August 31, 2011 —
The 5.8 magnitude earthquake that struck Virginia on August 23 wreaked a bit of damage nearby: the tip of the Washington Monument was cracked. But its vibrations were felt as far away as Detroit and Canada. That’s according to information from the U.S. Geological Survey’s feature called “Did you feel it?” located at http://earthquake.usgs.gov/earthquakes/dyfi/events/se/082311a/us/index.html .
Here in the Upper Delaware Valley, a lot of people felt it, and others didn’t. At 93 Erie Ave, in the building that houses The River Reporter, three staffers felt a slight bit of movement, but several others did not. An upstairs tenant indicated that the movement set her rocking chair in motion.
Rowena Lohman, an earthquake scientist with Cornell University, said that whether or not a person felt the earthquake is related to what they were doing at the time. She said, “I felt it, and ’m about 300 miles away [in Ithaca], but that’s in part because my window faces southward, and I was sitting at my office quietly at the time basically looking toward the earthquake, and I felt ‘boom, boom,’ and other people in my building didn’t feel it. People on the ground floor often don’t feel it as much as people higher up. We had people on the 11th floor of this building on campus who actually felt the building swaying, so they really felt it.”
Lohman said that when such large earthquakes occur on the East Coast, it’s not unusual to have them felt across a much larger expanse of land than earthquakes on the West Coast of comparable size. She said the earth’s crust in California is much younger and hotter, therefore “it absorbs energy much better; it’s mushy in a way,” whereas on the East Coast the crust is “much older; it’s cold, so when you hit it with something like an earthquake, it rings like a bell.”
She added that in the east, judging not only by modern seismic readings but also historical records of such things as where church steeples fell, there had been five or six earthquakes of this size over the past couple of centuries.
Locally, some people wondered whether this event could be connected in any way to hydraulic fracturing.
Lohman said, “There’s not any reason right now to draw that sort of conclusion. I think you’re right that people are going to look at that just because the stakes are so high. But these earthquakes have occurred for a long time. There was a similar one almost in the same place where this earthquake was in 1897. So we know that they happened long before we did hydrofracturing.
“When we do anything to the earth such as mining coal, making dams, anything where we’re moving mass from one spot to another, we do cause earthquakes; that’s been observed for a long time. But they’re very, very small, and they’re restricted to a very small area. In California, for instance, there’s a lot of extraction of oil and ground water, and there are earthquakes associated with each of those, but it hasn’t resulted in the sort of thing that we saw in Virginia.”