Earth Hazards of the South-Central United States

Natural hazards are events or processes that have significant impacts on human beings and the environment. Extreme weather conditions or geologic activity can cause substantial short-term or long-term changes to our environment. These changes can influence many aspects of the world around us, including crops, homes, infrastructure, and the atmosphere. The 4.6-billion- year-old Earth has experienced many naturally generated hazards, while other events are byproducts of human activities, created during mineral and energy extraction or in construction practices that modify the landscape.

The South-Central U.S. is subject to a variety of earth hazards. Most famously, the area happens to have just the right combination of conditions for tornados that cross the region and hurricanes that impact the Gulf Coastal Plain. Modifications of the Mississippi River and its mouth, as well as the Gulf coastline, have exacerbated the impacts of storms and floods. Limestone, gypsum, and salt deposits are responsible for significant areas of karst topography and sinkholes. Like many parts of the country, landslides from expansive soils and exposure to radioactivity from radon are present, depending upon the nature of the local bedrock. Perhaps most surprisingly, despite being far from a plate boundary, certain areas of the South Central are at risk from large earthquakes due to occasional movement along large ancient faults, and from smaller earthquakes associated with injection of wastewater into the Earth that promotes movement along smaller faults.

Earthquakes have occurred in each of the South-Central states and the table below shows the magnitudes of some of the most notable earthquakes in the region.

The greatest hazard potential is in the area of the New Madrid seismic zone (NMSZ), a 240-kilometer (150-mile) set of subsurface faults thought to have formed during the breakup of the supercontinent Rodinia in the late Precambrian (about 750 million years ago).

The next largest quake to have occurred along the NMSZ was a 6.6-magnitude quake that occurred on October 31, 1895. The quake, centered in Charleston, Missouri, damaged almost every building in the city. Even today, areas in the NMSZ continue to experience earthquake activity, which is closely monitored by seismologists. There are ancient, seismically inactive subsurface faults in many other parts of the country, and it is unclear why seismic activity remains so high along the faults in the NMSZ, which are now far from North America’s plate margins. Most of the dozens of annual earthquakes that occur in the NMSZ are very small—too small to notice except with sensitive equipment.

The seismic activity around the Nemaha Uplift is associated with faulting known as the Humboldt fault zone, which, like the NMSZ, lies along a Precambrian basement and ancient rift system.

Recently, Oklahoma has experienced an unusual amount of earthquake activity, with numerous earthquakes of magnitude 3 or 4 and a few above magnitude 5.

Only 89 earthquakes occurred in the state between 1970 and 2009, but since then the incidence has increased dramatically, rising from 48 earthquakes in 2010 to 611 in 2014 alone. The seismic activity in these instances has been linked to the high-pressure injection of wastewater from oil and gas extraction operations into the ground. The pressure of the water increases the likelihood that a rupture might occur along an otherwise locked fault. Concerns exist that additional activity along offshoot faults from the Nemaha Uplift near Oklahoma City might be even more serious. Similar instances of induced seismic activity have occurred elsewhere in the South Central, perhaps most famously with cases associated with injection wells near Dallas-Fort Worth. These wells have been used to dispose of wastewater from the extraction of natural gas in the Barnett Shale.

This information on earthquake risk can lead to better designs for high-risk infrastructure like dams, high-rise buildings, and power plants—and it can also be used to inform the public of potential hazards to lives and property.

Heavy rain, snowmelt, groundwater percolation, and water level changes along coastlines, earthen dams, and the banks of water bodies are conditions under which landslides can occur. These flood-related conditions are associated with precipitation, runoff, and saturation of the ground. Hazards occur from mudflows themselves, but also from backwater flooding, dam failure, and debris that rushes downstream and causes further erosion.

Soils that weather from shale, volcanic ash, or bentonite are rich in clay, which may contain minerals that can absorb water and swell up to 1.5 to 2 times their original volume. That amount of expansion can exert enough force to cause damage, such as cracked foundations, floors, and basement walls. An estimated nine billion dollars of damage to infrastructure built on expansive clays occurs each year in the United States.

Soil creep is a slow kind of landslide that occurs when certain types of clay in the soil on a hillside absorb water, expanding and causing the soil to swell. As the clay dries and contracts, the particles settle slightly in the downhill direction. This process can cause fences and telephone poles to lean downhill, while trees adjust by bending uphill.

While soils can swell by absorbing water, they will also shrink when they dry out, resulting in subsidence that damages landforms and infrastructure. Fissures may develop in the soil, allowing for the deep penetration of water when floods or runoff occurs. This produces a cycle of shrinkage and swelling that puts repeated stress on rock layers and human structures. While expansive soils can be found all over the US, every state in the South Central has bedrock units or soil layers that are possible sources, with Louisiana’s coastal plain and Oklahoma’s Cretaceous shales being the most susceptible.

Significant or repeated changes in moisture, which can occur in concert with other geologic hazards such as earthquakes, floods, or landslides, greatly increase the hazard potential of expansive soils. The key to reducing this hazard is to keep the water content of the soil constant. There are also chemical stabilizers, including lime, potassium, and ionic agents, that can reduce the potential for soil volume changes by increasing the clay’s structural stability.

Much of the South Central is underlain by karst and carbonates. Central and southern Missouri, including the Ozark Mountains and their extensions into northern Arkansas and northeastern Oklahoma, are famous for caves and karst in Ordovician and Carboniferous limestone and dolomite. Missouri is home to over 6000 limestone caverns, many of which are prominent tourist attractions.

In Missouri, karst is also associated with exceptionally large springs such as Big Spring, Greer Spring, and Maramec Spring.

Other karst formations are found in the Arbuckle Mountains of south central Oklahoma and the Limestone Hills in southwestern Oklahoma. Soluble gypsum and salt deposits near the surface in western Oklahoma and the Texas panhandle can also cause karst and dissolution problems.

Because karst terrain is very porous and fractures easily, groundwater pollution can be a serious problem. Contaminants that might otherwise be filtered through the sedimentary rock are quickly transported into aquifers by runoff. The hazards of pollution are increased by rampant industrial, agricultural, and residential development over karst features. This is occurring rapidly in northwest Arkansas and in Missouri, where 59% of the state sits atop thick layers of carbonate rock.

The Coastal Plain of eastern Texas and Louisiana is dotted with many Jurassic subsurface salt domes that can collapse if salt is removed. For example, the Bayou Corne sinkhole in Assumption Parish, Louisiana, is a site where an underground salt dome collapsed in 2012 (learn more here).

Before its collapse, the Bayou Corne sinkhole was preceded by months of seismic activity and the release of methane bubbles. It originally spanned one hectare (2.5 acres) but has since grown to over 10 hectares (26 acres). It is still growing, swallowing surrounding cypress swamp and endangering the nearby community, from which many of the residents have been evacuated. Scientists believe the sinkhole was created by the salt dome cavern being excavated too close to the massive salt deposit’s outer face, making it incapable of maintaining pressure.

A similar event occurred in Daisetta, Texas, a town also located on the edge of a major salt dome. In May 2008, a 330-meter-wide (1080-foot-wide) sinkhole caused by partial collapse of the dome swallowed a parking lot, construction equipment, and a small stand of trees over the course of a single day before filling with water.

These types of situations present a growing hazard that will be studied by geoscientists for years to come.

Salt karst can also be a hazard through its association with pathways for fluids, such as the flow of natural gas, which is sometimes stored in salt caverns. For example, in 2001, the Yaggy storage field—consisting of dozens of caverns in the Permian-age salt deposits near Hutchinson, Kansas— leaked 4 million cubic meters (143 million cubic feet) of natural gas, leading to multiple large-scale explosions within and around the city. In this case, the leak was associated with drilling errors as well as with faults and fractures in rocks overlying the salt formation.