Hampton Roads' sandy soil helped quake resonate

Virginia is full of faults and bits of volcanic islands, all left over from when the continents collided millions and millions of years ago, then pulled apart.

Those faults in the ancient, hard rock of central Virginia played a role in Tuesday's magnitude-5.8 earthquake, and geologists were in the field Wednesday, trying to choose between really old and really new.

The question is whether the quake can be traced to one of the ancient fault lines that parallels the Blue Ridge Mountains or whether it occurred in a fault that, until now, has been undetected.

"The more we look at Virginia, the more faults we find," said David Spears, Virginia's state geologist, who on Wednesday was near the epicenter in Louisa County.

Easier to understand is why the quake was felt so strongly in Hampton Roads, which sits on sand and aquifers - think Jell-O on a plate.

The ground shook from Georgia to Canada, and west as far as Indiana. The most severe damage was around Mineral, in central Virginia, where chimneys toppled, store shelves were dumped and walls cracked.

Like a ripple in a pond, the energy from the earthquake undulated outward, said John Hole, associate professor of geophysics at Virginia Tech. "The rocks in eastern North America are more rigid, so they absorb less of that energy as it goes through them," he said. "You are therefore able to feel it more over a longer distance."

The Coastal Plain, from Richmond to the sea, is made of layers of sand and clay and water. Anywhere from 200 to 5,000 feet below is the same bedrock that underlies western Virginia, Hole said. When an earthquake moves that rock, the sand and water layers jiggle like gelatin, he said, exaggerating the movement and leading to the swaying motion felt across Hampton Roads.

Likewise, the taller the structure, the more exaggerated the motion at the top, which is likely why the Washington National Cathedral and Washington Monument suffered damage near their peaks.

But the question still is why an earthquake of such magnitude happened in Virginia at all.

West of Richmond, the known ancient faults all run roughly northeast to southwest, and at least some of them are still active, said Christopher M. Bailey, chairman of the geology department at the College of William and Mary.

"The question is what are the stresses that are causing these ancient cracks and zones of weakness to continue to slide," he said.

In most of Virginia, deep rock layers have been melted and reformed many times.

"They're hard, they're old and they've had a complicated history," Bailey explained.

Spears said: "Half a billion years ago, the edge of North America was actually at the foot of the Blue Ridge Mountains."

When the continents collided, they wrinkled and fractured the land. When they separated again to form the Atlantic Ocean, more faults were created.

The earthquake appears to have been centered somewhere between two known fractures called the Spotsylvania fault and the Chopawamsic fault.

"There's still some adjustment happening to the Earth's crust in central Virginia," Spears said.

Those adjustments cause earthquakes. Most of them are minor, and not even noticeable. Tuesday's quake was extraordinary for the region, he said, so unusual for Virginia that crews were out Wednesday looking to see whether it had created any cracks in the ground or had liquefied soils.

"We haven't found any," he said before noon. "But we're not done."

In Williamsburg, Bailey was in the second hour of a faculty meeting when the earthquake abruptly ended it.

He was able to report to a colleague: "Our discussion was interrupted by a motion from the floor."

Diane Tennant, (757) 446-2478, diane.tennant@pilotonline.com