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A REAPPRAISAL of thE 1783 SEISMIC SEqUEnCE And RELAtEd SEISMoGEnIC fAULtS In thE CALAbRIAn ARC �SoUthERn ItALy��M.S. Barbano, C. PirrottaDipartimento di Scienze Biologiche, Geologiche e Ambientali, Università di Catania, Italy

Introduction. According to the most recent earthquake Italian catalogues, five devastating earthquakes struck the southern Calabria between February 5 and March 28, 1783 (Boschi et al., 2000; Rovida et al., 2011). The 1783 sequence caused a geomorphological crisis that changedThe 1783 sequence caused a geomorphological crisis that changed the Aspromonte and Serre Mts landscape triggering ground fracturing, deep seated gravitative deformations, landslides, liquefactions and inducing a substantial change in the fluvial network geometry with deviation and dam of fluvial channels, as well as the development of numerous lakes and other hydrological anomalies (Cotecchia et al., 1996).

Several authors deal with these earthquakes for different purposes such as earthquake catalogues, tsunamis and seismogenic fault individuation (e.g., Boschi et al., 2000; Graziani et al., 2006; Jacques et al., 2001; Galli and Bosi, 2002). There is a general agreement that some of the 1783 earthquakes are associated with the main NE-SW striking west-dipping active swarm of normal faults running along the Calabrian Arc. This hypothesis is also supported by the present-day WNW-ESE-trending crustal extension documented by focal mechanisms of earthquakes (CMT and RCMT Catalogues; Neri et al., 2004), geodetic deformation velocities (Mattia et al., 2009; D’Agostino et al., 2011) and structural studies (Tortorici et al., 1995; Jacques et al., 2001; Ferranti et al., 2008). Nevertheless some of the events are not associated with faults and some sources are not well defined. Although a lot of coeval reports are available for the 1783 seismic sequence, the overlap of many shocks in very narrow time and space makes the interpretation of historical data difficult, because historical accounts describing damage in several localities do not distinguish the shocks.

An accurate revision of the 1783 seismic crisis has been carried out with the main purpose of better separating the effects of the different shocks and understanding the seismogenic faults associated with these earthquakes. We show results for some minor shocks, whereas for the main five shocks we report published data.

The 1783 seismic sequence: a reappraisal. The devastating seismic sequence occurred in Calabria and in northeastern Sicily started on February 5 and was followed by a long series of earthquakes that migrated from the Messina Strait toward NNE, along the axis of the Calabrian Arc. The aftershocks lasted for about ten years. More than 180 localities were totally destroyed or almost collapsed and there were more than 30,000 victims.

Historical sources, testifying the effects of the 1783 earthquakes, come from different kinds of documents: chronicles, newspapers and reports (for a comprehensive reference list see Boschi et al., 1995). Among the numerous sources, the most complete works are those of Sarconi (1784), Vivenzio (1783, 1788) and De Dolemieu (1784). The first is the final report of the mission sent by the Bourbon government in Calabria and Messina for a systematic survey of the damaged sites, the second holds assembled data from several accounts and the third is the report of observations made during Dolemieu’ trip in Calabria between February and March 1784.

There are also several useful memoirs describing detailed effects in single locations, as well as dates and hours for felt shocks (Fig. 1), such as Corrao (1784), Gallo (1783, 1784) and Torreani (1784) for Messina, Roscitano (1783) and Palestino (1895) for Reggio Calabria, Minasi (1783, 1785) for Scilla; Masci (1783) for Santa Sofia, Serrao (1785) for Castelmonardo, De Leone (1783) and Augusti (1784) for Catanzaro and Vivenzio (1788) for Monteleone.

In order to separate shocks, we analyzed the description for single localities and compared the corresponding with the felt time of other sites. After having unraveled a tangle of multiple dates, hours and spurious connections between several earthquakes we identified collections of historical data dealing with single seismic events. This analysis allowed identifying more events than the five destructive shocks usually reported for the 1783 catastrophe.

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Results. The February 5 shock occurred at 19.10 Italian time (12.10 GMT) with a felt area extended from the whole of Sicily to most of southern Italy (e.g. Boschi et al., 2000). The area with the strongest effects (I = 11) is located in the Tyrrhenian side of the Calabrian Apennines, a region corresponding to the Aspromonte and Gioia Tauro Plain (Fig. 2)

The February 6 shock, which occurred at 7.30 Italian time (00.20 GMT), increased damage in Scilla and Messina already damaged by the previous earthquake Intensities reported in reported in Fig. 2 (Boschi et al., 1995) are probably overestimated, indeed damage in Scilla were mainly due to the following devastating tsunami that struck it and Torre Faro (e.g. Graziani et al., 2006).

The next day shock, on February 7, at 20.30 Italian time (13.10 GMT), moved to

a north-easterly direction compared to the one of previous earthquakes, towards the Mesima Basin (Fig. 2) on the Tyrrhenian side of the Serre Mts. The most affected area (I = 10-11) was that between Melicuccà and Vazzano (Fig. 2). Damage observed in the villages of the Aspromonte and Gioia Tauro Plain (e.g. Boschi et al., 2005), already badly damaged by the shocks on February 5 and 6, are not likely related to the same event. Indeed from the analysis

Fig. 1 – Localities described by different authors for the 1783 Calabria seismic sequence. Moreover Sarconi (1784), De Dolemieu (1784) and Vivenzio (1783, 1788) report information for the whole Calabria. Some of the localities described by Sarconi are shown.

Fig. 2 – Maximum intensity distribution of the main shocks of the 1783 seismic sequence and some other events studied in this work. For the bigger events only values of I > 8.5 are reported. Pink circles are the February 5 intensity values, dark green circles for the February 6; the blue ones for the February 7; the light green for the March 1, the red ones for the March 28 and the full grey ones for the minor shocks studied in this work. Yellow stars are the epicenters of the major earthquakes. Note: for the February 5 event, the anomalous intensity along the Tyrrhenian coast (Bagnara and Palmi); the localities also struck by the 6 and 7 February at 15 GMT (Fig. 3 a) very close to the previous ones; the large distribution points for the March 28 shock: Faults associated with the main events: ADCF = Armo-Delianuova-Cittanova Fault; SECF = Santa Eufemia-Calanna Fault; SF = Scilla Fault; GF = Gioia Fault; NF = Nicotera Fault; MEF = Mesima Fault; LCF = Lamezia Catanzaro Fault.

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of the sources consulted for the sequence, it is clear that on the evening of February 7, after the third main shock above mentioned, at 22 Italian time (15 GMT), another earthquake of intensity 7-8 damaged the towns of Scilla (Minasi, 1783), Messina (Gallo, 1784; Torreani, 1783), Sant’Agata di Reggio (Palestino,1877) and was strongly felt in Roccavaldina (Corrao, 1784), Monteleone [nowadays Vibo Valentia: Vivenzio (1788)], Santa Sofia (Masci, 1783) and Soriano Serre (Mercalli, 1897). The epicentre of this quake is located between Reggio Calabria and Scilla (Fig. 3a). On the beach of Reggio the soil sank and abundant spring water flow in the Giunchi area in the northern historical centre of the city (Roscitano, 1783). In most of the catalogues (e.g. Boschi et al., 2005; Rovida et al., 2011) these localities are mentioned together with the earthquake at 13.10 (GMT) earthquake of the same day.

The March 1 strong earthquake was preceded by a shock on February 27 (I = 6-7), located northernmost. It caused lesions to country buildings between the Angitola river and Borgia

Fig. 3 – Some minor events of the 1783 seismic sequence: a) the February 7 at 22 Italian time (15 GMT) shock; b) the February 27 shock c) the April 26 shock, d) the July 30 1783 shock.

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(Girafalco, Borgia and Francavilla Angitola), and was strongly felt in Catanzaro (De Leone, 1783; Grimaldi, 1784), Castelmonardo (nowadays Nuova Filadelfia, De Rossi, 1889), Santa Sofia (Masci, 1783) and largely observed in Monteleone (Vivenzio, 1788) (Fig. 3b).

The shock on March 1, at 08.30 Italian time (01.40 GMT), was felt in a wide region between Messina and Scalea, the area of the maximum effects (I = 9.0) moved further north (Fig. 2), along the Apennines. The most affected villages were: Polia (nowadays Trecroci), Poliolo, Castelmonardo and Francavilla Angitola.

The quake on March 28 was felt throughout southern Italy. The destruction area was located in the Catanzaro basin, with damage both in the Ionian and in the Tyrrhenian side (Fig. 2). It is the northernmost greatly damaged area of the entire seismic period. For this shock a I = 11 maximum intensity has been estimated, but most of localities were already strongly damaged by the preceding shocks,

On April 26 and 27, two shocks (intensity 7 and 6.5, respectively) damaged some quarters in Milazzo (Vivenzio, 1783), which had not been significantly hit by the previous shocks (Fig. 3c).

Finally, on July 30 a strong shock (Fig. 3d) caused moderate damage in Gerace (I = 7) (Mercalli, 1897), Catanzaro (I = 6) Reggio Calabria (I = 6) and Cinquefrondi (I = 6) and was strongly felt in Monteleone (I = 5) as well (Vivenzio, 1783).

Damage distribution and seismogenic faults. The February 5 1783 earthquake seems to be the strongest event occurred in southern Calabria in historical times (Mw 7.0, Rovida et al., 2011). It has been attributed to either the Cittanova fault (Galli and Bosi, 2002) or the Cittanova and Sant’Eufemia faults (Jacques et al., 2001) or the Gioia fault (Basili et al., 2008).

Long and continuous fractures, described by contemporary witnesses, are considered as the superficial expression of the seismogenic fault. This ground fracturing well fits with the northern sector of the Armo-Delianuova-Cittanova fault (Fig. 2). Furthermore the configuration of the macroseimic field, the observed deformation of the drainage (Pirrotta et al., 2013), the length of the fault (20 km), which is compatible with an earthquake of Mw 7.0 according to regression relations for normal faults (Wells and Coppersmith, 1994), are well-suited with the Cittanova fault (e.g. Galli and Bosi, 2002).

The February 6 shock was less strong than the previous one and the damage in Scilla was mainly due to the following large tsunami. This shock was probably triggered by the Scilla fault (Ferranti et al., 2008; Barbano et al., 2014). Based on fault length and slip, estimated from footwall uplift (Ferranti et al., 2008), and using regression relations for normal faults (Wells and Coppersmith, 1994), a Mw = 6.1/6.9 can be estimated for this event, but probably this value is overestimated.

It has been observed that for high-angle normal faults such as the Apennines ones, apart from local site-amplification effects, maximum intensity is distributed along an elongated area parallel to the seismogenic fault, above the projection of the fault plane on the surface. The macroseismic field of the February 5 earthquake (Fig. 2) well fit with the Cittanova fault, but in Bagnara, Palmi and Seminara damage (Vivenzio, 1783) were exceptionally high with respect to the distance from the Cittanova fault. It is likely that damage at Bagnara is partially due to the peculiar location of the city on a narrow alluvial delta between a cliff and the sea.

Another explanation could be that, since damage and fatalities for most of localities are reported for the whole seismic sequence and the February 6 shock occurred close in time to the February 5 shock, destruction and fatalities in Bagnara are the effect of the sum of the two events. Observing the macroseismic fields of these two events it is evident (Fig. 2) that intensities observed on the coastal area (Bagnara and Palmi) are compatible with the second shock. Moreover damage in Bagnara could have also been increased by the February 7 event at 22 Italian time (15 GMT).

This suggestion is also supported by the hypothesis that some fatalities at Bagnara could be due to the tsunami even if there is no report of tidal wave at Bagnara for the February 6 tsunami.

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The February 7 damage distribution clearly suggests the activation of the Mesima fault (e.g. Tortorici et al., 1995). The magnitude estimated from intensity is 7.0 (Rovida et al., 2011).

The relationship between the February 27, March 1 and March 28 1783 damage and the near known faults is poorly understood. The March 1 event appears to be linked also to a further north rupture of the Mesima fault. Whereas the 28 March 1783 macroseismic distribution is widely scattered; it is evident from the large felt area and the small gradient between different intensities that the last quake had a deeper source with respect to the previous events. This observation can account for the lack of coseismic rupture. The magnitude estimate for this event is 6.8-7.0 (Rovida et al., 2011). The intensity distribution suggests an E-W oriented fault, therefore it is likely linked to one of the strike-slip faults of the Catanzaro basin (Ghisetti and Vezzani, 1982).The Lamezia-Catanzaro fault (Fig. 2) suggested by Moretti (1982) seems to be far from the maximum damaged area and therefore it is like that the seismogenic structure of this quake should be searched southernmost. The April 27 and 28 shocks that damaged Milazzo seem to be related with a Sicilian source probably the Tindari fault. Lastly the 30 July shock may be associated with one of the Ionian faults that did not slipped during previous earthquakes.

Conclusion. The analysis of the 1783 sequence highlights that macroseismic fields of the single event can be overestimated because earthquakes occurred close in space and time and observed damage is the cumulative effects of multiple events. Furthermore effects observed in some localities are reported for the strongest events, but they could be due to minor earthquakes.

During the 1783 seismic sequence ruptured both Apennine structures and transversal faults. Most of earthquakes are linked to Apennine structures but, at the end of the main sequence, transversal structures ruptured as well. In this framework, the role played by the transverse structures is fundamental in limiting the extension of the main faults. The March 28 earthquake may be associated to a stress transfer toward larger depth, where accumulated stress is slowly released because rheological properties at deepest levels retard slips. The 1783 seismic sequence seems to be like, apart from the highest magnitudes, to the 1997-98 central Italy seismic sequence (Mw = 5.9). Indeed most of the Umbria-Marche earthquakes occurred along Apennine structures, but when seismic activity decreased, in March 26, 1998 a Mw = 5.3 deeper event occurred. The epicentre of this shock was located at the northern end of the Apennine faults associated with the Umbria-Marche seismic sequence, at a depth of 46.6 km (Pino and Mazza, 2000).

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