El Centro, Calif. - A magnitude-5.1 earthquake struck the seismically active California-Mexico border region Tuesday, shaking a wide area of Southern California. There were no immediate reports of damage. 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 the state's southeastern corner. The region on both sides of the border has been shaking with aftershocks since a magnitude-7.2 quake struck in northern Baja California on April 4.