1811–1812 New Madrid earthquakes
The first shock was on December 16, 1811. It had a magnitude of 7.5-7.9 on the Richter scale. There also was an aftershock of magnitude 7.4 on the same day. They were the most powerful earthquakes to hit the contiguous United States east of the Rocky Mountains in recorded history. We use the word "contiguous" because we wish to exclude Hawaii and Alaska, neither of which was then part of the United States.