discrepancy between the observed and predicted low-frequency S angles evidences strong SPL contamination between 20°and 60°. Both polarization stacking and conventional amplitude stacking techniques allow the detection of seismic phases that are buried within the noise on single seismograms, but polarization stacking additionally yields polarization angle and particle motion linearity.