Abstract
Ground motion records during recent 3 strong earthquakes disseminated from NIED are used to investigate seismic site amplification from bedrock to ground surface to be correlated with ratios of corresponding S-wave-velocity in bedrock, Vs(SUB)b(/SUB), and that in surface layer, Vs. Equivalent S-wave velocity Vs in an equivalent surface layer is introduced and determined from peak frequency of amplification ratio. A clear positive correlation exists between amplification ratio of measured motions (surface/base) and the Vs -ratio. Furthermore, amplification ratio calculated for bedrock modified by the amplification of measured motion is also correlated with the Vs -ratio much better than Vs-ratio using average Vs in top 30 m. The empirical formulation thus developed may be applicable to different earthquakes and site conditions including bedrocks with b Vsb =400-3000 m/s
