Two New York scientists headed to Maryland late yesterday hoping to make detailed measurements of any shocks that might follow Sunday's small earthquake, the third and most powerful to hit the Columbia area in the past week.
A colleague estimated that there is a 50-50 chance that another small quake will occur. Other researchers said there is no reason to believe that the recent tremors are the prelude to a larger, destructive earthquake.
Over the next few days, the research geologists from Columbia University's Lamont-Doherty Earth Observatory will bury orange crate-size seismometers in foot-deep holes scattered around the Howard County city.
Dr. John Ambruster and Dr. Leonardo Seeber want to record any more tremors from what scientists increasingly suspect is an ancient fracture, or fault. Scientists have not been able to analyze the recent quakes in detail because there are no working seismometers in Maryland.
"If that fault continues to pop away -- hopefully, with not much larger events than we've had in the past -- that would give us the opportunity to really pinpoint which geological fault or feature is causing that activity," said Dr. Klaus Jacob, another Lamont-Doherty seismologist.
In the 28 years prior to 1990, Dr. Jacob noted, Maryland experienced only one earthquake. But in the past three years there have been six small tremors, all of them within a 20-mile radius of the Granite-Woodstock area of western Baltimore County and northeastern Howard County.
Dr. Jacob estimated that there is a 50-50 chance that one or more small earthquakes will hit the Columbia area. They could be so tiny, though, that area residents would not feel them.
Other scientists added that while small shocks are possible, a major quake is unlikely.
"The overwhelming probability is that we're not going to see anything [major] follow on this," said Dr. Randall Updike of the U.S. Geological Survey in Reston, Va. "There's just no historic evidence to suggest that is going to happen."
In California, which lies on the seismically active edge of one of six huge plates that make up most of Earth's crust, the small Columbia quakes might be regarded as the prologue to a larger shock.
But in the East and Midwest, which lie in the middle of the North American plate, small earthquakes often occur in clusters of two or three and then subside. "Some old fault, once every 500 years, relieves some stress," Dr. Updike said. "It has a little bump."
Dr. James P. Reger of the Maryland Geological Survey said he doubts the recent Columbia earthquakes mark "the opening of a new era of earthquake activity. Maryland is still the quietest state on the East Coast between Georgia and Maine. We don't see any reason to revise that picture at this time."
Dr. Jacob cautioned that the current picture could change.
"At this point there's no way for us to know whether something larger is to follow," he said. "If someone tells you that they do know, I wouldn't trust them."
The latest tremor, measuring 2.7 on the Richter scale, occurred at 11:29 p.m. Sunday with its epicenter estimated at the intersection of Route 32 at U.S. 29, Dr. Updike said.
The shock was the most powerful of the recent temblors.
"One thing that this quake seems to have had that the others didn't is a rumbling like a freight train going by," said Dr. Reger, who has talked with many Columbia residents and has distributed 2,000 earthquake questionnaires to area supermarkets. "One person described it as sounding like driving over rumble strips. And they said they felt a little bit more vibration this time."
In all three quakes, some residents have heard loud pops or explosive sounds.
On Thursday, a 2.0 earthquake hit Columbia at 7:54 p.m. Its epicenter was roughly the same as Sunday's.
The 2.5 jolt that hit Columbia at 9:32 a.m. on Wednesday, March 10, had its epicenter about five miles away, near the intersection of Routes 108 and 175.
All three earthquakes are thought to have originated about three miles underground, which suggests that they might have been triggered by movement of the same relatively shallow geologic feature.
"It makes you start thinking that there is likely a fault at some depth," said Dr. Reger. "If it is, we don't know how big it is or how old it is."
If they catch an earthquake in the act, Dr. Jacob said, the Lamont-Doherty scientists hope to be able to pinpoint the cause of the shocks.