Five things to know about the earthquake off Ocean City — and earthquakes in general

Jan. 17--Earthquakes are uncommon on the East Coast, but from time to time Marylanders are reminded there's always the possibility of feeling the earth shudder beneath one's feet.

Just such an event happened Tuesday around 6:30 p.m. when the U.S. Geological Survey reported a 4.7 earthquake 136 miles off the coast of Ocean City. State geologists said they believe an underwater landslide might have triggered it, or that it could have been caused by what they called "reactivation" of an ancient fault.

Here are five things to know about earthquakes and the one Tuesday night.

Though they are rare, earthquakes can and do happen on the East Coast

While the Maryland Geological Survey cites numerous fault lines in the state, director Richard Ortt explains that East Coast faults are not the same as the tectonic plate faults like on the West Coast.

"We don't have plates moving under one another," Ortt said. "We have something called intra-plate faults. There are weak spots in the earth's crust that form where continents were torn apart or pushed together millions of years ago. Most of our faults are inactive, but there are some stressors in the faults that will re-energize and release tension."

Before Tuesday's earthquake, the Baltimore region last experienced an earthquake in November 2017, when a magnitude 4 quake centered in Dover, Del., was felt by many across Baltimore.

Quakes are measured on a scale

According to the USGS, an earthquake's severity is measured both for intensity and magnitude. Intensity refers to the observable effects of a quake and can vary from location to location. Magnitude measures the amount of seismic energy, or vibrations that travel through the earth, released at the center of the earthquake.

The standard for magnitude measurement is called a Richter scale, which is logarithmic -- a term that means the Ocean City earthquake measuring at 4.7 on the Richter scale was 10 times more powerful than a 3.7 earthquake and 100 times more powerful than a 2.7, Ortt said.

Scientists use devices called seismographs to measure magnitude. The Maryland Geological Survey has two seismic sensors, one in Baltimore County and another in Garrett County, Ortt said.

Maryland's sensors are part of a larger network of regional seismographs that help scientists determine a more accurate location when earthquakes occur. Tuesday's 4.7 earthquake off the coast of Maryland was registered on 127 sensors across the world, including in places as far away as Kazakhstan, Ortt said.

Seismic activity detected within 800 miles of Maryland are listed on the Maryland Local Quake web page.

The greatest magnitude on record is 8.9, registered during two separate earthquakes off the coasts of South America in 1906 and Japan in 1933. The famous 1906 earthquake in San Francisco registered a magnitude of 8.3, according to the Maryland Geological Survey.

Earthquakes can be felt hundreds of miles away

While the 4.7 earthquake took place more than 100 miles off the coast of Maryland, its tremor could be felt on land.

Dozens of people reported feeling the earthquake to the USGS. The responses came from locations across the East Coast, including the Baltimore area; Ocean City; Berlin; Dewey Beach, Del.; Norfolk, Va.; the Outer Banks in North Carolina; and even as far away as Massachusetts.

Other notable earthquakes in Maryland history --

Reporting an earthquake is not complicated

Because earthquakes are more unusual in Maryland, residents might not be able to discern the difference between tremors and, say, a man-made disturbance, Ortt said.

"If you felt it, it could be anything from a sonic boom from a jet, from a quarry making a blast to a very, very bad car accident," Ortt said. "You don't always know what that feeling is."

That's why Ortt recommends residents use regular emergency channels to report a suspected earthquake.

If the tremor is indeed the result of a seismic event and causes damage to the foundation of a building or house, that information should be reported to insurance companies and local government officials, he said.

You can also report whether you felt the earthquake -- and how intense it was -- to the USGS.

Fact or fiction: Did Mexico's soccer fans cause earthquake when winning goal was scored? --

East Coast earthquakes could be dangerous, but probably won't be

It's hard to predict the risk of a devastating earthquake because seismic predictions are based primarily on history, Ortt said.

The last earthquake to cause extreme damage in the eastern part of the United States took place in 1886 near Charleston, S.C., and registered a magnitude of 6.5 to 7, the Maryland Geological Survey website states. The quake was felt over an area of 2 million square miles, including in Maryland.

However, Ortt said it is important to remember that modern building codes mean many buildings are constructed these days to withstand seismic activity.

Also, the odds of a tsunami are unlikely, he said, in part because the tidal waves are typically caused by plates sliding beneath one another or by large-scale landslides on the ocean floor -- both of which are unusual along the East Coast.

The National Oceanic and Atmospheric Administration and the National Weather Service maintain a Tsunami Warning System online.

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Baltimore Sun reporter Christina Tkacik contributed to this article.

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