The Great New Madrid Earthquakes were a series of three powerful earthquakes and aftershocks that occurred in the winter of 1811-1812. The epicenter is believed to be near the city of New Madrid, Missouri, and the quakes were felt throughout the Mississippi River Valley and beyond.

Each of the three main New Madrid quakes was classified as major, meaning a magnitude of 7-7.9. Magnitude measures the energy of the seismic wave, or vibration, released by an earthquake. Each whole number increase in the scale means a tenfold increase in wave amplitude, and a release of 31 times more energy. So, a 7.5 quake releases 31 times the energy of a 6.5 quake.