Open File Report Task5, Characterization of site effects for the Colfiorito, Città di Castello and Val d’Agri areas: predictability and site transfer functions.

Abstract

The effect of local site amplification has been recognized as an important factor in ground motion assessment and is nowadays frequently studied. A common approach to evaluate ground shaking is to first estimate ground motion parameters at rock sites and then to correct them introducing site transfer functions derived from experimental data and from numerical modelling. The site transfer functions to be used to modify ground motion evaluated at rock sites can be evaluated starting from strong motion, weak motion and microtremor data. According to the amount and quality of the available data the transfer functions are evaluated for specific sites, in order to be used as a punctual information, or as representative of an average local condition in selected areas. All the available geological and geotechnical data must be collected to put some constraints on the obtained results and to permit numerical modeling to be compared with experimental results. The obtained transfer functions can be introduced in scenario studies convolving rock seismograms by the pulse response of the upper layers for different situations considered as representative of the geology of the studied areas. The capability of describing local site effects is strongly affected by the amount of seismological, geophysical and geotechnical data available. This is particularly true if numerical modelling needs to be performed and if the contribution of non linear soil behaviour has to be taken into account. For the three areas investigated in the framework of the project, the different amount of available data and information guided the performed studies and the obtained results. For Colfiorito test site, the availability of strong motion data recorded during the largest events of the Umbria Marche sequence (1997-98) yields well constrained information for specific sites. For Città di Castello the collection of weak motion and microtremor data allowed to reconstruct the geometry of the sedimentary basin underlying the city and to define zones with homogenous site response where to evaluate site transfer functions in a 1D approximation, including non linear behaviour. For Val D’Agri area, the lack of seismic and geotechnical data did not allow to describe in detail the site response in the sedimentary basin. In this case some sample sites with a known uppermost geological structure were selected as representative of the seismic response of the basin. For them, microtremor data were collected to put some constraint on the transfer functions computed in a 1D approximation. In this case it was not possible to consider the non linear soil behaviour.