Gulf quake rates a shrug

Experts offer some reassuring opinions about the Sunday temblor, but say it's too insignificant to investigate.

Published September 12, 2006

A fault nobody knew existed produced the strong earthquake Sunday in the Gulf of Mexico.

It doesn't appear on any geological maps.

No seismic monitoring occurs in that part of the gulf, so scientists are uncertain where the fault is or where the quake took place.

They expect they'll never know because, for one thing, they aren't going to look.

"There are faults in most places around the world, thousands of them, hundreds of thousands maybe, that we know very little or nothing about, and this was one of them," said Don Blakeman, a geophysicist with the National Earthquake Information Center in Golden, Colo.

"We knew of no causative faults in the Gulf of Mexico that could produce a quake of this magnitude. Now we know there is one down there somewhere, but it's not a big deal."

The word Monday from experts across the country is that Floridians should chalk up Sunday's quake as an oddity and nothing to stress over. It might happen again. It might not. But chances are it will never be more serious than this or have any serious ramifications.

"This fault is not likely to contain enough elastic strength to cause any problems along the Florida coast," said Paul Wetmore, a professor of seismic geology at the University of South Florida. "It will never slip with enough energy to cause a tsunami."

As of Monday evening, Sunday's quake had produced no aftershocks.

"I wouldn't be at all surprised to see some, and in fact, I expected to see some before now," Blakeman said. "We've seen shallow quakes before that didn't produce much in the way of aftershocks, but it's still early yet."

Sunday's quake measured 6.0 on the Richter Scale, where every whole number represents a change in strength by a magnitude of 10. A 6.0 quake feels 10 times stronger than a 5.0, 100 times stronger than a 4.0 and 1,000 times stronger than a 3.0.

Scientists don't know exactly what triggered Sunday's strong tremor. They speculate that pressure on the Earth's relatively thin crust had built up for decades or centuries and released at a fault, or weak point in the crust.

Stresses build as the plates that make up the crust move against one another. There are seven major plates and six minor ones fitting together like a jigsaw puzzle. They are in constant motion, sliding against one another vertically and horizontally, pushing together with crushing force, pulling apart and leaving gaps or weak spots. Occasionally one plate will dive under another.

It's a bit like a subterranean demolition derby in very slow motion. The plates move from two to 10 centimeters a year - about the same speed as human fingernails grow.

Most quakes occur along the boundaries of these plates, but not all of them.

The area of the Gulf of Mexico where Sunday's quake is believed to have occurred is well north of the nearest plate boundary.

Florida sits on the North American Plate, a huge slab of rock that makes up all of the United States, Canada, Greenland, the bottom of the Atlantic Ocean two-thirds of the way to Europe and far eastern Asia.

One of the plate's edges runs up the west coast of the United States and Canada and follows the southern Alaska coast out past the Aleutian Islands, explaining why that is such a seismically active stretch.

The closest a plate boundary comes to Florida is the southern edge, which crosses the Caribbean east to west on a line that takes it between Haiti and Cuba.

Sunday's event occurred in a general area about 500 miles north of that boundary. Scientists refer to it as a mid-plate quake.

"It neither changed or added much to our knowledge of such events," said Eugene Schweig, a geologist with the U.S. Geological Survey.

However, Schweig said the quake was "a bit odd because the way the fault broke suggests it was squeezed rather than stretched, and that's a little unusual."

Schweig, too, said Sunday's quake likely was as bad as a gulf quake could get.

"Even this quake was unlikely," he said.

Asked why there wasn't more interest in a comprehensive mapping of the Earth's faults, Schweig said, "Because most of them aren't very interesting."

"A fault just means a break in the Earth's crust," he said. "The major ones have surface features we can see. Most of the rest aren't big enough or deep enough to generate quakes.

"But, as you discovered Sunday, there are some we don't know about that are quite capable of generating quakes."