Wednesday, August 24, 2011

Earthquakes and the East Coast

As you've probably heard by now, there was a 5.8 earthquake that struck in rural Virginia, southwest of Washington DC, on August 23rd. You can read more details about the event here, via Yahoo! News.

Here in California, a 5.8 might only be cause for some dinner conversation. Back East, though, earthquakes work a little differently....

Naturally, we see a lot more earthquakes through California than they see elsewhere in the country. This is because we sit astride the San Andreas fault system, one of the largest and most active tectonic boundaries in the world. That doesnt mean that other parts of the country are "earthquake-free", though. Fault systems of various sizes exist pretty much throughout the United States. In the Midwest, for example, the New Madrid fault system runs through southern Missouri, and had a major earthquake in the early 19th century.

Many fault systems run along the eastern seaboard. Though not as active today as the faults along the West Coast, hundreds of millions of years ago these faults were created when what is now Africa collided with North America, pushing up the Appalachian mountains, which at the time may have rivaled the Himalays in height. Though those mountains have eroded over time to the modest heights they have today, some of those faults created still remain, and do still occasionally trigger earthquakes.

When an earthquake occurs along the East Coast, it moves through the ground differently than it does here. The ground in California is riddled with fault lines of all sizes, cracked like a broken windshield. The bedrock along the East Coast is much less broken-up, with time and heat and pressure sealing many old fault lines and annealing the bedrock together.

When a vibration, like an earthquake, travels through the ground in California, it's like trying to ring a cracked bell; the vibration doesnt travel very well, or very far. Along the East Coast, however, the bedrock is more like a solid bell, and when an earthquake "rings" that bell, the vibration can travel much farther, which is why such a relatively small quake was still felt from Georgia to Toronto.

The Easter quake of 2010 near El Centro (magnitude 7.2) released about 25 times as much energy as yesterday's Virginia quake. The Japanese quake (9.0) released over 1500 times as much energy. Fortunately, though, it looks like the most that the Virginia quake did was rattle the nerves of some Easterners who arent used to a little temblor now and then :)

No comments: