Sunday's 7.2-magnitude quake, centered just south of the U.S. border near Mexicali, was one of the strongest earthquakes to hit region in decades, shaking at least 20 million people. It had a shallow depth of 6 miles (10 kilometers).
The human toll was minimal in large part because the energy from the quake moved northwest of Mexicali toward a less populated area, said Jessica Sigala, a geophysicist from the U.S. Geological Survey.
"We were just kind of lucky that the energy went the other way," Sigala said. "With every earthquake, the earth starts moving a certain direction. It started south of Mexicali and the rupture moved northwest."
Sunday afternoon's earthquake hit hardest in Mexicali, a bustling commerce center along Mexico's border with California, where one man was killed when his home collapsed just outside town and another died when he into the street in panic and was struck by a car.
Across the border in Calexico, police patrolled streets littered with shattered glass Monday, and a downtown area containing was closed because of damage.
Scientists measured about 100 aftershocks early Monday morning, said seismologist Kate Hutton at the California Institute of Technology in Pasadena.
They caused no new damages in Mexicali, said Alan Sandoval, a civil protection inspector.
At least 100 people were injured in Mexicali, 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 because of the structural damage to the building, which also was without electricity and water. Some patients were taken to private clinics but others were in tents.
It was unclear how long the emergency generators powering the private clinics could last. Sandoval said the most critical patients would be transported to hospitals in Tijuana and the coastal town of Ensenada.
Sandoval said the Easter holiday delayed damage assessments for Mexicali, as did landslides that slowed traffic on the toll road into the city.
The parking garage at Mexicali's city hall collapsed but no one was injured.
Scientists said the main earthquake probably occurred on a fault that has not 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 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.
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.
"You've got a lot of cracks. You've got a lot of broken glass," he said. "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.
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.
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.
In Tijuana, the quake caused buildings to sway and knocked out power in some areas. Mexican 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 the area'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 as far away as Las Vegas, where 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.
Associated Press Writers Christopher Weber, Andrew Dalton, John Antczak and Alicia Chang in Los Angeles; John S. Marshall in San Francisco; Matt Reed and Katie Oyan in Phoenix; Sue Major Holmes in Albuquerque, N.M.; and Mariana Martinez in Tijuana, Mexico, contributed to this report.