Rare quake causes panic in eastern US

[Rivergirl]

http://www.theage.com.au" onclick="window.open(this.href);return false; online

One of the strongest earthquakes to strike the US east coast in decades has rattled offices in downtown Washington and caused panicked evacuations from skyscrapers as far away as New York.

The Pentagon, the US Capitol and Union Station in the nation's capital were all evacuated on Tuesday after the 5.9-magnitude quake, which was shallow with its epicentre only one kilometre underground.

The disruption to mobile phone services in the hour after the quake added to the sense of panic in a country preparing to mark the 10th anniversary of the September 11, 2001 terror attacks.

Seismologists from the US Geological Survey said the epicentre was 47 kilometres from Charlottesville, Virginia, and 139 kilometres from Washington, where Reagan National Airport was also evacuated.

"This is one of the largest earthquakes on the east coast in quite a while, in many decades at least," USGS spokeswoman Lucy Jones told CNN. "It's not unprecedented. But it's one of the largest we've had there."

The quake was felt as far away as Martha's Vineyard, around 800 kilometres away, off the coast of Massachusetts, where President Barack Obama was playing golf during his vacation.

In New York, thousands of people poured onto the streets as evacuation procedures put into place after September 11 attacks were activated.

"I was in the street when the ground shook and I looked up to see the building shaking like a tuning fork," Mary Daley told AFP below one Wall Street skyscraper.

Police in New York herded people toward local parks away from tall buildings as precautionary checks were started.

"I was on the 20th floor of the court building and it shook like mad. Everyone is scared," said Dan Ramater as he stood in Center Street where traffic jams quickly built up in the jittery chaos.

Manhattan District Attorney Cyrus Vance had just started speaking at a press conference about the sensational Dominique Strauss-Kahn sexual assault case when the building started shaking.

Fatima Richardson, 28, who was sitting on the steps of courthouse on her lunch break said: "You could see the building moving. I was just freaking out."

Blocks away, Juan Ramos, an office worker in Lower Manhattan was dazed and confused after giving blood.

"I saw my cup of coffee shaking but I thought nothing of it. I had just donated blood so I thought I had not recovered my equilibrium," he said.



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[daviescr]

There was a pretty rare cluster of shallow quakes in Southern Colorado yesterday too - very odd goings on!

[Rivergirl]

Yes I noticed that too Chris. Strange

[Ken]

It's always been my belief that earthquakes, even those on different tectonic plate boundaries, often (but not always) occur in clusters within the space of several days. Some of the more memorable sequences happened leading up to the big quake near Sumatra that caused the Boxing Day Tsunami, the Chilean quake and this year's Japan quake (although the Christchurch quake occurred about 1 month earlier so may be a coincidence). Although there's big quakes that also occur without any other major quakes elsewhere in the preceding few days, the occurrence of clustering still seems to happen often enough to justify ongoing research. Here's a list of significant earthquakes that have occurred over the last few days:

Magnitude 5.8 VIRGINIA August 23, 2011
•Magnitude 5.3 COLORADO August 23, 2011
•Magnitude 4.6 COLORADO August 22, 2011
•Magnitude 7.0 VANUATU August 20, 2011
•Magnitude 7.1 VANUATU August 20, 2011
•Magnitude 6.3 NEAR THE EAST COAST OF HONSHU, JAPAN August 19, 2011

The research into this type of clustering is still ongoing but here's some preliminary findings from some recent research into a similar phenomenon:

Using data from these seismometers, researchers from the U.S. Geological Survey and Georgia Institute of Technology examined the locations and timing of tremor activity following large distant earthquakes. In some cases, they found evidence that triggered slip and its associated tremor migrated along the length of the fault, and persisted long after the passage of seismic waves from the distant earthquake.
The scientists hypothesize that distant earthquakes can act as a trigger for ongoing episodic creep events, sometimes altering their timing.
In particular, triggered creep episodes could provide a physical explanation for the time delay commonly observed between passing seismic waves and distantly generated earthquakes.

[daviescr]

There's just been a couple of moderate quakes in the Norwegian Sea - M5.3 and M4.8.

I agree with you Ken, it seems a logical step to me - almost like the whole earth is 'settling' after the Great Tohoku Quake in March. I did read somewhere recently that there are many tenuous long-term after effects from mega-quakes.