Modeling seismic surface waves through time-frequency analysis and group delay spectrum

For many years, engineers and seismologists have recognized the destructive potential of seismic surface waves generated within sedimentary basins. With their large amplitudes and long periods, basin surface waves are an important consideration for high-rise buildings, bridges, nuclear power plants, and other long period structures. Identification of these wave types in a seismogram is an important step for the understanding of wave propagation phenomena. Since in most seismograms, different types of waves with different frequencies may appear simultaneously, separation of waves is more effectively achieved when a time-frequency analysis is performed. In this work, we propose a new time-frequency analysis procedure to identify and extract Rayleigh and Love waves from three-component seismograms. Exploiting the advantage of the absolute phase preservation by the Stockwell Transform, we construct time-frequency filters to extract waves based on the 'Normalized Inner Product' (NIP). The novelty and advantage of the proposed procedure is that it does not require making any assumptions regarding the direction of propagation of the surface waves, but in fact such direction is a byproduct. Furthermore, it is shown that the NIP is a more stable parameter in the time-frequency domain when compared to the instantaneous reciprocal ellipticity, and thus it avoids smoothing (and with it, altering) the data. The procedure has been successfully tested with real signals, in particular to extract Rayleigh and Love waves from seismograms of one aftershock of the 1999 Chi-Chi earthquake. With the proposed procedure we found different directions of propagation for retro-grade and pro-grade Rayleigh waves, which might suggest that they are generated by different mechanisms.