GNGTS 2019 - Atti del 38° Convegno Nazionale

38 GNGTS 2019 S essione 1.1 to identify evidence of seismic effects on ancient Medieval buildings or structures uncovered during archaeological excavations. Archaeoseismic evidence of the 847 earthquake comes from two archaeological sites located along the Volturno Valley, namely San Vincenzo al Volturno Abbey near Isernia in northern Molise and the Basilica of Santa Maria at Alvignano in northern Campania (Fig.1). San Vincenzo Abbey showed structural modifications shortly after the earthquake documented by English excavations in 1990’s (Hodges, 1993), as well as some still visible evidence of deformation and collapse. The most significant evidence is the collapse of a large wall in a ~NNE-SSW direction, which connected the workshops area with the eastern corridor in the Abbey, in addition to minor damages as cracking in walls (Marazzi, 2014). In the Basilica of Santa Maria (High Volturno Plain) we surveyed deformed arches, pillars tilted along a NE- SW axis, and partial rebuilding of the apsidal area. Furthermore, at this site we performed an integrated geophysical survey, consisting of ERT and GPR measurements, to explore the subsoil and to study local site conditions, which could have influenced co-seismic ground motion. The integrated survey was specifically designed to acquire information on the physical and geometrical features of the shallow subsurface beneath the Basilica of Santa Maria and to explain the tilt of the pillars and collapse of the apse. The integrated interpretation of the acquired data, calibrated by the stratigraphy of available boreholes, documents the presence of altered pyroclastic deposits, which could have certainly enhanced site effects at Alvignano. Indeed, the lateral and vertical variations in resistivity values observed down to the maximum exploration depth (~ 10 m b.g.l.) suggest a significant inhomogeneity of the investigated deposits, which is compatible with different degrees of argillification. This inhomogeneity could have amplified the site effect during seismic shaking and caused the vertical crashing of the left aisle arches. Analysis of damage descriptions and archaeological reports indicates that the 847 seismic event documented by historical sources damaged a wide area between Latium, Campania and Molise, with destruction of the town of Isernia and damages in Rome (CFTI5Med, Guidoboni et al. 2018). For the Isernia settlement, unfortunately, archaeological data relative to destruction and following reconstruction of buildings have not been found. Conversely, historical sources report total destruction of the town. Although historical sources did not explicitly mention damage in Rome, the traces of the 847 AD earthquake have been largely documented in the archaeological excavations and seismological literature (Galli and Molin 2014, Galadini et al. 2018). Starting from the documented effects of earthquake shaking, a wide area of damage Fig. 1 - a) Map of damaging effects area of the 847 earthquake: the black dots indicate the localities damaged by the seismic event, the Roman numbers report the intensities in MCS (after CFTI5Med 2018).