Human-enhanced tsunami hazard in large lakes

IntroductionWorldwide, land use has drastically accelerated erosion1,2,3,4, resulting in increased sediment yield and accelerated delta progradation in downstream lakes5,6,7, which are especially vulnerable because of their steeper slopes, confined basins, and smaller spatial extents. Paleoenvironmental records show that since the Holocene, human activities such as agriculture, pastoralism, urban expansion, deforestation, and burning have been the main drivers of amplified soil erosion globally8,9,10,11,12,13,14,15, increasing sediment fluxes to deltas by up to 100-fold in some regions16. These abrupt and extensive sediment inputs have profoundly reshaped delta morphologies5, overloaded subaquatic slopes, and weakened structural stability, making deltas more prone to collapse under seismic and gravitational stress17,18.Driven by intensified land use, multiple lacustrine delta collapses have been documented globally17,19,20,21,22,23,24,25,26,27,28. However, the role of anthropogenic sedimentation in tsunami hazards remains critically understudied, especially in tectonically active regions, where sustained sediment loading destabilizes deltaic slopes, preconditioning them for catastrophic failure during seismic events that could generate large tsunamis24,29,30,31.In this work, we examine the tsunamigenic consequences of long-term human-driven sediment accumulation in Lake Iseo (60.9 km²), one of the largest lakes in northern Italy. By integrating geophysical, sedimentological, historical, and archeological data with advanced numerical modeling, we reconstruct two medieval delta collapses and assess their tsunamigenic potential. This multidisciplinary investigation highlights how sustained anthropogenic pressure can lead to repeated delta collapses, which are capable of generating devastating tsunamis over relatively short timescales. Our findings underscore the urgent need to incorporate human-driven sediment dynamics into tsunami hazard assessments in densely inhabited lacustrine environments, particularly those in tectonically active regions.ResultsThe human footprint on Lake Iseo’s DeltaLake Iseo (Italy) is located at the downstream end of Val Camonica (Fig. 1), one of the largest valleys of the southern European Alps. The Lake Iseo catchment (1777 km2) is characterized by steep topography and is drained by the Oglio River (Fig. 1), which inflows the lake from the north, supplying substantial sediment that forms a large delta. Extensive human activity in the catchment is well documented by archeological, historical, palynological, and paleoenvironmental studies32,33,34,35,36,37.

Since the Roman period, intensified deforestation, agriculture, and pastoralism have significantly increased hillslope erosion, resulting in a threefold rise in sediment flux to the lake compared with earlier Holocene background levels32.

1: Location and geomorphological context of Lake Iseo.a Location of the study area in the central Southern Alps, northern Italy. b Topographic map showing the Val Camonica watershed (black dashed line), along with the extent of Lake Iseo and adjacent basins, the locations of the main cities, and the epicenters of the 1117 and 1222 CE earthquakes. c The detailed bathymetry of Lake Iseo, showing the location of core SEB18 (core data in Supplementary Fig. 2), some of the seismic survey lines in white A–C (corresponding seismic profiles in Supplementary Fig. 1), the onshore core sites collected in this study, the Oglio River Delta, and the locations of the two medieval delta collapses.The Val Camonica region has experienced moderate to high seismic activity due to the complex collisional and postcollisional phases of the Alpine orogeny38. Several destructive earthquakes have been recorded over the last two millennia39. Notably, two major medieval earthquakes, the 1117 CE Verona earthquake (Mw 6.5)39,40 and the 1222 CE Brescia earthquake (Mw 6.0)39,41, affected the Lake Iseo region, with estimated macroseismic intensities of approximately VI–VII42 and V–VI43, respectively. They triggered massive subaqueous delta collapses in Lake Iseo17 and induced minor slope failure, likely along the escarpment between Monte Isola and the eastern shore44. According to detailed sediment core and seismic reflection analyses, these delta collapses are recorded as the thickest mass-wasting deposits (MWDs, or megaturbidites) in the deep basin of Lake Iseo17. The 1117 CE earthquake produced an event deposit (MWD7 in Supplementary Fig. 1), which reached a maximum thickness of ~7 m and an estimated volume of \(6.2\,\times \,{10}^{7}{m}^{3}\) derived from the interpolated thickness (Fig.

2c). This exceptional event is linked to approximately six centuries of human-driven sediment accumulation at the Oglio River Delta following the last delta collapse in the 6th century (Supplementary Fig. 3)17. After the collapse of the 1117 CE, continued sedimentation from ongoing human activity rebuilt and preconditioned the delta slopes for another collapse, which occurred approximately one century later during the 1222 CE earthquake and produced a somewhat thinner layer (MWD6 in Supplementary Fig.

1) with a maximum thickness of ~2 m and a volume of \(1\times {10}^{7}\,{m}^{3}\) (Fig.

2f).

This rapid rebuilding is supported by a first-order estimate based on the mean deep-basin sedimentation rate (~3 mm yr−1)17 over the ~18 km2 depositional area (below 210 m water depth), which indicates that ~5.7 Mm³ of sediment accumulated between the 1117 and 1222 CE events.

These two events illustrate how increases in sediment supply from human activities may amplify the likelihood of delta collapses in seismically active regions.Fig. 2: Landslide modeling and its validation with geological records for the 1117 CE and 1222 CE events.a, d Initial reconstructed landslide volumes. b, e Simulated landslide thickness evolution over time. c, f Comparisons between modeled deposit thicknesses and those interpolated from seismic profiles are shown for the a–c 1117 CE event and d–f 1222 CE event. Background satellite imagery in (a, d): Google ©2026 CNES/Airbus, Maxar Technologies.Reconstruction of catastrophic tsunami eventsTo assess the tsunami hazards associated with both medieval delta collapses, we reconstructed the landslides and resulting tsunamis by quantifying the event deposits and conducting high-resolution numerical modeling. The reconstruction was based on high-resolution LiDAR topographic and multibeam bathymetric datasets acquired in 2002 by Regione Lombardia and the Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS) within the Italian National Geological Mapping Program (CARG)45,46.

Landslide source volumes were then estimated as the difference between the reconstructed post-failure surface and the modern bathymetric surface. The reconstruction was iteratively refined until the estimated source volumes matched the volumes of the mapped mass-wasting deposits identified from seismic reflection profiles and sediment-core data in the deep basin, yielding source volumes of 6.2 × 107 m3 for the 1117 CE event and 1.0 × 107 m3 for the 1222 CE event (Fig.

2a, d).

We then simulated landslide dynamics using a viscoplastic rheological model assuming a single-event failure mechanism.

This unit is overlain by thick homogeneous deposits interpreted as suspension settling associated with basin-wide seiche oscillations47, with an upper fine-clay interval representing the final stage of suspension settling.Details of the modeling approach and parameter selection are provided in the Methods section.

The temporal evolution of landslide thickness, from initiation to final deposition in the deep basin, is shown in Fig.

2b, e. Both landslides originated at the Oglio Delta, but their final depositional geometries differ slightly. During the 1222 CE event, a greater portion of the mass propagated farther into the basin compared to the 1117 CE event, likely due to greater accommodation space, as the 1117 CE deposits had already filled the northeastern deep basin with substantial sediment a century earlier.

Moreover, the 1222 CE landslide had relatively high water content and a large proportion of coarse sediment (Supplementary Fig.