It struck at the shallow depth of 0.9km (0.6 miles) and its epicentre was near the city of Ridgecrest, about 240km north-east of Los Angeles.
Seismologist Dr Lucy Jones said the quakes could continue. "This is an earthquake sequence," she said at a press conference. "It will be ongoing."
"Every earthquake makes another earthquake more likely," she added, saying there was a 10% chance of a similar or even larger quake following in the next week.
However, Dr Jones said it was not likely the quake would trigger shocks on other fault lines.
The earthquake was felt as far away as Las Vegas in the neighbouring state of Nevada and over the border in Mexico.