It is well known that the less reliable part of the present Catalogue of Italian earthquakes is that related to the medieval period; mistakes in the chronology, inaccuracy in the location of the phenomenon described, generalized use of works belonging to the sixteenth-seventeenth-century historiography led to wrong locations and to duplicate some events.

The cases here described constitute some examples of analysis of the medieval period that could certainly be multiplied in the advancing of research. Some events have been duplicated or triplicated because of the incorrect transposition in universal time of the datings reported in the chronicles according to the ancient habits, and because of the passages of information from the different works of erudition up to the modern catalogues as it has been here evidentiated in the schemes of relation between the sources and the subsequent works.

The cases exemplified involve earthquakes located in Rome and dated in the ENEL-PFG Catalogue (18) 1017 and 1020; nevertheless, the analysis prompted the conclusion that it was a single, but quite doubtful, event; the earthquake of S. Damiano D'Asti of 1275, of Milan dated July 28, 1276 and of Verona dated July 20, 1277 were all part of a single seismic event.