2019 Volume 19 Issue 4 Pages 4_52-4_70
We explained seismic responses of vertical ground motions of S-wave part by considering obliquely incident S wave at bedrock. Vertical component of surface and borehole ground motions for vertically incident P wave and obliquely incident S wave were theoretically calculated for two different velocity structure models: Contrast model (velocity changes discontinuously between seismic bedrock and sediments), and Mirage model (velocity changes continuously between seismic bedrock and the surface). The result shown that the borehole-to-surface spectral ratios for the P- and S-wave incidents into the Contrast model were similar; however the spectral ratios into the Mirage model were significantly different. These results indicated that the obliquely incident S wave is required to evaluate seismic response for the Mirage model. We tested the S-wave incident approximation to explain seismic responses of vertical component seismograms recorded at KiK-net station SRCH08. The borehole-to-surface spectral ratios of the P- and S-wave parts of the observed vertical component seismograms were clearly different. To make an appropriate velocity structure model for SRCH08, we inverted receiver functions and borehole-to-surface spectral ratios of horizontal components. The observed borehole-to-surface spectral ratios well agree to the spectral ratios calculated for the obliquely incident S wave rather than the ones for the vertical P-wave incident. The synthetic seismograms for the oblique S-wave incident also coincide with the observed records in the time domain.
