The footprint of an ancient forgotten earthquake: a VI Cent. A.D. event in the European Western Southern Alps.

Low-deforming regions are characterized by long earthquake recurrence intervals; thus, it is fundamental to extend back as much as possible the record of past events. Evidence from single sites or proxies may be not compelling, whereas a more substantial picture may be obtained from the integration of paleo- and archaeo-seismic evidence at multiple sites, eventually supplemented with historical chronicles. Here, we document deformations observed in a stratigraphic sequence (i.e., Via Manzoni Site) and in an archaeological site (Roman Baths) in the city of Como (N. Italy). We perform stratigraphic and sedimentological analyses on the sedimentary sequences at via Manzoni and we document Earthquake Archaeological Effects at the Roman Baths by means of Structure from Motion and field surveys. We interpret the observed deformations as due to earthquake ground shaking and provide constraints on the lower threshold for the triggering of such evidence. Radiocarbon datings and chronological constraints from the archaeological site allow to bracket the time of occurrence of the deformations at the VI century AD. We move toward a more regional view to infer possible seismogenic sources by exploiting a dataset of published paleoseismic evidence in Swiss and N. Italy lakes. We perform an inverse grid search to identify magnitude and location of an earthquake that can explain all the positive and negative evidence consistent with the time interval of the event dated at Como. Our results show that a so far undocumented earthquake (minimum Mw 6.32) with epicenter located at the border between Italy and Switzerland may account for all the observed effects. Our study calls for the need to refine the characterization of the local seismic hazard, especially considering that this region seems unprepared to face the effects of a potential earthquake similar to the VI century AD one. [ABSTRACT FROM AUTHOR]