Assessing the completeness of Italian historical earthquake data

The assessment of the completeness of historical earthquake data (such as, for instance, parametric earthquake catalogues) has usually been approached in seismology – and mainly in Probabilistic Seismic Hazard Assess- ment (PSHA) – by means of statistical procedures. Such procedures look «inside» the data set under investiga- tion and compare it to seismicity models, which often require more or less explicitly that seismicity is station- ary. They usually end up determining times (Ti), from which on the data set is considered as complete above a given magnitude (Mi); the part of the data set before Ti is considered as incomplete and, for that reason, not suit- able for statistical analysis. As a consequence, significant portions of historical data sets are not used for PSHA. Dealing with historical data sets – which are incomplete by nature, although this does not mean that they are of low value – it seems more appropriate to estimate «how much incomplete» the data sets can be and to use them together with such estimates. In other words, it seems more appropriate to assess the completeness looking «out- side» the data sets; that is, investigating the way historical records have been produced, preserved and retrieved. This paper presents the results of investigation carried out in Italy, according to historical methods. First, the completeness of eighteen site seismic histories has been investigated; then, from those results, the completeness of areal portions of the catalogue has been assessed and compared with similar results obtained by statistical methods. Finally, the impact of these results on PSHA is described.