GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 another multi-faulted colluvial wedge, datable between 5720-5590 BP and ~4850 BP, the latter being the age of the faulted paleosol mantling the wedge. Results from Preci-Campi segment This fault could be as long as 15 km (there are uncertainties about its northern tip), and it also bounds a Quaternary basin, deeply incised and emptied by the Campiano stream. Along this fault the carbonate slickenside outcrops less sparsely than in the Norcia basin, whereas it is completely buried below the alluvial aprons. We dug trenches 14-15-16 (#4, #5 and #6 in Fig. 1), which are the first paleoseismological trenches opened across this fault. Due to the absence of robust morphotectonic evidence moving away from the rocky slope, before trenching we performed multi-spaced ERT along the supposed fault trace that revealed the presence of huge alluvial coverage over the possible fault trace. Indeed, during trenching (site #5), these deposits were so loose and unstable that the digger did not succeed in excavating more than 3 m at depth, and thus we did not reach the fault seen in the ERT. Conversely, trench 14 is located across the carbonate slickenside, following the demolition and removal of the buildings that were destroyed by the 2016 earthquakes in Capo del Colle (#4 in Fig, 1). Here we investigated the remains of a wall and of its foundation, belonging to a 16th century house that was built above the carbonate slickenside. Both show the results of faulting and dragging along the fault plane that happened surely after 1488-1650 AD, an AMS age of detrital charcoals sampled in the foundation ditch. Obviously, this age places the faulting in the 1703 earthquake. Successively, the house was restored in the 18th century, the tilted/faulted masonry was levelled and the wall rebuilt with smaller stones in unconformity over the older ones. The last trench (#6 in Fig. 1) was opened at the foot of the limestone hill where the medieval Campi settlement was founded. The 65-m-long excavation interested the apex of an alluvial fan, where the ERT analyses showed the presence, at depth, of a vertical contact between terrains with different resistivity that matches a strongly reworked scarp at the surface. The digger excavated down to 6 m of depth inside a faintly layered, homogenous and monochromatic gravel succession, where it was very hard to see any lateral differences, both continuous or fragile, i.e., warping or faulting. Next to the scarp, after several days of cleaning and drying, at the light of the sunset, the wall finally revealed a subvertical fault with centimetric offset, passing upward into a vertical fracture filled by oriented clasts. The AMS age (1155-1265 AD) of the gravel matrix at the bottom of the trench, besides evidencing the high sedimentation rate of the fan during the past millennium (5 mm/yr), provides a rough post quem term for this faulting. As far as the little offset of the gravels, it is worth noting that this site is next to the northern tip of this splay - where the fault presents a right stepping of hundreds meters toward the hillslope - making thus possible that the coseismic slip locally tends to zero.