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.
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The U.S. Geological Survey said the quake struck at 5:38 p.m. and was centered 18 miles 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.