Historical lake tsunamis in Italy highlight ongoing risk

On January 3, 1117 AD, a significant earthquake struck northern Italy, triggering waves that reached up to 50 meters (164 feet) in height on Lake Iseo. A 12th-century manuscript housed in the Vatican documented the event, stating, “The lake rose so much that very few of those living in the villages nearby escaped this danger.”

Located at the base of Val Camonica and fed by the Oglio River, Lake Iseo has experienced centuries of erosion due to farming, deforestation, and grazing since Roman times. This has resulted in a threefold increase in sediment reaching the lake, creating a steep and unstable underwater slope.

Geological evidence indicates two earthquakes triggered these events: the first in 1117 AD and another on Christmas Day in 1222 AD. The 1117 earthquake displaced approximately 62 million cubic meters of sediment, while the 1222 event moved around 10 million cubic meters.

Computer models simulating the 1117 event suggest the tsunami wave stretched 5.5 kilometers (3.4 miles) – 22 times the lake’s greatest depth of 250 meters (820 feet). The wave reportedly reached the lake’s northern shores within a minute, surging up to 50 meters (164 feet) high and flooding low-lying areas up to 2.7 kilometers (1.7 miles) inland. The 1222 event generated waves approximately 13 meters (43 feet) high.

While the risk lessened for several centuries, including during the Black Death, sediment buildup increased again from the 17th century onward, peaking during the industrial era. Lake Iseo is now heavily developed, and another land collapse could generate a large wave impacting its northern shores in under a minute, leaving little time for warnings.

Researchers also note that Lakes Garda and Como are experiencing similar sediment accumulation and slope instability, putting over 370,000 people at risk. The study emphasizes the need for proactive hazard assessment, stating, “These findings reveal a persistent, human-amplified mechanism of lacustrine tsunami generation and intensity.”