Alaska's 1958 Megatsunami

The 1958 Lituya Bay megatsunami occurred on July 9 at 22:15:58, following an earthquake with a moment magnitude of 7.8 and a maximum Mercalli intensity of XI (Extreme). The earthquake took place on the Fairweather Fault and triggered a rockslide of 30 million cubic metres (40 million cubic yards, and about 90 million tons) to fall from several hundred metres into the narrow inlet of Lituya Bay, Alaska. The impact was heard 50 miles (80 km) away, and the sudden displacement of water resulted in a megatsunami that washed out trees to a maximum elevation of 520 metres (1,710 ft) at the entrance of Gilbert Inlet. This is the most significant megatsunami and the largest known in modern times. The event forced a re-evaluation of large wave events, and recognition of impact, rockfall and landslide events as a previously unknown cause of very large waves.

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, and the accounts given by eyewitnesses, 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, as an almost immediate and many times larger second slide, a ratio comparable with other events where this "dual slide" effect is known to have happened.

Lituya Bay has a history of megatsunami events in modern times — the 1958 event is one of many evidenced, but due to its remoteness was the first for which sufficient data was captured at the time, to confirm the nature of the event.

Rockfall from the Earthquake

The earthquake caused a subaerial rock fall in the Gilbert Inlet. Over 30 million cubic meters of rock fell from a height of several hundred meters into the bay, creating the megatsunami. Two people from a fishing boat died as a result of having been caught by a wave in the bay. In Yakutat, the only permanent outpost close to the epicenter at the time, infrastructure such as bridges, docks, and oil lines all sustained damage. A water tower collapsed, and a cabin was damaged beyond repair. Sand boils and fissures occurred near the coast southeast of there, and underwater cables that supported the Alaska Communication System were cut. Lighter damage was also reported in Pelican and Sitka.

After the earthquake it was observed that a subglacial lake, located northwest of the bend in the Lituya Glacier at the head of Lituya Bay, had dropped 100 ft (30 m). This proposed another possible cause to the production of the 100-foot (30 m) wave which caused destruction as high as 1,720 feet (520 m) above the surface of the bay as its momentum carried it upslope. It is possible that a good amount of water drained from the glacial lake through a glacial tunnel flowing directly in front of the glacier, though neither the rate of drainage nor the volume of water drained could produce a wave of such magnitude. Even if a large enough drainage were to take place in front of the Gilbert Glacier, the run-off would have been projected to be on the opposite side in Crillon Inlet. After these considerations it was determined that glacial drainage was not the mechanism that caused the giant wave.

https://en.wikipedia.org/wiki/1958_Lituya_Bay_megatsunami