The lifelines seismic risk assessment is based on a fuzzy classification on parameters related to geometry (effective length distribution L) and constructive details (material type C) along with urban (U) and economic (E) parameters. Then a functional value G = f(L, C, U, E) is constructed as an indicator of risk of the element/system under investigation. In the present study a new functional Fg is introduced in a GIS environment. The application of spatial analysis methods provides a better understanding of the possible failure patterns and trends in water distribution networks. The Fg parameter takes into account the influence of geological and seismotectonic environment in a local scale, constructed in a way to reflect the amplification factor A (X), where X local coordinate of the cell, which experimentally could be estimated. The Fg= f(A(X), U(X), L(X)) is strongly affected by the local geotectonic conditions as well the spatial distribution of population and the geometrical lattice of pipe line system. The proposed index was applied in the city of Chania, Crete island, as the functioning of the infrastructure during and especially after a destructive earthquake, in the front of the Hellenic Arc, is of vital importance for the society and contributes to the rescue and emergency operations. We propose Fg mainly for systems where site effects are important and present a significant spatial inhomogeneity while at the same time the examined urban system has no constructive variability it time and space.