And the overall impact of the quake, emanating from a previously unknown fault three miles beneath Louisa County, Va., was the biggest ever east of the Rocky Mountains, the U.S. Geological Survey said.
The assessments came as the Washington area marked the one-year anniversary of the 5.8-magnitude quake that terrified residents last Aug. 23.
Government officials gathered near the Washington Monument to provide the latest scientific knowledge of the event, and bells were rung at the damaged Washington National Cathedral around the time it struck last year.
The quake, centered near the town of Mineral, Va., was “the most significant . . . to ever strike the U.S. east of the Rocky Mountains,” Marcia McNutt, director of the U.S. Geological Survey, said during the review by officials on the Mall.
“More people felt this earthquake than any earthquake in U.S. history,” she said.
And it had a bead on Washington, McNutt said.
“The energy that was radiated out from this earthquake was not in a concentric pattern,” she said. “The energy actually went out in an asymmetric manner to the northeast and southwest.
“In a sense, this earthquake took aim, and Washington, D.C., was in its path,” she said.
It also did about $300 million in damage from New England to the Carolinas.
At least $15 million of that was incurred at the monument, which has been closed for repairs for a year and could stay closed until 2014.
The National Park Service is about to select the contractors who will do the repairs, even as the lengthy seismic study detailed the damage.
The injury, especially to the monument’s pyramidion, was a rare event, according to the report, prepared for the Park Service by the engineering firms of Wiss, Janney, Elstner Associates and Tipping Mar.
“The occurrence of another earthquake capable of causing more damage to the pyramidion than occurred on August 23 is judged to be extremely unlikely,” the report said.
“The shaking on the National Mall . . . was roughly 10 to 20% stronger” than a worst-case event that might be expected over the benchmark course of 2,475 years, the report said.
“A future 2,475-year event is more likely to have similar or less damage potential” to the pyramidion, the report said. “The pyramidion therefore appears to have just experienced its 2,475 year event. . . . ”
The report used computer modeling to examine what exactly happened to the 555-foot, 91,000-ton monument during the quake, which lasted less than a minute.
Experts also modeled the effect of future, potentially more serious quakes, and they found that the effect would be no worse, or even less. For example, a 7.5-magnitude earthquake, which was centered around Charleston, S.C., in 1886, did no recorded damage to the monument, the report noted.
The damage was concentrated in the pyramidion because the top of the monument experienced the most “swaying” during the quake and because of the pyramidion’s complex construction.
Two-hundred sixty-two, 3,300-pound external marble slabs are hung on an intricate internal structure of stacked stone ribs and protruding teeth. The report found that 24 of the “teeth” were fractured enough to require repair.
Several of the panels cracked, one seriously, the report said. A part of one panel fell inward about 2 inches and might have fallen into the interior had the quake lasted longer.
But because of the angle at which the panels rest, the report said, there was “less likelihood” that a panel could fall outside the monument. The report suggested repairs, which are already under consideration, but cautioned against fixes that might reduce the structure’s flexibility.
Some “restraints have the potential to alter the means by which the pyramidion successfully resisted the . . . [earthquake] and could have unintended detrimental consequences during future earthquakes.”
Another heavily affected building was the National Cathedral, where an estimated $20 million in damage was done, especially to the soaring limestone towers.
Officials there commemorated the anniversary with prayers, the ceremonial pealing of the cathedral bells and the blessing of a newly carved crocket stone that was set into a pinnacle damaged in the quake.
Sean Callahan, the mason who carved and then set the replacement stone in place Thursday, said he found it gratifying to be able to be part of the effort to help restore the cathedral’s pinnacles to the their pre-quake condition.
The earthquake caused huge stone finials, gargoyles and crockets to shake loose and crash down.
But the Rev. Francis H. Wade, interim dean of the cathedral, said Thursday’s anniversary was not a day to dwell on burdens but rather one to remember blessings.
The quake, he said, “reminds us just how fragile this world is.”