Medieval Lake Tsunamis Reveal a Hidden Human-Made Hazard Lurking Beneath Alpine Shores

Eight centuries ago, the waters of Lake Iseo in northern Italy rose without warning and smashed into its shores with waves taller than a five-story building. New research published in Nature Communications shows that these medieval tsunamis were not purely natural disasters. They were amplified by something far more insidious: two thousand years of human land use that loaded the lake’s delta with sediment until it became a loaded gun, primed to fire the moment an earthquake struck.

The study, led by Muhammad Naveed Zafar of Université Savoie Mont Blanc and colleagues across France, Italy, Switzerland, and the United Arab Emirates, reconstructs two catastrophic delta collapses in Lake Iseo with unprecedented detail. By combining seismic reflection profiles, sediment cores, historical manuscripts from the Vatican Library, archaeological evidence, and state-of-the-art numerical modeling, the team demonstrated that earthquakes in 1117 and 1222 CE triggered massive subaqueous landslides at the Oglio River Delta, generating tsunamis with maximum run-ups of roughly 50 meters and 13 meters respectively. These are among the largest landslide-induced tsunamis ever inferred in any lake setting worldwide.

The mechanism behind this hazard begins on the hillsides of Val Camonica, one of the largest valleys in the southern European Alps. Lake Iseo sits at the downstream end of this catchment, which drains 1,777 square kilometers of steep terrain through the Oglio River. Since the Roman period, intensified deforestation, agriculture, and pastoralism have dramatically accelerated hillslope erosion. Paleoenvironmental records show that sediment flux to the lake tripled compared with earlier Holocene background levels. All that extra sediment built up and steepened the delta at the lake’s northern margin, overloading the subaqueous slopes and making them increasingly susceptible to collapse during seismic shaking.

The geological evidence for what happened next is preserved in the deep basin of Lake Iseo as thick mass-wasting deposits, sometimes called megaturbidites. The 1117 CE Verona earthquake, magnitude 6.5, produced a deposit reaching a maximum thickness of about 7 meters and an estimated volume of 62 million cubic meters. This exceptional event represented roughly six centuries of accumulated human-driven sediment that had piled up at the delta front since the previous collapse in the sixth century. Just over a century later, the 1222 CE Brescia earthquake, magnitude 6.0, triggered a second collapse that left a thinner deposit about 2 meters thick with a volume of 10 million cubic meters.

What makes the second event particularly striking is how quickly the delta rebuilt itself. Using a mean deep-basin sedimentation rate of about 3 millimeters per year across an 18-square-kilometer depositional area, the researchers estimate that roughly 5.7 million cubic meters of sediment accumulated between the two earthquakes. That figure is conservative, because it only counts fine-grained material preserved in the deep basin and excludes coarser sediment stored at the delta front where the failure actually originated. The accommodation space carved out by the 1117 collapse likely enhanced sediment trapping, accelerating the rebuild. In other words, the delta had barely recovered before it was loaded and primed to fail again.

To translate these geological deposits into wave heights and inundation distances, the team employed a two-stage numerical approach. First, they simulated the landslide dynamics using BingClaw, a two-layer depth-averaged viscoplastic debris-flow model based on Herschel-Bulkley rheology, specifically designed for cohesive subaqueous landslides. The model represents the failed mass as an upper plug layer moving en bloc and a lower shear layer where deformation occurs once applied stress exceeds the yield strength. Yield strength values were constrained by observational runout distances from seismic profiles, with sediment density and water content taken directly from core data. The 1117 slide used a yield strength of 25 pascals and a bulk density of 1,600 kilograms per cubic meter, while the 1222 slide, being wetter and coarser, used a lower yield strength of 7 pascals.

Second, the time-dependent bathymetric changes from the landslide simulation were fed into GeoClaw, a nonlinear shallow-water equation model running on a 10-meter grid. The results are sobering. The 1117 event produced wave heights exceeding 50 meters in the northern part of the lake, inundating nearby shores within just one minute and propagating up to 2.7 kilometers inland. The 1222 event generated waves peaking at about 13 meters with inundation extending roughly 1 kilometer inland. Monte Isola, the large central island and the largest inhabited lake island in Europe, acted as a natural barrier that split the incoming waves and partially shielded cities behind it. Even so, the town of Iseo still experienced maximum wave heights of about 5.5 meters in 1117 and 2 meters in 1222, arriving roughly 7 minutes after the landslides began.

The numerical reconstructions find powerful corroboration in written history. A twelfth-century manuscript preserved in the Vatican Library describes the 1117 event in vivid terms, stating on folio 184r that the lake overflowed so much that very few inhabitants of nearby villages escaped the danger, and that many castles and fortresses collapsed miserably with the destruction of their inhabitants. Archaeological evidence adds further constraints. The Pieve church of Sant’Andrea d’Iseo underwent major reconstruction shortly after 1117, and the simulations indicate the tsunami reached the shoreline in front of the church with a maximum wave height of about 3.5 meters and an inundation distance of 165 meters, sufficient to flood the church area. Extensive structural damage documented around the lake shore following the 1222 earthquake supports the second tsunami as well.

The implications extend far beyond one Italian lake. A global synthesis of 632 lake-watershed records shows that human-driven soil erosion has now reached its highest level since the Industrial Era, progressively overloading deltas worldwide. Thick megaturbidites in the deep basins of nearby Lakes Garda and Como suggest those steep, sediment-overloaded slopes also collapsed during medieval seismic activity. Further afield, Lake Geneva, Lake Sils, Lake Brienz, and Lake Lucerne in the Alps have all experienced recurrent tsunamigenic delta collapses during periods of intensified erosion. Outside Europe, lakes Eklutna, Skilak, and Kenai in south-central Alaska suffered large tsunamigenic delta collapses during the 1964 megathrust earthquake, and delta collapses have been identified as the primary source of repeated lake tsunamis in New Zealand.

Today, the shores of Lake Iseo are among the most densely developed in the Italian Alps, and the large perialpine lakes of northern Italy together host more than 370,000 permanent residents and attract tens of millions of visitors annually. The researchers emphasize that warning times for near-field locations would be negligible, with waves reaching the northern shore in less than a minute, leaving no realistic window for evacuation even with an early warning system. The team argues that tsunami hazard assessments in densely inhabited lacustrine environments must now incorporate human-driven sediment dynamics, particularly in tectonically active regions. The hidden legacy of land use, they conclude, means that today’s land management decisions will shape the disasters of tomorrow, and communities around Lake Iseo and similar settings worldwide face a low-probability but high-consequence threat that has been quietly building for two millennia.

Subject of Research: Human-accelerated erosion of lake deltas increasing earthquake-triggered tsunami hazard, reconstructed from Lake Iseo sediment records and numerical modeling

Article Title: Human-enhanced tsunami hazard in large lakes

Article References: Zafar, M. N., Sabatier, P., Dutykh, D., Rapuc, W., Paris, R., Piccin, A., Vuillemin, P., & Anselmetti, F. S. (2026). Human-enhanced tsunami hazard in large lakes. Nature Communications, 17(1), Article 10531. https://doi.org/10.1038/s41467-026-78272-8

Image Credits: AI Generated

DOI: 10.1038/s41467-026-78272-8

Keywords: lake tsunamis, delta collapse, subaqueous landslides, Lake Iseo, soil erosion, land use, paleoseismology, numerical modeling, Alps, natural hazards, sediment flux, hazard assessment