Strong earthquake in a low seismicity area of the European Southern Alps during Roman Times – A lacustrine paleoseismic evaluation

Hazard assessment of moderate to strong earthquakes (moment magnitude, M-W > 5.5) in slowly deforming regions, such as the European Southern Alps, is hindered by recurrence intervals that exceed the timeframe covered by historical records and a lack of paleoseismic data revealing earthquake source parameters. Estimations of the maximum possible magnitude (M-max) in a region are often solely based on theoretical models and lack validation by long paleoseismic records. A rare example of a potential M-max event is formed by a coseismic surface rupture impacting a Roman age building in Egna/Neumarkt (South Tyrol, Italy) which occurred during the 3rd century CE, as proposed by an archaeoseismological study. Our study tests this hypothesis by examining the sedimentary record of Kleiner Montiggler See/Piccolo Lago di Monticolo, a lake located only 10 km from the Egna site, for evidence of seismic shaking. A multiproxy analysis of sediment cores allowed us to disentangle different depositional processes and to identify an event layer, which we interpret to be caused by a seismic seiche. Using a radiocarbon-based age-depth model, the event was dated to similar to 222 years CE (8-450 years CE, 95 % probability range), coinciding with the identified fault offset, and therefore supporting the hypothesis of a Roman Age surface rupturing earthquake in Egna. We constrain the minimum local intensity of this earthquake by considering the sedimentary characteristics of the event layer in Kleiner Montiggler See, the post-seismic response of the lake system, and the absence of lacustrine sedimentary evidence for other strong historical earthquakes in the broader region. We evaluate different potential magnitude scenarios for the Egna earthquake by using ground motion modelling and comparing the results with paleoseismic records from nearby lakes. This allows us to infer that an M-W of 6.5 is the most likely scenario for this event, which is consistent and slightly below the M-max estimates for the study area. This empirical evidence for a strong earthquake close to the considered M-max within its tectonic domain highlights the potential hazard posed by unrecognized seismogenic structures in regions with apparent low seismic activity. This emphasizes the need for long paleoseismic records in such regions to refine hazard assessment of strong earthquakes.