What’s the Deal with Quakes in East Tennessee?
By Stephanie Osborn
There’s been a lot of agitation and media reports (some legit, some less so) on the series of earthquakes in East Tennessee, and how this means the New Madrid fault system is about to go off again, and all kinds of stuff like that. Sarah and our mutual friend Jim W. and I have been discussing it, and I’ve been answering questions on social media and whatnot, so a quick tag-up with Sarah resulted in me sitting down to write this, to hopefully spread a little more understanding and lay some fears to rest.
So here’s the deal. According to the USGS, on 12 December, a magnitude 4.4 quake occurred near Decatur, TN. Epicenter location was 35.614°N 84.740°W, and it was about 9km (about 5.6mi) deep. OHMIGOSH! We had an UNPRECEDENTED quake in East Tennessee and it’s still rumbling!
So here’s a list of the quakes in the immediate area since the 4.4 quake (including the 4.4).
|4.4||11km NNE of Decatur, TN||2018-12-12|
|3.0||11km NNE of Decatur, TN||2018-12-12|
|2.5||11km NNE of Decatur, TN||2018-12-14|
But nobody even noticed that third quake! What’s up with that?
Well, it turns out that wasn’t all the quakes in the system, either. Here’s the rest of the quakes in the region for the last 30 days, as of 17 December, the time of this writing.
|2.7||10km SW of Calhoun, GA||2018-11-23|
|2.7||12km NW of Sweetwater, TN||2018-12-08|
|2.6||4km W of Blaine, TN||2018-12-13|
|3.0||4km ESE of Mascot, TN||2018-12-16|
Now, these quakes are not all clustered immediately around Decatur. But they are all along the edge of the mountain range, within ~100 miles or less of Decatur. But there’s something else important about them. They’re all magnitude 3.0 or less.
Why is that important? A 3.0 is just on the threshold of something that humans can even FEEL. Anything smaller than a 3, humans generally don’t even notice, and it must be detected by seismograph systems. Even many 3s go unnoticed, depending on location, underlying rock strata, building structure, etc. (Often they are completely unnoticed by those standing on the ground. Sometimes even larger quakes are unnoticed by those standing directly on the ground. I can speak from personal experience on this.)
So there were quakes going on in the area for some time (and likely longer than the USGS’ website is showing, since I’d have to request access to their database to go back earlier than 30 days ago, and that’s not going to be a quick process), and nobody even noticed, BECAUSE THEY WEREN’T BIG ENOUGH TO FEEL. (This is actually pretty typical of the fault zones associated with the Reelfoot Rift, as well.)
Are there any Reelfoot Rift quakes in that same timeframe? Well, sure.
|3.1||9km NW of Tiptonville, TN||2018-11-22|
|2.9||9km NW of Tiptonville, TN||2018-11-22|
|2.5||8km NE of Pinckneyville, IL||2018-12-09|
Note that the first two are squarely in the New Madrid Fault Zone, and the last is in the associated Wabash Valley Fault Zone.
OMIGOSH! They’re right! The East Tennessee quakes ARE setting off the New Madrid!
There were also quakes on the San Andreas, but nobody’s claiming the East Tennessee quakes affected IT. But it’s on the same tectonic plate! But there are no more structures connecting the New Madrid to the E.TN. faults than there are to the San Andreas.
Because the faults in the Appalachian orogenic zone are not only a long way away from the New Madrid system, they weren’t even created in the same era.
Many of the Appalachian faults are much, much older, and that’s one reason they’re largely inactive — the impetus to the original mountain-building/folding/faulting (this process is called “orogeny”) was the FORMATION of the Rodinia supercontinent (Appalachians 1.0, as some geologists waggishly make it, and they were likely huge) and the Grenville Orogeny, some 1,250,000,000-980,000,000 years before the present (YBP), due to the slamming together of tectonic plates. [Appalachians 1.0 are largely gone, eroded away by now.] The other sides of this orogeny — the other plates involved in the collision — include the Kibaran Orogeny in Africa (formed the Kibara Mountains in the Congo) and the Dalslandian or Sveconorwegian Orogeny in the northwestern (Nordic) part of Europe (now visible principally only as the Caledonian nappe system, which is a group of overthrust sheets).
The Reelfoot Rift, on the other hand, underlying the New Madrid and Wabash Valley fault systems, formed during the BREAKUP of Rodinia, some 750,000,000YBP. (For lots of information on this, I recommend my ebook, Rock and Roll, available for Kindle:
The more recent orogeny (aka Appalachians 2.0), sometimes called the Appalachian or Alleghanian Orogeny, began during the formation of Pangaea, some 480,000,000YBP, again, due to the slamming together of tectonic plates. The resulting range likely reached heights comparable to the Himalayas! (By comparison, the Laramide Orogeny, which formed the Rockies, ran from roughly 80,000,000YBP to possibly as late as 35,000,000YBP, so they are MUCH younger and higher, due to having less time to erode.) Like the Grenville Orogeny, the other side of this collision formed the so-called Anti-Atlas Mountains of northwest Africa.
This makes the faults in East Tennessee thrust, or more properly, overthrust faults. A thrust fault is where the upper fault block (“hanging wall”) overrides the lower (“foot wall”). But the Free Dictionary defines “overthrust fault” as a geological term: “A low-angle thrust fault in which displacement is on the order of kilometers,” that is, a nearly-horizontal thrust fault.
Example thrust fault, Cactus Hills, Saskatchewan, Canada:
Note the roughly 45º angle of the fault.
Example overthrust fault, Glarus overthrust fault, Swiss Alps (red line denotes fault):
Note the virtually horizontal angle of the fault.
So. Between the faulting and the also-prevalent FOLDING of the Appalachian Mountains, if you stretched it all out, some scientists have estimated that Tennessee would be easily 1/3 to 1/2 again as long as it is.
Most importantly: Thrust faults, regardless of angle, are COMPRESSION faults.
But the Reelfoot Rift, the actual geological structure that causes both the New Madrid and Wabash Valley quake zones, is a TENSION/EXTENSION rift/aulacogen system. Upwelling in the mantle from a hotspot directly under the center of the Rodinia supercontinent (which insulated the mantle, CAUSING the hotspot), combined with the “conveyor belt” effect of the resulting convection, as the hot magma spread out, moved away, and sank back into the mantle, literally tore apart the overlying crust. (The Southern Oklahoma Aulacogen may have been associated with this same breakup, though other geologists speculate it may have formed during the breakup of Pannotia, another supercontinent, younger than Rodinia.)
So. This means that the two systems — the Appalachian fault systems and the New Madrid/Wabash Valley fault systems — have COMPLETELY different ‘modes,’ if you will, as well as different forms and times of origin. More, they are separated by some 300mi, with no real geological structures connecting them.
That said, it’s my personal theory that the tectonic plates do act (to a limited degree) as flat springs, which means that significant distortion at one side of the plate can produce stress at the opposite side; Jim and I have talked about this, and watched that stress ping back and forth from plate boundary to plate boundary in the form of very large quakes, for quite a few years now. So it’s just conceivable that stresses transferred from the shifting in East Tennessee (which is largely just rebounding from the last ice age, according to the geological studies I’ve read) could make themselves known in additional stresses on the multiple faults of the Reelfoot Rift.
But frankly, these East Tennessee quakes are NOT large, but pretty small quakes. Most of the quakes that are occurring are getting no attention whatsoever, because they are below the threshold of being felt. And even the largest, the 4.4, is generally considered a minor quake most places. I don’t really expect a whole lot of stress transfer from these. Translated, I don’t expect any increased New Madrid activity on their account.
So what’s causing them? Leftovers.
The rocks are most likely still slowly rebounding from ancient stresses, including the various continental collisions that created the mountains, as well as the dissolution of the giant ice sheets from the last ice age. (And those ice sheets are VERY heavy — yes, they even make the continents sag.) More, since all those ancient stresses are no longer being applied, that rebound is apt to be slowly tapering off with time.
Yes, the Decatur TN 4.4 quake was felt over a large area — for the same reason the big temblors on the New Madrid were felt from the Front Range of the Rockies, up and down the East Coast, up into Canada, and down into the Caribbean — the rocks in this region are cold and unbroken, and will transmit the quake energies long distances.
But that doesn’t necessarily mean that a true Big One is coming in East Tennessee, as there would be on the New Madrid or the San Andreas, because the stresses on those two systems still exist, but the stresses on the Appalachian faults are gone. And as I said, they aren’t likely to transfer any significant stress to the New Madrid.
So do what you need to do to stabilize your homes (psst — brick/masonry chimneys are especially vulnerable) and their furnishings against falling over in case there’s a few more minor shakes, then consider it the interesting geological event it is, and move on.
Check out Stephanie’s latest Division One novel, book 9 of the series, Head Games, now available for preorder in print and ebook, coming in January 2019:
Two galaxies will go to war—if Fox can’t save Chairbeing Entiyti from assassins.
While Alpha One handles a Nazi zombie apocalypse, an assassin squad catches the Galactic Coalition President by surprise, leaving him at death’s door. Director Fox rushes to his old friend’s side, leaving Echo in charge of the Division.
But when the belligerent Persis Federation arrives from the Andromeda Galaxy, the Ennead orders Alpha One on the tricky first-contact mission, hoping to avert intergalactic war. Unfortunately, the Persan premier chooses Omega as his newest concubine, leaving Echo trapped between love and morality, and duty and orders.
On Earth, Ennead member Ordik Adita co-opts the Division directorship, revamping standard procedures, and enforcing his rule with an iron fist. A coup seems in progress.
The question is…how big is it?
Books in the Division One series, to date:
5) Trojan Horse
9) Head Games
With more on the way!