A powerful earthquake swayed buildings from Los Angeles to Tijuana, killing two people in Mexico, blacking out cities and forcing the evacuation of hospitals.
The 7.2-magnitude quake, which was centred just south of the US border near Mexicali, Mexico, was one of the strongest earthquakes to hit the region in decades.
It struck at 3.40pm yesterday (11.40pm Irish time), according to the US Geological Survey (USGS).
"It sounds like it's felt by at least 20 million people at this point," USGS seismologist Lucy Jones said. "Most of Southern California felt this earthquake."
The quake was felt the hardest in Mexicali, a bustling commerce centre along Mexico's border with California, where authorities said the quake was followed by at least 20 smaller aftershocks, including three of magnitudes 5.1, 4.5 and 4.3. The initial quake had a shallow depth of six miles.
"It has not stopped trembling in Mexicali," said Baja California state civil protection director Alfredo Escobedo.
He said a man was killed when his home collapsed just outside of Mexicali, while a second man died when he panicked as the ground shook, ran into the street and was struck by a car.
At least 100 people were injured, most of them struck by falling objects. Power was out in virtually the entire city and the blackout was expected to last at least 14 hours.
All 300 patients had to be evacuated from Mexicali General Hospital to private clinics because the building had no electricity or water.
The parking garage at Mexicali's city hall also collapsed, Mr Escobedo said, but no-one was hurt.
There were growing reports of damage just across the border from Mexicali in Calexico, California, a city of about 27,000 people. The Calexico City Council met and declared a state of emergency, but there were no reports of injuries.
Law enforcement vehicles guarded streets in Calexico, where windows were shattered and bricks and plaster had fallen from some buildings.
Calexico police Lt Gonzalo Gerardo said most of the damage occurred in the city centre where buildings that were constructed in the 1930s and 40s and not retro-fitted for an earthquake of this magnitude.
"Downtown is going to remain closed until further notice. I honestly doubt that it will reopen soon," he said. "You've got a lot of cracks. You've got a lot of broken glass. It's unsafe for people to go there."
Strong shaking was reported across much of Southern California. The earthquake rattled buildings on the west side of Los Angeles and in the San Fernando Valley.
More than 100 miles west of the epicentre, San Diego's Sheraton Hotel and Marina was briefly evacuated after minor cracks were discovered in the floors, said Fire-Rescue Department spokesman Maurice Luque. All guests were later allowed to return.
Susan Warmbier was putting away her groceries in the San Diego suburb of Chula Vista when her husband asked: "Is the house moving?"
"We turned and we looked at the house, and it was actually moving. You could see it slightly moving left to right," she said.
Elsewhere in San Diego, there were reports of shattered windows, broken pipes and water main breaks in private buildings, but no reports of injuries. Coronado Bridge over San Diego Bay was briefly closed by the California Highway Patrol as a precaution.
Across the border in Tijuana, Mexico, the quake caused buildings to sway and knocked out power in some areas.