Geophysical prospecting for the pre- and early-historical reconstruction of the subsurface underneath the Paleochristian Basilica of Santa Maria di Compulteria (northern Campania, Italy)

Integration of archaeoseismic observations, geological and geophysical surveys and a critical review of historical written sources contributed to shed light on the effects of the 847 CE earthquake that struck a large area of southern-central Italy. New archaeoseismic evidence of a strong earthquake which occurred around the middle of the ninth century CE comes from two Medieval archaeological sites along the Volturno Valley, between Campania and Molise regions. Historical documentation and its evidence include the tilting of pillars in the Basilica of Santa Maria near Alvignano (northern Campania region) and a collapsed masonry wall in the Abbey of San Vincenzo al Volturno near Isernia (northern Molise region). At Alvignano, a site so far unrecorded in seismic catalogues for the 847 earthquake, geoelectrical and georadar investigations were used to explore the subsoil and study local site conditions, which could have influenced co-seismic ground motion. Interpretation of geophysical surveys calibrated by stratigraphy of available boreholes document the presence of altered pyroclastic deposits, which likely enhanced site effects at Alvignano. Analysis of damage historical descriptions and of archaeological reports indicates that the 847 seismic event damaged a wide area between Latium, Campania and Molise, with destruction of the town of Isernia and damages in Rome. Because the damaged area for this medieval earthquake is loosely defined in literature, the present study represents a contribution to better define the shaking area and provide new hints on the extent and location of the possible seismogenic source.