GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 In 1968, from January 14th to 25th, a seismic sequence was recorded with more than 300 events affecting the Belice valley area (De Pan fi lis and Marcelli, 1968; Bottari, 1973), six of which with a Mw ranging from 5.1 to 6.4 (Fig.1b) (CPTI15, Rovida et al., 2021; Azzaro et al. 2020). The sequence highlighted the activity of compressive structures of the SFTB and caused severe damage to fourteen villages, some of which were later completely rebuilt in different sites (Gibellina, Poggioreale, Salaparuta and Montevago), whereas others were rebuilt in the same place with different urban layout. The 1968 seismic sequence is the only significant one that occurred in the instrumental period in western Sicily. After the 1968 events, the seismicity localized onland is characterized only by sparse low magnitude earthquakes. Other potential sources located offshore produced damaging earthquakes along an E-W contractional belt running about 50 km off the northern Sicily coast (Cuffaro et al., 2011; Barreca et al., 2014), such as the several historical events that hit Palermo (Guidoboni et al., 2003). Excluding the offshore sources, the Italian historical catalogue does not list any other moderate or large earthquake that occurred in the region. Possible events that caused the collapse of historical buildings occurred before 1000 CE, indications of which are still preserved in the site of Selinunte (Guidoboni et al., 2002; Bottari et al., 2009). These occurred in 370-300 BCE and 330-500 CE, causing collapses in Selinunte and probably in other sites, as well as the Selinunte thermal bath located in the Sciacca territory. Archaeological evidence of seismic destruction The most frequent damage observed in the northern area of the Agora of the Acropolis refers to oriented collapses of the central portion of the northern stoa (Erdas, 2004; Parra, 2006; Ampolo and Parra, 2018; Olvito and Serra, 2016), more precisely the back wall and the arch located in front of it, which collapsed one above the other (Fig. 2a, b). During the collapse, the back wall suffered a rotation of about 45° (Olvito and Serra, 2016). The stratigraphy was perfectly preserved until the archaeological dig, when the two structures fell down on the paved floor of the stoa square, causing cracks in the paved floor of the square. Also dipping broken corners in the column drums observed in the eastern wing of the stoa (Fig. 2c) probably due to seismic oscillation, as well the offset in steps, have been observed in the whole of the agora. In addition, basal drums of octagonal columns built to support the first floor of the stoa were found rotated in situ (Fig.2e). As a consequence of the collapse, we noted a shock breakout in flagstone in the northern stoa of the Acropolia.