Alaska Megatsunami Recorded as Second Largest in History

At 5:26 a.m. Alaska Daylight Time on August 10, 2025, approximately 64 million cubic meters of rock — equivalent in mass to 24 Great Pyramids of Giza — detached from a mountainside at the toe of South Sawyer Glacier and crashed into the narrow waters of Tracy Arm Fjord, about 70 kilometers south of Juneau [1][2]. The resulting wall of water, measured at 481 meters of runup on the opposite fjord wall, was taller than all but 14 of the world's tallest buildings [3]. It registered on seismic detectors around the globe [4].

It was the second-largest tsunami ever recorded, trailing only the 524-meter wave that struck Lituya Bay, Alaska, in 1958 [1]. And unlike Lituya Bay, which was triggered by a magnitude 7.8 earthquake, the Tracy Arm event was set in motion by something quieter: a glacier that had been retreating for decades finally pulled away from the slope it had been holding in place [3][4].

No one died. No ships were in the fjord. But more than 20 vessels, including large cruise ships carrying thousands of passengers, typically pass through Tracy Arm daily during summer [5]. The wave struck roughly three hours before the first cruise ships would have entered the waterway [6].

What Happened: Anatomy of a Megatsunami

A megatsunami is defined not by its open-ocean behavior but by its runup height — the elevation to which water is pushed up a slope or shoreline. Megatsunamis are typically confined to enclosed bodies of water such as fjords, bays, or lakes, where displaced water has nowhere to go but up [7].

The Tracy Arm event began with a landslide of between 64 and 100 million cubic meters, depending on the estimate used [1][2]. The initial breaking wave reached approximately 100 meters in height and traveled at speeds exceeding 70 meters per second — over 150 miles per hour [3]. When it struck the opposite wall of the fjord, roughly two miles away, the water surged upward to 481 meters [3].

The tsunami then converted into a seiche — a standing wave oscillating back and forth within the enclosed fjord — that persisted for days [4]. At the nearest tide gauge, 100 kilometers from the landslide, waves of 36 centimeters above normal tide levels were recorded, and they continued for hours [2].

Source: USGS / Science (2026)

Data as of May 5, 2026CSV

For comparison, the 1958 Lituya Bay megatsunami reached 524 meters, but was triggered by a magnitude 7.8 earthquake that sent 30 million cubic meters of rock into the bay [7]. The 2015 Taan Fjord event, caused by a mountainside collapse at the terminus of Tyndall Glacier, reached a runup of 193 meters from approximately 76 million cubic meters of material [8][9].

The Mechanism: How a Retreating Glacier Pulled the Mountain Down

The study published in Science in May 2026, led by Daniel Shugar of the University of Calgary, provides a detailed forensic account of what caused the slope to fail [3][4].

South Sawyer Glacier had carved Tracy Arm Fjord over millennia, steepening the valley walls in the process. The glacier's mass also served as a buttress — physically holding the slopes in place. As the glacier thinned and retreated, it exposed rock that had not been unsupported since the last glacial advance [3][4].

Shugar described the mechanism in concrete terms: "As that glacier retreated ... 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" [4].

The glacier had been retreating steadily for decades, but the final trigger was acute. In the spring of 2025 alone, South Sawyer Glacier retreated roughly 500 meters [3][4]. Between August 2 and August 5, 2025 — just days before the collapse — the glacier underwent a period of rapid calving that removed the last buttressing support from the unstable slope [5].

Using Interferometric Synthetic Aperture Radar (InSAR) satellite imagery, the research team tracked slope movements across Alaska and found that "a huge majority" of shifting slopes correlate with thinning glacier bases [4].

The Warning Signs That Went Unmonitored

Retrospective analysis of seismic data revealed that the slope gave substantial warning before it collapsed — warnings that no one was watching for [5][6].

Several days before the failure, small seismic signals — equivalent to magnitude 1-2 events — began occurring at the site at a rate of approximately one per hour [5]. Six hours before the collapse, the frequency increased sharply, with signals arriving every 30 to 60 seconds. Two hours before failure, the seismic activity became continuous vibration [5]. At approximately 5:00 a.m., about 26 minutes before the landslide, a distinctive low-frequency "hum" of continuous seismic energy was detected [6].

None of this was monitored in real time.

"Alaska has sophisticated processes for monitoring earthquakes and observatories for monitoring volcanoes," said Michael West, director of the Alaska Earthquake Center. "But this tremendous and dangerous landslide went undetected, and that is a hazard Alaska needs to come to terms with" [5].

Researchers estimate that had a monitoring program existed, progressive alerts could have been issued to ships in the vicinity at least one day before the failure [6]. Once a large landslide initiates, detection and measurement is possible within approximately two minutes, and ships farther down Tracy Arm could have received roughly 10 minutes of warning [6].

A Gap in the Warning Infrastructure

The Tracy Arm event exposed a structural gap in how the United States monitors landslide-generated tsunami hazards. The National Tsunami Warning Center (NTWC) in Palmer, Alaska, is designed to detect earthquake-generated tsunamis, issuing initial bulletins within minutes of a seismic event [10]. Japan's system can acquire an earthquake source model within six minutes and complete wave simulations in under two [10]. Chile uses 46 coastal sea level stations, 5 DART buoys, and 170 GNSS receivers to generate threat assessments in under eight minutes [10].

But these systems are built to detect the seismic signatures of earthquakes, not the precursory microseismicity of an unstable slope. There is no operational landslide-monitoring system at this scale in the United States [5][6].

The authors of the Science study proposed a three-tiered alert system modeled after avalanche and volcanic monitoring [6]:

• Yellow Alert: Elevated hazard from weather and rapid glacier retreat in a general area
• Orange Alert: Immediate concern based on exponential seismic increases, warranting vessel exclusion
• Red Alert: Event-in-progress notifications using NOAA's existing rapid message dissemination systems

Building such a system, the researchers acknowledged, would require cooperation across state and federal agencies and strengthened monitoring and communication networks [6].

The Barry Arm Precedent — and Its Unfinished Business

Tracy Arm was not the first time scientists warned about landslide-tsunami risk in Alaska's fjords. The Barry Arm landslide in Prince William Sound has been under observation since at least 2020, when satellite imagery revealed that a 3.7-square-kilometer mass of rock — approximately 500 million cubic meters, or 6.5 times larger than the Tracy Arm landslide — had been creeping toward the water [11][12].

Analysis of airborne and satellite data showed the landslide had been moving since at least 1957, accelerating between 2009 and 2015 as Barry Glacier thinned and retreated beneath it. The mass moved 183 meters toward the fjord during that period [11].

In 2021, Congress appropriated $4 million for landslide hazard assessment in Prince William Sound, with $2.2 million going to a cooperative agreement between the Alaska Division of Geological & Geophysical Surveys (DGGS) and the USGS Landslide Hazards Program [12]. Monitoring infrastructure was installed, including seismic sensors, infrasound arrays, and high-resolution lidar surveys [12].

But that monitoring program now faces uncertainty. Federal employment disruptions and budget proposals have affected the agencies responsible for Barry Arm oversight [13]. The Trump administration's FY2026 budget request proposed a 39% cut to overall USGS funding [14]. While congressional appropriators largely rejected the most severe cuts, preserving $1.42 billion for the USGS, the proposed elimination of entire mission areas and reductions to the Earthquake Hazards Program have raised questions about the long-term stability of monitoring at sites like Barry Arm [14].

A later USGS assessment suggested the potential tsunami from a Barry Arm failure would likely be less severe than initially modeled [12]. But the Tracy Arm event demonstrated that a slope can fail with little visible warning, and that the consequences of such failures in narrow, enclosed waterways can be extreme.

The Climate Question — and Its Complications

The Science study presented the Tracy Arm landslide as a direct consequence of glacial retreat driven by climate change [3]. Researchers found that slopes across Alaska are destabilizing in patterns that track with glacier thinning, and that the frequency of such events may be increasing by an order of magnitude compared to a few decades ago [1][5].

"The likelihood of similar large-scale events has increased substantially across the North as glaciers retreat and permafrost degrades," the study's authors wrote [5]. Leigh Stearns, a glaciologist at the University of Pennsylvania who was not involved in the research, noted that these landslides are occurring "precisely at the terminus of a retreating glacier" [4].

But attribution is not straightforward. Southeast Alaska is subject to glacial isostatic rebound — the slow uplift of land as it recovers from the weight of ice-age glaciers — at rates averaging about 15 millimeters per year in the Juneau area [15]. This uplift can itself fracture and destabilize slopes [15]. Tectonic activity in the region is significant; the 1958 Lituya Bay event was earthquake-triggered, and large earthquakes in 1899-1900 in Yakutat Bay are believed to have contributed to slope failures decades later [15][16].

Research on the interactions between climate, volcanism, and isostatic rebound in Southeast Alaska has found that "a direct link between regional climate change, isostatic adjustment, and the initiation of volcanism remains to be demonstrated" because of the difficulty of obtaining high-resolution, well-dated records [15]. The same caution applies to landslide attribution: disentangling the relative contributions of ongoing post-glacial rebound, tectonic stress, and recent climate-driven ice loss remains a work in progress.

The historical record also complicates frequency claims. Four or five megatsunamis are believed to have occurred at Lituya Bay alone during a 150-year period between the early 1800s and 1958 [7]. Whether today's events represent a genuine increase in frequency or reflect improved detection through satellite imagery and seismic networks is a question the available data cannot fully resolve.

Source: OpenAlex

Data as of Jan 1, 2026CSV

Academic research on landslide-generated tsunamis and glacial interactions has increased substantially in recent years — from one or two papers per year before 2017 to a peak of seven in 2025 — suggesting that scientific attention, if not event frequency, has risen sharply.

Economic Fallout and the Cruise Industry's Response

The immediate economic consequence of the Tracy Arm event has been felt in Alaska's cruise tourism industry. At least three major cruise operators — Carnival Cruise Line, Holland America Line, and Royal Caribbean Cruises — have modified their 2026 itineraries to avoid Tracy Arm Fjord entirely, redirecting passengers to nearby Endicott Arm and Dawes Glacier [17].

Royal Caribbean cited "current waterway conditions" as unsuitable for cruise ship navigation [17]. Alaska State Seismologist Mike West noted that after a major collapse, "the earth is getting used to its new arrangement," making follow-on activity a reasonable expectation. But West also cautioned that other Southeast Alaska fjords may carry similar unnamed risks [17].

The broader question of economic and insurance liability remains largely unaddressed. The National Flood Insurance Program (NFIP) covers flood damage caused by tsunamis, but the program was not designed for the low-probability, extreme-magnitude wave events that characterize megatsunamis in fjord environments [18]. Building codes and land-use regulations in coastal Alaska communities do not specifically address landslide-tsunami risk of this scale. Federal disaster frameworks, including FEMA's Stafford Act authorities, are structured around post-event response rather than pre-event mitigation for geologically novel hazards.

The coastal town of Seward, population 2,800, has direct experience with landslide-generated tsunamis: a 1964 earthquake-triggered submarine landslide produced waves reaching 17 meters locally [18]. In August 2024, another landslide-generated wave damaged infrastructure at a lodge and a Kenai Fjords National Park camping area [18]. In Skagway, a 1994 underwater slide produced a localized tsunami that killed a dock construction worker [18].

These communities face an asymmetric risk calculus: the probability of any single catastrophic event remains low, but the potential consequences — particularly for vessels carrying thousands of passengers through narrow fjords with no warning infrastructure — are severe.

What Comes Next

The Tracy Arm megatsunami did not kill anyone, damage any structures, or sink any ships. By the narrow metric of immediate harm, it was a non-event. By every other metric — the scale of the wave, the proximity to routine cruise traffic, the absence of any monitoring system that could have detected the precursory signals, and the documented link to accelerating glacial retreat — it was a near-miss of historic proportions.

The study's authors and the Alaska Earthquake Center have called for the development of a landslide-tsunami detection network, expanded slope-stability assessments across Alaska's fjord regions, and integration of landslide hazards into existing maritime warning systems [5][6]. Ezgi Karasözen of the Alaska Earthquake Center framed the challenge directly: "The scale of what we are dealing with in Alaska is unprecedented. We have a spectacular landscape, but the hazards that come with that are also very real" [5].

Whether those recommendations translate into funded infrastructure, regulatory changes, and operational warning systems will depend on federal budget priorities, interagency coordination, and the willingness of the cruise industry and coastal communities to invest in monitoring a hazard that may not strike the same fjord twice — but will, according to the available science, strike somewhere again.