Earthquakes Have Been Rare in Maryland - Until Recently

March 14, 1993|By DOUGLAS BIRCH

The tiny temblors that struck Columbia last week did little more than startle schoolchildren, rattle bottles of sparkling water and shake the suspensions of minivans zipping through that tranquil suburban community.

But while they were more a source of entertainment than terror, the tremors also helped demonstrate how little anyone knows about earthquakes in this region.

Maryland recorded just one quake between 1963 and 1990. Since then the state had its wrist smacked repeatedly. Five small earthquakes -- including the two this week -- have hit in the past three years, all of them clustered in a 20-mile radius of the hamlet of Woodstock in Howard County.

Maybe the recent spate of quakes means Maryland will get shaken up a little more often in the future. Maybe the local geography is headed back to sleep after a couple of fitful snores. No one knows for sure.

Scientists think it's unlikely that the area will ever get slammed with an earthquake on the scale of those that periodically rock California. Large quakes are rare in the eastern United States, and the earthquake hazard in Maryland and Delaware is considered the lowest of any state on the East Coast, according to one 1990 federal study.

There hasn't been anything close to a large quake, defined as an event measuring more than 6.0 on the Richter scale, detected in Maryland during the 362 years of European settlement.

But a few hundred years is a heartbeat in geologic time. Unlikely does not mean impossible.

"One of the truisms of geology is that just because you've never had a large devastating earthquake doesn't mean you can't have one," said Dr. James P. Reger, chief of the environmental geology program of the Maryland Geological Survey.

Geologists can't even be certain what caused the earthquakes that jarred Columbia like some motel guest on a coin-fed vibrating bed.

Some scientists say the restless shifting of ancient geologic fractures, or faults, are to blame. Some say other forces are at work, that Maryland is a no-fault earthquake zone.

Local quakes are little understood because they have not been studied very closely. There aren't many to study in the first place.

The United States Geological Survey in Reston, Va. says Maryland has had just 28 quakes since a 30-second-long tremor stuck south of Annapolis on April 25, 1758. Dr. Reger has compiled a slightly different list totaling 27.

By all accounts, the state's recorded earthquakes have been pretty puny. Wednesday's tremor registered a magnitude of 2.5 on the Richter scale. There was a lot of rumbling and shaking. "But it's not the kind that causes damage even if it's right under a building or facility," said Dr. Walter Hays of the U.S. Geological Survey.

Thursday night's temblor was a meager 2.0 -- the kind that sways chandeliers and jostles some people, but not much else.

The largest quake in the state's history, which shook the rural community of Phoenix in northern Baltimore County on Nov. 26, 1939, was the equivalent of a 4.0 magnitude event -- generally strong enough to break some windows and crack some plaster, but without the muscle to topple brick chimneys or shove heavy furniture around.

(A similar-sized quake struck near Lancaster, Pa. on Easter Sunday 1984. It rattled windows as far away as Baltimore. But it caused little damage.)

By comparison, the Loma Prieta earthquake of 1989 that killed 63 people in the San Francisco Bay area registered 7.1 on the Richter scale -- more than 30,000 times more powerful than Maryland's 1939 Phoenix quake.

At least ninety percent of all earthquakes occur along the restless boundaries between the six major slabs or "plates" that divide the Earth's crust. These plates fit together to form a kind of jig-saw puzzle covering the planet, and grind against each other as they drift over the semi-molten the rock about 60 miles below the surface.

California has a lot of earthquakes because it sits atop one of these plate boundaries. The eastern United States has few earthquakes because the closest plate boundary is in the middle of the Atlantic ocean.

While earthquakes east of the Rockies are not numerous, they can be extremely destructive. The biggest quakes ever recorded in North America struck New Madrid, Mo. and shook a huge swath of the Mississippi valley in the winter of 1811-1812. Some of the New Madrid quakes may have packed the power of an 8.7 magnitude earthquake.

The largest earthquakes in recorded history, including one off the coast of Honshu, Japan in 1933, were probably 8.9 shocks.

Small earthquakes in this region can do more damage, because the underlying geology helps them transmit more of their power over a wider area.

An earthquake centered in Fallston, Harford County in March 1883 -- which would not have rated much more than about a 3.0 on the Richter scale -- reportedly caused "heavy damage" to buildings near Patterson Park, according to an account in The Sun.

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