Worldwide, human-induced erosion enhances sediment delivery to lake deltas, increasing their susceptibility to collapse. In Lake Iseo, northern Italy, two millennia of intensified land use tripled sediment flux, oversteepening the delta and preconditioning it for failure. Here we show from geological records and numerical simulations that earthquakes in 1117 CE (Mw 6.5) and 1222 CE (Mw 6.0) ...