The warning noted that no areas had yet experienced any waves, but that anyone near a coastline should be alert to the possibility of danger.
Smartphone users received warnings urging them to move to higher ground immediately, the San Francisco Chronicle reported.
“A series of powerful waves and strong currents may impact coasts near you,” the warning said.
“You are in danger. Get away from coastal waters.”
The paper said tsunami warning sirens were activated along a stretch of the coast, including in Bonny Doon in Santa Cruz County.
The USGS, which monitors seismic activity, said the quake struck at 10.44am local time.
Media reports suggested that the quake had been felt across the region, including in the Bay Area, where some people said they had felt strong shaking and rolling waves underfoot.
The US West Coast is the confluence of a number of the Earth’s tectonic plates, and tremors are not uncommon.
The area has been struck by a number of major earthquakes, including a 1994 quake that hit Northridge, in the Los Angeles area, killing dozens of people and injuring thousands more, as it wrought billions of dollars of damage to homes and infrastructure.
The San Francisco earthquake of 1906, which also caused a tsunami, is thought to have killed upwards of 3000 people, some of whom died in fires that erupted after the powerful tremor.
-Agence France-Presse
