A scientist studying the spate of earthquakes that struck Columbia said yesterday that the shocks are probably trailing off.
"Most likely this is a waning event," said Leonardo Seeber, a research associate at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y. "It's in its disappearing phase."
Based on seismometer readings of small quakes that occurred Tuesday and Wednesday, Mr. Seeber and another Lamont-Doherty seismologist, John G. Armbruster, think they are close to pinning down the source of the recent shocks.
They probably originated about three-quarters of a mile northwest of the intersection of Route 32 and U.S. 29, in the Clemens Crossing neighborhood, Mr. Seeber said.
He said the fracture, or fault, causing the earthquakes is shallow by geologic standards -- about 1,300 feet to 1.2 miles underground.
Earlier reports from the United States Geological Survey estimated the depth of the quakes at 3 miles.
The shallow fault, Mr. Seeber said, explains why residents of Columbia reported feeling tiny shocks in the 1.0 to 2.0 range. Such quakes normally go unnoticed when they occur deep in the ground.
The scientist said the series of shocks may have begun before March 10, when the first one was recorded. Several residents reported feeling earlier ground movement, he said, and they "sound like good reports."
Measurements with Lamont-Doherty's network of seven seismometers have been consistent with his "working hypotheses" that the recent quakes come from a fault linked to an enormous igneous dike, Mr. Seeber said.
The feature is a long, narrow ridge of rock that formed after molten material forced its way up through cracks in the surrounding crust 170 million years ago, when the Atlantic Ocean was opening as the continents drifted apart.
The underground ridge, he said, probably extends to near Lancaster, Pa. It is thought to be the source of an earthquake there on Easter Sunday 1984, which registered 4.1 on the Richter scale.
Mr. Seeber said that the Columbia area could now return to its seismic slumber "for 100 years." But no one can be sure.
The series of earthquakes has also raised, slightly, the odds that a stronger earthquake might occur.
Before, he said, the risk was "very, very, very low. Now it's just very low."