USGS is still studying the August 23 east coast quake. They've concluded that this was the most widely-felt earthquake in U.S. history.

How exactly they define "widely-felt" I don't know. Surely the 1811-12 New Madrid earthquakes would have been felt more widely, if there were more people around back then to feel them. That earthquake epicenter in 1811-12 was the frontier of the U.S. at that time.


http://www.dmme.virginia.gov/DMR3/va_5.8_earthquake.shtml

August 23, 2011 1:51pm; 5.8 Magnitude Earthquake
Louisa County, Virginia

Virginia and much of the East Coast experienced a widely-felt earthquake at 1:51 p.m. eastern daylight time on Tuesday, August 23, 2011. According to the U.S. Geological Survey, the epicenter of the quake was located near Cuckoo, in Louisa County. With a magnitude of 5.8, this is the largest Virginia earthquake recorded by seismometers. 24 aftershocks have been reported by the USGS and the area is currently being monitored by geophysicists from several leading science institutions. Click here for more detailed information on the 5.8 magnitude earthquake.

[b]The U.S. Geological Survey is now reporting that this is the most widely-felt earthquake in U.S. history.[/b]