Medieval Farming May Have Set the Stage for Lake Tsunamis in Italy, Study Finds

Tsunamis are typically associated with oceans, but they have also historically appeared in giant lakes near human farms. For a recently published study in Nature Communications, researchers studied how human land use near Lake Iseo in northwestern Italy increased the risk of massive tsunamis in the 12th and 13th centuries. The analysis examined known historical events, as there are records of powerful earthquakes and tsunamis striking the region in 1117 and again in 1222. The team warned there is reason to believe these lake tsunamis may return, not just to Italy, but elsewhere in the world.

According to the study, Lake Iseo has been a longtime hotspot for human agriculture and pastoralism. Since the Roman period, the area has experienced a threefold rise in sediments fed into the lake as a result of human-driven erosion, the paper added, causing abrupt changes in the lake's border with the Oglio River delta. Study co-author Denys Dutykh wrote that the delta grew quickly and its underwater front became steep and overloaded, that it had collapsed once already in the sixth century, and that six centuries of this erosion loaded it again. When the quakes hit, it shook these overloaded areas, sending massive amounts of sediment into the lake's deep basin and pushing the water above it into tsunami waves.

Historical and archaeological records show that two powerful earthquakes struck the Lake Iseo region in 1117 and 1222. Later estimates by modern scientists put the quakes at magnitudes of 6.5 and 6.0, respectively, according to the paper. Dutykh, an applied mathematician at Khalifa University of Science and Technology in the United Arab Emirates, explained that one manuscript reported how, on January 3, 1117, the lakes also rose so much that very few of those living in the villages nearby escaped this danger.

Using sediment cores and seismic surveys, the team measured how much material each collapse moved, then simulated how the mud slid along the lake floor and how the resulting waves surged across the lake and onto land. Dutykh explained that the simulated 1117 waves were about 3.4 miles (5.5 kilometers) long, roughly 22 times the lake's greatest depth of about 820 feet (250 meters). Using these measurements, the team determined that the wave in 1117 would have reached the shores near the delta within a minute and flooded the low plain of the Oglio up to 1.7 miles (2.7 kilometers) inland. Dutykh added that at the town of Iseo, the water rose by about 5.5 meters (18 feet), 6 to 7 minutes after the collapse, and that the smaller collapse in 1222 raised waves of about 13 meters (43 feet).

In his column, Dutykh explained that the pressure on the land briefly eased after 1222, thanks to the Black Death, but spiked again from 1661. The erosion has simultaneously rebuilt the delta, but the team suspects that its underwater slopes are still shaky and vulnerable to landslides caused by earthquakes. Anselmetti said their results indicate that human interventions not only cause short-term environmental changes, but could also create a geological legacy that persists for centuries and amplifies natural hazards. He said there have been records of similar events at lakes in Switzerland, Alaska, and New Zealand, suggesting that this mechanism exists worldwide, and the team advised in the paper that authorities will need to carefully consider the hidden legacy of human land use when assessing natural hazards.