1958 Lituya Bay megatsunami
 Anchorage
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| Date | July 10, 1958 UTC |
|---|---|
| Origin time | 06:15:58 UTC |
| Magnitude | 7.8 Mw |
| Depth | 35 km (22 mi) |
| Epicenter | 58°22′12″N 136°39′54″W / 58.370°N 136.665°W |
| Type | Strike-slip |
| Areas affected | Lituya Bay, Alaska |
| Max. intensity | XI (Extreme) |
| Tsunami | 525 m (1,722 ft) runup |
| Casualties | 5 |
Coordinates: 58°38′35″N 137°33′54″W / 58.643°N 137.565°W
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 destroyed vegetation up to 1,722 feet (525 m) above the height of the bay and a wave that traveled across the bay with a crest reported by witnesses to be on the order of 98 feet (30 m) in height. 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.
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