Estimation of horizontal-to-vertical spectral ratios (ellipticity) of Rayleigh waves from multistation active-seismic records

The horizontal-to-vertical spectral-ratio (HVSR) analysis of ambient noise recordings is a popular reconnaissance tool used worldwide for seismic microzonation and earthquake site characterization. We have expanded this single-station passive HVSR technique to active multicomponent data. We focus on the calculation of the HVSR of Rayleigh waves from active-seismic records. We separate different modes of Rayleigh waves in seismic dispersion spectra and then estimate the HVSR for the fundamental mode. The mode separation is implemented in the frequency-phase velocity (f-v) domain through the high-resolution linear Radon transformation. The estimated Rayleigh-wave HVSR curve after mode separation is consistent with the theoretical HVSR curve, which is computed by solving the Rayleigh-wave eigenproblem in the laterally homogeneous layered medium. We find that the HVSR peak and trough frequencies are very sensitive to velocity contrast and interface depth and that HVSR curves contain information on lateral velocity variations. Using synthetic and field data, we determine the validity of estimating active Rayleigh-wave HVSR after mode separation. Our approach can be a viable and more accurate alternative to the empirical HVSR analysis method and brings a novel approach for the analysis of active multicomponent seismic data.