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Hurricanes, sure, but an earthquake in gulf?

Most earthquakes occur on faults near where giant sections of the Earth’s crust collide or slip past one another. Sunday’s 6.0 earthquake in the Gulf of Mexico was unusual because it was far from the nearest active tectonic plate boundary.

By MIKE DONILA
Published September 10, 2006



Oliver Kuglar had his toes in the sand and was reading the Sunday paper when he felt the ground at Indian Rocks Beach shake.

The 45-year-old Largo resident knew immediately it was an earthquake. He experienced one years ago in California.

“There was no breeze. It was a beautiful, typical Florida morning and then my chair started rocking left to right for about six seconds,” said Kuglar, who was 25 feet from the water. “I knew it couldn’t be the wind, and there was no one behind me playing a trick.”

Officials with the U.S. Geological Survey said an earthquake hit the Gulf of Mexico at 10:56 a.m., about 260 miles from St. Petersburg. It measured a magnitude 6.0 and lasted for 15 to 20 seconds. No damage was reported.
Jessica Sigala, a seismic analyst with the National Earthquake Information Center in Golden, Colo. called the earthquake “strong.”

She said a quake of that type and magnitude was unlikely to cause waves large enough to affect the coasts.

She said it would take a quake about 32  times stronger — a magnitude of at least 7.0 or greater  — to cause a tsunami. There is no record of a quake that large ever hitting Florida.

Another temblor, which measured magnitude 5.2, hit the same spot in late February but went largely unnoticed except by experts.

Don Blakeman, a geophysicist with the earthquake center, said it is unusual for a quake to occur in the gulf.He said that he doesn’t think another will hit any time soon but did say this particular quake might not be over.“

There is a possibility that an earthquake this size and in shallow crust like this could have some aftershocks ... but they’d be smaller than the main earthquake,” said Blakeman, adding that the quake originated about 6.2 miles deep in the Earth’s crust.

Sunday’s quake occurred 329 miles southeast of New Orleans at 26.327 degrees north and 86.571 degrees west. It was felt throughout the Tampa Bay region, in parts of Alabama and as far away as Kentucky, the Carolinas and the north Georgia mountains. Almost 4,000 people had contacted the National Earthquake Information Center by Sunday evening.

Despite some rumors, said Mike Stone, spokesman for the Florida Division of Emergency Management, no tsunami warning was issued Sunday. The Coast Guard did issue a mariner’s warning, however, advising boaters to use caution while on the gulf, said Operations Specialist First Class Josh Sharman.

“We’re monitoring the situation,” said Sharman, who is based in St. Petersburg.

Residents said pictures rattled on the walls, kitchen countertops shook and books on coffee tables moved. Some said they were a little nervous; others didn’t know what to think.

“We looked out the windows,” said Chris Kelly of Pinellas Point. “We didn’t see any neighbors making noise, there were no trucks, there wasn’t a sonic boom. Whatever it was, though, it was very noticeable.”

Dorothy Conley, of Redington Beach, was standing in the street when she felt it. “It just moved. It was that moving feeling that we experienced,” she said.

Sandy Oestreich, the former vice mayor of Redington Beach, said she was talking on the phone with her daughter in New York when the “keys that were hanging started shaking and clinking.”

Most earthquakes occur on tectonic plate boundaries, Blakeman said.

The Earth’s surface is put together by 13 large bodies of rock called tectonic plates that each fit together like a jigsaw puzzle, said Rafael Abreu, a geophysicist with the National Earthquake Information Center. But they are not static. They often move against, apart or past each other. The movement causes quakes.

The gulf, though, is not near a tectonic boundary, so Blakeman said “that’s highly unlikely” that another large quake would occur.

He said he’s not sure what caused Sunday’s quake but “the general answer is the movement of the plate produces stress throughout the interior and sometimes that stress is released in the form of an earthquake.”

Blakeman also said he seriously doubted human activity, such as the recent drilling for oil in the gulf, had anything to do with the quake.

“I’ve never seen anything this size caused by man-made” actions, he said.

When asked what type of effect an earthquake might have on oil rigs in the gulf, he said it would depend on how close the rig was to the quake’s epicenter and what type of sediment was holding the structure.

Most deep-ocean oil drilling rigs do not rest on the floor. Instead, they are anchored to the bottom and float on the surface, making it far less likely they would be damaged.

Sunday’s quake occurred in an area of the gulf where the bottom is covered in a layer of sediment a mile deep that originally flowed out of the mouth of the Mississippi River, said Paul Wetmore, a seismic geology professor at the University of South Florida.

Within the pile of sediment are sections that settled out in different densities, and the boundaries between them are full of “a lot of little faults,” Wetmore said. They don’t slide past each other smoothly, either, he said. They stick.

When the different sections of bottom sediment come unstuck and push past each other, that’s when an earthquake like the one Sunday occurs, Wetmore explained.

“The stress that was building up to cause that earthquake was probably building for hundreds of thousands of years,” he said.

An earthquake in that area of the gulf is unlikely to produce a tsunami, Wetmore said. Generally a tsunami results from one plate in the Earth’s crust shoving over another, with the upward thrust creating a massive wave. But the gulf contains no areas where that would happen, he said. The closest is in the Lesser Antilles, near Puerto Rico.

Meanwhile, Kugler, who oversees marketing and sales for Resort & Club at Little Harbor, will be watching the water.
“I want to make sure there’s nothing out of the ordinary,” he said. “I think we’ve all learned from the last several years about what to look for and to be careful.”

Times staff writer Craig Pittman and eesearcher Tim Rozgonyi contributed to this report.

[Last modified September 10, 2006, 22:58:04]


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