Defining seismogenic sources from historical earthquake felt reports

We present a method that uses macroseismic intensity data to assess the location, physical dimensions, and orientation of the source of large historical earthquakes. Intensity data contain a great deal of information that can be used to constrain the essential characteristics of the seismic source. In particular, both the seismological theory and its practice suggest that the orientation of the source of significant earthquakes is reflected in the elongation of the associated damage pattern. A plausible and easily manageable way of describing a seismic source is by representing it as an oriented rectangle, the length and width of which are obtained from moment magnitude through empirical relationships. This rectangle is meant to represent either the actual surface projection of the seismogenic fault or, at least, the projection of the portion of the Earth crust where a given seismic source is likely to be located. The systematic application of this method to all the M > 5.5 earthquakes that occurred in the central and southern Apennines (Italy) in the past four centuries returned encouraging results that compare well with existing instrumental, direct geological, and geodynamic evidence. The method is quite stable for different choices of the algorithm parameters and provides elongation directions that in most cases can be shown to be statistically significant. In particular, the resulting pattern of source orientations is rather homogeneous, showing a consistent Apennines-parallel trend that agrees well with the NE-SW extension style of deformation active in the central and southern portions of the Italian peninsula.