Defining seismogenic sources from historical earthquake felt reports

We present a method that uses macroseismic intensity data to assess the loc ation, physical dimensions, and orientation of the source of large historic al 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 th e elongation of the associated damage pattern. A plausible and easily manag eable way: of describing a seismic source is by representing it as an orien ted "rectangle," the length and width of which, are obtained from moment ma gnitude through empirical relationships. This rectangle is meant to represe nt either the actual surface projection of the seismogenic fault or, at lea st, 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 th at occurred in the central and southern Apennines(Italy) in the past four c enturies returned encouraging results that compare well with existing instr umental, direct geological, and geodynamic evidence. The method is quite st able for different choices of the algorithm parameters and provides elongat ion directions that in most cases can be shown to be statistically signific ant. 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 act-ive in the central and s outhern portions of the Italian peninsula.