The Hermosa Earthquake was an event, a large natural disaster that struck the west coast of the planet Earth's North American continent in the year 2047.
As a result of the earthquake, a flood of water ended up submerging the city of Los Angeles under several meters of the Pacific Ocean. The city's ruins would end up providing the basis for the vital Hermosa Reef, a coral reef. Chakotay and Kathryn Janeway observed the city as a place that would one day be destroyed when they visited the past to the year 1996. (VOY episode: "Future's End")
Some believed that Los Angeles, particularly Hollywood, was already a ghost town before the Hermosa Earthquake. (ST short story: "The Immortality Blues")
The area would be renamed Los Angeles Island after the quake. (TOS novelization: The Motion Picture)