The earthquake that rattled Anne Arundel County late Saturday occurred in bedrock nearly 7 miles down — twice as deep as initially reported — and its light shaking likely spread farther than it would have elsewhere because it traveled upward and outward through Maryland's sandy coastal plain.
The 2.2 magnitude quake is estimated to have occurred 6.8 miles beneath the Laurel area just after 10 p.m., and its shaking was strongest just to the east, in Millersville and Crownsville, according to the U.S. Geological Survey.
While the temblor originated at the edge of the rocky Piedmont, within stone dating to the Appalachian Mountains' formation 500 million years ago, the shaking was strongest where the formations meet the softer sediment that covers the Eastern Shore and most areas east of Interstate 95. That spongy layer carried the more significant tremors of the 5.8 magnitude Virginia quake that shook the Atlantic coast in 2011.
"This sand kind of acts like Jell-O and allows those waves to be felt across a much larger area," said Richard Ortt, director of the Maryland Geological Survey.
Any earthquake under a magnitude of 3 is difficult to sense, according to the USGS, felt only by people "under especially favorable conditions." Earthquakes on par with the August 2011 quake centered in Mineral, Va., are felt far and wide and typically cause at least slight damage to buildings.
There are nearly 1 million earthquakes of magnitude 3 or less around the world each year, according to the USGS.
But Saturday's quake prompted some concern around Central Maryland. The Maryland Emergency Management Agency received reports of a possible explosion or truck accident. The USGS received a handful of reports from residents as far away as Maine, New York and North Carolina.
"It was felt farther away than you would expect a 2.2 to be felt," Ortt said.
Reports described a loud boom followed by the ground shaking, said Chas Eby, a spokesman for the emergency management agency. There were no reports of injuries or significant structural damage, he said.
Geologists can't explain why the quake happened because they have little data about the Earth's crust at such depths.
Ortt speculated it could be traced to the compression that formed the Appalachians, or expansion that occurred when North America split from Africa.
"In this region of Maryland, more than likely there are unknown, unmapped faults," he said.
While fault lines in seismically active areas, such as California's San Andreas fault, are studied and monitored more closely, there is little reason to drill for more information in Maryland when the formation that produced Saturday's quake might do little more than rattle some picture frames once a decade.
About five dozen earthquakes are known to have rattled Maryland since 1758, when one of the strongest of them shook Annapolis with a magnitude between 3.5 and 3.7. A quake centered in Phoenix in northern Baltimore County matched that intensity in 1939. A 3.4 magnitude quake centered near Germantown woke up much of the Mid-Atlantic about 5 a.m. July 16, 2010.
But with damage from seismic activity so rare here, there is little justification to probe for evidence of fault lines beneath the state. Such research is typically only done ahead of drilling for oil or other resources or constructing major infrastructure such as highways.
"There's probably little reason for us to go ahead and spend 2 to 5 million dollars for a corer to actually find out that piece of information if we even think it's there," Ortt said. "If it's off by 10 or 15 feet you might not even see it."