Intensity assessment from documentary data. Criteria and procedures in the daily practice of seismologists

A major problem in assessing macroseismic intensity from historical records is the lack of a standardized procedure for processing documentary sources and supplying unequivocal intensity estimates. In fact, most macroseismic, scales do not include detailed descriptions of procedures for intensity assessment and many divergent interpretations of intensity values arc obtained by different operators. In order to disclose these problems an experiment was made in Italy in 1994, in the framework of GNDT "Seismicity" Working Group. A set of selected documents concerning origil1al descriptions of effects produced by earthquakes which had occurred in Italy from 15th to 20th century, was supplied to a number of independent researchers. Each of them was invited to assess on this basis the intensity values and to explain his intellectual path by describing it in detail. This documentation was finally collected and, during a meeting devoted to this topic, the researchers involved were invited to discuss their personal experiences. The discussion indicated that the discrepancies between the estimate obtained by the different authors from the same documentary source are, in many cases, greater than 1 degree. This implies that the effect of personal choices may play an important role in intensity assessment and makes intensity estimates performed by the different authors less comparable. At present no definitive conclusions about these problems can be drawn but the topics focused during the meeting may represent a useful basis for a wider discussion to define a standardized procedure for thc assessment of intensity from documentary data.