"We learn geology the morning after the earthquake". 

-Ralph Waldo Emerson

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What a Friday!  First, a quick apology to the geology of New Jersey.  I'm sorry...you DO have a reason to exist. Now that that is out of the way, let's talk rocks. 

 

As most of you know and many of you actually felt, we had a decent sized earthquake (as far as East Coast quakes go) on Friday and continue to experience smaller aftershocks as the ground's new disposition continues to settle itself.  For my part, I was with my second period class working in the lab when I felt what were clearly seismic waves rumbling through the building.  Turning around to look at the display of my seismic station, I was quite surprised by the size of the initial waves compared to what I'm used to seeing and my students got to see one of those rare moments when Schmidt's emotions got the better of him and my excitement bubbled over almost instantaneously.  

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Within one minute of that initial incident, my phone systematically melted down like it did for the last sizable quake in August 2011 when that was kicked off by Mike Massimo of Marple Newtown S.D. texted "Uh...what was that?"...still one of the funniest texts I've ever received given the context.  This year's "first to text" goes to two geologists, Jason Schein of Elevation Science Institute and Stephen Williamson (UD Class of 2014) who sent "Did we just have an earthquake ?!?!" (note the punctuation) and "Did you just feel that?" respectively.  My excited, expletive-laden responses will remain off the STORMcast.  Their texts were immediately followed by literally dozens of other emails and texts.  Way too many people have my email address and phone number.  Sorry if I didn't get back to you all but I was like an unsupervised kid in a candy store at the time and was caught up in the moment of gathering whatever data I could in the opening minutes.  Those first two minutes waiting for USGS to post where it happened were brutal.  Now on the back end, I'm flattered (I think) that so many of you think of checking with me first when you think the world is going to end, so...thanks?  

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If this is the extent of your interest, you can now check a box saying you felt a pretty legitimate earthquake for being East Coasters and move on towards the next apocalyptic event, the eclipse, on Monday.  If you've got a bit more interest and would like to know why this happened, read on. 

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Our Geologic Setting and its (Deep) Roots

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I think it is important to realize that the earthquake itself was not really much of a surprise to geologists of the region.  If you were to ask geologists about where they thought a quake was most likely in the Mid-Atlantic, you would have probably found this one happened right in the zone the geologists thought it would.  Granted, this type of activity is rather rare and usually not of this magnitude but its causes and patterns are well known and documented. 

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As some of you probably know, the Earth's crust is composed of moving plates which move around the face of the planet and occasionally crash into one another but then rift apart later.  These movements produce the majority of the planet's earthquake and volcanic zones and are studied in a field known as tectonics. 

 

The East Coast of the United States, like the rest of the continent, has undergone considerable tectonic tumult and change over the past 500 million years. During an era of geologic time known as the Paleozoic Era which lasted from about 540-250 million years ago, much of the basic East Coast geology we see today was created including the various provinces of the Appalachian Mountains that were created in several different events as various ancient continental blocks crashed into us.  At that time, the young Appalachians would have resembled the higher, sharper pinnacles of the Alps or Andes more than the heavily weathered peaks we see today.  These collisions from our southeast caused immense strain on the preexisting rock which in turn them to fail and fracture in a more or less southwest to northeast direction and set up some of the region's oldest fault zones. Towards the end of the Paleozoic Era (about 290 million years ago), the famous supercontinent Pangaea was created and directly linked the East Coast with what is now the northwest corner of Africa.  Again, more faults from the car crash between two massive continental blocks.

 

After about 50 million years of relative Pangaean happiness however, new plate boundaries began to form and the supercontinent began to disintegrate with one of its biggest breakups being a north/south split between what are now North America and Africa.  This split, known as the Central Atlantic Magmatic Zone (CAMP), opened up like a giant zipper and created the fledgling Atlantic Ocean.  However, the rifting also created a period of intense volcanism that was one of the worst the planet had ever experienced.  Eastern PA and northern New Jersey were both very close to ground zero for this hellish volcanic landscape and the lavas left behind can be seen today as a massive series of scars that run through the Gettysburg region, past Reading (PA), through northern New Jersey and into the Palisades Formation of New York where spectacular lava basalts line the Hudson River.  When this rifting subsided around 180 million years ago, there was a widening ocean between us and Africa and yet another set of faults that were created by the rending of the newly separated plates.

 

Considering all of this trauma, the idea of East Coast faults should not really be a surprise at all.  What makes our faults a bit different from those of the West Coast though is that we are no longer along the edge of a tectonic plate while theirs are.  It's a very different tectonic situation.  We are situated smack in the middle of the North American tectonic plate and have been geologically quiet since Pangaea rifted apart 180 million years ago.  Our faults (the scars) are still present and represent weak points in the regional rock layers but the stress that made them move is pretty much extinct so their continued activity is pretty rare and usually not intense.  Plates do flex and bend though and every so often, rock just reaches it maximum stress point and snaps.  This is an earthquake and, when they occur far from plate boundaries like the ones in NJ and PA, they are known as "intraplate" earthquakes. 

 

In this case, the quake is being attributed to the Ramapo Fault (right), a long, shallow fault system that winds through PA from near Pottstown, through New Jersey and into New York along the Hudson River. It was created in the tectonic events discussed earlier and has smaller faults branching off of the main line.  The fault system is loosely associated with what is called the Lancaster Seismic Zone in PA, one of the only parts of the state that has any seismic risk at all and even that is pretty small.

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So, there you have it.  Your geologic moment for the weekend and an explanation of how Friday's events unfolded geologically...for the past 500 million years.  I hope you found this informative.  Please pass it along so I stop hearing about people who think the quakes are associated with Monday's eclipse. NO...BAD DOG.  Come on...even Fox News got this one right.

 

From your master of disaster; have a nice weekend everybody and enjoy the eclipse on Monday!

 

rs

1630
 

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