There’s some value in looking at earthquakes state-by-state, not least because residents tend to associate themselves with political entity rather than a geological one (And after all, that’s how the USGS arranges its earthquake histories). But there are problems with that approach too; and the southern-central state of Missouri is a classic example.
The trouble is that state boundaries are lines drawn on the map by geographers (where they follow topography) or politicians (who seemed to find it simpler to use a ruler than to read the instructions set out for them by Mother Nature).
And earthquakes don’t recognise state boundaries — or, more accurately, state boundaries aren’t drawn around earthquake zones.
Missouri on the Map
Missouri, which manages to fall into both cartographic camps (lots of straight lines but that little bit in the south east where the border is river-defined) is a classic example. Or should I say that it’s the New Madrid Seismic Zone (NMSZ) which is the classic example — underlying not just the Show Me State but also its neighbours south and east — Arkansas, Illinois and Tennessee.
In the course of the series of Decoded articles on earthquakes by state, we’ll keep coming back to the NMSZ, both because of its location and because of its seismic hazard. But it’s fitting that we’re dealing with the New Madrid Seismic Zone in the most detail in this article because it’s a Missouri city which gives the zone its name. Also, by no means coincidentally, the region is notorious for the occurrence of what’s possibly the largest earthquake in the east-central states. (I’ll let Missourians fight it out with their southern neighbours on the specifics of the quake).
Missouri Seismicity and Seismic History
In its page on Missouri, the Netstate website welcomes visitors to “a land of fertile plains, rolling hills, well-watered prairies and historic rivers.” This sounds a little too gentle for the kind of country ripped and torn by earth movements, and for much of the state, that’s a reasonable assumption.
A look at the map of noteworthy seismicity — earthquakes of at least magnitude 2.5 in the area in the past century — clearly shows this, with a focus of earthquake activity in the south east and in neighbouring states, and little or nothing elsewhere. The sharp-eyed among you will be able to trace the outline of the NMSZ on the map. Note it well — it’s an area which has shaken spectacularly in the past and may well do so in the future.
The New Madrid Earthquakes of 1811-12
The largest earthquake shown in the map in Missouri is a relative tiddler of M5.0 – big for the area but certainly not so in world terms. That earthquake took place in 1977. But go back a further 166 years years and it’s a different story.e
“On the 16th of December, 1811, about two o’clock, A.M., we were visited by a violent shock of an earthquake, accompanied by a very awful noise resembling loud but distant thunder,” wrote local resident Eliza Bryant, a few years after the event, “but more hoarse and vibrating, which was followed in a few minutes by the complete saturation of the atmosphere, with sulphurious vapor, causing total darkness. The screams of the affrighted inhabitants running to and fro, not knowing where to go, or what to do – 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 minutes, owing as is supposed, to an irruption in its bed — formed a scene truly horrible.”
That was bad enough, but just weeks later, in January 1812, another local, George Heinrich Christ, was in despair: “What are we gonna do? You cannot fight it cause you do not know how. It is not something that you can see,” he wailed.
And the nightmare was’t over because a few more weeks on, George Heinrich must have felt he was subject to some kind of Biblical punishment. “I do not know if our minds have got bad or what. But everybody says it. I swear you can still feel the ground move and shake some.”
What Made The Earth Move?
George Heinrich and Eliza, and presumably other residents too traumatised or illiterate to record it, had experienced a series of earthquakes which must have been among the largest on record in the central US. Estimates indicate that the three earthquakes may have had magnitudes of M7.5, M7.3 and M7.5 again.
Although earthquake records don’t go back much further, there’s geologic evidence to suggest that this sequence of events isn’t unique. “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,” warns the USGS.
The underlying cause (pun intended) of such large earthquakes in an area which ought by rights to be tectonically stable, is an ancient, and consequently deeply buried, tectonic feature – a major rift system. The Reelfoot Rift, notes Yeats, “may have formed as long ago as Late Precambrian” (which puts it at around 540 million years old) and it has clearly been periodically active ever since. As a result, we must consider the Rift, in geological terms, to still be active.
Has the New Madrid Seismic Zone Calmed Down?
All the evidence suggests that the NMSZ isn’t finished yet. Yeats observes that the 1811-12 events might be part of a cluster which is winding down; but equally, they may not be. The USGS has cautious words for residents, civil authorities and seismologists alike.
“It was the consensus of this broad group of scientists that (1) the evidence indicates that we can expect large earthquakes similar to the 1811–12 earthquakes to occur in the future with an average recurrence time of 500 years and that (2) magnitude 6 earthquakes, which can also cause serious damage, can be expected more frequently than the large 1811–12 shocks. Based on this history of past earthquakes, the USGS estimates the chance of having an earthquake similar to one of the 1811–12 sequence in the next 50 years is about 7 to 10 percent, and the chance of having a magnitude 6 or larger earthquake in 50 years is 25 to 40 percent.”
Missouri: A State With Two Faces
Missouri is split. The NMSZ is seismically active and the rest of the state is calm. But bear in mind that a major earthquake in the south east of the state would be felt well beyond the immediate area – and a large tremor could produce shaking which would cause extensive damage.
Missourians — and your neighbours — beware.
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