a number of earthquakes that happened near the town of new madrid, in late 1811, and early 1812 are known as the 1811–12 new madrid earthquakes today. the first of these happened december 16, 1811. it probably 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. until 2018, they are the most powerful earthquakes to hit the contiguous united states east of the rocky mountains in recorded history. the earthquakes and the seismic zone of occurrence are named for the mississippi river town of new madrid. the town was part of the louisiana territory at the time. today, it is in the us state of missouri.