Earthquakes, Too?

Karla Wall Thursday, September 21, 2017 Comments Off on Earthquakes, Too?
Earthquakes, Too?

Louisiana Has A Long List of Natural Disaster Risks.  Are Earthquakes On It?

By Karla Wall

It’s the peak of hurricane season as I write this, and the Texas and Louisiana Gulf coasts have taken their first major hit in over 10 years. While Mother Nature let us have that decade or so free of storms, she made up for that lack of attention and then some with Hurricanes Harvey and Irma, the latest record-breaking weather events.

The annual Gulf storms are just one of the natural disaster risks we living here on the coast face — river flooding, violent thunderstorms with strong straight-line winds and flash flooding, ice storms that are rare and thus ill-prepared-for. There’s a long list.

You might be surprised to learn that earthquakes are on that list. In fact, there was a 3.8-magnitude quake in Sulphur in 1983.

How big a risk is a major earthquake here? Could SWLA ever experience a temblor on the scale of Haiti’s or Fukushima’s catastrophic events?

Earthquake 101

To answer that, let’s begin by examining what an earthquake is, and what causes one to occur.

A 2001 report on earthquakes in Louisiana released by the Louisiana Geological Survey describes an earthquake as a “sudden, sometimes violent trembling or shaking of the ground caused by the release of stored energy in the rocks beneath the earth’s surface.”

The earth underneath us is constantly in motion. Tectonic plates, on which continents are situated and which also run underneath oceans, are constantly in motion, and have been since the earth was formed. In fact, the plates were configured very differently eons ago — not into the seven continents and the ocean floors we know today; eons from now, the plates will have moved into another configuration, and the earth’s surface will look completely different than it does now.

When two plates in motion rub together — either laterally (side against side) or vertically (one plate moving under another) along an area of weakness, or fault, the stress on the rocks along that fault line build up to the point that it has to be released. The rock breaks, the pressure is released, and waves of energy radiate out from the point of the breakage, called the focus (the point on the earth’s surface at directly above the breakage is called the epicenter, usually named after the city or landmark situated on that point).

Those are the “seismic waves” felt during an earthquake — the rattling and shaking that can be powerful enough to collapse buildings and bridges, or only just strong enough to simply knock a picture off a wall. They may not be felt at all.

There are such fault areas all over the world, with the “hot spots” being the Pacific Rim area (western U.S. and Asia), South America and volcanic islands such as Haiti.

Growth Vs. Motion

Are there faults in Louisiana? There are many of them. There is a system of faults in the Baton Rouge area, including the Baton Rouge fault and the Denham Springs-Scotlandville fault; the Michoud fault in the eastern New Orleans area; and numerous others in the South Louisiana region. There are also faults in northwestern Louisiana.

These faults, called growth faults, were created by the rapid accumulation of river sediment along the coast in earlier geological periods; they aren’t the “plate faults” or “continent transform faults” found where two tectonic plates collide — the Pacific Rim, for instance (think the San Andreas fault in California).

What’s the difference? Plate faults are areas where motion and collision occur — bigger stresses on the rock, thus larger and more powerful earthquakes. Growth faults are areas in which earthquakes can occur, but on a much smaller and gentler scale.

“Faults associated with plate boundaries involve (the earth’s) crust and mantle,” explains XXX John Johnston. “They have the capacity to release very significant amounts of energy. On the other hand, Louisiana has normal faulting, with only minimal to moderate capacity to release significant energy.”

So, to answer the question of whether or not Louisiana would ever experience a significant quake: The chances are “extremely minimal,” says Johnston. Another LSU geologist, Richard McCulloh, says that, while quakes frequently occur in this area, there won’t be one much more powerful than a 4.0 on the Richter scale.

History Of Louisiana Quakes

A record of Louisiana earthquakes published in a 2003 Louisiana Geolgical Survey report on earthquakes bears this out. According to the report, there have been 43 earthquakes recorded in Louisiana since 1843, and only one was stronger than 4.0 — the Donaldsonville earthquake, which occurred in October of 1930 and measured 4.2 on the Richter scale.

The largest earthquake recorded in Louisiana, the quake was strong enough to register on a seismograph in Washington, D.C., and was heavy enough to cause structural damage to buildings in the New Orleans/Baton Rouge/Hammond area.

Going much further back, there were earthquake events in May, 1842, centered in Catahoula, La., which were felt for about 2-3 seconds over a 1,350-sqare-mile area south of Baton Rouge. That was followed by events in 1882, 1886 and 1905. There were quakes in 1927 and ‘29, both under 4.0 on the Richter scale, and three quakes in the 1940s.

In November, 1958, a 15-second quake centered in the New Orleans area occurred, strong enough, according to the LGS report, to “rattle windows and doors.” That was followed by another quake, on Nov. 19, with duration of 10 seconds.

In 1959, an earthquake was felt in a 3,000-square-mile area in SWLA, from Cameron north to Dequincy and east to Lake Arthur. The quake reportedly rattled objects in Creole and Grand Chenier.

From April to August of 1964, a series of small and shallow quakes occurred along the Texas/Louisiana border in the Toledo Bend area. This was around the time that Lake Sam Rayburn was being filled in, and the Toledo Bend Dam was under construction.

In 1983, the previously-mentioned Sulphur quake registered 3.8 on the Richter scale, powerful enough to move furniture and dislodge pictures and other objects off walls.

The state’s most recent quake occurred in December, 2005 — a 3.0 event centered in the Baton Rouge/Hammond/New Orleans area.

And according to National Geographic, an earthquake occurred in the Gulf of Mexico in September of 2006, at a point about 330 miles southeast of New Orleans, in an area called the Cuba fracture zone. Registering a 6.0 on the Richter scale, the quake was felt in parts of Florida, Georgia, Alabama, Mississippi and Louisiana. The quake was too weak to cause much damage, or to create a tsunami.

Earlier, in February, 2006, a 5.2 quake occurred in the same zone.

According to the National Geographic article, the earthquakes occurred not along two plate boundaries, but in a mid-plate section. The quakes were probably, according to the U.S. Geological Survey, caused by long-term stress originating from far out in the Atlantic, in an area where tectonic plates are in motion causing the seabed to spread.

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