Reports last week indicated that Southern California had been shaken by a 5.1 magnitude earthquake. After further review, however, the USGS downgraded the quake to a 4.5 and has opined that the tremor was most likely an aftershock from the April 4, 2010, 7.2 magnitude quake, which rattled buildings in Baja.Continue Reading Recent Earthquake Activity Causes Many to Wonder About Coverage