A magnitude 4.1 earthquake struck Delaware on Thursday in a rare seismological occurrence for the U.S. Northeast, officials said, with the temblor’s strength equaling the estimated magnitude of an 1871 quake that was believed to be the largest ever in the state.
The quake, previously reported at magnitude 5.1 and then at 4.4, was centered in the Bombay Hook National Wildlife Refuge, according to a statement from the Delaware Emergency Management Agency. It was less than 10 miles (17 km) from the city of Dover and less than a mile (0.8 km) from Donas Landing.
The largest quake ever recorded in Delaware was a magnitude 3.8 temblor in 1973, according to the Delaware Geological Survey.
Earthquakes in Delaware do not occur on the edge of a tectonic plate, as is more common in places such as California, where fault lines between plates generate earthquakes. Generations of California residents have been bracing for the so-called “Big One” along the San Andreas Fault.
Instead, the Delaware temblor occurred far from the edge of a plate, said Thomas Pratt, a research geophysicist at the U.S. Geological Survey who is based in Virginia.
“There’s a lot of speculation, but we don’t have a good answer for why these earthquakes are occurring in the middle of the plates,” Pratt said.
The latest quake was downgraded to a magnitude 4.1 after data came in from several monitoring stations, U.S. Geological Survey geophysicist Rafael Abreu said by telephone.
It was felt in Philadelphia in the neighboring state of Pennsylvania, some 53 miles (85 km) from the epicenter.
The quake was shallow, only 5 miles (8 km) deep, which would have amplified its effect, and some people reported feeling light shaking in areas around New York City and Baltimore, according to the USGS website.
Many social media users also confirmed feeling the temblor and #earthquake had quickly risen to the top of trending topics on Twitter with more than 11,000 tweets mentioning the hashtag.