Alaska's Giant Tsunami Left Warning Signs, and Scientists Say a Warning System Is Within Reach

Keypoints:

• A landslide on 10 August 2025 sent a tsunami 1,580 feet up the wall of Alaska's Tracy Arm fjord
• It is the second-highest tsunami ever recorded and the largest not triggered by an earthquake
• The slide dropped about 64 million cubic meters of debris into the water
• Small tremors, glacier retreat and heavy rainfall preceded the collapse
• Tracy Arm was not on any hazard watch list at the time

Scientists are detailing the warning signs of one of the largest tsunamis on record. A landslide in Alaska's Tracy Arm sent water and debris 1,580 feet (481 meters) up the opposite wall of the fjord. Only the 1958 Lituya Bay tsunami has been measured higher. That one was triggered by an earthquake. Tracy Arm is the largest tsunami recorded that was not.

Timing makes the story stark. A cruise ship left the fjord on the evening of 9 August 2025. Passengers had just photographed South Sawyer Glacier. About twelve hours later, a mountainside collapsed into the water. The slide dumped about 64 million cubic meters of rock into the fjord and created an initial wave about 328 feet (100 meters) high. Nobody was hurt.

Michael West and Ezgi Karasözen, seismologists at the Alaska Earthquake Center, say the event left clues. For several days before the collapse, instruments recorded microseismicity, which means movements too small for people to feel. The retreat of Sawyer Glacier had weakened the slope. Heavy late-summer rainfall may have helped tip it. These signs are being treated as possible precursors.

The study appeared in Science and was led by geomorphologist Dan Shugar of the University of Calgary. It shows the water stripped from the fjord walls down to bare rock. Seismic waves from the event traveled around the world for about 36 hours. Satellites also captured the scar.

The hazard map had a blind spot. Monitoring in Alaska had focused on Barry Arm, another fjord with a slope that is already moving. Tracy Arm, a popular route for cruise ships and boaters about 50 miles south of Juneau, was not on any watch list. Several cruise lines have since changed their itineraries.

The lesson reaches beyond one fjord. Karasözen notes that many Alaska fjords combine steep slopes with retreating glaciers. Those conditions could produce similar landslides. Researchers see a chance to build alerts from tremor patterns and glacier changes.