Observations of long period Rayleigh wave ellipticity

We analyse phase shifts and amplitude ratios of horizontal to vertical components of T~ 150 s fundamental Rayleigh waves. In the ray picture of surface wave propagation, it is expected that at a single station this ratio will be constant, being controlled only by the seismic velocity structure beneath the seismic station. In contrast, we observe substantial variability in the amplitude ratios measured at a number of stations from the Global Seismic Network. We discuss possible causes for the anomalous ratios, concluding that they probably arise from effects associated with small scale heterogeneity distinctly distributed across different tectonic domains and having scale lengths shorter than a wavelength.