MEXICALI, Mexico – Aftershocks rattled the southwest Mexico-U.S. border on Monday morning 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 region in decades, shaking at least 20 million people.It had a shallow depth of 6 miles (10 kilometers). 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."