Columbia, Pa. quakes traced to ancient fractures Expert points to 'Rock Hill Dike'


A seismologist says that some Eastern earthquakes, like the cluster that shook Columbia two months ago, probably occur when stress builds up in weak areas of Earth's crust -- the same areas that fractured 200 million years ago when North America and Europe pulled apart and opened up the Atlantic Ocean.

John G. Armbruster, a researcher with the Lamont Doherty Earth Observatory, said these fractures filled with molten rock, which gradually hardened into ribbon-like veins called "dikes" that are found all along the East Coast.

Today, the seismologist said, some earthquakes may occur as the crust stretches and compresses, building up pressure in these previously shattered areas.

"Dikes come up where rock is the weakest," said Mr. Armbruster, who talked about his views yesterday with colleagues at a meeting of the American Geophysical Union the Baltimore Convention Center.

"And the rule is that zones that were weak in the past are still weak."

Mr. Armbruster and Leonardo Seeber, a colleague at Lamont Doherty, studied 14 tiny earthquakes that hit Columbia in March and early April.

They also reviewed data from five small earthquakes that shook Sinking Spring, Pa., between May 10 and May 13.

Both events, Mr. Armbruster said, may have occurred near a single geologic structure that he calls the "Rock Hill Dike."

The quakes, he pointed out, showed some striking similarities.

The epicenters of the tremors in Columbia and Sinking Spring were unusually shallow, caused by slippage along a fault only 600 feet below the surface.

Earthquakes usually occur a mile or more underground.

Both tremors consisted of what the scientist called a "swarm" or cluster of quakes of similar size.

In both cases, most of the tremors measured between 2.0 and 2.7 on the Richter scale. None was large enough to do much more than rattle some dishes.

By comparison, the Loma Prieta earthquake of 1989 that killed 63 people in the San Francisco area registered 7.1.

He said it isn't possible to determine whether larger earthquakes are likely, or even possible, along the Rock Hill dike. But Mr. Armbruster said further research might provide the answer.

No one has so far rushed forward with evidence to support the theory, developed by Mr. Armbruster and Mr. Seeber, that some earthquakes cluster around dikes.

"I don't know of another case anywhere in the East where people have associated other dikes with other earthquakes," Mr. Armbruster said.

But no one seems to be sure how to explain any East Coast earthquakes.

West Coast quakes, seismologists agree, are caused by slipping and sliding along fractures in the earth, called faults, that occur where the huge plates that divide the planet's crust meet and grind against each other.

But the Eastern United States is in the middle of the North American plate.

Eastern earthquakes generally do not creates breaks at the surface, telltale evidence of slippage along a fault. And some known Eastern faults don't seem to produce earthquakes at all.

Charles K. Scharnberger, a seismologist with Millersville State University in Pennsylvania, thinks there may be a relationship between dikes and some earthquakes.

"It makes some sense," he said. "You can make a case for it. But it's not generally accepted as part of the common knowledge of seismologists."

And not all Eastern earthquakes can be explained by the theory, he pointed out, since not all of them occur near dikes.

The title of Mr. Armbruster's talk last night was: "The Mouse That Roared: The March-April 1993 Earthquake Sequence in Columbia, Md."

In 22 years as a seismologist, he said, he had never seen such interest generated by small earthquakes. "The Columbia sequence of earthquakes was very well designed for keeping the media interested and keeping people on edge," he said.

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