Alaska Tsunami Explained: Water 1,580 Feet Up a Mountain
🕐 9 min read | 🌍 Natural Wonders
🔒 Key Takeaways
- On July 9, 1958, a magnitude 7.8 earthquake on Alaska's Fairweather Fault dropped roughly 30 million cubic metres (40 million cubic yards) of rock into Lituya Bay's Gilbert Inlet.
- The resulting splash wave stripped forest from the opposite mountainside up to a trimline 1,720 feet (524 m) above sea level, with sustained water surge measured across the 1,500–1,600-foot band — including the widely cited 1,580-foot mark.
- The main wave then raced the 7-mile length of the fjord at an estimated 100 mph (160 km/h), snapping 1,000-year-old spruce trees like matchsticks.
- Only five people died — two in the bay itself — because Lituya Bay is one of the emptiest coastlines in North America; three fishing boats were caught, and two crews survived the ride.
- Geologists have found at least four older trimlines in Lituya Bay (1854, 1874, 1899, 1936), proving this fjord generates giant waves roughly every 25–50 years.
On a calm summer night in 1958, a slab of mountain the size of a small city fell into a remote Alaskan fjord — and the water answered by climbing 1,580 feet up the slope on the far side, scouring the forest down to bare rock. The Lituya Bay megatsunami remains the tallest wave ever documented on Earth, and a fisherman anchored in its path watched the whole thing happen in the moonlight. What he described sounded impossible until geologists walked the trimline and realized he had understated it.
What Happened in Lituya Bay on July 9, 1958
At 10:15 p.m. Alaska time on 9 July 1958, a magnitude 7.8 earthquake ruptured the Fairweather Fault directly beneath the head of Lituya Bay, a T-shaped glacial fjord on the Gulf of Alaska coast. The violent shaking destabilised a near-vertical rock wall above Gilbert Inlet, and an estimated 30.6 million cubic metres of schist and gneiss — roughly 40 million cubic yards — detached and fell as a coherent mass from an average height of about 600 metres. It struck the 220-metre-deep water of the inlet almost like a piston rather than a scatter of debris, and the displaced water had nowhere to go but up the opposite wall. Witnesses on boats more than six kilometres away reported a wall of water rising against the mountainside, glowing pale in the twilight. The surge scoured the slope bare, then collapsed back and propagated outward as a tsunami wave 30–60 metres high that swept the entire length of the bay. Within four minutes it had reached the narrow entrance at La Chaussee Spit and spilled into the open Pacific. By morning, 10 square kilometres of mature coastal forest had simply ceased to exist.
The Fairweather Fault: Alaska's Fastest-Moving Scar
The Fairweather Fault is a transform boundary where the Pacific Plate grinds northwest past the North American Plate at roughly 46–50 millimetres per year — one of the fastest strike-slip rates on the planet, comparable to or exceeding the San Andreas. Lituya Bay is not merely near this fault; the fault runs straight through the bay's head, and the two arms of the T, Gilbert and Crillon inlets, are literally carved along the fracture zone. That geometry means every major rupture shakes vertical rock walls that are already oversteepened by glacial erosion and weakened by deep weathering along the fault trace. The 1958 earthquake produced up to 6.5 metres of horizontal offset and about a metre of vertical displacement, which geologists mapped by the fence lines and stream channels it sliced apart. The fault's rapid slip rate means strain rebuilds quickly, so large earthquakes here recur on timescales of decades to a couple of centuries rather than millennia. In other words, the trigger mechanism is reloaded and ready far more often than most of the world's tsunami sources.
🤔 Did You Know?
The 1958 Lituya Bay wave was taller than the Empire State Building, the Eiffel Tower, and the Sydney Harbour Bridge stacked one on top of the other — and one fishing boat surfed over a tree-covered sandbar on its crest and lived.
The Physics: How a Rockslide Throws Water 1,580 Feet Uphill
A tectonic tsunami lifts a broad column of ocean by a metre or two and converts it into a long, low wave train; a landslide-generated megatsunami does something entirely different. When a dense rock mass hits confined water at 100–150 kilometres per hour, it transfers enormous kinetic energy into a tiny volume, producing an impact crater in the water surface and a vertical jet that rides straight up the opposing slope. Laboratory experiments by Hermann Fritz and colleagues, published in 2001, reproduced the Lituya event at scale and confirmed that a slide impacting at around 110 metres per second could generate runup in the 500-metre range. Independent hydrodynamic modelling by Mader and Gittings in 2002 reached the same conclusion using the full Navier–Stokes equations, settling decades of scepticism that the measured trimline was real. Crucially, runup height is not wave height: the 1,580-to-1,720-foot figure describes how far water and debris were driven up a steep slope, not the depth of water standing in the bay. The confining walls of a narrow fjord act like a funnel, preventing the energy from dissipating sideways the way it would in open ocean. That is why megatsunamis are overwhelmingly a fjord, lake, and reservoir phenomenon rather than an oceanic one.
Three Boats, Five Deaths, and the Men Who Surfed a Megatsunami
Three fishing vessels were anchored in Lituya Bay that night, sheltering from Gulf of Alaska swell. Howard Ulrich and his seven-year-old son aboard the Edrie saw the wave coming, paid out anchor chain, and rode up and over the crest; Ulrich's radio call — describing a wave towering over the trees — is one of the few real-time accounts of a megatsunami ever recorded. Bill and Vivian Swanson's Badger was lifted stern-first and carried backwards over La Chaussee Spit, clearing treetops Swanson estimated at 24 metres, before being dropped into the open Pacific where the boat sank and the couple were rescued from a dinghy. The third vessel, the Sunmore, was overtaken near the entrance; Orville and Mickey Wagner were never found. Three more people died when the earthquake collapsed a beach on Khantaak Island near Yakutat, bringing the official toll to five. The staggeringly low casualty count is purely an accident of geography — Lituya Bay sits inside what is now Glacier Bay National Park, with no permanent settlement for dozens of kilometres. An identical event in an inhabited fjord would rank among the deadliest natural disasters in history.
Reading the Trimline: How Scientists Measured the Wave
USGS geologist Don J. Miller flew over Lituya Bay within days and spent the following years mapping what he called the trimline — the brutally sharp boundary between untouched forest above and scoured bedrock below. On the spur opposite the slide, the trimline reached 524 metres (1,720 feet) above sea level, with the zone of complete scour and water-driven destruction concentrated through the 1,500–1,600-foot band that gives the commonly quoted 1,580-foot figure. Miller dated the destruction by counting growth rings on surviving trees at the trimline edge, many of which bore abrasion scars from debris carried by the water. Soil was stripped to bare rock in places, and mature Sitka spruce more than a metre in diameter were sheared off or uprooted entirely and rafted into the bay. His 1960 USGS Professional Paper remains the definitive field record, and it also documented four distinct older trimlines at lower elevations. Later LiDAR surveys and repeat aerial photography have refined but never overturned his measurements. Sixty-plus years on, the regrowing forest still traces the wave's high-water mark as a visible change in tree height and species from the air.
Why Lituya Bay Is a Megatsunami Factory
Lituya Bay is not unlucky — it is structurally designed to do this. It is a deep, narrow, glacially carved fjord with near-vertical walls rising more than 1,000 metres, sitting astride one of Earth's fastest-slipping faults, at the foot of retreating tidewater glaciers that have recently unloaded their lateral support from the rock. Miller identified evidence of giant waves in approximately 1854, 1874, 1899 and 1936, the last of which reached about 150 metres (490 feet) and was witnessed by a family living temporarily in the bay. That gives a recurrence interval on the order of 25 to 50 years for waves large enough to destroy forest. The bay's single narrow entrance, barely 300 metres wide and crossed by a shallow bar, traps wave energy inside rather than letting it radiate away. Glacial retreat since the Little Ice Age has exposed thousands of vertical metres of oversteepened, debuttressed rock — a process geologists call paraglacial adjustment. Each decade of warming leaves more of these walls unsupported and waiting for the next earthquake.
Could It Happen Again? Barry Arm and the Thawing Slopes
In 2020, scientists identified an actively creeping landslide on the flank of Barry Arm in Prince William Sound, roughly 500 kilometres east of Lituya Bay, where retreat of Barry Glacier has removed the ice that once propped up an enormous rock mass. Estimates suggest a sudden failure could generate a wave capable of reaching Whittier, a community of several hundred people and a major cruise terminal, within about 20 minutes. Alaska now operates a dedicated monitoring programme there using satellite radar interferometry, seismometers and GPS to track millimetre-scale slope movement. The precedent is not hypothetical: in 2015 a landslide of about 180 million tonnes into Taan Fiord at Icy Bay produced runup of 193 metres (633 feet), and in 2017 a rock avalanche into Karrat Fjord, Greenland, generated a wave that destroyed homes in Nuugaatsiaq and killed four people. Warming is accelerating this hazard worldwide by thawing the permafrost that cements high-alpine rock faces together. Lituya Bay, once dismissed as a freak event, is now understood as the clearest warning we have of what deglaciating coastlines can do.
Final Thoughts
The Lituya Bay megatsunami of 1958 is not a geological curiosity frozen in the past — it is a working blueprint for what happens when warming climates, retreating glaciers and active faults meet in a narrow fjord. Walk the bay today and you can still read the wave's height in the treeline, a 1,580-foot scar that nature has not yet erased. Next time you see a photo of a serene Alaskan inlet, ask yourself how high the water has climbed there before.
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Frequently Asked Questions
What was the tallest wave ever recorded?
The tallest wave ever documented was the Lituya Bay megatsunami of 9 July 1958 in Alaska, which drove water up a mountainside to a trimline of 1,720 feet (524 metres), with the main zone of total scour centred around 1,580 feet. No larger wave has been directly measured anywhere on Earth, in ocean or lake.
How did anyone survive the Lituya Bay tsunami?
Two of the three boats in the bay survived because of their position and sheer luck. Howard Ulrich rode the Edrie over the crest after letting out anchor chain, while Bill and Vivian Swanson's Badger was carried backwards over a forested spit into the open ocean, where they escaped by dinghy before the boat sank.
Can a megatsunami happen in Alaska again?
Yes — geologists consider it likely. Lituya Bay itself has produced destructive waves roughly every 25 to 50 years, and an actively moving landslide at Barry Arm in Prince William Sound is currently monitored because a sudden collapse could send a damaging wave toward Whittier in about 20 minutes.
What is the difference between a tsunami and a megatsunami?
A conventional tsunami is generated by seafloor displacement during an earthquake and travels across oceans as a long, low wave. A megatsunami is caused by a landslide, rockfall, glacier calving or impact dumping mass into confined water, producing extreme local runup heights but losing energy quickly with distance.
📚 Further Reading & Research Sources
The following journals and institutions publish peer-reviewed research on the topics covered in this article:
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U.S. Geological Survey / National Park Service — public domain aerial imagery of Lituya Bay, Alaska
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