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!
Cheers,
Donna
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.
OMG. That graphic had me pounding the floor. Hilarious!
ReplyDeleteBut in all seriousness, I'm glad it wasn't any worse, Donna. Oklahoma has been suffering quite a few, and they believe fracking is the cause there, too.
I actually "survived" a 4.3 here at home. It was at least a decade ago, and I was working on my computer late at night (imagine that!) when I heard a rumbling like someone was running through the house. My first thought? Intruders? I jumped up and looked into the living room. Not intruders....earthquake! Earthquake? In New Mexico? Wait. We don't get earthquakes...or hurricanes or even very many tornadoes. Earthquake? Really?
The local news confirmed next morning that we had, in fact, had a tremor. It wasn't catastrophic, but it did do minor damage and we had have several cracks in the walls of the house that needed repair. Sort of a badge of honor. Never been through an earthquake before. Hopefully, I never will again.