The How's of a Local Earthquake

By: Stephen Bowers Email
By: Stephen Bowers Email

The small earthquake that shook northern Lauderdale county early Friday morning brings about more questions than answers. The primary question is why in the world did an earthquake hit Mississippi?

It's easy to think earthquakes don't hit here, but believe it or not they do happen. They're usually very small and often go unnoticed, but according to the U.S. Geological Survey small earthquakes they do happen in our part of the world. In fact, data show at least 15 small earthquakes in our area of East Mississippi and West Alabama since 1990.

Most powerful earthquakes occur along fault lines. Fault lines are boundaries where two or more of earth's tectonic plates come together. What is a tectonic plate? Imagine all of the world's oceans and land masses sitting atop huge graham crackers. Those graham crackers are floating, not necessarily on water but on softened butter. They don't move quickly by any means. Any given point might move a centimeter in a century or less.

Those graham crackers, with all of the oceans and land on top of them, can collide or move apart or just simply drift passed each other. Remember they move very slowly, so it takes time for stress to build on the edges, or fault lines. When the stress builds, one graham cracker will crack and force itself to dive beneath another. That alone can cause a massive earthquake. The thrusting of one graham cracker beneath the other can also generate huge ocean waves called tsunamis if this process happens beneath one of the oceans.

Remember the graham crackers that are drifting passed each other? Sometimes they bump into each other and lock up. Eventually, the earth's crust will snap so the plate (or our graham cracker) can keep moving. That "snapping" can also cause an earthquake. This is very much like southern California's quakes. They can be quite powerful and damaging but usually are little less powerful than the thrust earthquakes mentioned earlier.

As those graham crackers keep pushing together, some stress can also build up in the center of the graham crackers. That stress can make the graham cracker crumble in the middle, which is how Friday morning's Lauderdale county quake happened. These are small and rarely even felt.

Then there is the New Madrid Seismic Zone, which runs along the Mississippi River from northwestern Mississippi into Arkansas, western Tennessee, southern Missouri, western Kentucky, and southern Illinois. This is an area where two tectonic plates actually drifted apart about 750 million years ago, leaving an especially weak area within the plate (our graham cracker). The fault itself is deeper than most, but when the fault ruptures its earthquakes are epic. We know about earthquakes in this zone back to the 1600s. Most recent earthquakes occurred in the early 1800's, about 200 years ago, and were about 8.0-magnitude quakes. One of them was strong enough to shake the Liberty Bell all the way in Boston. Are major earthquakes still possible in that area? Yes. When will they happen? Hopefully not anytime soon, but there's really no way to tell. Will we feel it in Meridian? Probably, but the strongest shaking is most likely no closer than the northwest corner of Mississippi.

It's just another natural disaster to add to our resume of tornadoes, hurricanes, ice storms, and snow storms.

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