Modern techniques of treating damage patterns (intensity) to retrieve information on the 6 May 1976 M 6.4 earthquake

We used all the macroseismic data of the Friuli, 6 May 1976 earthquake to: i) retrieve geometric and kinematic information on the source, by doing automatic nonlinear geophysical inversions; ii) to analyse site effects. The inversions were performed with the KF model of radiation of S body waves in the 10- to 100-km distance range from the source, using a genetic algorithm with niching (NGA). A solution with a N-dipping fault was obtained, with strike angle 266 degrees +/- 10 degrees, dip angle 53 degrees +/- 8 degrees, rake angle 71 degrees +/- 11 degrees. For the site effects we used the type of soil, the depth of the bedrock, the simplified impedance classes and the NEHRP classification. A detailed analysis on the Gemona fan in the epicentral area was also done using intensities from 69 districts of the town. The general analysis confirms amplification on soft soils at great epicentral distances. The analysis of Gemona showed a striking correlation between the trend of macroseismic data and the contour lines of the topography of the fan, with maximum intensity (X-XI, i.e. 10.5) toward the apex of the fan and minimum intensity (VI-VII, i.e. 6.5) in the Friulian Plain beneath it.