GNGTS 2013 - Atti del 32° Convegno Nazionale

On the other hand, neither important historical earthquakes nor moderate instrumental epicentres are known to be occurred within the Sulmona Plain or can be attributed to this fault system. The only indication of possible, local seismic activity is provided by the archaeoseismic investigations carried out by Galadini and Galli (2001), who hypothesized that a large magnitude event hit several settlements in the Sulmona Plain around the middle of 2nd century AD (Fig. 1; see also Ceccaroni et al. , 2009). According to these authors the earthquake was likely generated by the Mount Morrone fault rupture. Notwithstanding all these indications, the lack of certain (i.e., directly dated through radiometric analyses) post Last Glacial Maximum (LGM)-Holocene deposits affected by the fault prevented any conclusive assessment regarding the recent activity of this structure. Therefore, in order to cast light on this debated issue, following aerial photo and LiDAR interpretation, geological field surveys, high-precision topographic levelling, and geoelectrical investigations (ERT), we have finally decide to open a first set of three paleoseismological trenches across the western basal fault. The Mount Morrone fault system. The Mount Morrone fault system is expressed at surface by two main, discontinuous fault splays which roughly parallel the western slope of the massif, for a total length of 22 km (Fig. 1). The eastern one runs mainly at 1100-1200 m a.s.l., and it is evidenced by an impressive, up to 50-m-high rock fault scarp within the Jurassic limestone (upper inset in Fig. 1). In the hanging wall, it affects a thick sequence of slope debris which likely formed during the cold and arid phases of the LGM (Galadini and Galli, 2000). In turn, the western splay runs along the base of the foothill, roughly between 300-400 m a.s.l., from the northern outskirts of Popoli to the Pacentro village. In some sector of the slope the western fault is evidenced by a rock-fault scarp (e.g., Roccacasale strand), with an outcropping, smoothly polished plane (slickenside) carved in the carbonate footwall (e.g. in Boncio et al. , 2012). However, most of its surficial trace is buried below several, active and coalescing alluvial fans which mantle the whole piedmont of Mount Morrone. All together, these fans form an alluvial apron, the distal limb of which rests over the upper terrace of the Sulmona Plain (350-380 m a.s.l.). Several quarries opened in these fans allowed the observation of metre-scale displacements of the Late Pleistocene deposits, even if these always concerned secondary faults, i.e. N-S transfer faults between en echelon strands (Vittori et al. , 1995). Fig. 2 – ERT profile across the northwestern, basal splay of the Mount Morrone fault system. Arrows indicate the possible fault position. Blue area match very low resistivity deposits, i.e. silty-clayey colluvia. Upper inset: view looking N of the slope where we have made ERT 1 and trench 1. 55 GNGTS 2013 S essione 1.1