A magnitude 4.1 earthquake centered in Delaware shook parts of Washington, D.C., and other parts of the Mid-Atlantic on Thursday in what one expert called an "exceptionally rare" occurrence for the region.

The U.S. Geological Survey (USGS) confirmed Thursday that the earthquake struck northeast of Dover about 4:47 p.m., and was felt as far southwest as Washington and as far north as parts of New York and Pennsylvania. There have so far been no reports of injuries or damage.

The quake was initially classified as a magnitude 5.1 event, but was downgraded by the USGS after it ended.

Dale Grant, a geologist at USGS, called Thursday's event "exceptionally rare" for the region in an interview with The Washington Post.

“We’re just getting the information in now. I wish I could be of more help," he told the Post.

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Washington Mayor Muriel Bowser (D) confirmed reports of the ground shaking in the city on her Twitter account.

"No, you weren’t imagining that the ground was shaking," she tweeted, linking to instructions from D.C.'s security agency.

"Did you feel the ground shake? See @USGS update on the earthquake ENE of Dover, DE. Be prepared to 'Drop, Cover and Hold on' in the likely event of aftershocks," the agency tweeted Thursday afternoon.

No, you weren’t imagining that the ground was shaking. https://t.co/JXRm8org4T — MurielBowser (@MurielBowser) November 30, 2017

Social media lit up with the hashtag #earthquake in the minutes after the quake, with reports of tremors coming in from states across the eastern seaboard.

Did anyone in CT feel it? Now a 5.1 magnitude earthquake near Dover, Delaware. — Ryan Hanrahan (@ryanhanrahan) November 30, 2017

The makers of the Paris-based LastQuake app tweeted Thursday that the tremors were felt as far away as Poughkeepsie, N.Y., which is northeast of New York City. About 20 million people live in the area affected by the quake, according to the company.