Tracy Arm landslide produced 1,580‑ft megatsunami

A fjord megatsunami sounds like movie stuff. But this one was real, and the new part is that researchers have now reconstructed how an Alaskan mountainside collapse in Tracy Arm produced a 1,578-foot runup — the second-highest ever measured. The bigger point is not just that the wave was enormous. It’s that the slope seems to have been quietly primed by glacier retreat, then failed in a place full of summer tourist traffic. ### What actually happened? Early on August 10, 2025, a huge rock mass above South Sawyer Glacier gave way and plunged into Tracy Arm fjord, about 80 miles south-southeast of Juneau. The collapse happened at 5:26 a.m. local time, generated seismic signals detected around the world, and shoved enough water across the fjord to blast vegetation off the opposite wall. (science.nasa.gov) ### How big was the wave? The key number is 481 meters — about 1,578 feet. That is the maximum runup, meaning the highest point water and debris reached on the far wall, not a freestanding vertical wall of water moving down the fjord. The initial breaking wave was still enormous — about 100 meters high and moving faster than 70 meters per second. (usgs.gov) ### Why do people call this a “megatsunami”? Because this was not a normal earthquake tsunami spreading across an ocean basin. It was a local, landslide-driven water displacement event in a narrow fjord. Think of dropping a mountain into a bathtub, except the bathtub is miles long, the splash strips forests to bare rock, and the basin keeps sloshing for more than a day. That long sloshing is called a seiche, and Tracy Arm kept “ringing” after the first impact. (science.nasa.gov) ### Why did the slope fail there? The cleanest explanation is loss of support. South Sawyer Glacier had retreated rapidly, exposing open water and removing ice that had been buttressing the slope near its base. Researchers also point to moderate rainfall before the collapse, plus a burst of small seismic events in the days beforehand, as likely contributors that pushed an already weakened mountainside toward failure. (science.org) ### Was climate change really part of this? Basically, yes — as a precondition, not as a cartoon instant trigger. The new Science paper says the landslide was preconditioned by glacial retreat caused by climate change. That matters because the hazard is indirect: warming shrinks glacier support, changes drainage and stress in the rock, and leaves behind slopes that may still look stable until they suddenly are not. (science.nasa.gov) ### Why was this such a near miss? Tracy Arm is a cruise-ship and glacier-sightseeing route. Researchers say roughly 20 cruise ships, tour boats, and kayakers can pass through the broader area on a summer day, but this collapse hit around 5:30 a.m., before most traffic entered the narrow fjord. A few hours later, the story could have been about mass casualties, not geomorphology. (science.org) ### Could anyone have seen it coming? Maybe not predicted to the hour, but turns out there were clues. The slope produced days of microseismicity that increased in rate and magnitude until about an hour before failure. That does not mean easy forecasting yet, but it does suggest a practical path: combine satellite monitoring, glacier-change tracking, and local seismic networks to flag slopes that are entering the danger zone. (news.climate.columbia.edu) ### So what’s the real lesson? The lesson is that retreating glaciers do more than raise sea levels or change scenery. They also remove structural support from steep valley walls and create new water-filled basins where rockfalls can turn into tsunamis. Tracy Arm is the scary version of a broader Arctic problem — old stability assumptions can expire quietly, then fail all at once. (science.org) ### Bottom line This was a giant wave, but it was also a systems warning. In places where glaciers are pulling back fast, the landscape is not just melting — it is being mechanically rearranged. Tracy Arm shows that the next disaster may start long before the collapse, in the slow disappearance of the ice holding everything up. (science.nasa.gov)