MadSci Network: Earth Sciences

Re: Do all seismic events occur at faultlines?

Date: Tue Mar 10 08:26:34 1998
Posted By: Bill Raatz, Ph.D., Exploration geologist, Stratigraphy/sedimentology, ARCO International Oil & Gas Co.
Area of science: Earth Sciences
ID: 888542318.Es

Hi L. Norris,

Sorry my answer is a little late.

Most major earth movements do occur because of slippage along faults. Landslides, mudslides, and volcanic rumblings come to mind as exceptions, but these don't concern you in Florida. You are aware down there of sink holes. These are collapse features that occur when water is removed from the bedrock and unsupported limestone caverns collapse, sometimes swallowing houses. These are dramatic but local events.

Most true earthquakes occur at plate boundaries. The earth's skin is composed of a number of rigid plates that move somewhat indepedently of one another. Where these plates meet the earth's surface is moving in different directions and so earthquakes occur. If the two plates are both thick continental plates, they bulldoze into one another and form large mountains, such as India colliding with Asia forming the Himalayas. If a thick continental plate collieds with a thin dense oceanic plate, the oceanic plate will subduct (go under) the continental plate. This is happening along the west cast of the U.S and South America. The movement of the subducting slab causes earthquakes and as it melts the molten material rises to form volcanoes (such as Mt St. Helen's).

None of this is happening in Florida, however. The oceanic plate forming the eastern Atlantic Ocean and the North American plate are joined and moving together, forming what is called a passive margin. Passive margins are stable, evidenced by the lack of volcanoes and major earthquakes on the east coast. So how did you get your earthquake??

Here I am guessing a little. The east cost and Florida area was not always so stable. The Appalachian Mts. attest to a time when major plate collision occurred. The last major event happened about 250 million years ago when all of the major continents collided and for a time formed one giant supercontinent called Pangea. Europe and Africa were attached to the eastern U.S. and caused mountain building. Before the collsion, Florida was actually part of Africa, but when the continents split back apart, Florida stayed with North America. So, there are a large number of old, largely dormant faults, folds, and sutures along the east coast. Although the area is stable, there are still some stresses that build up over time, and my guess is that some of this stress built up enough to temporarily reactivate an old fault zone.

I can't say more due to my lack of local knowledge. If you want more specifics I would write to the Florida Geological Survey or one the the University geology depts.

Bill Raatz

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