Mild earthquake felt across region

By Mike McPhate

* Q&A recap: Mike Blanpied of the USGS *

* Forum: Discuss your experience this morning *

* Your take: The Weather Gang compiles best comments *

* Live map: Track earthquakes around the world *

* Audio: USGS official, residents discuss quake *

This post was updated at 1:56 p.m.



Crews were doing basic inspections of bridges throughout Maryland and the District after a 3.6 magnitude earthquake jolted the region early Friday, transportation officials said.



So far, so good.



Besides a natural gas leak at a maintenance depot in the Gaithersburg-Rockville area that may have had nothing to do with the earthquake, officials have found no evidence of damage after residents awakened to wobbling homes at precisely 5:04:47 a.m.



"There's not a single tree branch down," Montgomery County Fire and Rescue Assistant Chief Scott Graham said. "This was a nonevent, thank God."



Still, many residents dialed 911 to report the rumbling.







A zoomed view of the wooded area at the epicenter of Friday's quake in Germantown. (Bill O'Leary/Post)

At Montgomery County's emergency center, as many as 100 confused people called in during or just after the earthquake. "We were inundated with calls from people not knowing what it was," said Gloria Jackson, a communications supervisor at the center.



Carroll Ripley of Germantown said his first guess was that an explosion had been set off or perhaps a plane soared low overhead. "The whole house shook," he said.

"I went straight outside to do a perimeter check -- in my underwear.”

The quake’s epicenter was in the Germantown-Gaithersburg area near I-270 and Route 119 (39.167°N, 77.252°W), according to USGS. It was 3.1 miles deep.

The fault's slide "probably only lasted a second or two," Mike Blanpied, a USGS geologist, told Post readers during an online chat. The resulting vibrations, he said, may have been felt for 10 seconds or more.

Story: Reporter Michael Rosenwald describes scene at epicenter

Graphic: See an intensity map at USGS Web site

Amy Vaughan, a geophysicist with the USGS, said the quake was the largest recorded within about 45 miles of Germantown since a database was created to track such activity in 1974. The largest earthquake before Friday morning's was a 2.7 tremor in 1993, Vaughan said. There was a 2.6-magnitude tremor in 1990 and quakes measuring 2.5 in 1997, 1993 and 1974.



"It’s not something that’s completely out of the ordinary," Vaughan said.

Q&A recap: Mike Blanpied of USGS







Audio: USGS official, residents discuss quake.

At the same time, quakes measuring above 3 are extremely rare in this area. Vaughan said the number of people reporting the quake to the USGS highlighted how unusual it seemed to residents of the Mid-Atlantic states.



Within about two hours of the quake, more than 6,700 people had reported it on the agency's Web site. Most were from the District, Maryland, Virginia and Delaware, but reports came from as far away as New York and Georgia.

Many found it thrilling."It felt like the earth was churning," said Linda Wheeler of Monrovia in Frederick County. "I knew it was an earthquake right away because it was unlike anything I've ever experienced. It was actually kinda cool."David Hayes of Boyds was getting dressed when he said he heard a loud rumbling. “I opened my front door and yelled 'earthquake' toward my neighbor. Great way to start the day!” he said.

Reader forum: Discuss your experiences



The first question journalists asked President Obama Friday morning, when he appeared outside the White House to discuss the Gulf of Mexico oil spill, was whether he felt the earthquake at 1600 Pennsylvania Ave. NW.



No, the leader of the free world answered with a slight smile, "I did not."



Geologists note that earthquakes can occur anytime and anywhere. They are a natural phenomenon resulting from the movements of continental plates along faults under the Earth's surface. "These faults just naturally exist and over time they build up pressure," Vaughan said.



When the pressure is released at a high enough intensity, an earthquake results.



The earliest recorded earthquake in Maryland occurred in Annapolis on April 24, 1758, and lasted a full 30 seconds, according to a history posted on the USGS Web site.



There have been other tremors reported periodically in the region through the years. None caused any significant damage, according to the Web site.



Story: Recall the Annandale quake of '08? See Post's coverage.

The confusion following Friday's quake led to some misplaced theories on its cause. Dean Miletich of Frederick said he was taking out the trash when the rumbling started.



“When I came back into the garage, everything on the shelves was shaking,” he said. “It sounded like a deep rumble. When I came back inside, my wife had woken up and asked me, 'What did you do?’ ”



-- Staff writers Debbi Wilgoren and Jonathan Forsythe contributed to this report

Web site: U.S. Geological Survey