Thursday, April 19, 2012

[Geology2] How the Virginia earthquake packed such a punch

Last summer's Virginia earthquake continues to reverberate in the scientific community.

The 5.8-magnitude tremor August 23 wasn't huge by global standards, but punched above its weight. Scientists were struck by the way the tremor near the town of Mineral managed to damage the nation's capital 84 miles away, putting cracks in the Washington Monument and toppling a spire at the National Cathedral. So: How'd it do that?

They're also studying the ongoing aftershock sequence. The Mineral event isn't truly over, because the fault is still generating small quakes, though fewer and fewer. Herein lies a research opportunity for understanding enigmatic East Coast tremors — how long does it take for the earth to calm down?

And Mineral is also a warning. No one was shocked when a moderate quake hit the Central Virginia Seismic Zone — that's precisely where you might expect an earthquake. But it was much bigger than any recorded tremor in the zone. The previous standard was a roughly 5-magnitude event in 1875, its strength estimated from the damage it caused.

The obvious implication is that other seismic zones in the east are capable of generating earthquakes more powerful than anything in the historical record.

"The Virginia earthquake suggests that what we've seen in the past is not as bad as what we could potentially see in the future," said John Ebel, a Boston College geophysicist who spoke with reporters in a conference call Wednesday from a meeting of the Seismological Society of America in San Diego.

Based on what happened in Virginia, "we have a seismic hazard that we really have to take seriously" in other major East Coast cities, including Philadelphia, New York, Providence and Boston, Ebel said.

'Highly stressed' rocks

East Coast earthquakes are inherently mysterious. There are no tectonic plate boundaries nearby — no big slabs of crust grinding and sliding and heaving past one another. The Mineral quake occurred on an unmapped fault.

Scientists have studied the aftershocks and have been able to model the fault and understand how the earthquake had such strong effects in Washington. This earthquake broke in such a way that it essentially threw a haymaker at the nation's capital.

The Mineral quake nucleated about five miles below the surface on a fault near Interstate 64. The rupture propagated to the northeast. An earthquake doesn't generate waves like a pebble dropping into a pond, but rather can send them preferentially in one direction. In this case, the way the fault ruptured led to intense shaking in Washington and along the Chesapeake.

All of the East Coast is theoretically vulnerable to a major earthquake, but such events are assumed to be very rare. Martin Chapman, a Virginia Tech geophysicist who has been studying the Central Virginia Seismic Zone since the 1970s, said Wednesday that a quake the size of August's occurs roughly every 400 or 500 years in that area.

"It's something that I've been sort of expecting at some point, but I didn't expect to get a magnitude 5.8 in my professional lifetime," Chapman said.

One obvious policy implication is that the public needs to be better informed about the risks of earthquakes, he said. Even though Central Virginia has a known seismic zone, many residents were caught off guard by the August quake.

"The scientific evidence suggests that much of the earth's crust is in a condition where the rock is close to failure everywhere," said U.S. Geological Survey geophysicist Bill Ellsworth. "Even in the East Coast, the rocks are highly stressed."

In this newly released footage from the National Park Service, tourists run down the steps of the Washington Monument as debris falls from the ceiling the moment the earthquake hits. (No audio)

Dispute over fracking

Ellsworth has spent much of the past two weeks dealing with a different issue: Whether some small earthquakes have been induced or triggered by fluid injections in deep wastewater wells in the central United States.

His research strongly suggests a correlation. There are thousands of deep wastewater wells in the United States that are used for the disposal of fluids that are a byproduct of the oil and gas industry. There has been a sharp increase in small earthquakes in the nation's midsection in recent years, Ellsworth found, and, in a much-quoted abstract prepared in advance of the seismology conference, he said the change in seismicity is "almost certainly man-made."

The link between industrial activity and minor earthquakes has long been established, but the topic is politically sensitive at a time when the oil and gas industry is increasingly using hydraulic fracturing to liberate natural gas from shale deposits.

Some of the fluids injected into the deep wastewater wells come from hydraulic fracking, but, contrary to some initial news accounts, the fracking itself hasn't been linked by Ellsworth to the earthquakes.

The White House has been boasting of its record of increasing natural gas production. After the flurry of media reports seemed to suggest that fracking could be triggering quakes, the top deputy at the Interior Department, David Hayes, weighed in with a blog post to clear up any misinformation.

"We just do not see that there is a connection between hydro-fracking and earthquakes," Ellsworth reiterated Wednesday in the call with reporters.

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