GNGTS 2019 - Atti del 38° Convegno Nazionale

28 GNGTS 2019 S essione 1.1 had a few damage, were very heavily damaged (I 8-9). At S. Margherita, S. Ninfa and Partanna heavy damage occurred (I 8); finally, severe damage (I 7-8) also affected Menfi and Salemi (Fig. 3b). Many localities around were moderately or slightly damaged. On 16 January, another M w 5.3 shock caused the collapse of the few buildings left standing in Gibellina, Salaparuta and Montevago (I 9-10); very heavy damage (I 8-9) occurred at Poggioreale and S. Margherita, whereas Partanna, Menfi, Santa Ninfa and Contessa had further partial collapses (Class A buildings, I 8). The damage area extended to the North as far as Palermo (I 6) (Fig. 3c). On 25 January, the last M w 5.2 event raised to the ground the remains of Gibellina and Salaparuta (I 10) and, partially, Montevago (I 9-10). Poggioreale and S. Margherita remained struck at a lesser degree (I 9) as well as Partanna and S. Ninfa (I 8-9); Menfi did not modify the previous damage scenario (I 8) but moderate damage appeared in several localities little damaged by the previous earthquakes (Fig. 3d). In conclusion, the intensities in the localities of the epicentral area increased day by day (Tab. 1), reaching the maximum as early as 15 January (Partanna and S. Ninfa), 16 January (Contessa Entellina) and in most of them on 25 January. Table 1 - Intensity (EMS-98) and damage progression in the most struck towns of the Valle del Belìce . Locality 14 January 15 January 16 January 25 January Gibellina 7 9 9-10 10 Salaparuta 7 9 9-10 10 Montevago 5-6 8-9 9 9-10 Poggioreale 6-7 8-9 8-9 9 Santa Margherita 5-6 8 8-9 9 Partanna 6 8 8 8-9 Santa Ninfa 5-6 8 8 8-9 Salemi 4-5 7-8 7-8 8 Contessa Entellina 7-8 8 8 Calatafimi 5-6 6-7 7-8 Macroseismic parameters. We calculated epicentres andmagnitudes from themacroseismic data points (MDPs) reported in Fig. 2 through the Boxer method (Gasperini et al. , 2010); the magnitude M w is obtained by using the last calibration adopted in the CPTI15 catalogue (Rovida et al. , 2016). Results in Tab. 2 show that macroseismic parameters are increasingly affected by the cumulative effect of damage during the sequence; in practice, epicentre and magnitude of the first earthquake (14 Jan.) can be considered quite reliable, and little less those of the event occurred on the next day (15). This occurs because the epicentres differ significantly each other while those of the following shocks (16, 25 Jan.) vary within the errors. The comparison with the instrumental parameters (ISC data) suggests the same conclusion also for the magnitude values, which diverge considerably from 16 January onwards. In conclusion, we note that: i) the 14 January shock appears located north of the alignment Gibellina-Poggioreale, near the instrumental epicentre; ii) the following events moved southward, close the most damaged towns of the Valle del Belìce. The epicentre apparent migration along an E-W direction is probably due to the position of the localities (i.e. coverage on the macroseismic map). As a result, though the actual location of the 1968 earthquakes presents wide margins of uncertainty, the epicentres proposed appear closer to the instrumental ones than the previous ones indicated in CPTI15. Regarding the magnitude overestimation of the earthquakes during the sequence shown by the increasing values of M w in Tab. 2, we tried to solve the problem through the computation