Remembering the Tallest Known Mega-Tsunami That Destroyed a Bay in Alaska

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Damage from the 1958 Lituya Bay megatsunami can be seen in this oblique aerial photograph of Lituya Bay, Alaska as the lighter areas at the shore where trees have been stripped away. The red arrow shows the location of the landslide, and the yellow arrow shows the location of the high point of the wave sweeping over the headland. Credit: Wikipedia

At 10:15 pm on July 10, 1958, the ground shook violently as an M7.8 earthquake occurred along the Fairweather Fault in Southeast Alaska. The strong shaking set loose an enormous rockfall, in which 40 million cubic yards of material (9 Superdomes!) tumbled down a steep slope into the narrow inlet of Lituya Bay. The impact was heard 50 miles (80 km) away and the displaced water traveled outward as a landslide tsunami, stripping away trees and soil up to an elevation of 1720 feet (524 meters): the tallest known tsunami.

Photos by D.J. Miller, United States Geological Survey

The areas of destroyed forest along the shorelines are clearly recognizable in photos as light-colored areas rimming the bay. Three lives were lost as a fishing boat anchored in the cove at lower left was carried over the spit in the foreground; a boat underway near the entrance was sunk; and a third boat, anchored near the lower right, rode out the wave. This was the largest and most significant megatsunami of modern times; it forced a re-evaluation of large-wave events and the recognition of impact events, rockfalls, and landslides as causes of very large waves.

Part of the south shore of Lituya Bay showing the trimline, with bare rock below. Credit: Wikipedia

A 2010 model examined the amount of infill on the floor of the bay, which was many times larger than that of the rockfall alone, and also the energy and height of the waves. Scientists concluded that there had been a “dual slide” involving a rockfall which also triggered a release of 5 to 10 times its volume of sediment trapped by the adjacent Lituya Glacier, a ratio comparable with other events where this “dual slide” effect is known to have happened. Lituya Bay has a history of megatsunami events, but the 1958 event was the first for which sufficient data was captured at the time.

The effect of the tsunami still visible in 2010. Differently-aged vegetation is visible on the ridge separating Lituya Glacier from the main part of the bay – looking north from the head of the bay, Lituya Glacier to the right. Credit: Wikipedia

Landslide tsunamis can happen very quickly, and even in the absence of earthquake shaking. If you spend time in steep coastal areas, make sure to know natural warning signs of a tsunami: strong coastal shaking, a loud roar from the ocean or coastal cliffs, appearance of a wall of water, or sudden retreat of water past normal low tide. If something seems “off,” don’t wait for an official warning: immediately go to high ground or inland, and check for more information once you are somewhere safe.

Photos by D.J. Miller, United States Geological Survey
Lituya Bay, AK

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