On August 10, 2025, at approximately 5:26 a.m. local time, a massive landslide of more than 64 million cubic meters of rock and debris (a minimum estimate) collapsed from a steep slope adjacent to the retreating South Sawyer Glacier into Tracy Arm fjord.

A massive landslide in Alaska’s Tracy Arm fjord (near Juneau, in an area popular with cruise ships) triggered a landslide-generated megatsunami with a run-up of about 1,580 feet (481 meters) on the opposite fjord wall.

The landslide was preconditioned by glacial retreat, which removed buttressing support from the unstable rock slope.

It generated a megatsunami with an initial breaking wave roughly 100 meters (328 feet) high that crossed the fjord at speeds exceeding 70 m/s (over 150 mph).

The wave’s maximum run-up reached 481 meters (1,580 feet) on the opposite fjord wall—about 3.3 km across from the landslide source. This is the second-highest tsunami run-up ever reliably recorded, behind only the 1958 Lituya Bay event (~524–530 m / 1,720–1,730 ft).

The tsunami stripped vegetation and soil down to bare rock along large sections of the fjord walls, with run-up and erosion effects observed tens of kilometers away (e.g., up to 35 m at Williams Cove and 16 m in Holkham Bay). It was even detected on tide gauges as far as Juneau (~130 km tsunami travel distance).

Tracy Arm is a popular destination for cruise ships and smaller tour boats/kayakers, with up to six large vessels per day possible in peak season (some carrying thousands of passengers). The event occurred early in the morning, before typical boat traffic, so no injuries or vessel damage were reported.

Several cruise lines have since adjusted or avoided itineraries in Tracy Arm for safety reasons due to ongoing instability risks.

The landslide produced seismic signals equivalent to a ~M5.4 earthquake, detectable globally.

A long-period seiche (standing wave) in the fjord persisted for up to ~36 hours.

Researchers emphasize the need for better monitoring and early-warning systems for landslide-tsunamis in glaciated fjords, as climate-driven glacier retreat increases such hazards in Alaska and similar regions.

This event provides valuable data for modeling and hazard assessment in steep coastal environments visited by tourists.

This event, detailed in a landmark Science paper (published May 6, 2026), stands as a striking example of cascading hazards in a warming world.

A massive landslide in Tracy Arm fjord (southeast Alaska, near Juneau) on August 10, 2025, generated the second-highest tsunami run-up ever reliably recorded: 481 meters (1,580 feet).

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A 481-meter-high landslide-tsunami in a cruise ship–frequented Alaska fjord

The paper “A 481-meter-high landslide-tsunami in a cruise ship–frequented Alaska fjord” (Shugar et al., Science, 6 May 2026, DOI: 10.1126/science.aec3187) is a comprehensive, open-access forensic analysis of the 10 August 2025 Tracy Arm event.

It integrates satellite remote sensing, DEM differencing, seismic/infrasound data, field surveys, eyewitness accounts, and numerical modeling. Lead author Dan H. Shugar (University of Calgary) collaborated with a large international team including experts from USGS, Alaska Earthquake Center, and others.

Core Event Parameters

Landslide: A rock wedge slide (~63.5 ± 2.7 × 10⁶ m³ subaerial minimum volume; total likely higher due to submarine component) from a slope adjacent to the South Sawyer Glacier terminus. Headscarp ~1,025 m asl; wedge geometry with planes dipping ~44° and ~46°. It transitioned rapidly into a rock avalanche, impacting the glacier and releasing large icebergs.

Tsunami: Initial breaking wave 100 m (328 ft) high crossing the fjord (>3 km wide) at >**70 m/s (157 mph)**. Maximum run-up 481 m (1,580 ft) on the opposite wall (~3.3 km from source), stripping vegetation/soil to bare rock and creating a visible trimline. Effects propagated tens of km down-fjord (e.g., ~35 m at Williams Cove).

Seismic: Main landslide equivalent to M5.4 (long-period waves detected globally). A distinct ~66 s period seiche signal persisted up to **36 hours** — only the second documented multi-day landslide-induced seiche observable globally.

This ranks as the second-highest reliably recorded tsunami run-up (behind Lituya Bay 1958, ~524–530 m, which was earthquake-triggered with smaller volume ~30 × 10⁶ m³).

Glacial retreat was the dominant preconditioner. South Sawyer Glacier retreated dramatically over decades (multiple km since the mid-20th century, with accelerated thinning of 11–19 m/yr in frontal zones recently). This “debuttressed” the oversteepened slope (previously supported by ice), exposing the failure plane progressively in July–early August 2025.

The paper attributes regional warming (and thus glacier retreat/snowline rise) overwhelmingly to anthropogenic forcing using climate model ensembles. Moderate rainfall preceded the event but was not extreme. No major tectonic earthquake trigger (unlike Lituya Bay). Smaller precursor slope failures occurred in 2017 and July 2025. No clear pre-failure creep/deformation visible in high-res imagery weeks prior — it failed abruptly.

Microseismicity: Increased activity detected regionally at least 24 hours prior, with exponential rise in rate/magnitude in the final ~6 hours (events every 30–60 s). Rate decayed 1–2 hours before failure, possibly transitioning to continuous slip. These were small, repeating stick-slip events

This pattern offers hope for monitoring but is challenging in remote areas with sparse networks. The paper highlights potential for improved early-warning via denser seismic arrays or real-time processing.

Researchers reconstructed landslide motion from seismic data and used it as a tsunami source in simulations. Models successfully reproduced observed run-up, wave speeds, and propagation, including energy splitting and amplification at certain shorelines. The confined fjord geometry focused energy dramatically. A long-lived seiche (standing wave oscillation) trapped in the fjord explains the prolonged seismic signal.

Supplementary materials detail methods (e.g., high-res DEMs from SkySat/WorldView, field RTK surveys) and include animations.

The study is exceptionally well-documented with before/after satellite imagery (NASA/USGS), figures, and supplementary data (including Zenodo auxiliaries). It balances rigorous geoscience with clear hazard communication.

Published:  Science (6 May 2026)

DOI: 10.1126/science.aec3187

Authors: Dan H. Shugar, Katherine R. Barnhart, Mira Berdahl, Jacqueline Caplan-Auerbach, Göran Ekström, Aram Fathian, Marten Geertsema, Stephen P. Hicks, Bretwood Higman, and Michael E. West

Abstract

Early in the morning of 10 August 2025, a >64 × 10⁶ m³ landslide struck Tracy Arm fjord in Alaska. The landslide was preconditioned by glacial retreat caused by climate change. The resulting 481 m runup megatsunami followed an initial 100-m-high breaking wave traveling >70 m s⁻¹. The landslide was preceded by several days of microseismicity, which increased in rate and magnitude until ~1 hour before failure. The landslide produced globally observed long-period seismic waves equivalent in size to a M5.4 earthquake. A long-period (~66 s) global seismic signal, produced by a landslide-induced seiche trapped within the fjord, persisted for up to 36 hours, the second time a days-long seiche has been thus observed. With fjord regions increasingly visited by cruise ships, and climate change making similar events more likely, this unanticipated, near-miss event highlights the growing risk from landslides and tsunamis in coastal environments.