"In the next 24 hours, there's a 56 percent chance of a 5.0 aftershock. And probably there will be eight or nine 4s in that time period," said Caltech seismologist Kate Hutton. "In the next seven days, we would expect to see about 22 magnitude-4s in this aftershock sequence and maybe another two magnitude-5s."
The stronger aftershocks, according to Hutton, you probably won't notice.
"Well I think none of these aftershocks that have been projected are likely to be strong enough to be felt in L.A. The chances of a quake larger than 7.2 are way down below one percent," said Hutton.
The seismic waves from the earthquake rattled much of Southern California and parts of Arizona and Las Vegas. Caltech estimated that at least 20 million people felt the earthquake.
"Oh man, it was bad, big time. We started running," said Mexicali resident Rosa Vasquez.
The quake hit at 3:40 p.m. on the Laguna Salada fault, which stretches 43 miles along the U.S.-Mexico border. The last time it produced a major quake was 1892.
The quake was centered about 30 miles south of Mexicali in an area that had been rocked with magnitude 3.0 quakes all week.
Mexicali bore the real brunt of the quake, with much of the infrastructure heavily damaged. Officials said 45 building collapsed or were significantly damaged in the quake, and two people were killed.
The main hospital in Mexicali was damaged, and patients were evacuated. They will not be allowed to return until the building is evaluated. Meanwhile, hospital beds were set up underneath nearby tents.
Mexican President Felipe Calderon is expected to declare the city a disaster zone when he tours the area, freeing up federal funds.
Things are slowly returning to normal in Calexico, along the Mexican border, with a massive cleanup continuing in the downtown area, where the city sustained the most damage. There were piles of glass and broken concrete and tiles. Authorities said most of the damage occurred in the downtown area, where buildings were constructed in the '30s and '40s and not retrofitted for an earthquake of this magnitude. Due to such construction, many buildings are in partial or full collapse.
"The noise was deafening," said Monica Guajardo in Calexico. "The noise coming from the alarms, the buildings sort of making noise, the palm trees swaying, the kids cryin, everything. It was really, really strong."
City crews were out on Monday collecting light poles, which bent over in the shaking. The entire downtown shopping area has been red-tagged because of the structural damage.
"There was substantial damage down to our most oldest part of town," said Calexico Fire Chief Peter Mercado. "We had our hands full for the last few hours, but we're trying to sustain and stabilize and really get a good grasp as for the damage, but we won't have a good idea until tomorrow morning."
Calexico is a major retail center, catering to Mexican shoppers from Mexicali.
According to Calexico's fire chief, about two dozen people were injured during the shaking, but most of those injuries were minor.
Mexicali is still struggling with the effects of all the shaking. Calexico's fire chief said his counterpart in Mexicali is asking for an emergency hookup of water from the U.S. side.
Water mains in Mexicali were destroyed, so there wasn't enough water to fight fires on Sunday afternoon that broke out in the city. Many structures burned to the ground.
"We want to help them, but I think that at this time right now we want to take care of our city," said Calexico Mayor David Ouza.
According to the Tijuana Civil Protection Director Alfredo Escobedo, the powerful earthquake killed a man whose home collapsed in northern Mexico. Escobedo also said that another person was killed when the person panicked as the ground shook, ran into the street and was struck by a car.
Many residents were just sitting down to Easter gatherings when the earthquake hit. The shaking lasted between 40 to 50 seconds.
"The cars were just moving, like they were just toy cars. Our vehicle was parked parallel against the sidewalk. When it was over, the car was facing (almost perpendicular)," said Alonzo Lucero of Calexico.
The quake was one of the strongest to hit Southern California in decades, and high-rise buildings in Los Angeles and San Diego rocked back and forth when it hit. There were reports of shattered windows, broken pipes and water main breaks in private buildings in San Diego, but there were no reports of injuries.
The quake knocked out power to 346 California Edison customers in Rancho Palos Verdes. Customers in Huntington Beach, Montebello and Compton reported having "flickering lights."
According to Jennifer Ramp of San Diego Gas & Electric, nearly 2,400 customers lost power in Dana Point. In San Diego County, more than 600 customers lost power in Borrego Springs when the earthquake struck, Ramp said. Utility officials said that all Southern California residents who experienced any power interruption regained it within minutes.