Time- and frequency-dependent polarization analysis: anomalous surface wave observations in Iberia

Abstract

S U M M A R YA time-and frequency-dependent polarization method is applied to synthetic and observational data in order to investigate effects of lateral heterogeneity on surface wave polarization. It is shown that interference of surface wave signals can produce artifacts in the polarization as determined for the dominant signal present in the data.The time-and frequency-dependent polarization technique is applied to surface waves recorded in Iberia to investigate the character of the recorded wavetrains. Rayleigh waves with a southwest backazimuth recorded in the south of Spain appear strongly attenuated relative to other locations in Iberia. By analysing their polarizations, it can be inferred that these surface waves are highly distorted, implying that scattering mechanisms must be operative in the south of Spain. Strong scattering is one of the mechanisms that might explain the observed amplitude reduction, although other mechanisms like intrinsic scattering or defocusing may also contribute. With the present data and the present technique it is not possible to determine the relative contributions of each of these mechanisms.

Off-great-circle propagation of intermediate-period surface waves observed on a dense array in the French Alps

“…They confirmed the presence of significant refraction and multipathing. On data recorded in Iberia, Paulssen et al (1990) performed a time‐ and frequency‐dependent polarization analysis and suggested that the anomalous surface wave observations could be due to the interference of surface wave signals. More recently, Levshin et al (1994) studied polarization anomalies observed in the former Soviet Union for surface waves crossing northern and central Eurasia, and showed how a regional structure showing lateral heterogeneities in the crust and upper mantle can cause large deviations from great‐circle propagation.…”

Regional velocity structure in northern California from inversion of scattered seismic surface waves

“…On the other hand, since surface waves sample primarily the highly heterogeneous crust and uppermost mantle, they are subject to strong path effects, manifested in the form of lateral refraction and multipathing. Since regional seismic structure can vary sharply over the scale of one wavelength and teleseismic wavefields often exhibit serious distortions in phase and amplitude [e.g., Bungum and Capon, 1974;Paulssen et al, 1990], ray theory combined with the assumption of "plane" incoming wavefields are only an approximate means of performing regional surface wave tomography [Friederich et al, 1994] (hereinafter referred to as FWS94). Ray theory is justified in most global tomographic studies [e.g., Trampert and Woodhouse, 1995; Ekstr•m et al, 1997], where the minimum modeled wavelength of Earth structure is much larger than the wavelengths of the propagating waves that sample it.…”

Theory and Observations: Normal Mode and Surface Wave Observations

“…In the interactive analysis of Plešinger et al (1986), one seeks the local coordinate system for which certain component products are zero. Paulssen et al (1990) expanded the basic timefrequency analysis for the three-component seismograms and presented the time and frequency-dependent polarization of anomalous surface waves observed at stations on the Iberian Peninsula. Paulssen et al (1990) expanded the basic timefrequency analysis for the three-component seismograms and presented the time and frequency-dependent polarization of anomalous surface waves observed at stations on the Iberian Peninsula.…”