A new study (Vega et al. 2026) shows that patterns of reported structural damage in Medellin are probably caused by deep-seated deformation driven by a series of ancient landslides under the city.
Medellin is the second largest metropolitan area in Colombia, with a population of around 4 million people. It has grown rapidly, expanding into the surrounding hillsides, with many unplanned and informal communities on steep slopes. Landslides are a common problem.
The rapid rate of growth
A new study (Vega et al. 2026) shows that patterns of reported structural damage in Medellin are probably caused by deep-seated deformation driven by a series of ancient landslides under the city.
Medellin is the second largest metropolitan area in Colombia, with a population of around 4 million people. It has grown rapidly, expanding into the surrounding hillsides, with many unplanned and informal communities on steep slopes. Landslides are a common problem.
The rapid rate of growth has been accompanied with many reports of structural failures in buildings, with a general (and not unreasonable) assumption that these are associated with poor construction quality. But a fascinating new study (Vega et al. 2026) in the journal Landslides challenges this assumption in a most interesting way. The headline from the study is that there is a strong correlation between areas that have a high density of reports of structural damage and ground deformation driven by large, deep-seated landslides.
Vega et al. (2026) have used InSAR to map ongoing displacements across Medellin. In three key areas (Doce de Octubre, Manrique and Villa Hermosa) they detected high rates of ground deformation. They were able to show that these areas correspond to mappable deep-seated landslides. An example is in the Manrique neighbourhood of Medellin:-
Google Earth image of the Manrique neighbourhood of Medellin in Colombia.
Interpretation of pre-urbanisation imagery suggests that the topography underlying Manrique includes a series of deep-seated landslides. The InSAR data indicate that these areas are actively deforming, and these deformation zones correspond to areas with a high density of reports of structural damage. Interestingly, the density of damage reports does not correlate with the style of construction of the buildings.
Vega et al. (2026) also note that many of the recent acute landslide events in recent years also lie within these areas of high underlying ground deformation. Fore example, the 24 June 2025 landslide that killed 27 people lies within an area highlighted by the InSAR analysis.
This study highlights two key things for me. First, it is a novel and interesting application of InSAR in an urban setting, allowing the underlying processes that are driving structural damage in the city to be understood. Second, the study highlights the underlying vulnerability of Medellin to deep-seated slope processes. As the climate continues to change, and human processes modify the landscape and the groundwater, management of these slopes would seem to be a high priority.
In the early morning of 10 August 2025, a mountainside collapsed into the waters of Tracy Arm Fjord in southeastern Alaska.
This massive landslide produced a tsunami that reached 481 meters on the opposite side of the fjord—higher than all but the world’s 14 tallest buildings—and registered on seismic detectors around the globe. For days after the slope collapsed, the waters of the fjord churned with a standing wave known as a seiche.
This drone video shows a man paddling throu
This massive landslide produced a tsunami that reached 481 meters on the opposite side of the fjord—higher than all but the world’s 14 tallest buildings—and registered on seismic detectors around the globe. For days after the slope collapsed, the waters of the fjord churned with a standing wave known as a seiche.
This drone video shows a man paddling through the iceberg-filled Tracy arm fjord in the aftermath of a landslide. Credit: Bill Billmeier
This event was the second-largest tsunami ever recorded and the largest not linked to an earthquake. A new paper published in Science presented strong evidence that the Tracy Arm landslide was instead the result of the rapid retreat of South Sawyer Glacier, itself a consequence of global climate change.
“It’s like if you have a kid and they said they cleaned their room but really all they did was throw everything in the closet. As soon as you open that door, everything falls out.”
Nobody was harmed by the rockslide or tsunami, but cruise ships were scheduled to visit the fjord later that morning. If the collapse had happened just a few hours later, it could have been disastrous.
“While the [South Sawyer] Glacier is in the fjord, it’s supporting those valley walls, like the buttresses on a cathedral,” said Daniel Shugar, a geomorphologist at the University of Calgary who led the study. “As that glacier retreated over the last few decades, it retreated just past the spot that did fail. It’s like if you have a kid and they said they cleaned their room but really all they did was throw everything in the closet. As soon as you open that door, everything falls out.”
This animation shows an overhead view of the 10 August 2025 Tracy Arm landslide. Credit: Patrick Lynett, University of Southern California
In other words, the glacier that carved the fjord in the first place was also holding its slopes in place, and the ice’s retreat under warming temperatures exposed rock that became vulnerable to crumbling. The proximate cause of the landslide might have been something else—as Shugar noted, rainfall is plentiful in that part of Alaska, which could have weakened the fjord’s walls further—but it might also have been a combination of small, individually insignificant factors. In any case, the removal of that glacial “closet door” was what made the collapse and tsunami possible.
“We know that steep slopes are very sensitive to the things that climate [change] is exacerbating, whether it’s losing permafrost, glacier retreating, or more water in the soil,” said glaciologist Leigh Stearns of the University of Pennsylvania, who was not involved with the Tracy Arm study. “Often, we think of glacier retreat as a long and continuous thing, but [it] can trigger sudden catastrophic events.”
This aerial photo shows the highest run-up resulting from the 10 August 2025 landslide-triggered tsunami in Tracy Arm. It was captured during a U.S. Geological Survey field reconnaissance overflight on 13 August 2025. Credit: John Lyons/U.S. Geological Survey.
The Tracy Arm tsunami, like the record-setting Lituya Bay 524-meter megatsunami in 1958, was so dramatic in part because it happened in a fjord. The steep sides of the relatively narrow channel concentrated the energy generated by the rockfall into water.
A drone video shows the tsunami-affected part of the fjord, including the highest run-up area and the landslide itself. Credit: Bill Billmeier
Unlike Lituya Bay, which resulted from an earthquake, Tracy Arm provided very little seismic warning before the slope collapsed, requiring forensic work to determine what caused it.
Shugar noted that South Sawyer Glacier had retreated by roughly 500 meters in the spring of 2025 alone, on top of the general trend of shrinking and thinning over the decades. And it’s not alone: Interferometric synthetic aperture radar (InSAR) images taken by satellites indicate that many slopes in Alaska and beyond are in motion, pointing to potential future danger.
“Not every single one, but it seems like a huge majority of [shifting slopes] are above the lower parts of thinning glaciers,” Shugar said. He described this phenomenon as “debuttressing,” as in losing the glacial buttress holding a slope up. He added, “I think in the next 5 years or so, we’ll probably have a much better understanding of just how and how quickly slopes respond to that debuttressing.”
Threats, Hazards, and Climate Change
“We were unbelievably lucky that the [tsunami] occurred with the timing that it did, and not 5 hours later.”
Most tsunamis are set in motion by earthquakes and travel across the open ocean, wreaking their destruction when they reach shallower water near coasts; the word “tsunami” means “harbor wave” in Japanese. The Tracy Arm tsunami joined the ranks of other landslide-driven tsunamis, like the ones in Taan Fiord (Alaska) and Dixon Fjord (Greenland), in being linked to human-driven climate change. Beyond the immediate impact of the waves, this category of hazard requires rethinking potential risks from abrupt catastrophes like debuttressing as well as slower effects such as sea level rise.
“The risk to any particular cruise ship [from a tsunami] on any particular day is very low,” Shugar said. “We were unbelievably lucky that the [tsunami] occurred with the timing that it did, and not 5 hours later. The risk certainly still could be increasing as we build new settlements, new mining camps, or new oil and gas infrastructure.”
Both Shugar and Stearns highlighted the importance of learning lessons from Tracy Arm and related events.
“Climate is a threat multiplier, and the research is really forcing us to look at these cascading hazards,” Stearns said. Tracy Arm “is one example of this: Small slow changes can trigger big events. Hopefully, we don’t need so many disasters to spur some change.”
A drone video shows Sawyer Island in the Tracy Arm Fjord and evidence of the tsunami on the fjord walls. Credit: Bill Billmeier
This aerial photo shows the north side of Alaska’s Tracy Arm Fjord in the aftermath of the 2025 landslide and tsunami. The lighter-colored rock is the exposed surface, where the mountainside collapsed and fell into the water. The foot of South Sawyer Glacier is visible at lower right; in decades past, the ice extended much farther and was thick enough to hold the rock slopes in place. Credit: Cyrus Read/U.S. Geological Survey
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