Aviation officials had warned of possible US military activity.
Videos posted online show a bright flare rising high, before a wide, bright blue flash emerges in a cone shape. Many videos continue to track the light for several minutes.
On Friday, the Federal Aviation Administration (FAA) said night-time flights to and from Los Angeles International (LAX) would avoid flying over the Pacific Ocean to the west of the airport, the second busiest in the US.
The FAA did not disclose the reason for the change, but indicated that military activity in the area would take place between Friday and Thursday.
Media in California confirmed that the light came from an unarmed Trident missile fired from the USS Kentucky navy submarine.
The platinum-selling singer Josh Groban was one of those who tweeted his shock at the light: