Effect of transient seismic noise on estimates of H/V spectral ratios

The horizontal-to-vertical (H/V) spectral ratio of seismic noise has become a widely used tool in microzonation over the last decade. However, attempts to provide standards for seismic-noise analysis have only recently been made. One point often debated is whether only the stationary part of the recorded signal must be used or also the transients. Until now, no systematic analysis has been carried out to clarify this point. In this study, we compare H/V spectral ratios obtained using stationary noise with those calculated without any a priori selection of the signal. Results show that transients have no (or very little) effect on the H/V ratio. Furthermore, we filter the seismograms using a wavelet-packet transform method, to perform H/V spectral-ratio calculations for only the transients. Results show a large variability in the H/V shape that we explain, by means of numerical simulations, as being due to source type and distance from the receiver relative to the thickness of the sedimentary cover.