The 1780 seismic sequence in NE Sicily (Italy): shifting an underestimated and mislocated earthquake to a seismically low rate zone

Abstract

The southernmost sector of the Italian peninsula is crossed by an almost continuous seismogenic belt capable of producing M~7 earthquakes and extending from the Calabrian Arc, through the Messina Straits, as far as Southeastern Sicily. Though large earthquakes occurring in this region during the last millennium are fairly well known from the historical point of view and seismic catalogues may be considered complete for destructive and badly damaging events (IX £ I o £ XI MCS), the knowledge and seismic completeness of moderate earthquakes can be improved by investigating other kinds of documentary sources not explored by the classical seismological tradition. In this paper, we present a case study explanatory of the problem, regarding the Ionian coast between the Messina Straits and Mount Etna volcano, an area of North-eastern Sicily lacking evidence of relevant seismic activity in historical times. Now, after a systematic analysis of the 18th century journalistic sources (gazettes), this gap can be partly filled by the rediscovery of a seismic sequence that took place in 1780. According to the available catalogues, the only event on record for this year is a minor shock (I o = VI MCS, M w = 4.8) recorded in Messina on March 28, 1780. The newly discovered data allow to reinstate it as the mainshock (I o = VII-VIII MCS, M w = 5.6) of a significant seismic period, which went on from March to June 1780, causing severe damage along the Ionian coast of North-eastern Sicily. The source responsible for this event appears located offshore, 40-km south of the previous determination, and is consistent with the Taormina Fault suggested by the geological literature, developing in the low seismic rate zone at the southernmost termination of the 1908 Messina earthquake fault.

Incipient extension along the active convergent margin of Nubia in Sicily, Italy: Cefalù-Etna seismic zone

“…The 1823 earthquake near Cefalù, for instance, may have occurred off Cefalù along the south‐Tyrrhenian contractional seismic belt (Figure 1) with a magnitude greater than that hypothesized solely on the basis of the macroseismic evidence observed in the Cefalù area [ Guidoboni et al , 2003; Jenny et al , 2006]. However, even considering possible underestimates of historical earthquake magnitudes and their mislocations, it is widely accepted that in the last millennium there should be no gaps of information on destructive earthquakes in Sicily, especially for earthquakes occurring onshore [ Neri et al , 2006; Azzaro et al , 2007]. In synthesis, the above‐reported data and notions suggest that the occurrence of destructive earthquakes of magnitude over 6.4 in the Cefalù‐Etna seismic zone can be excluded since at least 1300 A.D. and, plausibly, since 1000 A.D. (http://zonesismiche.mi.ingv.it/).…”

“…Moreover, the time window of instrumental seismicity (i.e., 1981-2007) may be too short to show all seismic structures and their entire length. This problem is only partially solved by the historical analysis (Figure 8a), whose limitations are discussed above and in the specific literature [e.g., Guidoboni et al, 2003;Azzaro et al, 2007]. The above-discussed uncertainties concerning the instrumental and historical seismicity affect the seismic potential analysis (Table 1), whose results should, therefore, be regarded as a preliminary estimate.…”

A new dataset and empirical relationships between magnitude/intensity and epicentral distance for liquefaction in central-eastern Sicily

“…Italian historical bibliography offers numerous accounts describing seismo-induced effects (Working Group CPTI04, 2004). The column Rt reports the source of the macroseismic observations: CFTI (Boschi et al, 2000); DOM (Monachesi and Stucchi, 1997) and Azz* refers to Azzaro et al (2007) which are reported in catalogues (Berardi et al, 1991;Galli and Ferreli, 1995;Romeo and Delfino, 1997;Boschi et al, 2000;Galli, 2000;Prestininzi and Romeo, 2000); these have also been used to draw maps of liquefaction-prone areas of Italy (Galli and Meloni, 1993) and, at local scale, of historical liquefaction-induced phenomena in the Catania area (Azzaro, 1999). This paper presents an updated dataset of liquefaction phenomena in central-eastern Sicily, realized through the revision of historical accounts, retrieved from the aforementioned catalogues and through an original research of historical primary sources.…”

Macroseismology: the lessons learnt from the 1997/98 Colfiorito seismic sequence

“…Since its formulation, the new EMS-98 scale has become more frequently used in Italy, although only after some initial perplexities (Molin, 1995;Tertulliani, 1995). This scale has been applied to a variety of cases, many of which have also included systematic comparisons with the MCS scale, which focuses on both the collection and interpretation of data collected in the field (Azzaro and Barbano, 1995;Azzaro et al, , 2004Azzaro et al, , 2006, and the analysis and revision of historical sources, also in terms of seismic histories and damage scenarios for hazard purposes (Azzaro et al, , 2007Barbano et al, , 2005. These 1997/98 earthquakes represent an important moment in the aforementioned process, since it was the first experimental «laboratory» involving a large group of Italian operators working in macroseismology.…”