A 3.6-magnitude earthquake that startled Marylanders from their slumbers early Friday morning might have been the strongest measured tremor on record for the state.
With its epicenter near Germantown in Montgomery County, the quake was felt by as many as 3 million people in the Mid-Atlantic region, according to the United States Geological Survey.
The 5 a.m. earthquake was felt as far away as south-central New Jersey, as well as in Washington, Northern Virginia, southeastern Pennsylvania and Delaware. People in Columbia, Owings Mills, Carroll County and Odenton reported the quake was enough to rattle household items and send pets into a tizzy.
Odenton resident Paul Muirhead said the temblor woke him up about 5:05 a.m. "I was startled from my sleep as if being shaken," he wrote in an e-mail. "Though there was hardly any light by which to see, I could hear items of mine — large and small — rattling on glass shelves."
It was one of the strongest quakes centered in Maryland since European settlement. An earthquake in Annapolis in 1758 has been estimated, from eyewitness accounts, at magnitude 3.5, said Jeffrey Halka, acting director of the Maryland Geological Survey. A 1939 quake centered in Phoenix in Baltimore County has been estimated at magnitude 3.5 to 3.7.
The Maryland agency's website lists a 3.1 shaking recorded in Hancock in 1978 as the strongest confirmed tremor centered in Maryland prior to this one.
A MarylandWeather.com reader in Frederick named "Jen" wrote, "Around 5 a.m. I heard a loud sound and my pet birds were screaming and thrashing against their cages. I didn't put two and two together until a co-worker gave me the news when I got to work."
A reader in Glen Arm reported: "To me, it felt like an ocean wave going under my bed from foot to head. I was already awake before it happened but had no idea it was an earthquake until the radio news reported it. I can now check off an item on my bucket list."
In Washington, not everyone was impressed. Lourdes Almeda, 55, a controller at a local firm, said the shake briefly disturbed her sleep in Gaithersburg. But after checking the house, she went right back to her slumber. "I come from California, so I'm used to it," she said. "A 3.6 earthquake is not strong for me."
Jessica Sigala, a geophysicist with the USGS's National Earthquake Information Center in Golden, Colo., said the center had not received any reports of significant damage or injuries. Officials there said they do not expect such small quakes to cause such problems.
Even so, highway officials in Maryland and Washington said crews were inspecting bridges for possible damage. Maryland State Highway Administration spokesman David Buck said no damage had been reported. A spokeswoman for the District of Columbia Department of Transportation said crews there were inspecting some older bridges and developing a list of others that will be evaluated in the wake of the quake.
Most Americans don't think of the eastern United States as being prone to earthquakes. The East Coast is far from major continental plate boundaries and fault zones, such as those on the West Coast and at the center of the Atlantic Ocean.
"That's not to say we don't have faults," Halka said. "But most are hundreds of millions or billions of years old, and they were much more active in the past, just like California is now."
"But since then, not much has happened," he said. Only "some minor readjustments to stresses in the Earth."
One of the possible explanations for Friday's quake, he said, is that the crust beneath Germantown is continuing to release stresses from the last Ice Age 12,000 years ago.
"With a mile of ice up in northern Pennsylvania and as far south as Long Island, you're going to have depression of the crust" from the weight of the glaciers, Halka explained. "And consequently, down in front of that, in Maryland, you're going to have a little bulge."
Since the glaciers to our north melted, the land that was under them has been slowly rising, while the crust to the south has been slowly dropping — about 1.1 millimeters a year in Maryland. That's why sea levels in Maryland are rising a bit faster than climate-related sea level increases alone can explain, Halka said.
It might have seemed a small quake to Californians and geophysicists, but the tremor was a rarity and a big deal for Maryland. The earthquake marked the second time within a month that residents here have felt the ground shake. On June 23, a 5.0-magnitude quake with an epicenter near Ottawa, Canada, sent shock waves along the East Coast.
Sigala said back-to-back earthquakes in this area are unusual, particularly because the Mid-Atlantic states do not fall along a fault line as does California. "We do have them [in the Mid-Atlantic], it's just not very often," Sigala said.
Although there are several places east of the Rockies where quakes are fairly common — including the New Madrid seismic zone centered in southeastern Missouri, and others in South Carolina and New England — most of the region can go years without a tremor.
But when big quakes do strike the Eastern states, they can be felt 10 times farther away than an equivalent quake on the West Coast. In 1886, when Charleston, S.C., was devastated by a great quake, the shaking was sharply felt throughout Maryland.
The USGS placed the epicenter of Friday's tremor about 3 miles beneath the Gunners Lake Village area of Germantown, just south of the Interstate 270 interchange with Middlebrook Road, at Exit 13.
Halka, who didn't feel it, said the main quake was followed about 11 minutes later by a second shake, measured at magnitude 2.0. That one was centered about 5 miles from the first, about 27 miles northwest of Washington. Each whole number increase in magnitude represents a 10-fold increase in strength.
"It could be an aftershock, but no one's done the analysis to figure that out," he said.
USGS officials estimate about 12,000 people experienced moderate shaking Friday morning, and about 300,000 felt light shaking. Millions of others may have felt at least a degree of shaking, USGS officials said.
President Barack Obama was not one of those millions, however. In a chat before his departure for Maine on Friday morning, Obama said he did not feel the earthquake in Washington, which was 20 miles from the quake epicenter.
Self-reporters in Gaithersburg and Jessup described the quake on the government agency's website as having a Modified Mercali Intensity as high as 4. An MMI of 4 is equivalent to the vibration of heavy trucks passing and can cause pictures to swing and household objects to rattle.
Alvin Borenstein of Randallstown said the quake woke up his entire household.
"All of a sudden, we felt the house shake," he said. "It woke my granddaughter up, it woke my son up, and it woke me and my wife up. The house shook. We didn't know what it was."
Of his 60 years in Randallstown, he said he's never had an experience like that one.
"You're looking up at the ceiling and your house is shaking," Borenstein said. "It was terrible. We were really, really scared."
Baltimore Sun reporter Jamie Smith Hopkins, Tribune Washington Bureau reporter Jennifer Martinez and the Associated Press contributed to this article.