New Yorkers in high-rise buildings were quick to evacuate after feeling the rumblings from today's (Aug. 23) magnitude 5.8 East Coast earthquake. If you were standing on the street level, however, you might have found yourself wondering why hundreds of people were streaming out of their buildings.
Why was the East Coast earthquake felt in skyscrapers, but not on the street?
It's difficult to say for certain; big quakes are relatively rare on the East Coast, so scientists don't have a lot of experience studying their characteristics and determining what is normal, and what is not. [Full coverage of the East Coast earthquake]
That said, there is enough known about East Coast geology to speculate. The Earth's crust is different on the East Coast than in Western states — it's colder and denser and, as a result, seismic waves tend to travel much greater distances along the East Coast. [Can Humans Cause Earthquakes?]
"If you have an earthquake centered around Virginia, the waves will spread out in a much larger area than they would if the earthquake originated in California," said Jack Moehle, a professor of engineering at the Pacific Earthquake Engineering Research Center at the University of California, Berkeley.
And this might be why buildings, but not the streets, rumbled today in New York City. The types of waves that travel the longest distances have low, relatively-subtle frequencies, and are known as long period waves. (Think of a blue whale's undulating tail compared to a hummingbird's rapidly beating wings.)
"A long period wave will tend to excite a building more so than a high frequency wave," Moehle told Life's Little Mysteries. "It might be quite natural that people at the base level might not feel the wave, but the building might resonate such that people inside it will feel it."