A Site-Response Model for the Vertical Component of Ground-Motion Prediction Equation Using a New Site-Response Parameter TVH

Abstract

This study presents an alternate site-response model to the existing ground-motion prediction equation (GMPE) from our previous study with some calibration to the magnitude term. We used a new site-response proxy TVH=4×max(30,HB)/VS30 that combines VS30 (the travel-time-averaged shear-wave velocity to 30 m depth) with the engineering bedrock depth HB. VS30 is available for many strong-motion recording stations. However, GMPEs using VS30 do not directly account for the response of the soil layers between 30 m and bedrock depth. Site period TS (four times the shear-wave travel time to the engineering bedrock depth) has also been used in recent GMPEs, with TS being considered a theoretically better parameter than a pseudosite period TVS30 (four times the shear-wave travel time to a depth of 30 m). Obtaining the shear-wave velocity profiles between 30 m and HB for a deep soil site can be expensive, whereas obtaining bedrock depth based on the geotechnical description of borehole data may be relatively easy. We used velocity profiles from the Kyoshin net and Kiban–Kyoshin net strong-motion networks and found that a pseudosite period TVH and TS have an excellent correlation with a small standard deviation at all spectral periods, suggesting that TVH is a suitable site-effect parameter. The poor correlations between TVS30 and TS and between TVS30 and TVH for sites with TS>0.4 s led to poor model performance at long spectral periods. We modified a GMPE by replacing TVS30 with TVH, and we made a minor modification to the moment magnitude term of the GMPE that was necessary for the new site term. The response spectra predicted by models using TVH, TS, and TVS30 at short spectral periods up to 0.6 s are generally similar. At long spectral periods, the spectra predicted by the TVS30 are much smaller than those from the other two GMPEs. For all spectral periods, TVH is an excellent substitute for TVS30.