Two earthquakes, in 1117 and 1222, each triggered a tsunami on Lake Iseo in northern Italy, and a new Nature Communications study concludes that centuries of human land use made the waves far worse than the shaking alone would have. Modeling by an international team indicates the 1117 wave exceeded 50 meters near the lake’s northern shore. The authors warn that a comparable event today would cause severe devastation along a densely built shoreline.
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The research combines sediment cores, seismic profiles, computer simulations, archaeological records and a medieval manuscript. It argues that the lake’s hazard cannot be read from faults and quakes alone, because the sediment that people sent into the water loaded the steep underwater slope that gave way.
Farming, mining and a delta that grew too steep
ScienceAlert’s account of the study, published on Nov. 4, says deforestation for pasture and mining, together with population growth, sent about three times more sediment into Lake Iseo each year over roughly two thousand years. The material piled up at the northern end of the lake, where the Oglio River enters, and built steep underwater banks. The lake is fed by Alpine snowmelt and reaches a depth of about 250 meters, which gave a collapsing slope a deep basin to fall into.
University of Bern geologist Flavio Anselmetti, a co-author, says Lake Iseo “offers a unique natural archive,” in remarks carried by Gizmodo. He describes the approach as joining geological traces with historical accounts to understand how the catastrophes unfolded.
1117 and 1222: two earthquakes, two collapses
The Verona earthquake of Jan. 3, 1117, which damaged cities across northern Italy, shook the overloaded delta and brought down its banks. The resulting landslide pushed water outward in a wave that, in the simulation, exceeded 50 meters in the north and reached up to 2.7 kilometers inland within about a minute. Gizmodo reports the quake’s magnitude at about 6.5; the Wikipedia entry gives a range of 6.5 to 7.0.
Gizmodo adds two scale details from the simulation. The 1117 wave was about 5.5 kilometers long, roughly 22 times the lake’s greatest depth, and the delta had already failed once before, in the sixth century. Each collapse moved a measurable volume of sediment, which the team estimated from cores and seismic surveys rather than from the historical record alone.
The Brescia earthquake of Dec. 25, 1222 produced the second event. That wave peaked at about 13 meters and reached up to 1 kilometer inland, smaller than the first but still catastrophic for the lakeside settlements of its day. The team puts the magnitude near 6.0, according to Gizmodo’s summary.
A Vatican manuscript and the cores that back the simulations
A 12th-century manuscript held in the Vatican Library describes the 1117 disaster in a way that matches the models, and the authors translate one line from it: “Many castles and fortresses also collapsed miserably.” Applied mathematician Denys Dutykh of Khalifa University in the United Arab Emirates, another co-author, told Gizmodo that judging the danger of a lake means reading its history as well as its faults. He cited a further passage saying the lakes rose so much that very few of those living in nearby villages escaped.
The paper builds on earlier fieldwork. A 2022 study in the journal Sedimentology, with Anselmetti among its authors, analyzed a 15.5-meter core taken from the deep basin in 2018. It found that event deposits make up about 61% of the sediment laid down over 2,000 years, and that the thickest mass-wasting layers line up with regional earthquakes around 700, 1117 and 1222. Periods of heavy sediment remobilization followed increased erosion in the watershed, driven mainly by human activity.
Monte Isola, today’s shoreline and the warning to planners
The lake’s central island, Monte Isola, acts as a natural barrier in the simulations, splitting the waves and partly shielding nearby cities. Iseo town still saw modeled maximum wave heights of about 5.5 meters in 1117 and about 2 meters in 1222, arriving roughly 7 minutes after the landslides. The shielding is partial: the modeled wave of more than 50 meters struck the northern shore, where the delta sits. The warning comes from what has happened since. Sediment has built up in the delta since 1222, the researchers say, and the underwater slopes remain vulnerable to earthquake-triggered landslides. They also note that global erosion is at its highest since the Industrial Era, which could make similar delta failures more likely at other lakes, including in Switzerland, Alaska and New Zealand.
The authors do not advise anyone to leave the lakeshore. Their recommendation, as reported by Gizmodo, is that hazard assessments weigh the long-term legacy of land use and sediment accumulation alongside fault activity. The full paper is available through its Nature Communications DOI, and the open question it leaves is how much sediment the Oglio delta holds today compared with 1117.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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