Assessment of the ground motion at reference rock sites in Italy

Giovanni Lanzano
Istituto Nazionale di Geofisica e Vulcanologia, Milano, Italy

According to the code prescription, the reference ground motion is associated with those sites where rocks or stiff soils outcrop and the VS,30 value exceeds a given threshold. However, this condition does not ensure that the ground motion is unaffected by site-effects, as requested in several engineering application such as the evaluation of the site response using empirical approaches and numerical 1-D/2-D analyses. In these cases, the amplification function of the reference sites must be flat with amplitude equal to one over a frequency range of engineering interest (Steidl et al. 1996).

In the last years, we worked to develop the reference-rock detection method based on the identification of a set of site parameters that are related to geophysical and seismological data and to geomorphological and installation features. The main results of our studies are i) a selection of well-tested site parameters with a specific weight in the reference site identification; ii) a list of reference rock sites in Italy, with an associated score; iii) a model for the generic-to-reference rock corrective factor, parametrized in terms of VS,30 and Îș0.