The 1958 Lituya Bay megatsunami was triggered by a magnitude 7.8 earthquake in southeastern Alaska.
The earthquake struck on July 9, 1958, along the Fairweather Fault, a major strike-slip fault near Lituya Bay. The shaking destabilized a mountainside beside Gilbert Inlet, sending roughly 30 million cubic metres of rock into the narrow fjord. That sudden displacement forced water up the opposite slope and produced an extraordinary wave run-up of about 524 metres.
This was not a typical ocean-crossing tsunami generated by movement of the seafloor. It was a landslide-generated megatsunami confined largely within the bay. The narrow, steep-sided geography amplified the wave, helping explain its remarkable height.
Some scientific summaries list the earthquake as magnitude 7.8–8.3, reflecting differences in historical magnitude estimates. Nevertheless, 7.8 is the conventional answer used in many references and is the single answer a fair quiz accepts. The event killed five people and remains the best-known example of an extreme landslide wave.