Earth Hazards of the Southeastern United States

Introduction

Natural hazards or earth 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 Southeast 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 and Atlantic Coastal plains. Modifications of the Mississippi River and its mouth, as well as the Southeast's 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 Southeast are at risk from large earthquakes due to occasional movement along large ancient faults.

Earthquakes

Earthquakes are much less common in the eastern U.S. than in the west. This is primarily because the east coast of North America is a passive margin; that is, it is located in the center of a tectonic plate rather than at an active plate margin. All earthquakes that occur in the eastern U.S. are therefore referred to as "intraplate" earthquakes. Eastern quakes, however, are typically felt over a much larger region than western quakes of similar magnitude. For example, in the Southeast an earthquake of magnitude 5.5 can usually be felt as far as 480 kilometers (300 miles) from where it occurred, and sometimes causes damage as far away as 40 kilometers (25 miles). This appears to be because the bedrock that makes up most of the eastern U.S. is older, colder, drier, and less fractured than rocks in the western US. As a result, although earthquakes here release the same amount of energy as other earthquakes, the shaking affects a much larger area because the seismic waves travel through denser, more solid bedrock.

Notable Earthquakes of the Southeastern United States

Southeastern U.S. Seismic Zones

There are four major seismic zones in the Southeastern states: the New Madrid Seismic Zone, the Southern Appalachian Seismic Zone, the South Carolina Seismic Zone, and the Virginia Seismic Zone.

New Madrid Seismic Zone

The New Madrid Seismic Zone (NMSZ, also called the New Madrid Fault Line or Fault System) is 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). Although this rift did not split the continent, it remains an underground weak point—most of the zone's seismicity is located 5–24 kilometers (3–15 miles) beneath the surface. Faults in the NMSZ are occasionally reactivated by the relatively small east-west compressive forces associated with continuing continental drift of the North American plate, making the area unusually prone to earthquakes.

The NMSZ has been the source of numerous earthquakes in western Kentucky, Tennessee, and Mississippi. Four of the largest earthquakes in North American history—the New Madrid Sequence—occurred in the NMSZ on three days over a period of three months: December 16, 1811, January 23, 1812, and February 7, 1812. The quakes, with estimated magnitudes between 7.0 and 8.0, occurred along the Mississippi River in southeastern Missouri and northern Arkansas. While much of the damage was confined to Missouri and Arkansas, the tremors shook the Mississippi Valley and much of the eastern United States, destroying buildings and warping the ground. In Kentucky and Tennessee, landslides occurred along the Mississippi River bluffs and ground subsidence brought on by soil liquefaction spread across the Mississippi River floodplain. Shaking was felt as far away as New Orleans and Boston, where it is said to have caused church bells to ring, and the waters of the Mississippi River appeared to flow backward for several days due to local uplift and waves flowing upstream. Hundreds of aftershocks followed over a period of a several years, and were felt regularly until 1817.

South Carolina Seismic Zone

The South Carolina Seismic Zone is located in the mid-coast area of South Carolina, and is believed to be caused by faults formed during the break-up of Pangea, beginning around 200 million years ago. These faults are very difficult to study, however, because they are buried beneath as much as three kilometers (two miles) of sediment. The South Carolina Seismic Zone was the site of the 1886 Charleston earthquake, which had an estimated magnitude of 7.3 and was the most damaging earthquake to occur in the Southeastern states. The quake damaged and destroyed hundreds of buildings, killed at least 60 people, destroyed kilometers (miles) of railroad tracks, and generated extensive craters and fissures for 1300 kilometers (810 miles) surrounding the epicenter.

The image below shows the intensities felt in surrounding areas after the 1886 earthquake in Charleston, South Carolina.

Southern Appalachian Seismic Zone

The Southern Appalachian Seismic Zone (and its extension, the Eastern Tennessee Seismic Zone) runs along the Appalachian Mountains, from easternmost Tennessee to northeastern Alabama. The faults in this zone formed as a result of continental collisions that created the Appalachian Mountains in the mid- to late Paleozoic.

Central Virginia Seismic Zone

The Central Virginia Seismic Zone is located in central Virginia, and has been a site of earthquakes at least since the 1700s. This zone is also a result of the continental collisions that created the Appalachian Mountains; its faults have been reactivated as a result of subsequent rifting and continental movement. Most recently, the Virginia earthquake of August 2011 (M5.8) occurred along a north- or northeast-striking fault in the Piedmont of Louisa County, 61 kilometers (38 miles) northwest of Richmond. No deaths and only minor injuries were reported, but minor damage to buildings was widespread and included the National Cathedral and Washington Monument in Washington, DC. The 2011 earthquake, along with a magnitude 5.8 quake on the New York-Ontario border in 1944, is the largest to have occurred in the US east of the Rocky Mountains since an 1897 temblor centered in western Virginia (M5.9). Research following the 2011 Virginia quake revealed that the farthest landslide from the epicenter was 240 kilometers (150 miles) away, by far the greatest landslide distance recorded from any other earthquake of similar magnitude (previous studies of worldwide earthquakes indicated that landslides occurred no farther than 58 kilometers [36 miles] from the epicenter of a magnitude 5.8 quake). It remains unclear exactly why this occurred.

Vulnerabilities and detection

In the Southeast, the Coastal Plain region (including the Mississippi Embayment, the Gulf Coastal Plain, and the Atlantic Coastal Plain) is considered the most vulnerable since many sizable cities in those areas have been built upon unconsolidated earth materials susceptible to liquefaction, a process by which water-saturated, unconsolidated sediment temporarily loses strength and behaves as a fluid when vibrated. This phenomenon is similar to that observed when you wiggle your toes in the wet sand near the water at a beach. Liquefaction brought on by an earthquake is capable of causing structures to collapse due to loss of support.

Networks of seismograph stations have improved geologists' ability to detect and accurately locate earthquake hazards, and specific fault zones are being studied throughout the Southeast. 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. The hazards associated with earthquakes are mainly related to collapsing buildings and other structures, fire related to broken gas lines and other utilities (and broken water lines preventing firefighting), and in some instances tsunamis—seismic sea waves. Tsunamis are not known to occur along the coastal areas of the Southeast—they tend to be more common along active plate boundaries.

Landslides

Landslides can also occur in areas with low relief, as with the mass movement of sediments in the Mississippi River drainage basin. 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. Human activity can also lead to mass wasting events, especially where excavation, blasting, or construction occur on unstable or steep surfaces. For example, in March 2015 a massive landslide occurred along the steep edge of fill below the runway of Yeager Airport in Charleston, West Virginia.

The airport was built on seven million cubic meters (nine million cubic yards) of engineered fill, due to the lack of suitable flat land for an airport. The landslide occurred after a spell of warm, wet weather following a freeze; it destroyed a church and damaged several homes, caused flooding, and blocked Keystone Road.

In September 2005, heavy rainfall from two consecutive hurricanes dislodged a deadly debris flow from the top of Fishhawk Mountain in Franklin, North Carolina. In only 30 minutes, the flow of debris cleared a 150–300-meter-wide (500–1000-foot-wide) swath of trees, rock, and dirt as it traveled 4 kilometers (2.5 miles) down Peeks Creek at roughly 50 kilometers per hour (30 miles per hour), destroying a housing community and killing four people in a matter of seconds.

In the Blue Ridge, where the bedrock contains many discontinuities (folded bedding planes, faults, joints, and cleavage) resulting from several episodes of mountain building (the Taconic, Acadian, and Alleghanian orogenies), rock slides and rockfalls are common, especially along transportation routes running east to west through the mountains. US Route 64 in Tennessee and I-40 where it crosses the Tennessee-North Carolina border are often impacted by rockfalls, leading to frequent road closures. Often, stretches of highway remain closed for periods of several months.

In some low-lying areas of the Coastal Plain, especially southern Mississippi, saturated soils and heavy rains can combine to cause soil liquefaction, which can result in laterally moving mudslides. This can be triggered by storm runoff or by rapid earth movement during an earthquake. Earthquakes along the New Madrid Seismic Zone commonly contribute to landslide incidence along the Mississippi River Valley in Tennessee and Kentucky.

Slumps and creep are common problems in parts of the Southeast with a wetter climate and/or the presence of unstable slopes or unconsolidated sand, especially in the Coastal Plain and the foothills of the Appalachians. Many areas in the Southeast contain expansive soils generated from clay-rich shales. Certain clay minerals can absorb water and swell up to twice their original volume. The pressures exerted through expansion of the minerals in the soil can easily exceed 22 metric tons per square meter (5 tons per square foot)—a force capable of causing significant damage to highways and buildings. An estimated $9 billion of damage to infrastructure built on expansive clays occurs each year in the United States, making swelling soils one of the costliest hazards. In addition, when the clay dries and contracts, the particles settle slightly in the downhill direction. This process can cause soil creep, a slow movement of land that causes fences and telephone poles to lean downhill, while trees adjust by bending uphill.

Human development can exacerbate this process when homes are built along river bluffs, disturbing vegetation that would otherwise stabilize the slope and adding water to the land in the form of yard irrigation or septic systems.

Expansive soils can be found all over the U.S., and nearly every state in the Southeast has bedrock units or soil layers that are possible sources.

Clay minerals that expand and contract when hydrated and dehydrated due to their layered molecular structure are generically referred to as smectite; soils that tend to form deep cracks during drought are often indicative of the presence of smectite. The Coastal Plain region has the highest risk of damage caused by swelling soils (clay), but the residual soils of some Paleozoic carbonate rocks may contain smectite, and some Paleozoic pyritic shale formations in the Valley and Ridge can form expansive minerals.

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.

Karst and sinkholes

Many parts of the Southeast are underlain by karst and carbonates. Karst areas, which are often connected to the natural groundwater circulation system, can also be subject to flooding when sinkholes are plugged with refuse or covered over by parking lots, driveways, or buildings.

Central and southern Kentucky, including most of the Interior Basin, is one of the most famous karst areas in the world. About 55% of the state is underlain by limestones and dolostones that are capable of developing karst, and 25% of the state already has well-developed karst features. This is evident in the large number of commercialized caverns advertised throughout the state, including Mammoth Cave National Park.

In Tennessee and northern Alabama, especially the Nashville Basin and Highland Rim, Paleozoic limestones are abundant, with significant karst and related dissolution hazards. Southern Alabama and Mississippi contain karst formations related to the dissolution of subsurface salt, and the entire state of Florida is underlain by karstic limestone.

Sinkhole formation commonly damages roads, buildings, and utilities, and it is a major environmental problem in the Southeast, including Kentucky, Tennessee, Alabama, and especially Florida, which has numerous lakes that were formed by sinkholes. 

In 2014, a sinkhole in Bowling Green, Kentucky received worldwide media coverage when it opened beneath the Corvette Museum, swallowing eight collectable sports cars.