"It sounds like it's felt by at least 20 million people," USGS seismologist Lucy Jones said. "Most of Southern California felt this earthquake."
Sunday afternoon's earthquake hit hardest in Mexicali, a bustling commerce center along Mexico's border with California, where authorities said the quake was followed by many smaller aftershocks, including five with magnitudes between 5.0 and 5.4. The initial quake had a shallow depth of 6 miles (10 kilometers).
"It has not stopped trembling in Mexicali," said Baja California state Civil Protection Director Alfredo Escobedo. Escobedo said one man was killed when his home collapsed just outside of Mexicali and another died when he into the street in panic and was struck by a car.
At least 100 people were injured in the city, most of them struck by falling objects. Power was out in virtually the entire city and the blackout was expected to last well into Monday, Escobedo said.
All 300 patients were evacuated from the Mexicali General Hospital to private clinics because the building had no electricity or water. But it was unclear how long the emergency generators powering the private clinics could last, and the patients might have to be moved again to hospitals outside the city, he said.
The parking garage at Mexicali's city hall also collapsed but no one was injured, Escobedo said.
Scientists said the main earthquake probably occurred on a fault that hadn't produced a major temblor in over a century. Preliminary data suggest the quake occurred on the Laguna Salada fault, which last unleashed a similar-sized quake was in 1892. Since then, it has sparked some magnitude-5 temblors.
U.S. Geological Survey seismologist Erik Pounders described the area as a "chaotic" system of faults that needed more research.
In Calexico, California, a city of 27,000 right across the border from Mexicali, the city council declared a state of emergency amid growing reports of damage. Law enforcement vehicles guarded downtown streets, where windows were shattered and bricks and plaster had fallen from some buildings.
Calexico police Lt. Gonzalo Gerardo said most of the damage occurred downtown, where buildings constructed in the 1930s and '40s were 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."
Rosendo Garcia, 44, 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, adding that homes in his trailer park were seriously damaged, including one knocked off its foundation.
A home for seniors in Calexico built in the early 1900s was evacuated and its residents moved to a Red Cross shelter. The Fire Department also brought some sick and elderly people to hospitals because of power outages and gas problems.
Lights shattered, ceiling tiles fell and shelves collapsed at a Subway 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."
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 and water sloshed out of swimming pools.
More than 100 miles (160 kilometers) west of the epicenter, San Diego's Sheraton Hotel and Marina was briefly evacuated after minor cracks were discovered in its floors, said Fire-Rescue Department spokesman Maurice Luque. All guests were later allowed to return.
Susan Warmbier was putting away 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, San Diego Fire-Rescue Department spokesman Maurice Luque said. Coronado Bridge over San Diego Bay was briefly closed 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.
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 fire department's chief of aquatic rescue. The beach filled up again shortly.
If the preliminary magnitude 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 even felt hundreds of miles away in Phoenix, a rarity for residents there. Jacqueline Land said the king-sized bed in her second-floor 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.
The quake was felt in the fire and medical dispatch center in downtown Las Vegas, but there were no reports of damage or injuries.
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.
Christopher Weber reported from Los Angeles. Associated Press Writers Andrew Dalton, John Antczak and Alicia Chang in Los Angeles, John S. Marshall in San Francisco, and Matt Reed and Katie Oyan in Phoenix contributed, Elliot Spagat in Calexico and Sue Major Holmes in Albuquerque, N.M., contributed to this report.