INFLUENCE OF THE UNCERTAINTY IN THE SOIL-ROCK SPECTRAL RATIOS IN THE DEFINITION OF UNIFORM HAZARD SPECTRA AT SURFACE LEVEL

Based on traditionally accepted hypothesis and verified by existing data, an expression is derived to calculate response spectrum at the ground surface if the response spectrum at the basement rock is known. The fundamental assumptions are with regards to the form of variation of the exceedance rates of spectral accelerations in the basement rock, and based also on the usual (lognormal) distribution forms of the uncertainties associated with the spectral amplification function. The resulting approach multiplies the mean of the amplification function in order to consider in a rigorous way its uncertainty.