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Earthquakes in California more mellow than on crusty East Coast

August 25, 2011 |  7:02 am

  Image: U.S. Geological Survey comparing shaking intensity around a 6.0 earthquake in central California in 2004 and this week's 5.8 earthquake in Virginia. The maps are to scale. Image prepared by USGS seismologist David Wald.
Why was Tuesday's moderately-powered 5.8 earthquake in Virginia felt as much as 500 miles away, shaking up Chicago, Atlanta, Boston and even Canada?

It's the rocks on the East Coast.

"The crust out in the East Coast is older, colder and harder, and does a better job transmitting the energy. So it's felt over a much wider area," said U.S. Geological Survey seismologist Lucy Jones.

In contrast, a stronger magnitude earthquake in central California in 2004 -- a 6.0 quake in Parkfield in Monterey County -- was felt only 200 miles away, as far away as San Francisco and Los Angeles.

In California, Jones said, "the bad news is we have a lot of faults. The good news is all those faults break up our rock. And just like a cracked bell doesn't resonate as well as a solid bell, our cracked-up crust [weakens] the waves as they pass through. It dies off pretty rapidly.

"But out there, the rocks are old and cold and hard, and do a lovely job of transmitting the energy," Jones said.

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-- Rong-Gong Lin II in Pasadena

Image: U.S. Geological Survey comparison of shaking intensity around a 6.0 earthquake in central California in 2004 and this week's 5.8 earthquake in Virginia. The maps are to scale. Credit: David Wald / USGS 

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