Shape analysis of isoseismals based on empirical and synthetic data.

We present an attempt to compare modeled ground-motion acceleration ®elds with macroseismic observations. Two techniques for the representation of the observed intensities by isoseismals, a smoothing technique and one which visualizes the local uncertainty of an isoseismal, are tested with synthetic and observed data. We show how noise in the data and irregularities in the distribution of observation sites aect the resolution of the isoseismal's shape. In addition to ``standard'' elongated shapes, we identify cross-like patterns in the macroseismic observations for two Italian earthquakes of strike-slip type; similar patterns are displayed by the theoretical peak acceleration ®elds calculated assuming the point source models given in the literature.