On the Accuracy of Surface-Wave Dispersion Measurements from Horizontal-Component Ambient Noise Cross Correlations

At present, both Rayleigh and Love waves can be extracted from cross-correlations of ambient noise in ambient noise tomography (ANT). It has been demonstrated that accurate Rayleigh-wave dispersion curves can be extracted from vertical-component noise when noise sources are evenly distributed in space. However, because the rotation of horizontal components alters the distributions of effective sources in horizontal-horizontal cross correlations between a pair of stations, it is still not completely sure if dispersion curves of Rayleigh and Love waves extracted from horizontal-component noise data are accurate for ANT. In this study, through numerical simulations, we demonstrate that surface-wave phase velocities extracted from the horizontal-horizontal cross-correlation functions (CCFs) systematically deviate from structural phase velocity even when noise sources are evenly distributed in space. The deviations of phase velocities quickly increase with decreasing station separations of CCFs when station separations are shorter than three wavelengths. Further analysis on field data recorded at seismic stations in the contiguous United States confirms the existence of the phase velocity deviations of Rayleigh and Love waves when they are measured from horizontal-horizontal CCFs. Because the deviations become smaller with increasing station separations and are less than 0.25% at station separations longer than three wavelengths, we suggest that it is best to only select those CCFs with station separations longer than three wavelengths in ANT when measuring phase velocities of surface waves from horizontal-component CCFs, such as Love waves from radial-radial CCFs.