We talk a lot about the stars above our heads here at Spacefreighters, but today I want to talk a bit about the Earth under our feet.
We had a little earthquake here in my new home of Marshall the other day, not much more than a tremor of about 2.2 on the Richter scale. I didn’t even feel it myself up on the back side of Bailey’s Mountain where I live, but my friends and neighbors were all astir with the news.
|Area of the 2011 Mineral VA quake|
I was reminded of 2011, when a magnitude 5.8 earthquake hit Mineral, VA, near Washington, D.C. I was home in Fredericksburg that day, and it was my first experience of the earth moving under me. At first there was just a deep rumbling, and I thought they were testing weapons at the Naval Surface Weapons Center in Dahlgren, some 30 miles away. (When the wind blew just right, you could hear the BOOMs from my old house.) Then it got louder, and I thought a truck was coming down the street. But it got louder, and deeper, and it didn’t stop, and I finally realized, This is a freakin’ earthquake! By the time I got out of the basement and into the street, though, the tremblor had stopped.
Jaded Californians made great fun of us for making much ado over nothing, but that quake was actually felt by more people than any other in U.S. history, according to the U.S. Geological Survey. It caused millions of dollars of damage to public buildings in D.C. and Virginia, including the Washington Monument and National Cathedral. Buildings at the local community college in Fredericksburg were closed for months for repairs. Fortunately, no one was hurt.
|Ha, ha! Very funny, guys!|
Though many were quick to blame the fracking in Pennsylvania for the quake, some things were already known to be unusual about the normally quiet geology of the area. There are several identified fault lines in the southeastern U.S., though few non-geologists are familiar with them. One of them runs along the Appalachian Mountains (no doubt the source of our little quake in Marshall). A number of worrisome fault lines lie along the Mississippi River near Memphis, Tennessee and Cairo, Illinois—the New Madrid fault. A series of devastating earthquakes, some of at least magnitude 8.1, along the New Madrid fault in December, 1811 and January,1812 caused the Mississippi River to run backwards, created the “bottomless” Reelfoot Lake in Tennessee and rang church bells in Boston.
The reason those quakes were felt so far from the source is the same reason the Mineral quake of 2011 was so damaging. Geologists have long known that the earth of the eastern U.S. reacts like a bowl of Jell-O in a quake, transmitting the force of the shaking for long distances.
Now a study has shed new light on the geological structure of the southeastern U.S., which provides not only an explanation of why that happens, but a theory of why quakes happen in this supposedly “inactive” region in the first place. This area is far from the edges of the North Atlantic tectonic plate and shouldn’t be active at all. But according to Berk Biryol, a seismologist at the University of North Carolina Chapel Hill and lead author of the new study*, pieces of the earth’s mantle have been breaking off from underneath the plate throughout the area and sinking back down into the Earth, leaving the remaining portions thinner and more quake-prone.
Biryol and his fellow researchers used 3-D imaging techniques to track the effect of seismic waves originating more than 2200 miles away on the deep rock layers of the Earth’s crust in the southeastern U.S. They mapped the layers and found to their surprise that the mantle, rather than being uniformly thick, as you would expect of older, more stable rock, was thick and old in some spots, thinner and “younger” in others. The group theorizes that some of the rock is breaking off and sinking into the molten layer below (known as the asthenosphere), only to have “fresh” rock flow up to replace it. The younger rock is not as dense as the older rock. The differences in the thickness and age of the plate leave it more open to the stresses that lead to earthquakes.
So, fasten your seatbelts, folks, it’s gonna be a bumpy ride!
Information for this blog taken from “Scientists find likely cause for recent southeast U.S. earthquakes,” by Lauren Lipuma, GeoSpace: Earth and Space Science blog, May 3, 2016.
*Study published in Journal of Geophysical Research – Solid Earth, a journal of the American Geophysical Union, May 3, 2016.