For central Virginia’s seismic zone, quake is an event of rare magnitude

August 23, 2011

For Virginia, the earthquake that struck Tuesday was the big one.

The state hasn’t suffered a quake of this size since the slightly larger one that rattled Giles County in 1897. “That’s the biggest earthquake in human history in Virginia,” said David Applegate, associate director for natural hazards at the U.S. Geological Survey. And the 5.8 quake Tuesday was as big as anything experts expect in the so-called Central Virginia Seismic Zone.

After all, this isn’t California or Japan, where vast tectonic plates are crashing into one another and jostling for position. While Virginia is an active seismic region and the nature of its bedrock can magnify the impact of tremors, the earthquakes it serves up are usually small stuff, barely noticeable temblors ranging in magnitude from 2 to 3.

The rarity of big quakes was cited by Dominion Power a few years ago when it asked for a permit to build a third nuclear unit at North Anna, Va., just 10 to 20 miles from Tuesday’s epicenter. The company cited a model that said the odds of an earthquake greater than a magnitude of 5.5 in central Virginia were slim, predicting about six such quakes over the next 10,000 years.

The 5.8 magnitude earthquake Tuesday disrupted electric power to Dominion’s two existing nuclear reactors, which quickly shut down and switched to four locomotive-size diesel generators. The reactors were built to withstand a 6.2 magnitude earthquake — four times greater than this one — and a Dominion spokesman said that the reactors and a nearby dam initially appeared to have survived intact.

But Allison Macfarlane, a professor of environmental science and policy at George Mason University, said that because Tuesday’s quake was shallow — just seven miles deep — it could shake the ground more than a deeper quake and threaten nuclear reactors and other buildings.

“We just learn new things all the time about the Earth. It just surprises us,” Macfarlane said.

No one understands the underlying geologic reasons for East Coast quakes. “What stresses are causing these infrequent but annoying earthquakes such as the one we just felt are not entirely understood,” said Marcia K. McNutt, director of the U.S. Geological Survey. “The earthquakes occur in the same regions again and again, activating old faults from long-ago geologic eras.”

While earthquakes along the East Coast tend to be smaller in magnitude than those in California, they tend to be felt over a much wider area.

The Central Virginia Seismic Zone sits in the middle of the enormous North American tectonic plate — an ocean of bedrock stretching from California to the middle of the Atlantic Ocean. Quakes generated in the middle of a plate tend to be weaker than the temblors jarred loose when two plates rub together — the potentially devastating strike-slip quakes seismologists say could hit California at any time.

Mid-plate quakes also tend to be smaller than the type of quake that rocked Japan on March 11. That 9.0 monster was unleashed when one tectonic plate slid under another — a so-called subduction zone quake.

Still, mid-plate earthquakes around the world regularly top magnitude 5, said Karen Fischer, professor of geologic sciences at Brown University.

That’s because faults, breaks in the bedrock, can be hidden just about anywhere.

On Tuesday afternoon, one of these faults some four to seven miles beneath the ground near Mineral, Va., came grinding to life, releasing a wave of energy that traveled outward at a “few miles per second,” said Mike Blanpied, associate coordinator for the USGS earthquakes hazard program.

The quake could have taken place in a previously undetected fault, but it could also be linked to one of two known faults on either side, said Callan Bentley, a geology professor at Northern Virginia Community College. He said the released energy traveled “along the grain” of the Appalachian Mountains to the northeast and southwest.

Once it hit, it reverberated, shaking the ground from St. Louis to New York, from Montreal to Atlanta, according to reports collected by the USGS Web site. The Nuclear Regulatory Commission said that while the North Anna plants were the only ones to shut down, nuclear power plants from the Carolinas to New Jersey felt the ground move slightly.

The geology of the eastern United States — which sits on top of old, cold rocks that efficiently transmit seismic waves — explains this huge shake zone.

In California, earthquakes — while more frequent — tend to travel a shorter distance. That’s because a cracked glass-like network of faults and breaks in the bedrock contains some of the seismic waves. But here in the mid-Atlantic, the older bedrock lacks such breaks, so it transmits vibrations quickly and efficiently.

“East Coast rock isn’t broken up as much,” said Mitchell Gold, a seismologist at the Lamont-Doherty Earth Observatory in Palisades, N.Y. “For the same size earthquake, we feel it further than they do out west.”

The rocks under the East Coast also tend to be colder than in California, and colder rocks transmit energy more efficiently, Gold added.

Within three hours, USGS specialists had classified the Mineral quake as a “reverse” earthquake. “There is probably a fault that’s inclined like a ramp,” said Blanpied. “As the rocks get squeezed together, the upper rocks climb up the ramp by a few inches.”

When enough pressure builds, the two great sections of rock suddenly slip. On Tuesday, the upper slab slipped up and the lower slab slipped down, the opposite of what happens in a “normal” earthquake, USGS seismologists said. Blanpied estimated the slabs moved in an area three to 10 miles wide on each side.

While unusual in the mid-Atlantic region, earthquakes larger than magnitude 5 are not uncommon in the country. Last year, the United States experienced 71 earthquakes of magnitudes between 5 and 6, according to USGS data. Most of those occurred in northern California, many off the coast and far from cities, said Richard Allen, director of the seismological laboratory at the University of California, Berkeley.

But while the reasons for those quakes are clear, experts are debating the ultimate cause of mid-plate quakes along the East Coast.

One camp argues that the bedrock here still holds residual stress from the last time the region was geologically active, some 250 million years ago. That’s when the continent was ripping apart from Europe and Africa to form the Atlantic Ocean.

Another camp points to deeper forces, rocks that flow in Earth’s mantle, beneath the crust. “These rocks are flowing all the time, and this continuous motion puts stress on the rocks above, the crust,” Fischer said, noting the idea was “highly speculative.”

Whatever the cause, seismologists caution that after a quake shakes a fault zone, stresses transmitted to surrounding rock can cause aftershocks. Several aftershocks shook central Virginia throughout Tuesday afternoon, including a 2.2 aftershock at 3:20 p.m. and a 4.2 at 8:04 p.m. But the risk of aftershocks decreases quickly, said Bentley.

“While this feels big to us, this was not that big of a quake,” he added. “We’re not going to see a huge number of aftershocks.”

Bentley said there is a remote chance that today’s quake was a “foreshock” — a harbinger of a bigger earthquake to come. But he labeled that as a very slim possibility.

And while seismologists cannot predict when an earthquake might next strike the region, they are certain of one thing: We’ll feel another again, someday.

“We can expect [magnitude] five or so in the mid-Atlantic region,” said Gold. “But we don’t expect them very often.”

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