The largest earthquake sequence the United States has ever experienced happened east of the Rocky Mountains starting out in Northeast Arkansas on December 16, 1811. On the Richter scale, it was ~7.7 and five hours later AK experienced an aftershock of a magnitude of ~7.0. A little over a month later on January 23, 1812 New Madrid, MO experienced an earthquake with a magnitude of ~7.5, and again on February 7, 1812 reaching a magnitude of ~7.7. By March 15, 1812 approximately 2000 aftershocks had been felt. Damage from the largest of these shocks was reported from 300 miles away. In an effort to recover, then Missouri Governor, William Clark, ask for Federal relief for the inhabitants of New Madrid County. 1815 marked the first ever disaster relief act in the United States with Congress awarding $50,000. Earthquakes recorded after 1811-12 happened on January 4, 1843 with a magnitude of ~6.0 near Marked Tree Arkansas and again on October 31, 1895 with a magnitude of ~6.3 to 6.6 near Charleston Missouri, (USGS). New Madrid, MO was founded in 1789 with about 400 hundred residents and the 1811, 1812 earthquakes were named after New Madrid as this was the only place with a sizeable population at the time. The New Madrid Fault System extends approximately 150 miles southward crossing five state lines. West coast fault zones are called transform faults in contrast, the New Madrid fault is called an intraplate zone. This means that there is a weak spot in the middle of the North American Plate with a series of faults in the Earth’s crust and is called the Reelfoot Rift, named after Reelfoot Lake in Tennessee, that was formed during the quake, (The Encyclopedia of Arkansas History & Culture). It wasn’t until 1909 that seismographs were available, so in the 1800s, reports came from survivors. Witnesses reporting the ground moved like a wave indicating that the quakes ranged between 6.5 and 8. Comparing the Mercalli Scale, (invented in 1902 by Guiseppe Mercalli), based on observation to the Richter scale, (invented in 1935 by Charles Richter), based on actual magnitude is very similar. During a 7.5 magnitude few structures remain standing; bridges are destroyed, fissures in ground, pipes are broken, there are landslides, and rails become bent. A magnitude of 8.0 a person can witness waves on the ground surface, lines of sight and level distorted, and objects can be seen thrown up in the air, (Missouri, DNR). Early Research
In the late 1970s, using oil-exploration equipment, seismologists bounced sound waves off the subterranean rock where the tremors were centered. It was discovered that along a large crack interlocking layers of rock were offset vertically and in some places as much as 3,300 feet. The theory was that sometime in the past the rock was uplifted, possibly by volcanism. As the volcanic flow cooled, part of the rock collapsed, creating sharp breaks. During magnetic and gravitational surveys, the underground rift was discovered along the fault zone. Scientists measured this rift to be at least 120 miles long, and 30 miles wide. They also believe this rift was created millions of years ago when the North American plate began to pull apart and molten rock rose to the surface. The rift halted and a weak area remains, (Time, 1979).
To further explain this weak area, tectonic forces stretched North America in Precambrian time. As the continent pulled apart, rock fractured to create two huge fault zones. The fault zones failed to develop into a divergent plate boundary, in result, weaknesses remain in the lithosphere. New Madrid lies at the intersection of these major faults. As the North American Plate glides over the asthenosphere, it may pass over irregularities, or bumps, in that plastic zone, causing slippage and earthquakes along these faults. Along with the 1811 and 1812 Madrid quakes, there has been evidence of quakes occurring in...