The New Madrid Earthquake Series of 1811-12

The recent minor tremors of the Mississippi valley recall the New Madrid earthquakes of 1811-12. Image credit: USGS

A series of minor earth tremors (maximum magnitude of just 2.6) have not so much shaken as shivered the Mississippi valley between Paducah and Memphis over the past 30 days.

Though of limited interest in themselves, these tremors call to mind one of the largest series of earthquakes on record in the contiguous United States – the New Madrid earthquakes of 1811 and 1812.

The New Madrid Seismic Zone

For most people, earthquake zones are associated with the planet’s major plate boundaries; and, generally speaking, this is true. In the United States, California and Alaska are the areas most at risk of large earthquakes, with smaller seismic events occurring elsewhere – in the Rockies and (more recently) a continuing swarm of minor tremors in Oklahoma. Most laymen therefore regard the eastern part of the US as more or less immune to major earthquakes.

In reality, this is far from the truth. An ancient plate boundary underlies a section of the Mississippi river valley from Memphis to just south of Indianapolis. That ancient rift, though deeply buried, is nevertheless active and is the source of current minor – and past major – seismic activity.

The New Madrid Earthquake Series of 1811-12

The location and seismic history of the New Madrid seismic zone. Image credit: USGS

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December 16 1811. Eliza Bryan, of New Madrid, was woken in the small hours of the morning by “a violent shock of an earthquake, accompanied by a very awful noise resembling loud but distant thunder…” This was the first, though not the largest, of the three ‘New Madrid’ earthquakes (though its epicentre was in fact some way to the north in Arkansas).

This was a tremor which, according the in the USGS, awakened people in New York City, Washington, D.C., and Charleston, South Carolina and “the ground surface was described as in great convulsion with sand and water ejected tens of feet into the air.”

The drama was not over. Just as the good people of the New Madrid area must have been beginning to put the disaster behind them, a second tremor struck, this time in the town itself. A fortnight later, again in New Madrid, the third and largest of the mainshocks struck. And even then a series of aftershocks followed, continuing the shaking and damage, destroying buildings and triggering landslides.

Two centuries ago, there was no way of measuring magnitude but estimates put the size of the largest tremor at between M7.5 and M8.7; even the smallest of these would have come close to matching the great San Francisco earthquake of 1906, while the higher end ranks the event as the second largest ever recorded in the United States outside Alaska.

Damage, Deaths and Lasting Impacts

Eliza Bryan’s account gives some idea of the terror. “The screams of the affrighted inhabitants running to and fro, not knowing what to do or where to go – the cries of the fowls and beasts of every species – the cracking of trees falling, and the roaring of the Mississippi, the current of which was retrograde for a few moments, owing, it is supposed, to an interruption in its bed – formed a scene truly horrible,” she wrote.

Damage caused by the New Madrid earthquakes. Image credit: USGS

The mechanics of the earthquake offer only the bare science behind dramatic events. There are no existing records of the death toll (though the USGS observe that the relatively sparse population at the time means that it may not have been large) but there’s no question that there was extensive damage, both locally and regionally.

And seismologist Robert Yeats suggests that the events were truly far reaching: “At the time New Madrid was the second largest town in the territory later to become the state of Missouri; it never fully recovered from the earthquakes.”

Future Vulnerability

The New Madrid earthquakes are not just extensively recorded in contemporary accounts; their story is told in the rocks themselves.

The USGS leaves residents with a stark warning:

“The geologic record of pre-1811 earthquakes reveals that the New Madrid seismic zone has repeatedly produced sequences of major earthquakes, including several of magnitude 7 to 8, over the past 4,500 years.”