Yes, that was an earthquake. It wasn't much of one. People conceivably have sneezed harder than the violence produced by Thursday afternoon's earthquake. But it was unusual, centered in the normally quiescent state of Delaware, and it could be felt as far as the nation's capital and its Virginia suburbs.
The tremor occurred at 4:47 p.m., centered about 6 miles northeast of Dover, Del., and about 5 miles deep. The U.S. Geological Survey gave varying estimates of the earthquake's magnitude, at one point rating it as high as a 5.1 but later downgrading it to a 4.1 — a modest event that did not appear to cause any damage, much less fatalities. The Delaware Emergency Management Agency told the Dover Post that there were no reports of damage or injuries.
Geophysicist Dale Grant of the USGS said an earthquake in Delaware is "exceptionally rare."
The earthquake was felt across a broad swath of the Washington-New York corridor, and into the Hudson River Valley and parts of Connecticut and Long Island, according to the USGS "Did You Feel It?" page. Almost everywhere, the shaking was reported by residents to be either "weak" or "light."
In comments posted to The Post's Capital Weather Gang blog, people in and around Washington reported rattling windows or the sense that a big truck had just passed down the street.
— EMSC (@LastQuake) November 30, 2017
A reader in Hyattsville wrote, "My toddler and I looked at each other like 'what was that?!?' House shuddered for about a second, 3 shakes. At first I thought the mailman had thrown a big package onto our porch (nope). Then I thought maybe a tree branch had fallen in our backyard (nope). The street was quiet, so not construction or a big truck or a low plane flying overhead. I correctly deduced it must have been an earthquake!"
Closer to the epicenter, the earthquake was more dramatic. A reader in downtown Dover wrote, "felt house shake violently….thought it was a gas explosion."
The magnitude scale is logarithmic, and a 5.0 magnitude event is 10 times more powerful than 4.0. The Aug. 23, 2011, earthquake centered near Mineral, Va., which damaged the Washington Monument and the National Cathedral, had an estimated magnitude of 5.8, and was one of the three strongest quakes east of the Rocky Mountains in the last century.
There is no known fault associated with this earthquake. The hazard maps produced by the USGS do not show Delaware having a significant seismic hazard. Those maps may have to be revised as a result of Thursday's tremor. There has been debate, sometimes quite acrimonious, within the scientific community about the usefulness of hazard maps, because earthquakes often happen in places of no known seismic hazard, or the ground shakes with greater intensity than had been imagined.
The East Coast has a limited history of major earthquakes, but experts say that citizens should not assume they are free of seismic hazard. Charleston, S.C., was hit by a devastating earthquake in 1886 that killed 110 people. Indeed, New York City is considered highly vulnerable over the long term because so many masonry structures lack the kind of reinforced steel that is required in places with more of a known earthquake risk.
Jason Samenow contributed to this report.