The Reelfoot Rift
At the end of 1811 the United States was just beginning its expansion across the Mississippi River. St. Louis, Missouri had been transferred from Spain and France seven years before, and was home to just a few thousand people. Lewis and Clark’s expedition had begun there, as it was then right on the unexplored frontier. So when the most powerful earthquakes in the history of the continental US hit not far away, there was relatively little damage and few casualties.
Most earthquakes happen along the edges of the Earth’s great continental plates. San Francisco and Los Angeles suffer because of the Pacific Plate twisting north as North America heads south. The same general effect causes quakes in Alaska, Chile, Japan, and India. Missouri is distant from any plate boundary, though, which makes its case more interesting.
Beginning about 150 miles southeast of St. Louis, a traveler enters the land between the St. Francis and Mississippi Rivers. This is the north end of the Mississippi Embayment, a cone-shaped lowland that reaches all the way to the Gulf of Mexico and guides the course of rivers throughout the south-central United States. It was here, at the town of New Madrid in the northern point of the cone, that the 1811-12 quakes occurred.
Discounting smaller aftershocks, there were three of them in all: one on December 16th, 1811; one on January 23rd, 1812; and the largest of the three on February 7th of the same year. An aftershock six hours after the first quake was large enough to count as a fourth main earthquake according to some. How bad were they? They happened before modern seismological instruments were invented, but the United States Geological Service estimates they were an 8.1, a 7.8, and roughly an 8.0 on the Richter Scale. Some estimates wander as high as 8.8 for the last of the three quakes. By comparison, the famous San Francisco earthquake was a single 7.8. Only a series of extremely strong quakes in Alaska from 1938 to 1965 have exceeded the New Madrid earthquakes anywhere in the United States. And if they were that big, there hasn’t been a set of such strong earthquakes in such a short period of time anywhere in the world in at least two thousand years.
On the other hand, the Richter scale doesn’t tell most people very much. It’s also commonplace to show the very much larger area over which the New Madrid earthquakes were felt as compared to San Francisco’s. But very few of us have any experience of large earthquakes that could be contrasted with these. Eyewitness accounts of how the quakes actually affected the landscape have a place here.
A thrust fault forced the flow of the river to reverse for a little while, strongly sweeping the river flatboats that happened to be in the vicinity some miles to the north; the winter ice dams on the Ohio River had broken not long before the final big quake in February, and as the frontier river traffic picked up for the coming spring many rivermen were drowned. Rapids were formed that dropped as much as ten or twelve feet, and took several days to erode away after the river returned to its normal flow. The typical oxbow lakes of the Mississippi were deranged and often drained. The area may have even gained a substantial body of water, Reelfoot Lake, where the ground subsided at one point. Trees were knocked sideways and continued growing straight, to be photographed a century later. The moist sand that generally underlies the soil in the region liquefied and, under pressure, flew up and out of the surface — infertile, these sand blows could be hundreds of meters wide and can still be seen dotting fields all over the region to this day. At the time, they rendered about 3.5 million acres (about 14,000 square kilometers) of land unfit for farming by changing their drainage pattern and waterlogging the whole area.
In particular, the liquefied sand made the ground so precarious when shaken that destruction of man-made objects was widespread. Every single building in New Madrid and the nearby town of Little Prairie was destroyed. The latter was abandoned entirely, though eventually the modern Caruthersville, Missouri was built there.
Partway between measuring the quakes and describing them is Jared Brooks. He lived in Louisville, Kentucky at the time, and being of a scientific bent in those pre-formal days of seismology he developed his own system of earthquake strength. His most powerful category he characterized as “Most tremendous, so as to threaten the destruction of the town, and which would soon effect it should the action continue with the same degree of violence.” He counted eight events like that from December to February, not even including the two initial quakes on December 13th, and remember that the hometown he thought in jeopardy of being destroyed was almost 400 kilometers from New Madrid. Brooks even rigged up some horizontal pendulums to prevent argument about “did you feel that?” temblors on the other end of the scale—and with their help counted 1874 quakes from the week of December 22nd to March 15th. Many contemporary accounts seem somewhat histrionic at first by claiming that the ground constantly shook and quivered like the surface of the ocean, but a bit of math shows that there was a quake noticeable in Louisville happening on average every ninety minutes for three months! It could well have felt continuous closer to the center of the action.
From the surface, there’s little sign of why the region suffered such enormous and continuous rumbles. On the west there’s a peculiar line of low hills, Crowley’s Ridge. Otherwise, the land looks normal. In the last hundred years, though, we’ve developed technologies to extend our senses down into the ground. Even with seismographs it’s tough to see much detail, but the culprit behind the New Madrid Earthquakes is big, if deep. The Reelfoot Rift extends for 150 miles through four states, no less than three miles underground and often more.
The rift is a weak spot in the center of North America, formed 750 million years ago by a mantle plume — a “hot spot” deep inside the Earth. The ancient subcontinent Laurentia, which would one day form the core of the United States, started to split down the middle as the hot rock underneath lifted and thinned the surface. This sort of action causes new oceans to form, but in the case of the Reelfoot Rift the underlying plume died away before this happened. All that was left was a flaw in the Earth’s crust where forces deep inside the planet could leak out. If you live along the Great Lakes or St. Lawrence River, you can thank another failed rift for them, though it produces much weaker earthquakes than New Madrid.
This is because when the North Atlantic started to form to the east about 180 million years ago, the new motion produced more changes under the Reelfoot Rift and the crust was weakened further. The area is now sensitive to pressures anywhere on the continent, a partial hinge in a plate of rock that extends for thousands of miles around.
At the time that it formed, the Proto-Laurasia and the rift were near the south-central part of Rodinia, a single landmass composed of all the world’s continents. A single ocean, Mirovia, lapped against it on all sides and covered three-quarters of the planet. What would one day be Missouri was a bit below the equator then but, despite being in the tropics, a person walking along the newly forming rift would have felt the cold. 750 million years ago, the Earth was in the middle of the worst ice age it ever had, far harsher than the more recent one that brought us wooly mammoths and Neanderthals. It’s even possible that the Earth was frozen over entirely, though it’s likelier there was still some open ocean in a thin band of the tropics.
When the Reelfoot Rift was forming, it would have been a shallow valley likely filled with snow most of the year — or even a glacier. To either side, the valley would have been framed by volcanoes powered by the mantle plume, similar to today’s Cascade Range — the mountains around Mount St. Helens. The whole area would have been slashed through with arroyos; 750 million years ago, life was still microscopic and limited to the ocean, so there was no ground cover to hold the soil in place when it rained, or the sun melted ice and snow.
Eventually, though, the Earth warmed up and the rift was covered up. The Appalachian Mountains formed to the east, and as they were rounded off by erosion their silt, sand, and pebbles buried it. The glaciers of the last ice age added more debris. The land was swampy until recently, when it was drained and turned back into farmland. But the sand on the rift is still soft, and still saturated with water. When the next big quake comes, the soil is perfect for transmitting the shocks a long distance. In 1812, the land was barely inhabited and the destruction far less deadly than it might have been. These days nearby metropolitan Memphis has over a million people, and Nashville throws in another million and a half not much further away. New Madrid itself has a coal-fired electrical power plant parked on the fault—its 812-foot smokestack, the taller of two, is locally believed to be right on top of the epicenter of the quakes.