On Tuesday, August 23, 2011, at about 1:50 p.m., as I sat at my upstairs computer writing a post for the Capital Weather Gang, a sharp jolt shook my room—and my house—in Rockville, Md. I yelled to my wife, who was downstairs, “Did you feel that?” She replied, “What do you think?”
We had just experienced what was later classified as a “moderate” 5.8 magnitude earthquake, centered near Mineral, Va., about 80 miles southwest of Washington and three miles below sea level. The quake occurred in what is known as the Central Virginia Seismic Zone.
As we know now, the quake was felt far and wide. On its “Did You Feel It?” site, the U.S. Geological Survey (USGS) reported around 148,000 responses from southeast Canada to Florida and from the East Coast westward to near the Mississippi River, although damage was limited to an area from central Virginia northeast to Southern Maryland and D.C. The USGS says that the quake radiated so far outward because eastern substrata is composed of much harder and more contiguous rock formations than in the West, allowing seismic waves to travel much farther.
Although Californians thought it was much ado about nothing, here in the mid-Atlantic, where we’re more accustomed to congressional rattling than the geologic kind, it was a shock—even causing panic in some places. In fact, it was the strongest East Coast tremor (temblor for the purists) since 1897, when a 5.9 magnitude quake hit Giles County, Va., still Virginia’s strongest earthquake on record. And as we now know, last year’s quake caused considerable damage in the Washington area and elsewhere, but fortunately, no deaths.
Following inspection, the Washington Monument, for example, was found to have numerous cracks and will be closed to the public and undergoing repairs until 2014. But at least it didn’t sink, as had been feared, since it was originally constructed on reclaimed marshland. Repairs are expected to cost more than $15 million.
In addition, Washington’s National Cathedral incurred severe damage to its spires, pinnacles, flying buttresses, and gargoyles, possibly totaling as much as $25 million.
For the record, the USGS says that the strongest known East Coast earthquake occurred in 1886, near Charleston, S.C. It registered an estimated magnitude of 7.3.
As we’ve reported previously, aftershocks from last year’s quake have been plentiful, with more than 450 recorded according to this recap by USGS. Most have been relatively benign. One of the strongest ones was a 3.1 magnitude quake centered eight miles south of Louisa, Va. Other than keeping citizens on edge, even this tremor was of little consequence, however.
The aftershocks continue to occur and scientists say it may be some time—even many years—before central Virginia’s seismic activity returns to the ”background” level prior to last year’s quake.
Some have wondered how the energy released by the August 2011 U.S. East Coast earthquake compares with that of the March 2011 Tohoku Japan earthquake, which caused the great Fukushima nuclear disaster. The answer, as you might expect, is that they’re not even in the same league.
The scale used by the USGS to measure earthquake severity is logarithmic in nature, meaning that for each numerical increase, the energy increases enormously. Therefore, according to Michael Blanpied, associate coordinator of the USGS Earthquake Hazards Program, the 9.0 magnitude Tohoku Japan earthquake was about 63,000 times more powerful than the 5.8 magnitude U.S. East Coast quake! Even the 7.9 magnitude Great San Francisco Earthquake of 1906 was not as powerful.
By comparison, the 9.2 magnitude Great Alaskan Earthquake of March 27, 1964 was the most powerful earthquake ever recorded in North America since 1900. Ironically, though, it was the weaker 7.4 magnitude Alaskan earthquake of April Fools’ Day, 1946 that managed to send a severe tsunami toward Hawaii.