Verification of horizontal-to-vertical spectral-ratio technique for estimation of site response using borehole seismographs

The City of Yokohama has established a high-density strong-motion seismogra ph network in 1997 to enable rapid evaluation of seismic hazard in case of disastrous earthquakes. It consists of 150 surface strong-motion seismograp hs and nine borehole strong-motion seismographs within the city area, which is approximately 20 km X 30 km wide. We use seismograms recorded by the bo rehole seismographs of the network from seven earthquakes that occurred aro und Yokohama. We calculate the S-wave spectral ratio between the surface se ismograph and the borehole seismograph and compare it with a synthetic spec tral ratio calculated from a velocity profile of the borehole. The agreemen t of observed and synthetic spectral ratios is excellent, which shows that we may use these borehole seismographs to test various techniques for estim ation of the site amplification. We then apply the horizontal-to-vertical ( HN) spectral-ratio technique for seismograms recorded at the surface and co mpare them with these spectral ratios. The results show that the HN ratio t echnique gives a good estimate of the surface amplification for frequency r ange between about 1 and 4 Hz, if the near-surface velocity structure is re latively simple and if the signal-to-noise ratio of seismograms recorded at the surface is adequate.