The 1958 Lituya Bay earthquake occurred on July 9, 1958, at 22:15:58 PST with a moment magnitude of 7.8 to 8.3 and a maximum Mercalli intensity of XI (Extreme).

The impact was heard 80 kilometers (50 mi) away,[7] and the sudden displacement of water resulted in a megatsunami that washed out trees to a maximum elevation of 524 meters (1,719 feet) at the entrance of Gilbert Inlet.

[8] The two arms that create the top of the T-shape of the bay are the Gilbert and Crillon inlets and are a part of a trench on the Fairweather Fault.

[6] The shock was felt in southeastern Alaskan cities over an area of 400,000 square miles (1,000,000 km2), as far south as Seattle, Washington, and as far east as Whitehorse, Yukon, Canada.

[8] The impact of the rockslide included the creation of wave run up that shaved up to 400m of ice off the front of the Lituya Glacier and eroded or completely eradicated its rocky deltas.

The wave caused damage to the vegetation up the headlands around the area where the rockfall occurred, up to a height of 524 meters, as well as along the shoreline of the bay.

[10] 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.

Two more persons, a fishing boat captain and his seven-year-old son, were struck by the wave and lifted hundreds of feet into the air by the swell.

[6] In Yakutat, the only permanent settlement close to the epicenter at the time, infrastructure such as bridges, docks, and oil lines all sustained damage.

Sand boils and fissures occurred near the coast southeast of there, and underwater cables that supported the Alaska Communication System were cut.

[16] At 22:15 hours PST on July 9, 1958, which was still daylight at that time of year, an earthquake with a magnitude of 7.8 struck the Lituya Bay area.

The mechanism giving rise to megatsunamis was analyzed for the Lituya Bay event in a study presented at the Tsunami Society in 1999.

[17] The rockfall also caused air to be dragged along due to viscosity effects, which added to the volume of displacement, and further impacted the sediment on the floor of the bay, creating a large crater.

The large mass of rock, acting as a monolith (thus resembling high-angle asteroid impact), struck with great force the sediments at bottom of Gilbert Inlet at the head of the bay.

The impact created a large crater and displaced and folded recent and Tertiary deposits and sedimentary layers to an unknown depth.

Also, the impact and the sediment displacement by the rockfall resulted in an air bubble and in water splashing action that reached the 1,720-foot (520 m) elevation on the other side of the head of Gilbert Inlet.

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, as well as the energy and height of the waves.

[9][19][17] A subsequent analysis to the 1999 one that examined the wider impact of the event found that the rockfall itself was inadequate to explain the resulting accounts and evidence.