New insights on the strongest historical earthquake in the Pollino region (southern Italy)

Abstract

The Italian seismic catalog (Rovida et al., 2011) portrays a significant gap of seismicity in the Pollino Range area, the southernmost segment of the southern Apennines at the boundary with the Calabrian arc. In this region, the only significant seismic event of the instrumental era occurred in 1998 north of the Pollino Range (Mw 5.6, Fig. 1). No seismic event with intensity greater than VIII on the Mercalli–Cancani–Sieberg (MCS) scale is present in the historical records, whereas strong earthquakes occurred immediately north (1836 M 6.0; 1857 M 7.0) and south (1184 M 6.7; 1836 M 6.2) of the area (Fig. 1). However, geologic data (Bousquet, 1973; Russo and Schiattarella, 1992) have shown clear evidence ofQuaternary faulting along two major normal seismogenic faults, the Pollino and the Castrovillari faults. Paleoseismological data (Michetti et al., 1997;Cinti et al., 1997, 2002) recognized the occurrence of large magnitude events during medieval times along those two faults, but no trace is left of these major events in the historical record. In the case of the Pollino area, the discrepancy between true seismic history and recorded seismic history is due to a combination of a documentary gap of the historical sources (Scionti et al., 2006) and to the low population and scarcity of settlements in the epicentral area (D’Addezio et al., 1995; Cinti et al., 1997). Furthermore, there is the sound possibility that such documentary limitation also might affect moderate (M 5–6) earthquakes, thus providing the opportunity for new assessments that could re-estimate their sizes. Noticeable recent examples of this come from Scionti et al. (2006), with the reestimation to Mw 6.2 of an earthquake that occurred 100 km southeast of the Pollino region, from Tertulliani et al. (2012) for the 1762 earthquake close to L’Aquila, and from Azzaro et al. (2007) for northeastern Sicily. We want to emphasize that there is recent growing interest in the question of reestimating the sizes of historical earthquakes (see Rong et al., 2011; Hough, 2013). Therefore, reducing the uncertainties in the locations and sizes of seismic events for areas like Pollino can be an important contribution toward a clearer picture of the seismic potential and the distribution of earthquake recurrence times that can affect the seismic hazard of a territory. The first attempts at in-depth analyses of the historical seismicity in the Pollino seismic gap date back to the first half of the 1990s (Storia Geofisica Ambiente [SGA], 1994; Valensise et al., 1994; Guidoboni and Mariotti, 1997), whereas more recent developments are by SGA (2000), Camassi and Castelli (2004), and Castelli and Camassi (2005). The seismic histories (the list of earthquakes affecting a site during historic time) of towns and villages of the area (Locati et al., 2011) do not contain earthquakes that occurred before A.D. 1600, neither local nor remote, which could have affected such localities. The oldest traces of earthquakes are associated with the 1638 M 7 event that occurred in central Calabria ∼100 km south of the Pollino area and produced damage in Castrovillari. The more ancient event documented in the study area is the 1693 earthquake, which was listed for the first time in a catalog in 2007 (Guidoboni et al., 2007). In this study, we focus on this latter event, which also appears as the strongest shock in the region and is characterized by a relative scarcity of information and few intensity datapoints. We gathered all potential primary sources of information available for the 1693 earthquake in the attempt to improve its intensity map. We also address two further issues in the reappraisal of the 1693 earthquake: (1) the temporal coincidence between 8 January 1693 Pollino earthquake and its aftershocks and 9–11 January 1693 catastrophic earthquakes of eastern Sicily, along with the consequent quest for a correct attribution of the information regarding the two seismic sequences; and (2) the occurrence in October 2012 (following a sequence of small earthquakes lasting three years) of an Mw 5.3 mainshock, which is by far the strongest event in the instrumental era in the area.