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.