Analysis of macroseismic data of seismic sequences: the case of 1783 Calabria earthquakes

The present study focuses on macroseismology and historical seismology and aims to improve the understanding of the 1783 Calabrian seismic sequence, one of the most significant and paradigmatic cases of Italian seismicity. The Calabrian Arc is one of the most active tectonic systems in the Italian peninsula and has been affected over the last four centuries by numerous high-intensity earthquakes. Among these, the 1783 seismic sequence is particularly relevant, as it includes three main earthquakes (5 February, 7 February, and 28 March) and at least two other significant events (6 February and 1 March). This research is based on the collection, cataloguing, and systematic analysis of historical sources, including archival documentation, periodicals, chronicles, and iconographic material. This analysis has enabled the integration of previously unconsidered information and a more comprehensive reconstruction of the 1783 seismic sequence. The revised dataset comprises 1193 macroseismic data points (MDPs) spanning 562 localities, representing an increase of approximately 33% compared to the reference study [Guidoboni et al., 2007]. Macroseismic intensities are assigned using both the MCS scale, to ensure continuity with the Italian seismological tradition and existing historical earthquake catalogues, and the EMS-98 scale, in accordance with the modern international methodological framework. To address issues such as cumulative damage and false damage attribution, the chronology of sources was used to isolate the effects of each event, giving greater weight to contemporary accounts to evaluate the reliability and consistency of the evidence. For the three main earthquakes, macroseismic parameters (epicentral coordinates and macroseismic magnitude) are estimated using the BOXER code (Methods 0 and 1) [Gasperini et al., 1999; 2010]. In addition, the cumulative intensity subtraction approach (CIS) [Graziani et al., 2022] was applied to isolate the contributions attributable to individual events and to exclude localities likely affected by cumulative damage from the parametrisation. The 5 February earthquake (438 MDPs) is well constrained by the intensity data, with information available from both the epicentral area and the far field. The analysis yields an epicentral intensity of 10 EMS-98 (10–11 MCS), a magnitude of Mw 6.73–6.90, and a relocation of the epicentre approximately 12–17 km north of the CPTI15 epicentre [Rovida et al., 2022]. The estimated source geometry is consistent with the Cittanova Fault, a high-angle W-NW dipping normal fault, in agreement with seismogenetic models proposed in the literature [Jacques et al., 2001; Galli and Bosi, 2002; Cucci, 2022]. The 7 February earthquake (327 MDPs) is characterised by an epicentral intensity of 10 EMS 98 (10–11 MCS). The dataset is strongly influenced by a cumulative damage scenario. Analysis of the full dataset locates the epicentre further south than the CPTI15 solution [Rovida et al., 2022] and yields a magnitude of Mw 6.61–6.91, whereas the application of the CIS procedure results in a relocation of the epicentre approximately 12–24 km to the north-east and a lower magnitude estimate of Mw 6.48–6.70. The estimated source geometry derived from the non cumulative data is consistent with the southern segment of the Serre Fault, a west-dipping extensional structure, supporting this seismogenetic interpretation [Galli and Bosi, 2002; Galli et al., 2007]. The 28 March earthquake (339 MDPs) shows an epicentral intensity of 10 EMS-98 (10–11 MCS) as well. The macroseismic pattern is strongly conditioned by the cumulative damage scenario. Processing of the full dataset locates the epicentre to the south-west of the CPTI15 epicentre [Rovida et al., 2022], defining a NW–SE oriented source with magnitudes ranging from Mw 6.82 to 6.98. Conversely, the CIS approach allows for the isolation of non-cumulative data, resulting in a lower magnitude estimate of Mw 6.67–6.83 and shifting the epicentre approximately 10 km to the north-east relative to the CPTI15 location. The source derived from this analysis shows an E–W geometry beneath the Catanzaro Basin, appearing consistent with the composite seismogenic structure of the Golfo di Caraffa–Squillace, a south-dipping, subvertical, left-lateral strike-slip fault, proposed by the DISS Working Group [2025]. Finally, for the 6 February (67 MDPs) and 1 March (22 MDPs) earthquakes, the available information did not allow reliable parametrisation in either case. The first is too strongly affected by cumulative damage and could plausibly represent a strong aftershock of the 5 February event, whereas the second is characterised by considerable uncertainty due to the fragmentary nature of the contemporary sources. A key aspect of the research involves the assessment of the effect of cumulative damage on the estimation of seismic parameters. The findings highlight the limitations of conventional methodologies, demonstrating how cumulative intensities can result in considerable inaccuracies in the estimates of seismic parameters. The results include a more robust and reliable reinterpretation of the 1783 Calabrian sequence, a refined reconstruction of the chronology of damage effects, the creation of a comprehensive thematic database on macroseismic effects, and a new characterisation of the major earthquakes within their seismotectonic context.