The 6 October 1762 Middle Aterno Valley (L'Aquila, Central Italy) Earthquake: New Constraints and New Insights

Abstract

The effort for reducing the uncertainties in the location and size of historical earthquakes, even moderate-size ones, is not a peripheral issue, as it plays a major role in the distribution of earthquake recurrence times that can affect the maps of seismic hazard of a territory. The L’Aquila area (Abruzzo, Central Italy) struck by the 6 April 2009 Mw 6.3 seismic event is a typical example of an earthquake-prone region, the location of which historical seismicity needs to be better located. Thanks to a large body of geological, seismological, and geodetic evidence, the deep source of the 2009 mainshock has been imaged as a ~15-km-long, northwest-trending, southwest-dipping, almost-pure normal fault coinciding with the Paganica fault system at the surface (Fig. 1). Conversely, so far, very few papers have addressed the issue of reconstructing the seismic history of the Paganica fault, charting the relationships between this fault and the adjacent ones, and finding out the causative source for each of the earthquakes listed in the Italian earthquake catalog (CPTI Working Group, 2004; Rovida et al., 2011). In particular, the 1461 M 6.4 and the 1762 M 6.0 earthquakes (Fig. 1) took place in the same general area and display a comparable magnitude with that of the 2009 earthquake. According to recently provided paleoseismological data (Cinti et al., 2011), the shock of the 1461 earthquake was the penultimate event that ruptured the Paganica fault. The case of the 1762 earthquake was more open to debate: Tertulliani et al. (2009) suggested that it could have had the same fault source as that of the 2009 shock (west of the catalog epicenter) on account of some similarities in the distribution of the maximum intensities and in the temporal evolution of the 1762 and 2009 sequences. At the same time, however, Tertulliani et al. (2009) were aware of, and cautioned against, the very short recurrence interval implied by their hypothesis. The issue of the too-short recurrence interval, along with some observations on the too-low throw rate measured across the Paganica fault, induced Roberts et al. (2010) to invoke the role of other neighboring sources and to propose for the 1762 shock an alternative location some 10 km east-southeast of the epicenter adopted by the catalog. Such a variance of opinions concerning the epicentral location of the 1762 earthquake is explained by the scantiness of the data set on which investigations must rely and which consists of the shaking intensities at three localities only. As Di Bucci et al. (2011) pointed out, such a data set made it impossible to associate the 1762 event to any fault “beyond mere speculation”. We are left from this stark and somewhat frustrating consideration to systematically gather all potential primary sources of information available for the 1762 earthquake in the area to attempt to improve its intensity map and to compare it with the 2009 one. In the following, we will (1) set out the evidence newly collected from archives in L’Aquila and Naples and from other repositories of contemporary sources; (2) explain the rationale that allowed us to turn demographic and historical information into data sensitive to macroseismic analyses; (3) show the new intensity map for the 1762 earthquake and discuss its main differences from the previous intensity data sets (Boschi et al., 2000 and the most recent versions of the Italian macroseismic database, i.e., Stucchi et al., 2007; Locati and DBMI Working Group, 2009; Locati et al., 2011); (4) provide a more accurate location and propose new source parameters for the event; and (5) discuss the constraints on its epicenter location and source parameters with the possible seismotectonic implications.