4.1 magnitude Delaware earthquake shakes Philly, South Jersey region

Yes, that was an earthquake that rattled your office window, jolted your desk and caused your Twitter feed to explode late Thursday afternoon.

The U.S. Geological Survey reported that a rare earthquake rumbled under the East Coast about 4:45 p.m. Thursday. The epicenter was six miles from Dover, Del., near Bombay Hook National Wildlife Reserve, and about five miles underground.

It was initially reported as a 5.1 magnitude earthquake.  However, it was downgraded to a 4.4, then a 4.1 shortly after 5:30 p.m.

At first, people throughout the area were puzzled by buildings that felt as if they were swaying, or windows seemingly ready to pop. Social media lit up with people reporting feeling the shaking for miles around Philadelphia and South Jersey. Reports say the quake was felt from Washington to New York.

The tremors were felt in Atlantic City, where Gino Martinelli, 67, said his house seemed to turn to rubber for about four seconds.

"I felt the whole house almost like waving," he said. "It lasted for approximately four seconds. I knew it was pretty strong, because we live where northeast winds hit, and this was way worse than that. I actually felt the whole house moving."

People reported that their pets suddenly bolted or jumped. Others recalled seeing cars rock.

The Philadelphia Office of Emergency Management said there were no reports of damage. SEPTA put speed restrictions in place for some Regional Rail and suburban trolley lines, while personnel performed precautionary inspections of tracks and bridges.

As far as earthquakes go, it was noticeable, but not that powerful.

"Relatively speaking, a 4.4 or 4.1 is relatively light," said Derrick Pitts, chief astronomer at the Fels Planetarium at the Franklin Institute, who also has a background in geology.

According to the USGS, earthquakes with magnitudes of about 4.5 or greater are strong enough to be recorded around the world. Several thousand are recorded each year. By comparison, really powerful earthquakes, such as the 1964 Good Friday earthquake in Alaska, have magnitudes of 8.0 or higher.

A powerful earthquake last struck the region in August 2011, but Philadelphia was further from the epicenter, which was in Mineral, Va. That 5.8 magnitude quake was brief, but it rumbled under the eastern seaboard, rattling nerves and damaging the Washington Monument.  It was the most powerful earthquake on the East Coast in 114 years.

Pitts said earthquakes in California often fall in the 6 or 7 range.

Earthquakes in the East are more likely caused by vertical "crustal readjustments" rather than the horizontal motion of tectonic plates that can occur in California, Pitts said. They strike the East Coast every few years and usually don't produce large aftershocks.

Still, the quake had an impact all along the seaboard. In Baltimore, Husam Albarmawi, a 30-year-old graduate student at the University of Maryland, rushed out of an apartment tower with his wife when they felt two separate jolts roughly 20 seconds apart in their 23rd-story apartment, according to the Associated Press.

"When we felt it, we looked at each other like, 'Are we losing it?"' Albarmawi said as he and his wife ventured back to their apartment. "It was actually pretty scary and pretty surprising."

Staff writer Amy S. Rosenberg contributed to this article.