Exelon Corp. (EXC) said Tuesday that four of its nuclear-power plants have declared "unusual events" following an earthquake in northern Virginia. Exelon'sLimerick plant near Philadelphia, Oyster Creek plant near Toms River, N.J., Peach Bottom plant near Lancaster, Pa., and Three Mile Island Unit 1 plant near Harrisburg, Pa., declared unusual events in the wake of a 5.9-magnitude earthquake centered northwest of Richmond, Va., the company said.