The California-Mexico border region was rocked by a magnitude-5.7 quake, rattling nerves from San Diego north to Orange County and Los Angeles.
The US Geological Survey said the quake was centred eight kilometres southeast of Ocotillo in Imperial County - about 137km east of San Diego. It struck about 4.26pm (NZT) and has been followed by dozens of aftershocks.
The San Diego County Office of Emergency Services made a round of calls to all cities in the county and found no reports of significant damage. Louis Fuentes, chairman of the Imperial County board of supervisors, said he had no immediate reports of damage.
"As soon as it hit, my wife said, 'Grab the baby.' My daughter ran out to the back yard," said Fuentes, who was in his garage in Calexico, about 50 kilometres east of the epicentre. "It thumped really hard."
The quake was an aftershock of the deadly Easter Sunday magnitude-7.2 quake that shook Baja California and Southern California, said Egill Hauksson, a seismologist at the California Institute of Technology in Pasadena. He said the epicentre of Monday's quake occurred in the same zone of the quake in April.
"Aftershocks can go on for months and years," he said.
Thousands of aftershocks have occurred since the Easter earthquake. At least 45 aftershocks were recorded immediately following Monday's 5.7 quake, with the largest measuring at magnitude-4.5.
A 5.7-magnitude earthquake "could break windows, it could throw things on the floor, it could create cracks on the wall, but we don't expect things to collapse," Hauksson said.
Fuentes said the chandeliers swayed at his home and metal objects banged but nothing fell off the shelves. Imperial County suffered significant damage in April's Easter Sunday quake.
"All the lamps, the liquor bottles and the TV hanging from the ceiling shook, but nothing dropped," said Marina Garcia, an employee at the Burgers and Beer restaurant in El Centro, about 50km east of Ocotillo.
The quake was felt as a gentle rolling motion in the Los Angeles area.
San Diego's Petco Park swayed during the quake, causing a momentary pause at the Toronto Blue Jays-San Diego Padres baseball game. The public address announcer asked that everyone remain calm. The crowd cheered.
David Eckstein of the Padres had just grounded out in the bottom of the inning when the stadium began shaking. The next batter, Chase Headley, stayed out of the batter's box for a few seconds, then stepped in.
San Diego County Sheriff's dispatch supervisor Becky Strahm said some of her colleagues reported things falling off their shelves, but there were no immediate reports of significant damage or injury.
The quake followed a series of temblors that struck Southern California over the weekend, including a pair of moderate earthquakes that rattled a desert area east of San Diego. Residents in downtown San Diego felt the ground rumbling during at least one of the Saturday quakes.
At least 20 aftershocks were recorded within 30 minutes of today's earthquake, with the largest measuring at magnitude-4.1.
More than 1000 people reported feeling the shaking, according to online citizen reports compiled by the USGS. The strongest shaking was felt in San Diego County.
