STUDY LINKS MEDIEVAL FARMING TO LAKE ISEO TSUNAMIS

Centuries of farming and deforestation partly caused two medieval tsunamis that struck Lake Iseo in Lombardy, Italy, a study in Nature Communications has found. Researchers used sediment cores, seismic profiles, archaeological records and a 12th-century Vatican manuscript to reconstruct the disasters. Earthquakes on 3 January 1117 and Christmas Day 1222 dislodged sediment built up at the mouth of the River Oglio, shifting about 62 million cubic metres in the first quake and 10 million in the second. The Vatican manuscript records: "The lake rose so much that very few of those living in the villages nearby escaped this danger."

Lake Iseo sits at the base of Val Camonica in the southern Alps and receives Alpine snowmelt. Before the Middle Ages, forests around the Oglio delta held sediment in place, and after the fall of the Roman Empire, forest clearance for pasture and mining roughly tripled the sediment reaching the lake, researchers estimate. That sediment built up at the lake's northern end, forming steep banks that researchers say are primed to collapse under seismic stress. Simulations suggest the 1117 wave reached about 50 metres and flooded the plain up to 2.7km inland, while the 1222 quake produced waves of about 13 metres.

Researchers warn that similar sediment build-up could still trigger a comparable tsunami on a European lake. Lake Iseo remains one of the most densely built lakes in the Alps, and a future sediment collapse at the delta could send a large wave toward its northern shore, researchers say. Pressure on the land eased for centuries after 1222, including during the Black Death, before rising again by the industrial era. An international team of scientists carried out the research.