The 7.2-magnitude quake struck at 3:40 p.m. PT, about 38 miles southeast of the border city of Mexicali, Mexico, according to the U.S. Geological Survey. It had a shallow depth of 10 kilometers. Three aftershocks of magnitudes 5.1, 4.5 and 4.3 followed within the hour and dozens of smaller temblors struck later.
"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 earthquake was felt the hardest in Mexicali, a bustling commerce center along Mexico's border with California.
Baja California state Civil Protection Director Alfredo Escobedo said a man was killed when his home collapsed just outside of Mexicali. He said the other man was killed 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. At least 20 aftershocks were felt in the city, he said.
"It has not stopped trembling in Mexicali," he said.
Power was out in virtually the entire city and the blackout was expected to last at least 14 hours, Escobedo said.
All 300 patients had to be evacuated from the Mexicali General Hospital to private clinics because the building had no electricity or water. But the emergency generators powering the private clinics might not last long and authorities might have to move patients to hospitals outside the city, he said.
At least 20 people were trapped in two stores when the electrical doors would not open but firefighters quickly freed them, Escobedo said. Authorities initially feared people were trapped in homes but Escobedo said that was not the case.
The parking garage at Mexicali's city hall also collapsed, Escobedo said, but no one there was hurt.
Several Landslides forced the closure of the toll free highway connecting Tijuana to Mexicali and slowed traffic on the toll road.
As darkness fell there were growing reports of damage on the U.S. side of the border in Calexico, a city of about 27,000, but no injuries. The Calexico City Council met and declared a state of emergency.
Law enforcement vehicles guarded downtown 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 downtown Calexico where buildings that were constructed in the 1930s and '40s and not retrofitted 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."
The southeast portion of the city lost electricity for about four hours.
Rosendo Garcia, 44, said he was driving his daughter home from work when the quake struck.
"It felt like I was in a canoe in the middle of the ocean," he said.
He said homes in his trailer park were seriously damaged, including one that was knocked off its foundation
His wife, Elvia Garcia, 47, said her refrigerator door flew open and the food fell out. Her TV and computer crashed to the ground. All the plates were broken.
"It was tremendous," she said.
The Fire Department responded to several calls to transport sick and elderly people to hospitals because of power outages and gas problems. A senior living center built in the early 1900s was evacuated and the people were moved to a shelter by the American Red Cross.
Lights shattered, ceiling tiles fell and shelves collapsed at a Subway sandwich restaurant in Calexico, said manager Rosie Arellano.
"Everything is shut down, the whole town," Arellano said. "All the stop lights and the street lights are out. We have no power."
More than 100 miles west of the epicenter, 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 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.
A heavy chandelier in the couple's dining room "was just swaying like someone had just jumped from it," she said. There was no sound from the quake itself, but she added, "All of the dogs in the neighborhood were just barking crazy."
Elsewhere in San Diego, there were reports of shattered windows, broken pipes and water main breaks in private buildings, but no reports of injuries, San Diego Fire-Rescue Department spokesman Maurice Luque said. 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. Families celebrating Easter ran out of their homes, with children screaming and crying.
"I grabbed my children and said, 'Let's go outside, hurry, hurry!"' said Elizabeth Alvarez, 54, who said she was just getting ready to leave her house with her kids in an eastern Tijuana neighborhood when the quake hit.
No tsunami warning was issued, but hundreds of people on Tijuana's crowded beach feared the worst and fled when they felt the ground shake, said Capt. Juan Manuel Hernandez, the city fire department's chief of aquatic rescue. The beach filled up again within an hour.
Scientists said the main earthquake probably occurred on a fault that hadn't ruptured in over a century. Preliminary data suggest the quake occurred on the Laguna Salada fault, which last broke in 1892 and unleashed a magnitude-7.2.
U.S. Geological Survey seismologist Erik Pounders describes the area as a "chaotic" system of faults that needs more research.
The main quake was initially reported as magnitude-6.9. The updated magnitude was still an estimate, but if it holds it would be California's largest temblor since the 7.3-magnitude Landers quake hit in 1992, Jones said. There were at least two other 7.2-magnitude quakes in the last 20 years.
The main quake was felt hundreds of miles away in Phoenix, where residents rarely feel the earth shake.
Jacqueline Land said her king-sized bed in her second-floor Phoenix-area apartment felt like a boat gently swaying on the ocean.
"I thought to myself, 'That can't be an earthquake. I'm in Arizona,"' the Northern California native said.
Mike Wong, who works at a journalism school in downtown Phoenix, said he was in his second-floor office when he heard "cracking sounds, like Rice Krispies," and felt the building sway. He said the swaying lasted for "just a few seconds," and he didn't notice any damage.
The quake was felt in the fire and medical dispatch center in downtown Las Vegas, but there were no reports of damage or injuries, according to Tim Szymanski, a spokesman for Las Vegas Fire and Rescue.
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, interrupting Easter dinners. Some stalled elevators were reported, water sloshed out of swimming pools and wine jiggled in glasses.
Power outages were rare, and mostly brief. Most of the 3,000 customers who lost power in southwestern Arizona, and the more than 5,000 who went dark in Southern California, regained power within minutes, utility officials said.
Clint Norred, a spokesman for the Yuma, Ariz., Police Department, said the quake was very strong there but he'd heard no reports of injuries or major damage. "In my house, it knocked a couple of things off the wall," he said.