Paleoseismic evidence of legendary earthquakes: The apparition of Archangel Michael at Monte Sant'Angelo (Italy)

Abstract

The Gargano promontory, a carbonate massif belonging to the Adria microplate, the foreland of both the Apenninic and Dinaric mountain chains, is a well-known seismogenic area, with historic earthquake up to Imax=X MCS (July 30,1627, M=6.7). Nevertheless, paleoseismic data on the region are still scarce, and no trench fault stratigraphy has been analysed along the major, E–W trending, right-lateral strike-slip fault system that cuts through the whole massif, the South Gargano Line (SGL). The over 60 km long on-shore trace of the SGL, the Mattinata Fault, is clearly segmented. This study focused on the eastern active fault segment, the Monte Sant'Angelo Fault (MSA), to verify the associated seismic potential.Analysis of contemporary descriptions of the August 10,1893 earthquake (M=5.2) coupled with field-work allowed identification of surface faulting along the MSA. Along the fault there is paleoseismic evidence of the occurrence of larger earthquakes. After collecting geomorphic evidence of paleo-earthquakes, we selected two trench sites at the base of two prominent fault escarpments, where the MSA is characterized by significant vertical components of fault displacement. The preliminary results demonstrate that: i) the recent vertical slip-rate of the MSA range between 0.2–0.3 and 0.7 mm/yr; ii) the MSA moves with incremental slip episodes, with vertical surface offset up to several decimeters; iii) based on the stratigraphic and geomorphic evidence of surface faulting observed and assuming a rupture length of around 20+ km, the seismic potential of the MSA segment would be similar to that displayed by the western SGL segment during the 1627 event.A large earthquake on the MSA is reported only in ancient legends, the most relevant of which being the one describing the apparition of Archangel Michael at Monte Sant'Angelo traditionally dated AD 493. Using a multidisciplinary approach, combining paleoseismic and historical data it has been possible to interpret the legend, and hereby identify an event of surface faulting before 8th century AD. A recent scarplet, up to 1 m high, on the MSA can be seen to be related to this legendary earthquake, which in turn seems to correspond to the maximum possible magnitude. Geologic phenomena observed during the earthquake at the site of the sanctuary appear to lie at the very origin of the veneration of the place.