things to think about

today I am thinking about: EARTHQUAAAAAKE
August 23, 2011, 3:59 pm
Filed under: Uncategorized

So I am sure by now you have heard about the !5.8 earthquake in Virginia! At least, if you are on the East Coast, youA map showing the intensity of the August 23rd earthquake in Virginia from the United States Geological Society are pretty obsessively blogging/tweeting/facebooking about it, and/or freaking out and trying to get through to your family. A 5.8 earthquake is a big deal when your mom is a few miles from the epicenter. But, geologically: not so so bad.

What does a 5.8 earthquake even mean? It is the logarithm of the amplitude of the waves felt by the seismograph, as corrected for the distance from the epicenter. What does that even mean? It means that however big the displacement the seismograph felt, you scale that number accordingly. It also means that each whole number step means the earthquake was 10 times larger than the previous whole number step: so a 5.8 is ten times bigger than a 4.8 and an 8.8 is 1,000 times bigger than a 5.8.  (Want to know a ton more about this? Like math about waves? The United States Geological Survey has a website for you!)

So a 5.8 earthquake isn’t nothing, but really, in the world of earthquakes, it’s pretty small potatoes. Damages to buildings will mostly happen when they are poorly made or very old. A water main burst at the Pentagon. That kind of thing.

So, if it’s so small, why did we feel it? It’s because this was an intraplate, rather than interplate, quake.

The U.S. east of the Rockies is, for the most part, pretty far from any boundary between the large plates that make up the earth’s crust. A lot of the dramatic earthquakes on the West Coast (or in Japan, for that matter) come from interplate activity – two plates getting stuck on each other, pressure building, until the pressure releases in a big burst. But here in the rest of the country, earthquakes are intraplate, caused by a fault inside of a larger plate.  It’s the same basic idea, but they’re harder to detect. Often scientists don’t even know about the faults until they earthquake. (That is probably not the right verb.)

These kinds of earthquakes also travel further. Ergo, this quake was in Virginia, but we were feeling it all over the eastern seaboard.

The big mother intraplate earthquake was the New Madrid Earthquake of 1812. It was a series of intraplate earthquakes felt across the eastern United States over a few days in December 1811 and January 1812. There weren’t seismographs back then, but estimates put it as a series of 4 earthquakes all at ~7.5 on the Richter scale. The descriptions are dramatic:

The earthquakes caused the ground to rise and fall – bending the trees until their branches intertwined and opening deep cracks in the ground. Deep seated landslides occurred along the steeper bluffs and hillslides; large areas of land were uplifted permanently; and still larger areas sank and were covered with water that erupted through fissures or craterlets. Huge waves on the Mississippi River overwhelmed many boats and washed others high onto the shore. High banks caved and collapsed into the river; sand bars and points of islands gave way; whole islands disappeared.

(taken from the United States Geological Survey’s website on the quake, where much of this info came from.)

So it’s not as if intraplate earthquakes are less potentially hazardous; just that they’re a little less common and a lot less predictable. (That should make you feel safe!) These New Madrid earthquakes were felt all over the United States – over an area 10 times larger than the giant San Francisco quake of 1906, which (again, without a seismograph, it’s hard to know for sure) was probably of a higher magnitude.

What does all this mean? It means we might keep feeling aftershocks, even if they’re relatively minor. It means that feeling it from far away doesn’t correlate directly to the size or damage. Especially since the Japan earthquake in March, it makes sense we are all feeling a little panic. But in this case, it is probably unnecessary. Hopefully everyone is fine, and if the Pentagon is a little soggy, well, serves them right.