Yesterday an earthquake measuring 5.8 on the Richter Scale struck central Virginia and was felt up and down the East Coast. I was sitting at my desk in RTP when I felt my office building begin to shake. At first I thought it was nearby construction activity, but then the slats in my window shade began to rattle and the shaking increased. I realized that I was experiencing my first earthquake. It was both a bit frightening and a little exciting. My next thought? I should be blogging about this, but, alas, my day job was getting in the way again. So, somewhat belatedly, here is your Common Science blog on earthquakes.
The Earth’s crust is made up of sections called tectonic plates which float over a liquid layer of molten rock. While we all learned this in 8th grade Earth Science, the general acceptance of the theory of plate tectonics only occurred 50 to 60 years ago. As the plates slide over, under, or past each other tremendous strain builds up. Eventually the strain becomes strong enough that the two plates move suddenly to release the strain; put simply, an earthquake. The energy released in an earthquake can be larger than an atom bomb explaining how it can shake the earth hundreds of miles away.
Earthquakes are a very common phenomenon with approximately 500,000 occurring around the earth each year of which about 5,000 are in the United States. Over 80% of all earthquakes in the world occur in the well know fault lines like San Andreas in Southern California or the ring of fire in the Pacific. In the US we tend to associate earthquakes with California and Alaska, but earthquake epicenters have been recorded in 39 different states.
Earthquakes are measured with the Richter Scale, invented by Charles Richter in 1935 at the California Institute of Technology. For my geekier readers, the Richter Scale is a logarithmic measure of the shaking amplitude of the earthquake. Therefore an earthquake measuring 5.0 on the Richter Scale has a shaking amplitude of 10 times one at 4.0. Earthquakes measuring over 7.0 are considered major earthquakes capable of causing major damage. Of the 500,000 earthquakes which occur around the world each year an average of 18 register at 7.0 or above. Earthquakes registering between 5.0 and 5.9 like yesterday’s are considered moderate meaning that they are able to cause major damage in a small area to poorly constructed buildings or slight damage to well constructed buildings. For reference here are the magnitudes of some familiar earthquakes:
· Tohoku Japan (2011) 9.0
· Chile (2010) 8.8
· San Francisco (1906) 8.0
· Krakatoa (1883) 8.8
Here in the Southern Part of Heaven we feel pretty secure; too far from the coast to sustain serious hurricane damage, too far south for blizzards, to far east for tornados, and too far from Los Angeles for earthquakes. It turns out that earthquakes are a bit more common in North Carolina than we might have thought. The earliest recorded earthquake in NC occurred in Bath in 1735. McDowell County recorded 75 earthquakes between February and April of 1874. (That had to be pretty darn unnerving.) The strongest earthquake on record in NC occurred on February 21, 1916 with a magnitude of 5.2 knocking down chimneys, breaking window panes, and muddying local streams.
Earthquakes in our part of the country tend to register less than 6.0 on the Richter Scale since our tectonic plates slip past one another rather and over or under each other which results in less strain being built up. This was not always the case. Our beautiful western mountains are the result a collision between the US and Africa 240 million years ago. For the moment there is no evidence for increased earthquake activity in North Carolina so we can rest easy….. at least until Hurricane Irene hits on Saturday.
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