GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 1.1 29 (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.