The footprint of a historical paleoearthquake: the sixth-century-CE event in the European western Southern Alps

Abstract. Low-deformation regions are characterized by long earthquake recurrence intervals. Here, it is fundamental to extend back the record of past events as much as possible to properly assess seismic hazards. Evidence from single sites or proxies may be not compelling, whereas we obtain a more substantial picture from the integration of paleo- and archeoseismic evidence at multiple sites, eventually supplemented with historical chronicles. In the city of Como (N Italy), we perform stratigraphic and sedimentological analyses on the sedimentary sequences at Via Manzoni and we document earthquake archeological effects at the Roman baths by means of structure from motion and field surveys. Radiocarbon dating and chronological constraints from the archeological site allow us to bracket the time of occurrence of the deformations to the sixth century CE. We interpret the observed deformations as due to earthquake ground shaking and provide constraints on the lower threshold for the triggering of such evidence. We move toward a regional view to infer possible relevant seismic sources by exploiting a dataset of published paleoseismic evidence in Swiss and N Italy lakes. We perform an inverse grid search to identify the 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 an earthquake (minimum Mw 6.32) with epicenter located at the border between Italy and Switzerland may account for all the observed effects; a similar event in the sixth century CE has not been documented so far by historical sources. 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 an earthquake size similar to the one inferred for the sixth-century-CE event.