In regards to the two earthquakes today--one near Trinidad, CO (5.3) and the other near Mineral, VA (5.8). Its worth checking out the USGS seismic hazard maps, as both earthquakes occurred where it was predicted earthquakes were likely. (The star shows where today's earthquake occurred.)
Colorado: http://neic.usgs.gov/neis/eq_depot/2011/eq_110823_c0005idz/neic_c0005idz_z.html
Virginia: http://neic.usgs.gov/neis/eq_depot/2011/eq_110823_se082311a/se082311a_z.html
For some more interesting discussion of today's earthquakes, see: http://live.washingtonpost.com/east-coast-earthquake.html?tid=sm_twitter_washingtonpost.
Although the east coast is an old plate boundary, it can still have minor earthquake activity. The New Madrid fault zone near St. Louis, MO is a zone with potentially large earthquakes. According to USGS "The geologic record of pre-1811 earthquakes reveals that the New Madrid seismic zone has repeatedly produced sequences of major earthquakes, including several of magnitude 7 to 8, over the past 4,500 years." The 1811-1812 earthquakes included three earthquakes with a magnitude likely more than 7.
It is good to prepare for how to respond to an earthquake (stop, cover and hold on), even if you live on the east coast. Learn more about earthquake "drills" from the Great Shake Out program, see http://www.shakeout.org/.