The 13th minor earthquake in the last four months went nearly unnoticed in Columbia early yesterday, prompting just five calls from residents to Howard County's Department of Fire and Rescue.
That's a far cry from the hundreds of panicky queries emergency operators received after the first few tremors, which began March 10.
No damages or injuries were reported as a result of the 2:30 a.m. earthquake, said acting Lt. Robert Wiseman, who described it as a "very minor tremor," registering 1.9 on the Richter scale.
Mr. Wiseman said three calls came from connecting streets in the Allview Estates development, near the intersection of U.S. 29 and Route 32, where the other earthquakes have been centered.
The other two calls came from the vicinity of that intersection, he said.
The tremor was strong enough to "rattle a few dishes" and alert people that "something happened," said Emery Cleaves, director of the Maryland Geological Survey.
"You can barely feel it," said Mr. Cleaves. "Only people in the immediate vicinity apparently felt it. It's a continuation of very low-level activity. This is another incident."
The dozen Columbia earthquakes have ranged from zero to 2.7 on the scale, which measures the magnitude of earthquakes.
By comparison, the earthquake that caused 63 deaths in the San Francisco area in 1989 registered 7.1.
The series of Columbia earthquakes took place over a four-week period in March and early April. Three earthquakes that occurred from March 10 to March 14, measuring between 2.0 and 2.7 on the Richter scale, generated between 300 and 700 calls each to the county's communications center, said Mr. Wiseman.
Calls to the center "markedly drop off" for tremors measuring less than 2.0, he said.
Mr. Cleaves said the Geological Survey received "one or two calls" after yesterday's earthquake.
"It hasn't been the avalanche of calls when they were first felt in March," he said. "People have been able to obtain information about [the earthquakes]. Hopefully, there's more curiosity than concern about this one."
Seismologists from a Columbia University earth observatory in New York who have studied the Columbia earthquakes said the tremors probably occur when stress builds up in weak areas of the Earth's crust.
They theorize that earthquakes may be occurring near a "dike," or a fracture below the Earth's surface, which is filled with molten rock and gradually hardens. Some earthquakes may occur as unstable areas of the crust stretch and compress, building up pressure in the previously shattered areas, said seismologist John G. Armbruster at a Baltimore meeting of the American Geophysical Union in May.