EL CENTRO, Calif.
USGS computers initially calculated the quake's magnitude at 5.1, but seismologists downgraded it a couple of hours later after reviewing data from hundreds of seismic stations across Southern California.
"With more stations reporting, you get a more accurate magnitude," said California Institute of Technology seismologist Anthony Guarino.
The aftershock was centered about 40 miles northwest of the magnitude-7.2 quake that struck Easter Sunday in northern Baja California, Mexico, said Susan Potter, a geophysicist at the USGS' National Earthquake Information Center in Golden, Colo.
"The 7.2 quake was shallow and large so we'd expect to see aftershocks for quite some time," she said.
In Mexicali, residents were frightened because memories were still fresh from the earlier quake, said Alfredo Escobedo, Baja California's state civil defense secretary. But no reports of injuries or damages were received, he said.
Inspection teams were sent to small communities on the outskirts of the city to check for damage.
Tuesday's quake was felt at San Diego's Petco Park, about 90 miles to the west of the epicenter.
At the Styles and Smiles salon in El Centro, shampoo bottles rattled but the haircuts and massage therapy sessions continued.
"We just try to keep the customers relaxed," owner Lourdes "Lulu" Castro said. "We're used to it. We were just like, 'OK, here it comes.'"