ASMI Archivio Storico Macrosismico ItalianoItalian Archive of Historical Earthquake Data

In 1968, six earthquakes with magnitude between 5.1 and 6.4 destroyed or heavily damaged several towns in the Valle del Belìce (western Sicily), causing some three hundred fatalities. There have been some critical issues in the intensity assessment however in the macroseismic studies produced over the years, since the MCS scale was used as an estimation of shaking rather than a representation of the damage scenario; in practise, intensity was assigned for each earthquake of the sequence disregarding the effect of the cumulative damage. The case-study of the 2016-17 earthquakes in Central Italy reflected the difficulty in estimating the intensity in localities repeatedly hit by strong shocks, producing macroseismic parameters (epicentre, magnitude) inconsistent with the instrumental ones. As for the 1968 Valle del Belìce sequence, the Parametric Catalogue of Italian Earthquakes CPTI15 reports the macroseismic parameters, owing to the inadequacy of the seismic network operating in Italy in that period. Aware of the issues related to epicentre locations and magnitudes calculated by the existing macroseismic datasets, here we propose a reappraisal of the 1968 earthquakes following a methodology tested during the 2016-17 seismic sequence in Central Italy. By a new analysis of the primary sources, we reconstruct the evolution of the damage scenario during the sequence and assess intensity by using the European Macroseismic Scale EMS-98; the new macroseismic parameters are finally compared with the ones of the CPTI15 catalogue. Lastly, we propose a procedure for minimizing the magnitude overestimation in case of seismic sequences, based on the computation of the total seismic moment M0.

The Catalogue lists earthquakes that occurred in Italy between 461 B.C. and 1997, and earthquakes that occurred in the general Mediterranean area between the VIII century b.C. and the XV century. This version (CFTI5Med) features: the retrieval and formatting of over 23,000 original bibliographic documents, transcribed or printed, nearly 50% of those utilized in the CFTI5Med. These documents are now available on-line as fully searchable pdf files; a full geological reinterpretation, georeferencing and reprocessing of over 2,300 descriptions of earthquake-induced environmental effects, which are now all available and searchable in a user-friendly web-GIS environment; the elaboration of a number of texts and commentaries that were missing from the CFTI4Med version of the catalogue; a totally re-designed and more efficient web- and web-GIS interface. The new Catalogo dei Forti Terremoti in ltalia includes 1,259 earthquakes that occurred in Italy (98 of which are currently considered false). No commentaries on the main earthquake effects were available in the previous version of the catalogue for 87 of such events as they had not been updated with respect to the information supplied in Guidoboni e Comastri (2005) for the 1000 AD-1500 AD time interval. The commentaries concerning the most significant effects of Italian medieval earthquakes have now been elaborated and are available along with the information on more recent events. For the remainder of the earthquakes first presented with the CFTI4Med - those that occurred in the general Mediterranean area away from the Italian coasts - the new catalogue provides only the felt reports and basic epicentral parameters.

The macroseismic studies of historical earthquakes have become, in recent years, of great interest and the Italian Geodynamic Project has promoted and encouraged them.

This renewed interest induced us to complete the researches on the seismicity of Sicily and Calabria -which started with the earthquake catalogue, Carrozzo et al (1975)- by compiling a catalogue of isoseismal maps for earthquakes which occurred in the period 1783-1978.

Data, already gathered during the preparation of the earthquake catalogue, have now been critically reinterpreted, so making possible the drawing of isoseismal maps of 142 events. Later on, as established among members of group «catalogo dei terremoti», a macroseismic study of earthquakes preceding 1783, with intensity greater than nine, will be carried out trying to find other original references and papers of contemporary authors.

The quality of data is not homogeneous. Only earthquakes of greater intensity have a remarkable number of information, nevertheless, earthquakes which had a poorer quality of information, but enough to define the macroseismic field, were included as well.

Criteria used in the catalogue

The degree of intensity has been assigned using the M.S.K.-64 scale.

This scale shows the advantage of giving separate criteria of assessment of the intensity, according to the description of three kinds of effects:

1. on buildings and man-made structures,
2. on ground and in the regime of ground water,
3. on people and their environment.

This permits the definition of the intensity with a good accuracy even when there are poor descriptions of the effects as often happened for old or low-intensity earthquakes.

The original description of damage by past authors, have been reinterpreted according to criteria of the M.S.K. scale; when this was not possible, due to lack of descriptions, the values of intensity, generally expressed according to the Mercalli scale, have been transformed into the corresponding M.S.K.-64 values, using the relationship.

I_M.S.K. = 0.94 I_M.M. + 0.08 (Cosentino-Lombardo, 1980)

As regards criteria used in drawing isoseismals, a simple contouring of points representing the estimated intensity was performed, this avoids any normalization of the macroseismic field and any subjective interpretation.

In some cases, mainly for events located in the etnean area, when there was lack of descriptions but it existed the isoseismals map drawn by the author according to his own survey, the mesoseismal area was faithfully reported.

The isoseismals of studied events have been grouped into 5 zones Calabria and Peloritani area, Sicilian coastal chain, Etna, Iblei and Belice with different seismic and structural characteristics, according to the preliminary results of studies on the seismotectonic zonation of Sicily and Calabria (Barbano et al., 1978; Barbano et al., in print).

The map in fig. A, besides, shows the macroseismic epicentres, estimated as barycentre of the mesoseismal area. The epicentres of earthquakes occurred in the sea were included as well, only if the trend of isoseismals allowed a rough location.