An approach to improve Rayleigh-wave ellipticity estimates from seismic noise: application to the Los Angeles Basin

We present an approach for seismic noise which improves Rayleigh-wave ellipticity estimates by reducing the influences from body waves and Love waves. The method requires three-component seismographs and uses the phase-shift information between the vertical and horizontal components. We select data that show the phase-shift of about 90° in order to separate Rayleigh waves from body waves and Love waves. In comparison to the usual H/V approach, the estimates for Rayleigh-wave ellipticity are systematically different by about 20percent because the existence of S waves and Love waves systematically increases horizontal amplitudes. The differences in the inverted S-wave velocity structure reach up to about 20percent in the upper 20km. The method is limited to a relatively low-frequency range, below 0.3Hz, as the 90° phase-shift peaks become difficult to identify above 0.3Hz. Despite this limitation in the frequency range, this approach can improve the S-wave velocity structure in the upper 5–10km, which should lead to a better prediction of long-period ground motions for periods of about 5–10 s. An improved prediction of such long-period ground motion is important for hazard mitigation in the world's metropolitan areas with high-rise buildings. This approach is applied to the structure in the broader Los Angeles basin in Southern California and improvement of the reference velocity structure in the upper 5–10km is demonstrated.