CONSERVATION CORNER
October 2, 2006
Earthquakes
by James L. Cummins

On September 10, 2006, 300 miles southeast of Biloxi, an earthquake rocked the Gulf of Mexico. It had a magnitude of 5.8 on the Richter Scale. Damage was very light. One-hundred years ago, the great earthquake of San Francisco, California, rocked the Pacific Coast. Even though most people don't consider Mississippi, the state is in a major fault zone for earthquakes.

In the winter of 1811 1812, three of the largest earthquakes in North America occurred in the Mid-South. All had magnitudes greater than 8.0 on the Richter Scale and altered the landscape significantly. The effect of these earthquakes caused church bells to ring in Boston, Massachusetts, which was nearly 1,000 miles away.

Since that time, two major damage level earthquakes have occurred in the Mid-South. An earthquake with a magnitude of 6.5 occurred near Marked Tree, Arkansas in 1843, and one with a magnitude of 6.8 happened near Charleston, Missouri in 1895. In addition, earthquakes of smaller magnitudes were felt in the past and continue to be felt today. Since the installation of seismic instruments throughout the Mid South, it is known that earthquakes happen all the time in the New Madrid Seismic Zone (about 150 per year), but most are too small to be felt by people. The largest earthquakes felt in this region (on land) in recent times occurred in 1976 (magnitude of 5.0 with a 4.5 after shock) and in 1990 (magnitude of 4.8).

National maps of earthquake shaking hazards provide information essential to creating and updating the seismic design provisions of building codes used in the United States. The 1996 U.S. Geological Survey shaking hazard map for the United States is based on current information about the rate at which earthquakes occur in different areas and on how far strong shaking extends from quake sources. Colors show the levels of horizontal shaking that have a 1 in 10 chance of being exceeded in a 50 year period. Shaking is expressed as a percentage of g (g is the gravitational acceleration of a falling object).

Although strong earthquakes are less frequent in the eastern United States, damage in this area could be catastrophic due to a powerful quake because most buildings and other structures have not been designed to withstand severe earth shaking.

Damage to buildings, dams, levees, highways and interstates, grain bins, power lines and similar structures only partly depend on the amount of energy released during an earthquake. Certain kinds of earth materials enhance their vibrations. In Mississippi, structures built on thick, loose sediments of river floodplains (i.e., the Delta) and filled areas are more likely to be damaged than structures built on bedrock. In fact, seismic intensity may increase as much as one full unit on the Modified Mercalli Intensity Scale if loose sediment is present. Also, loose sediment with a high moisture contact can turn to liquid when shaken.