Medieval Deforestation Set Up Italian Lake Tsunamis With Waves Over 50 Meters

Two huge tsunamis that struck Lake Iseo in northern Italy in the Middle Ages were partly the result of human activity, according to a new study in Nature Communications. Computer simulations suggest waves in the northern part of the lake exceeded 50 meters after an earthquake in 1117, as reported by ScienceAlert.

The researchers warn that the shores of the lake are now far more crowded, and that sediment has been piling up underwater again for more than eight centuries.

How cleared forests made the lake floor unstable

An international team combined sediment cores, seismic profiles, archaeological records, a medieval manuscript and data from the modern lake. They used the information to rebuild the events in computer models.

Lake Iseo sits downstream of the Italian Alps, which feed it with snowmelt. The tectonic activity that built the Alps also brings occasional earthquakes to the area.

Before the Middle Ages, forest surrounded the delta of the Oglio River, where it enters the lake. Dense networks of tree roots and fungi held soil in place and slowed the flow of loose sediment downhill. Underwater, the delta's slopes were gentle and tilted gradually toward the deepest point of the lake, which is around 250 meters.

That changed around the fall of the Roman Empire. In the following centuries, forests were cleared for pasture and mining and the human population grew. On average, three times more sediment washed into the lake each year, according to the report. The sediment built up at the northern end and made the underwater banks steep and prone to collapse.

What the simulations show about the two waves

When the Verona earthquake struck in 1117, the steepened banks gave way in an underwater landslide. According to the researchers' modeling, the displaced water would have flooded large stretches of shoreline with less than a minute of warning. Waves in the northern part of the lake would have exceeded 50 meters, and water could have reached up to 2.7 kilometers inland.

A 12th-century manuscript now held at the Vatican Library describes the disaster in a way that matches the modeling well, ScienceAlert reported. It says the lake overflowed so badly that very few people in nearby villages escaped, and that many castles and fortresses collapsed with their inhabitants inside.

The area was rebuilt and the same land-use practices resumed, according to the report. Just over a century later, the Brescia earthquake of 1222 triggered a second tsunami. The simulations put its peak waves at around 13 meters, with water reaching up to 1 kilometer inland.

Monte Isola, the island in the middle of the lake, acted as a natural barrier in both events. It split the waves and partly shielded the towns behind it. The town of Iseo was still hit, with maximum wave heights of about 5.5 meters in 1117 and about 2 meters in 1222, arriving about 7 minutes after the landslides. The study's authors describe the 1117 event as one of the largest landslide-triggered lake tsunamis ever inferred. The waves are model-based reconstructions, not direct measurements.

Why it matters for lakeshores today

Lake Iseo now has the most densely built shores of any lake in the Italian Alps, the report said. Sediment has also been accumulating for more than eight centuries since the 1222 collapse. The authors write that a tsunami of similar size would cause "severe devastation" today because of the dense population and development along the shore.

The warning reaches beyond one lake. The authors point to a global synthesis of lake studies showing that human-driven erosion has reached its highest level since the Industrial Era and is progressively overloading deltas around the world. On that basis, they say similar catastrophic failures may become more likely in the future. That is the authors' assessment rather than a forecast for any specific lake.

The findings rest on modeling informed by geological and historical evidence. They show how changes in land use can make a lake more vulnerable when an earthquake strikes, especially where people live close to the water.

Around Prague, based on reporting by ScienceAlert