Two earthquakes in northern Italy, in 1117 and 1222, triggered unusually large waves in Lake Iseo. An international research team has reconstructed both events and concluded that the earthquakes did not cause the tsunamis through fault movement alone. Instead, they destabilized and collapsed part of the Oglio River delta beneath the lake.
The first event is described in a 12th-century Latin manuscript preserved in the Vatican Apostolic Library. Its account says that the lake rose so dramatically that very few residents of nearby villages escaped the danger. The researchers compared this historical testimony with sediment cores, seismic profiles and numerical simulations.
According to their reconstruction, the magnitude-6.5 earthquake of 1117 caused approximately 62 million cubic meters of the delta to collapse. The magnitude-6 earthquake in 1222 triggered a second collapse involving around 10 million cubic meters. The resulting waves reached more than 50 meters in the northern part of the lake during the first event and about 13 meters during the second. They extended inland roughly 2.5 kilometers and 1 kilometer, respectively, and reached the northern shore in less than a minute.
The study links the delta’s instability to erosion accelerated by human activity. Deforestation, farming and grazing intensified around Lake Iseo from the Roman period onward, tripling the amount of sediment entering the lake compared with the earlier background level. That sediment accumulated in the delta, leaving it vulnerable to collapse when earthquakes struck.
The delta also recovered relatively quickly between the two disasters. The researchers estimate that at least 5.7 million cubic meters of sediment accumulated in the deep lake basin between the events, helping explain why another collapse was possible just over a century later.
The findings suggest that tsunami risk in lakes depends not only on seismic activity but also on how people manage surrounding land. Similar lake tsunamis have been documented in Lake Geneva in 563, Lake Lucerne in 1601, and in Alaska’s lakes in 1964 and Lake Brienz in 1996.
The risk remains relevant around the Alpine lakes. The shores of Lakes Garda, Como and Iseo are home to more than 370,000 permanent residents, excluding the millions of tourists they attract. A global review of 632 lake records also indicates that human-caused erosion is currently at its highest level since the industrial era. The researchers characterize such events as low-probability, high-consequence disasters: they are rare, but their impact can be severe enough to destroy structures, including castles described in the medieval account.
Sources: Xataka.