MEXICALI, Mexico - Aftershocks rattled the southwest Mexico-U.S. border Monday in the aftermath of a major earthquake that killed two people, blacked out cities and forced the evacuation of hospitals and nursing homes.

Sunday's 7.2-magnitude quake, centered just south of the U.S. border near Mexicali, was one of the strongest earthquakes to hit the region in decades, shaking at least 20 million people.

It had a shallow depth of 6 miles. But 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."

Building construction has also improved in northern Mexico, a region with a history of quakes, said Carlos Valdes, chief of the Mexican National Seismological Service.

"People see that it always shakes and have improved their construction capacity," Valdes said. "Then when the construction codes are implemented, there is stricter control, especially in larger structures."

Still, at least 45 businesses and dozens of homes were destroyed in Mexicali, a bustling commercial center along Mexico's border with California where the quake hit hardest, said Baja California state Gov. Jose Guadalupe Osuna.

A 94-year-old man was killed when a wall collapsed in his home in the city, and a homeless man died when the abandoned home he was living in collapsed in a farming community.

Most of the destruction was in small farming communities on the outskirts of Mexicali.

In the farming town of Guadalupe Victoria, the epicenter of the quake, roads were buckled by moist soil and some crevices were several feet wide.

People in the town of onion and asparagus farmers southwest of Mexicali are accustomed to earthquakes but nothing prepared them for Sunday.

At least 233 people were injured in Mexicali, most of them struck by falling objects, Osuna said.

The governor added that power had been restored to 75 percent of the city by midday and water service was expected to be restored later Monday. He said it would take time to quantify the damages.

"It's major damage. What interests me more is re-establishing public services and safeguarding life," he said.

Large cracks opened in the streets of Mexicali. Power lines fell, lamp posts were bent, bricks fell from buildings and furniture and merchandise was strewn around in shops and banks. Wells crumbled and the waters of a canal spilled into some streets.

Mexicali General Hospital was evacuated 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. Civil Protection Inspector Alan Sandoval said the most critical patients were being moved to hospitals in Tijuana and the coastal town of Ensenada.

The parking garage still under construction at Mexicali's city hall collapsed, but no one was injured.

Sandoval said the Easter holiday delayed damage assessments for Mexicali, as did landslides that slowed traffic on the toll road into the city.

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.

In Calexico, California, a city of 27,000 people across the border from Mexicali, police patrolled streets littered with shattered glass and closed off several blocks of damaged historic buildings. The city council declared a state of emergency.