A catastrophic rockslide in Alaska's Tracy Arm fjord on August 10, 2025 generated a wave that surged nearly 500 metres up a rocky shoreline — the second-highest tsunami ever recorded on Earth — yet went largely unnoticed by the public because it struck before dawn and caused no casualties, according to a new study published in the journal Science.
A Near-Miss of Historic Proportions
At 5:26 am local time on August 10, 2025, a wedge of rock estimated at more than 63.5 million cubic metres broke away from a mountainside above Tracy Arm fjord in southeastern Alaska. The mass plunged into the waters at the base of the South Sawyer Glacier, displacing an enormous volume of water and generating an initial breaking wave roughly 100 metres high that tore down the fjord at speeds exceeding 70 metres per second.
When that wave struck the opposite shoreline, it ran up the steep rock face to a height of 481 metres above sea level — a figure that places the event second only to the 1958 Lituya Bay tsunami, which reached 530 metres in the same general region of Alaska.
"It was the second highest tsunami ever recorded on Earth," said Aram Fathian, a researcher at the University of Calgary and co-author of the Science study. "But until now, almost nobody heard about it because it was a near-miss event."
No injuries or fatalities were reported. Scientists attribute the fortunate outcome largely to the early-morning timing. Tracy Arm is a popular destination for cruise ships and small tour boats, which regularly transit the fjord during daylight hours to view the glacier and surrounding wildlife. Had the collapse occurred later in the day, the consequences could have been severe.
Landslide Tsunamis vs. Earthquake Tsunamis
The Tracy Arm event belongs to a category of so-called megatsunamis driven not by seismic activity but by sudden mass movements into confined bodies of water. Earthquake-generated tsunamis typically produce runup heights of a few tens of metres when they reach the coast. Landslide tsunamis, while generally more localised, can be far more violent due to the direct displacement of the water column within a narrow fjord or bay.
Since 1925, scientists have documented 27 such events with runups exceeding 50 metres. The fact that two of the highest in recorded history have occurred along Alaska's coastline underscores the region's particular vulnerability.
Glacial Melt as a Contributing Factor
Researchers note that the collapse was linked to the ongoing retreat of the South Sawyer Glacier. As glaciers thin and recede, they remove the buttressing support they once provided to the rock walls above — a process that can destabilise mountain flanks and make large-scale rockfalls more likely. This dynamic is expected to intensify as global temperatures continue to rise, raising concerns about similar events in other glaciated fjord systems worldwide.
The study reconstructed the event using a combination of seismic data, satellite imagery, eyewitness accounts, and computer modelling, providing what researchers say is one of the most detailed analyses of a landslide megatsunami to date. Their findings are intended to improve hazard assessments for coastal areas near retreating glaciers and to inform safety protocols for tour operators and maritime authorities in at-risk regions.
Analysis
Why This Matters
- Tracy Arm is a major tourist destination visited by cruise ships and small vessels daily; a repeat event during peak hours could result in mass casualties with little or no warning time.
- The event directly links glacial retreat driven by climate change to an increased risk of catastrophic landslide tsunamis, with implications for fjord systems in Norway, Greenland, Patagonia, and New Zealand as well as Alaska.
- The fact that this near-record event received minimal public attention highlights a significant gap in hazard communication and monitoring for landslide-generated tsunamis compared to seismic tsunamis.
Background
Landslide-generated megatsunamis have a documented history along Alaska's coast. The 1958 Lituya Bay event — still the largest tsunami runup ever recorded at 530 metres — was similarly caused by a rockslide triggered by an earthquake along the Fairweather Fault. That event destroyed forest, capsized boats, and killed two fishermen, though the bay's remoteness limited the death toll.
Since the mid-20th century, researchers have catalogued dozens of events worldwide where mass movements into confined water bodies produced waves far exceeding those generated by earthquakes. However, systematic monitoring and early-warning infrastructure for this type of hazard remains far less developed than for seismic tsunamis, which benefit from global ocean-based sensor networks.
The retreat of tidewater glaciers in Alaska has accelerated markedly over recent decades. As ice thins, the lateral pressure it exerts on adjacent rock slopes diminishes, and water from meltwater infiltration can further weaken rock fractures — two processes that together elevate the probability of large-scale slope failures in glaciated fjords.
Key Perspectives
Researchers (University of Calgary / Science study authors): Emphasise that detailed reconstruction of the Tracy Arm event offers a rare opportunity to validate tsunami models and strengthen hazard assessments. They argue that the near-miss outcome should not breed complacency and call for improved monitoring of unstable slopes in glaciated fjord systems.
Tourism and maritime operators: The Tracy Arm corridor is economically significant, attracting substantial cruise and charter boat traffic throughout summer. Operators will face pressure to assess route safety and consider contingency protocols, though no regulatory changes have yet been announced in response to the study.
Critics/Skeptics: Some hazard scientists caution that predicting when any specific unstable slope will fail remains extremely difficult, and that early-warning systems for landslide tsunamis in remote fjords face major technical and logistical challenges. The cost of comprehensive slope monitoring across hundreds of kilometres of Alaskan coastline may be prohibitive without federal prioritisation.
What to Watch
- Whether the US Geological Survey or National Oceanic and Atmospheric Administration moves to expand slope-stability monitoring in Tracy Arm and comparable fjords following the study's publication.
- Upcoming tourist season activity in Tracy Arm (peak season: June–August) and any new safety advisories issued by the US Coast Guard or Alaskan maritime authorities.
- Further peer-reviewed research on the rate of glacial retreat at South Sawyer Glacier and other tidewater glaciers in southeastern Alaska, which will indicate how quickly the underlying risk environment is changing.