GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 2.1 GNGTS 2023 possible: beginning of the shock, maximum phase and end; degree of uncertainty of the time, total duration, on which source the clock was set... At least since 1892 different postcards were available for the instrumental and macroseismic recordings; in the latter, it is requested to describe the “different effects produced by the shock from which its intensity can be detected”. Since August 1893 another postcard model began to circulate which, leaving the rest of the form more or less the same, incorporated a guide for macroseismic observation in the footnote of the “effects of the shock” item: “To say whether the shock was felt by many or by a few people in a state of rest or motion; if it has produced a tremble of small or large objects, windows, doors; large or light cracks in a few or many well or badly built houses; partial or general ruins of buildings; numerous victims or not.” It is unclear how this guide was created, but probably it is a modified synthesis of De Rossi's scale. The next step takes place when (at least since 1909) another guide script is inserted: among the first queries, after reporting the time of the event, the local observer is asked to assign an intensity value in the Mercalli scale. This indication will remain in the following years, albeit with a different graphic relevance, for all the years that occupy us and at least until the 1940s. With this new indication, a discrepancy is created between the diagnostics listed in the postcard and those corresponding to the different degrees of the Mercalli scale, which could confuse the local observer, when interpreting the effects of the earthquake and reporting them on the pre-printed form. Beyond the diversities and continuities of the macroseismic postcards, it should be noted that there is a further change in the network of correspondents. The heterogeneous composition described in the last years of the 19th century was transformed during the first decades of the XXth century into a much more homogeneous profile composed largely of mayors, “podestà”, prefectural officers and municipal secretaries. Nor will the number of personnel of the Technical Office of the various municipalities be relevant among those who compile the postcards; and those sent by the staff of an observatory, priests, engineers or other professionals become an exception. At this point we can refer to the second layer: the compilation by the local observer. It is known that the information collected by the Seismic Service of the Royal Central Office of Meteorology and Geophysics (UCMG) was published by its Bollettino Sismico , in particular in the Microseismic and Macroseismic appendices. As has already been noted elsewhere (Caracciolo, 2021), Antonio Cavasino himself, service’ chief and editor of the Bulletin as well, had expressed criticisms of the effectiveness of the system of gathering data from local observers. In fact, the method of data collecting was based on information that spontaneously arrived from local correspondents and on explicit requests from the Seismic Service. These requests had the purpose of completing the information that arrived spontaneously, collected from the newspapers or arrived from an instrumental report (if it was estimated that the epicenter was in the peninsular area). In these cases, the Seismic Service sent an explicit request for news to the municipalities in the localities mentioned, or close to the area, accompanied by macro-seismic postcards and a copy of the Mercalli scale. However, not always and not from all municipalities the Seismic Service had received any answer.