Seismic Vertical Array Analysis: Phase Decomposition for S and Surface Waves

It is important for interpretation of the seismic responses of a sedimentary basin to distinguish S and surface waves on strong-motion seismograms. A vertical array analysis method that decomposes S and surface waves from a set of surface and borehole seismic records (horizontal components) is proposed. Each of horizontal components of the array records assumed to consist of S wave and only one kind of surface (Love or Rayleigh) wave. The Fourier transform of each phase at the borehole is expressed as the product of the Fourier transform of that phase at the surface and the transfer function of that phase. I apply this method to a record of nearby intermediate-depth earthquake observed at a vertical array in the Ishikari plain, Japan. The array analysis result for ground motion show short duration of the direct S wave and late arrival surface waves with long duration. The estimated records are in good agreement with synthetic seismograms calculated by a 2-D finite difference method for a sedimentary basin model assuming vertical incident plain SH and SV waves. The simulation indicates that the basin-induced surface waves are generated at the southwestern basin edge and arrive at the array in the expected times.