THE NEWS DESK
The News Desk is a collection of news, notes and breaking items affecting the Fredericksburg community.
Researchers study forces behind Virginia quake
By RUSTY DENNEN
Within hours after the magnitude-5.8 earthquake hit a year ago today near Mineral, geologists from around the country flocked to the site to begin studying the rare East Coast tremor.
A lot of work has been done, but there’s a lot more to do and years of data to analyze, says Wright Horton, a research geologist with the U.S. Geological Survey in Reston.
Horton is among those working to understand the event that caused $80 million in damage in Louisa County, shut down two reactors at the North Anna Power Station and damaged structures in the Fredericksburg area and as far away as Washington, D.C.
The USGS will be among the agencies gathering at 10:30 this morning on the National Mall for a press briefing on the impact of the quake, rebuilding and the likelihood of future events. They’ll discuss a national “Shake Out” campaign to engage the public on earthquake preparedness.
Horton co-authored an article this month in EOS, Transactions of the American Geophysical Union, titled “The 2011 Virginia Earthquake: What are Scientists Learning?”
For months, geologists worked on the ground to better understand the forces that caused the earthquake, and to study scores of aftershocks.
Beginning in March, they took to the air to map ground features with a laser mapping system known as lidar, which stands for light detection and ranging.
“Basically, you’re shooting lasers out of airplanes to make very accurate maps of the land surface,” he said in a telephone interview.
“We’re hoping that will reveal some subtle features that maybe are not so obvious walking on the ground.”
So far, “We haven’t found any direct surface expression of the fault that caused the earthquake, but plenty of shaking-type damage. There are some subtle features that need further investigation.”
The low-flying plane is equipped with other instruments. One measures the earth’s magnetic field, giving researchers a reflection of the rock properties underground.
“This is one of the techniques we’re using to come up with a 3–D image of the earth down to a depth to where the earthquake occurred” about three miles underground, Horton said.
Also on board: a gravity meter to measure subtle changes in the earth’s gravity—denser rocks have a stronger gravitational pull—and a gamma ray spectrometer. That helps identify rock properties near the surface by detecting minute amounts of radioactive uranium, thorium and potassium.
Data from flights in March and mid-July are still being analyzed. Horton says a variety of techniques are being used in combination.
“If lidar gives us a good impression” of a spot on the surface that could be a fault, “someone can dig a trench and see what it is.”
One surprising finding reported by geologists so far: The quake was not one jolt, but four, in the span of about a second.
Another discovery: Hard, dense rock in the eastern United States focused energy of the earthquake to spots far to the west and north of the epicenter, the tiny Louisa community of Cuckoo. For example, buildings in Culpeper, Fredericksburg and Washington were damaged.
The quake was felt from Florida to Canada.
According to the newsletter, “Considerable scientific uncertainty remains about the nature and scope of the earthquake hazard associated with the [Central Virginia Seismic Zone].”
Rusty Dennen: 540/374-5431
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