Shape of Empirical and Synthetic Isoseismals: Comparison for ItalianM= 6 Earthquakes

We present results from a comparative analysis of empirical and synthetic shapes for isoseismals of low intensity (I= IV–VI on the MCS scale) for six Italian earthquakes of ML = 4.5–6. Our modeling of isoseismals is based on a plane-stratified earth model and on the double–couple point source approximation to calculate seismograms in the frequency range f £ 1 Hz. With a minimum of parameter variation we demonstrate that the low intensity isoseismals provide information on source geometry. We strive to avoid subjectivity in isoseismal constructions by using the new Diffuse Boundary method, which visualizes isoseismals with their uncertainty. Similar results in this direction are known for large earthquakes (ML » 6 or greater) with extended sources and for the higher isoseismals (I ‡ VI on the MM scale). The latter studies disregard the earth structure, use a greater number of parameters, and therefore have greater possibilities for fitting the data than our approach.