Va. earthquake epicenter the focus of USGS study


Associated Press

Posted on July 12, 2012 at 3:30 AM

RICHMOND, Va. (AP) — Seismologists are taking a miles-deep examination of central Virginia to better understand the causes of an earthquake that rattled millions along the East Coast, closed the Washington Monument to visitors and took a nuclear power plant off line for months.

Provided a key piece of equipment arrives, a small aircraft laden with sophisticated measuring devices will begin flying Thursday over a 12-by-15-mile area — the epicenter of a magnitude 5.8 earthquake on Aug. 23, 2011.

The flights, which could last up to 10 days, will cover parts of Louisa, Goochland and Fluvanna counties to measure buried geological features and detect underground faults 7 to 9 miles deep.

The powerful earthquake, believed to have been felt by more people than any other in U.S. history, was centered 3 to 4 miles beneath Mineral in Louisa County.

Unlike the West Coast's much-studied San Andreas Fault, where the intersection of two tectonic plates have some Californians awaiting the Big One, the East Coast quake occurred in the center of the North American Plate. That plate stretches from the middle of the Atlantic Ocean across the U.S. to the Pacific Ocean.

"There's something else causing earth movement," said Anji Shah, a U.S. Geological Survey research geophysicist and lead scientist of the study. "So we don't know as much about them and we're trying hard to understand."

Shah likened the San Andreas Fault to two floating icebergs grinding up against each other. The Virginia quake, she said, would be akin to cracks or breaks occurring in the middle of the icebergs from the collision.

The August 2011 quake was felt from Georgia to Canada and damaged nearly 1,000 homes in Louisa County and permanently closed two schools. Dominion Virginia Power's North Anna Power Station also was shut down after measured ground movement was about twice the level for which the nuclear power plant was built. It was the first operating U.S. nuclear plant shut down because of an earthquake. The Nuclear Regulatory Commission gave the utility the green light in November to restart.

The earthquake also caused cracks in the Washington monument that will keep the 555-foot obelisk closed into 2014 for repairs.

The plane mapping the quake area — called the Central Virginia Seismic Zone — will carry equipment that will measure the earth's gravitational and magnetic pull and low-level radioactivity. The aircraft will map the area in a grid and fly as low as 400 feet. The USGS has warned residents and local officials of the low-flying plane.

"Our goal is to map the rocks beneath the earth's surface to better map the faults that are in the area," Shah said in an interview Wednesday. "In part, we'd like to map that faults that were responsible for the Aug. 23 earthquake and also those where the aftershocks are occurring."

Dozens of aftershocks have been felt since the August quake, including a magnitude 3.1 trembler in March that occurred within 10 miles of the epicenter.

"They seem to be decreasing but you never know," Shah said. "It's possible you could still get a larger one in the next month or so. It's possible."

So how does the measurement of magnetic fields and gravitational forces reveal what lies beneath the earth's surface?

Rock formations have different gravitational and radioactive readings that can signal where a fault occurs, Shah said. Once the data are collected, scientists will analyze them.

"They will hopefully give us a sense of where the major faults are and some of the details of those faults," Shah said. "It will give us a better sense of the general seismic hazard is and how it varies over the area."

They will not, however, provide the basis for an early warning earthquake system.

"Unfortunately, we can never predict earthquakes," Shah said. "Is it possible for an earthquake to happen again, sure? Earthquakes are unpredictable."

Instead, she said, the findings help local and state emergency officials better prepare for a future earthquake.


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