A series of earthquakes measuring up magnitude 5.4 occurred in southern California, Arizona and the Mexican border region on Sunday.
No damage or injuries were immediately reported, although buildings were evacuated in Brawley, 185km east-northeast of San Diego
The quakes were felt from San Diego and Orange County in California east into Arizona and swamped the measuring resources of the US Geological Survey.
The quakes began around 12.16pm (local time) with a magnitude 4.0 quake 26km north of El Centro, California. They lasted more than two hours.
The biggest shakes included a magnitude 5.4 with the same epicentre.
USGS says such a seismic swarm is a relatively rare occurrence there. There had been none since the 1970s and before that, the 1930s.