Earthquakes in South Carolina
The Palmetto state has a seismic past and will, no doubt, tremble again as quakes shake the ground beneath our feet. Earthquakes are not an uncommon occurrence in South Carolina. (See the Report on the most recent Earthquake in the Midlands of SC.) The South Carolina Geological Survey (SCGS) is interested in making South Carolinians aware of our seismic past, what causes our earthquakes, what is being done to reduce the losses that will result from future quakes, and what you should do before, during, and after an earthquake.
Charleston earthquake of 1886
Most people had gone to bed by 9:51 p.m. on Tuesday, August 31, 1886. They had no way of knowing that shock waves from a massive quake (see Figure 1), estimated magnitude of 7.6, would throw them out of bed. Imagine how bewildered and frightened they must have been as they darted for safety when ceilings gave way and tons of debris rained down about them. Survivors saw that many of Charleston's brick and masonry buildings had crumbled to the ground. Those structures that remained standing bore great cracks and other scars (see Figure 2). Sadly, about 60 people did not survive the quake. It was reported that ground shaking damaged structures as far away as 200 miles from Charleston. This quake was the strongest earthquake known to hit the Eastern Seaboard, and it shook with such force that it was felt over 21/2 million square miles (from Cuba to New York, and Bermuda to the Mississippi River).
Union County earthquake of 1913
Twenty-seven years after the 1886 Charleston earthquake and subsequent after shocks, another strong earthquake occurred in South Carolina. This quake was on the afternoon of January 1, 1913, at 1:28 p.m. near the town of Union in Union County with an estimated magnitude of 5.5 (Figure 1). Shock waves moved out from the western portion of South Carolina into adjacent Georgia and North Carolina, and even up into parts of Virginia. Fortunately, damage was minimal and no deaths resulted. This event is significant because it demonstrates that large, destructive earthquakes can strike the Piedmont region.
Prehistoric South Carolina earthquakes
In South Carolina, geologists have recently discovered evidence of at least five large paleoearthquakes during the past 5,000 years (Amick and Gelinas, 1991). As shown in Figures 3 and 4, paleoliquefaction features, considered earthquake induced, have been found in South Carolina's coastal area. During a strong earthquake, subsurface saturated sand becomes liquefied and this fluid mass can be ejected to the surface. The resulting liquefaction features, sand blows, vents/fissures, landslide, and differential compaction, are preserved in the soil as evidence of the earthquake's occurrence and strength. Given the necessary conditions, a magnitude 5.5 quake can generate liquefaction features that could cause damage to existing facilities and property.
What causes an earthquake?
An earthquake is the violent shaking of the Earth caused by a sudden movement of rock beneath its surface. Rocks respond to stress (squeezed or pulled apart) near the Earth's surface by breaking, and when rocks move along either side of a fracture, it is called a fault. The land around a fault may shift horizontally, vertically, or a combination of these motions (see Figure 5). The force that causes the stress within the rock is a result of movement of giant sections of the Earth's crust (see Figures 6 and 7).
Figure 6. Cross section of the Earth's crust showing oceanic crust sliding under continental crust, and mountains being built as a result of collision of two continents.
For hundreds of millions of years, the forces of continental drift have reshaped the Earth. Continental drift is based on the idea that the continents bumped into, and slid over and under each other and at some later time broke apart. Today, most people accept the theory that the Earth's crust is on the move, and we call this theory plate tectonics. The crust (lithosphere) is broken into about 12 enormous plates that float on hotter, softer rocks in the underlying mantle (asthenosphere). The Earth's heat drives convection currents in the asthenosphere, moving the plates past one another very slowly. Plates move mere inches annually, carrying the continents and ocean basins with them as they drift about.
The majority of earthquakes worldwide occur at plate boundaries when plates stick and then jump past each other. These quakes often are the ones that are the most destructive and well understood in terms of plate tectonics. The cause of earthquakes in South Carolina is not so clear. South Carolina's quakes are located within a plate rather than at a plate boundary. Perhaps the intraplate quakes felt in South Carolina are the result of stresses transmitted inward from the boundaries of the North American plate. In our state, quakes may occur along ancient plate boundaries where existing faults are reactivated as the tectonic stress is released.
In response to this threat, SCGS has been mapping faults and related geologic structures throughout the state. SCGS has published a map, similar to the very generalized one shown in Figure 8, showing where faults and other geologic structures in South Carolina are located. These and other maps are being produced to increase the public awareness of quake-prone areas.
In South Carolina, approximately 70 percent of the earthquakes occur in the Coastal Plain and most are clustered around three areas west and north of Charleston: Ravenel-Adams Run-Hollywood, Middleton Place-Summerville, and Bowman. These faults and other geologic structures related to the earthquakes are hidden by the thick sequence of sediments. Therefore, few clues to the causes of earthquakes in the Coastal Plain can be found at the surface. To unmask these hidden geologic structures, geologists are using geophysical techniques, recorded seismic activity (see Figure 9), or both. In the Piedmont, studies of surface geology are beginning to offer important clues to the causes of quakes in South Carolina.
Is there a major earthquake in South Carolina's future?
When will the next strong quake occur? The ability to accurately predict when and where earthquakes will occur is not yet available. South Carolinians need to realize that South Carolina faces the possibility of the occurrence of a strong quake having its epicenter within our borders. We also need to realize that a major earthquake anywhere in the Eastern United States could adversely affect us, causing damage.
SCGS is now conducting studies to evaluate the geologic response to earthquake-induced motion in an attempt to help reduce the risk to lives and property. An example of these studies is the map showing where the Coastal Plain is most susceptible to severe ground shaking (Figure 10). Once data about an area have been gathered, maps that show levels of risk for an area can be produced. Seismic-risk studies will become more important as a statewide earthquake hazard mitigation strategy is developed.
What you should do before, during, and after an earthquake
Learn first aid procedures.
Learn how to shut off all utilities in your home, and have the necessary tools on hand.
Keep emergency items on hand, including first-aid kit, supply of important medicines, flashlight, battery-powered radio, batteries, three-day supply of water and nonperishable food, and a fire extinguisher.
Use bolts or straps to secure heavy items that might topple over, such as a bookcase, china cabinet, or water heater.
Avoid placing heavy objects on shelves or heavy pictures on walls where they could fall onto a bed.
If at home, avoid windows and objects that could fall. If possible, get under a sturdy table, desk, or bed, or stand in a doorway.
If in a public building, avoid stairways and elevators and don't run for exits. Get under a desk or table and avoid outside walls, especially glass walls.
If outside, move to an area where you will be safe from falling trees, utility poles, or debris from buildings.
If driving in a car, avoid bridges, underpasses, and overpasses. Safely bring the car to a stop and turn on the radio for emergency broadcasts.
Check for injuries to others and provide assistance as needed.
Check for and extinguish fires, and check for and correct conditions such as spilled gasoline that could lead to a fire.
Check appliances and gas, electric, and water lines for damage, and shut off utilities that are damaged.
If outside, watch out for downed power lines.
Use the telephone only for emergencies
Be prepared for aftershocks.
For more information, contact your local chapter of the American Red Cross
(Information courtesy South Carolina Dept. of Natural Resources)