As earthquakes go, it wasn't much of one.
It was only a magnitude 1.8 on the scale geologists use. But its center was just a mile from Annandale, Va., in the suburbs of Washington, D.C. — enough to get the attention of scientists and local emergency officials.
It happened on Tuesday afternoon, and though it did no damage, hundreds of people called local news outlets saying they had heard a bang and felt a mild jolt.
"Of course, this was a very small earthquake, but we've had larger ones in this part of the country in the past," said Leonardo Seeber, senior research scientist at the Lamont-Doherty Earth Observatory of Columbia University in Palisades, N.Y.
Several others have made news lately: The normally quiet area around Reno, Nev., has had dozens of tremors since late April, the largest measuring 4.7.
Southern Illinois had one on April 18, measuring 5.2.
And there was one on April 30 in northern California, about 190 miles north of Sacramento, with a magnitude of 5.4.
What's going on? Nothing unusual, say scientists — unless the shaking is going on under your feet.
An Earthquake? In Virginia?
Virginia, like most of the world, is geologically stable. There hasn't been much seismic activity in the Washington suburbs since 1997, and the quake that happened then only had a magnitude of 2.5.
Below the level of 3.0, scientists refer to tremors as "microquakes," and say it's unusual for people to feel them. About a thousand of them happen somewhere on the planet every day.
"Quakes of this size are strong enough to be felt in dense population areas, especially in high-rise buildings," said Bruce Presgrave, a geophysicist with the U.S. Geological Survey in Golden, Colo., in an interview with the Associated Press.
Scientists say something of magnitude 5 is a "moderate" earthquake. A magnitude 4 is "light." A 3 is "minor" and below that is "micro."
"We can laugh at a 1.8," said Lamont-Doherty's Dr. Seeber, "but the issue is how small a quake in this part of the world can be damaging."
Seeber says that except in the geologically active parts of the U.S. along the Pacific coast, earthquakes tend to be rare and mild. But they also tend to take place less deep in the ground. And since there are fewer fault lines to separate sections of the Earth's crust, the vibrations from an eastern earthquake can be felt more widely.
The most violent earthquakes in U.S. history took place near New Madrid, Mo., a thinly-populated area south of St. Louis, in 1811 and 1812. Scientists estimate in retrospect that they were probably on the order of 8.0 in magnitude. They did not do much damage since few people were nearby — but they were strong enough to change the course of the Mississippi River, and accounts from the time say sidewalks cracked in Washington and church bells rang in Boston.
The great San Francisco earthquake of 1906, likewise, would probably have registered on modern instruments as somewhere between 7.7 and 8.3.
The scale used to measure earthquakes is one of magnitude. A 3.0 is 10 times as violent as a 2.0, a 4.0 is a hundred times more violent, a 5.0 is a thousand times more, and so on. So the microquake in Virginia was truly small, unusual only in that people in the populated area above felt it.
The problem, says Seeber, is that fewer earthquakes mean scientists know less about what it takes to do meaningful damage.
"In stable areas, there's less data, so we understand it less well," he said.
"I would point to this small earthquake as a symptom that we need to know more."