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Friday, April 18, 2008

Seismology 101

By now, everyone has heard about the earthquakes in Illinois today. Some of the Cotillion members were chatting about it, and I offered to explain the difference between a San Andreas fault-type earthquake and a New Madrid fault-type earthquake. Someone took me up on it, so I wrote something up. Then it was suggested that I put it on my blog. So... here it is, with a few more details and a couple of links.

First of all, here's a link from the USGS that all kinds of nifty info on earthquakes. From the world's least reliable source for facts, we have pages on earthquakes, the San Andreas Fault, and the New Madrid Seismic Zone.

In a nutshell, an earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. (That's straight from the wiki page on earthquakes.) They can be caused when tectonic plates run into each other, pull away from each other, slide against each other, or some combination of those (those are tectonic or interplate earthquakes). They can also happen at cracks in a plate, where stress builds up and is then released (intraplate earthquakes). Then again, volcanoes and even nuclear bombs can cause earthquakes (or seismic activity, at least).

Now, in the discussion of the difference between San Andreas and New Madrid quakes, we first need to define the seismic zones. The San Andreas fault is the place where two tectonic plates slide against each other. Every now and then, as they move, they'll get stuck, and when they "unstick" is when an earthquake occurs. (this is my "explain it to a kid version", not a geology term paper, ok?) The New Madrid seismic zone is more of a series of scars and mini-faults than a meeting of two plates. Bazillions of years ago, according to geologists like my uncle, the North American plate made two attempts to break apart. These periods of activity led to rifts and scarring of the plate. There is a main rift (basically along the Mississippi, more or less) but it's as active as a plate/plate fault would be.

Within the rifts, lots and lots of sediment piled up. When everything gets rumbly in the seismic zone, the sediment kind of acts like water (the energy travels like the ripples in a pond when you toss a rock in.). Or, imagine one of those big gongs (I got this example from someone on Gatewaypundit- it's a great explanation). You whack it, and it vibrates... and vibrates... and vibrates... it just keeps going and going.

Instead of a quick SNAP and tremors for a couple hundred miles or so (the Great San Fran earthquake- ~8.0 give or take on the Ricther scale- was felt from Oregon to LA), New Madrid quakes are more rolling, like an ocean wave (the 1812 New Madrid quake- ~7.9 on the Richter scale- was felt in New Orleans, houses were destroyed 150 miles away in St. Louis, and there were reports that sidewalks were cracked in Washington, DC.)

I have no idea how many earthquakes occur along the San Andreas (a bunch, I'm sure.) People just don't think about earthquakes in the middle of the US- that's where you're supposed to have tornadoes, right? Well, in the past 30-some-odd years, there have been thousands of quakes in the general area, but most of them are too small to feel. The problem is that, if there is a big quake along the main rift, it will probably take out a city.
There you have it. Keep in mind that all of this came from Geo1005 at the University of Oklahoma more than 15 years ago, curious research from time to time, and fact-checking some numbers this morning. If I'm WAY off, please gently explain it to me in the comments. I'm not a geophysicist, and I didn't stay at Holiday Inn Express last night.

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