Strong earthquake in Gulf – ‘rare’

The strong earthquake in the Gulf of Mexico that sent shocks through Florida on Sunday was rare in various ways, scientists said.

Don Blakeman, a seismologist with the U.S. Geological Survey’s National Earthquake Information Center in Golden, Colo., said earthquakes in the Gulf are unusual because the sea is not near the edge of a tectonic plate, the massive pieces of the Earth’s surface layer that are in constant movement.

Consequently, there have been only about a dozen earthquakes registered in the Gulf in the past 30 years, according to U.S. Geological Survey data. By comparison, areas such as Indonesia can register twice that many in a day.

Sometimes, as the huge tectonic plates of soil and rock move while floating atop a hotter portion of the Earth’s crust, they slide by each other in opposite directions, collide or move apart. In those instances, tension is built up and then released in the form of an earthquake.

But because the Gulf is not near the edges of the North American plate upon which it sits, the seismic movement had to occur in the interior of the plate itself.

Such midplate earthquakes are rare.

“Most earthquakes – 99 percent of them, I’d say – occur alongside the edges,” Blakeman said. “This earthquake is what we call a midplater, a quake that occurs in the middle of the plates. They are very infrequent.”

Scientists are still trying to understand how such midplate quakes come to be, but they think the tension generated at the edges of the plates sometimes travels inward, into the middle of the plate, where it is then released.

“Think about it as two cookies rubbing against each other,” Blakeman said. “If they rub hard enough, the force of the friction can probably be felt not just in the edges of the cookies.”

Chris Harrison, a marine geophysicist at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science, said the fact that there have been two quakes in the same region this year isn’t a troubling sign.

“I just examined a ring 200 miles from this quake and, since 1973, there’s been only eight others in that same region,” he said. “That’s a very small number. And they were all much smaller in magnitude, so this is quite a big earthquake for that area.”

Underwater earthquakes occur more frequently than the public is aware of, but usually near the plate edges, he said. This quake, he added, was far from any plate edge – a fact that, when combined with it’s relative strength, makes it noteworthy but not worrisome. The intensity of the earthquake – magnitude 6 – was unusual, said Blakeman, the Colorado seismologist. “Midplaters tend to be smaller,” he said.

Blakeman reiterated that the location of the quake puzzles him and his colleagues. “We just don’t have many earthquakes in the Gulf,” he said.