EL CENTRO, California (AP) – The seismically active California-Mexico border was rattled Tuesday by a magnitude-4.5 aftershock of the major earthquake that hit the region April 4. No damage was immediately reported.

The United States Geological Survey (USGS) said the quake struck at 5:38 p.m. and was centered 28 kilometers west-southwest of El Centro, the Imperial County seat in California’s southeastern corner.

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 64 kilometers 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, Colorado.