Cinematica attuale, comportamento sismico e sismologia storica della faglia di Monte Sant’Angelo (Gargano, Italia): la possibile rottura superficiale del “leggenadario” terremoto del 493 d.C.

Actual kinematics, seismic behaviour and historical seismology of the Monte Sant’Angelo fault (Gargano, Italy): the possible surface rupture of the “legendary” earthquake of the 493 A.D.

Abstract

Tectonic analysis is a fundamental tool to understand kinematics and behavior of active faults, and to assess of the seismic risk of an area. Structural-geological and morphotectonic analysis of the Monte Sant’Angelo fault clearly indicate it to be active, Detailed measurements of the morphology of the mountain slope and of the cumulative scarp allowed to kinematically characterize this fault with a vertical slip rate of 0,7 ± 0,2 mm/yr and a right-lateral slip-rate of 1 ± 0,2 mm/yr. Evidence of cumulated coseismic surface ruptures have been recognized along the main fault trace, and it was even possible to estimate a maximum magnitude of M = 6,9 and a recurrence time of 1860 ± 460 years for such earthquakes. Historically, similar earthquakes are not documented on this fault, and the area is in fact classified with a maximum felt intensity of VII-VIII MCS. Nonetheless, a strong earthquake at Monte Sant’Angelo is described in one of the most important medieval sacred legends, and traditionally dated around the 5th century A.D. By combining tectonic, archeological arid historical data, the most recent of the observed faulting events could be identified as the possible result of the earthquake reported in that legend. The main results of this study are therefore: 1) identification of a strong Italian earthquake (6<M<7) in the Roman – Middle Ages period, as well as of its epicenter; 2) determination of the kinematic and seismological parameters of the active fault implied in that earthquake; 3) individuation of its possible coseismic surface rupture. An interesting observation arises also from the recognition of some secondary fractures which, due to their particular location and shape, may have been interpreted as «the footprint s» of Archangel Michael, whose descent was believed to be the cause of the earthquake.