On January 3, 1117, a powerful earthquake shook northern Italy. Just 100 years later, in 1222, there was another earthquake. We know of 1117 thanks to a 12th-century Latin manuscript preserved in the Vatican Apostolic Library, which states “Lacus quoque adeo excreverunt ut in villis iuxta positis inhabitants paucissimi ab hoc periculo evaderent”, with few towns growing around lakes. To avoid danger. Come a kind of tsunami.
It can be assumed that earthquakes are responsible for both tsunamis, but it was not the only cause: a recent study maintains that humans also did their work.
what happened. We are talking about Lake Ico in northern Italy and an international research team has been able to reconstruct these two medieval tsunamis to reveal their causes. This was not a direct movement of the fault, but a collapse of the Oglio river delta under water. Simply put: the delta was poised to collapse from erosion due to human activity.
Why is it important?. Because the study maintains that tsunami risk in lakes depends not only on earthquakes, but also on what we do with the land, such as deforestation, farming or grazing, yesterday’s and today’s activities that increase erosion. Especially in the ICO, these activities have intensified since Roman times, resulting in sediment contributions to the lake tripling over previous background levels.
And they are made today, so the risk is still valid. A fact: currently, according to the population data of the Italian National Institute of Statistics, there are more than 370,000 permanent residents on the shores of Lake Garda, Como and Ico, and without counting the millions of tourists they attract.
Landslide modeling and its validation with geological records for the 1117 AD event. c. and 1222 AD. C., from the paper “Human-Enhanced Tsunami Hazards in the Great Lakes”.
Waves up to 50 meters. According to their reconstruction, for which they used sediment surveys, seismic profiles and numerical simulations and which contrasted with the aforementioned Guido Pisano manuscript, the 1117 earthquake (magnitude 6.5) collapsed about 62 million meters del meter. That one is 1222 (magnitude 6), the other is about 10 million. As a result, the first tsunami generated waves of more than 50 meters in the northern part of the lake, and the second, more than 13 meters, with landfalls of about 2.5 and 1 km, respectively. And be careful, because according to their calculations the waves reached the north shore in less than a minute.
That a collapse occurred again only a century later suggests that the delta was quickly rebuilt. The team estimates that between both events the amount of deep-bottom sediment was at least 5.7 million cubic meters.
It was not the first. It will (probably) never end.. This process of collapsing lake deltas is not exclusive to the Alps, indeed tsunamis have been documented in others such as in Lake Geneva in 563, in Lucerne in 1601 or, more recently, in Lake Alaska in 1964 or in Brienz in 1996.
And be careful, because a synthesis of 632 lake records worldwide shows something that stops all alarm: human-source erosion today is at its highest level since the industrial age. The “good” news is that research suggests that such events are a low-probability, high-consequence event. Rarely happens, but when it does it destroys even castles, as detailed in the Pisano Manuscript.
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cover Chris Weiher
