Ambient Noise Surface-Wave Polarization in Anisotropic Media: Theory, Measurement, and Interpretation

Xiongwei Liu, Michael H Ritzwoller

We develop an adiabatic theory and practical methodology for measuring quasi-Rayleigh and quasi-Love polarization from the complete ambient noise cross-correlation tensor. The formulation allows the polarization parameters at both stations of a pair to be estimated jointly and, under sufficiently adiabatic propagation, interpreted in terms of local anisotropic structure at those locations. A new empirical processing step approximates the theoretical source-group normalization to reduce source-related amplitude effects, providing the practical link between observations and theory. In geologically relevant limiting regimes, simple relations link diagonal carrier ($TT,ZZ$) and off-diagonal polarization-sensitive ($TZ,ZT$) waveforms, yielding information complementary to conventional phase-speed observations.

In numerical benchmarks, predictions of the polarization-sensitive waveforms are accurate when anisotropy varies sufficiently slowly relative to wavelength. Stronger or shorter-scale structural variations can increase conversion between the two wave types, degrading the adiabatic approximation and local interpretability of measured polarization.

We present the first measurements of surface-wave polarization from ambient noise cross-correlations in Alaska, focusing on the 20--30~s period band. For relatively short paths south of the Denali Fault, repeated polarization estimates are mutually consistent and broadly consistent between the two station endpoints, supporting a local interpretation. For paths crossing the fault, boundary-transient behavior develops, waveform fits degrade on average, and the polarization parameters change systematically, consistent with a breakdown of local interpretation across a strong structural boundary. Finally, in this example, adding a single polarization constraint to an inversion of Rayleigh- and Love-wave $2\psi$ and $4\psi$ phase-speed observations resolves a dip-orientation nonuniqueness that remains in the phase-speed-only inversion. The alternative dip solutions correspond to distinct three-dimensional orientations of the anisotropic fabric and may have important tectonic implications.