South Carolina experiences earthquakes despite being situated far from the nearest tectonic plate boundaries. The state occupies an unusual geological position within the interior of the North American Plate, making its seismic activity distinct from the highly active Pacific coastlines. While most tremors are minor and often undetected, the state has a history confirming the potential for powerful seismic events.
The Definitive History of SC Earthquakes
The reality of South Carolina’s seismic risk was cemented by the catastrophic event that struck near Charleston on August 31, 1886. The main shock delivered the most powerful and destructive earthquake recorded in the history of the southeastern United States. Seismologists estimate the magnitude of this tremor to have been between 6.9 and 7.3, reaching a maximum Modified Mercalli Intensity of X (Extreme) in the epicentral area northwest of Charleston.
The impact on Charleston was immediate and devastating, as nearly all the city’s buildings sustained some level of damage. More than 60 fatalities were attributed to the disaster, and property damage exceeded $5 million in 1886 dollars. The shaking was felt across an immense area, extending as far north as Boston, west to the Mississippi River, and south to Cuba. Evidence of liquefaction was widespread, with vast areas of land covered in sand and water ejected from the ground.
The Cause: Intraplate Tectonics
The mechanism for seismic activity in South Carolina is classified as intraplate tectonics, meaning earthquakes occur within the stable interior of a tectonic plate rather than at its boundaries. This region is not characterized by the grinding of two plates against each other, as seen on the West Coast, but by the reactivation of ancient geological structures deep underground.
The North American Plate is constantly subjected to forces exerted at its edges, such as the spreading of the Mid-Atlantic Ridge and compressional forces from the western United States. These distant plate movements generate immense regional stress that is transmitted inward across the continent.
This stress accumulation seeks out zones of weakness in the crust, which in South Carolina are ancient fault systems formed hundreds of millions of years ago during past continental collisions and rifting events. The rocks of the Coastal Plain province, in particular, contain a complex network of these buried faults. When the accumulated regional stress exceeds the strength of these old, pre-existing fault planes, the rock suddenly slips, releasing energy in the form of an earthquake. Current scientific modeling suggests the maximum horizontal compressive stress in the crust is oriented in a northeast-southwest direction, which drives the movement on these faults.
Current Seismic Zones and Frequency
South Carolina experiences constant, low-level seismic activity, with geologists typically recording between 10 and 20 earthquakes annually across the state. The vast majority of these events are minor, usually registering magnitudes of 3.0 or less, and only a small number of these are felt by residents.
Activity is concentrated in specific areas, primarily within the Coastal Plain. The most prominent of these is the Middleton Place-Summerville Seismic Zone (MPSSZ), situated approximately 20 kilometers northwest of Charleston. Approximately 70% of all instrumentally recorded earthquakes in the South Carolina Coastal Plain occur within the MPSSZ. The U.S. Geological Survey and the South Carolina Geological Survey actively monitor these zones, using seismographs to track the frequency and magnitude of tremors.
Preparing for Seismic Events
Given the confirmed seismic risk, preparedness is a practical step for residents, even though large-magnitude events are infrequent. The consensus safety action during an earthquake is to immediately perform the “Drop, Cover, and Hold On” technique. This involves dropping to the hands and knees, covering the head and neck, and holding onto a sturdy shelter until the shaking stops.
Securing heavy furniture and objects, such as bookshelves, water heaters, and large wall hangings, is a proactive measure that can prevent injuries and property damage during shaking.
Because standard homeowners insurance policies typically do not cover damage resulting from earthquakes, the South Carolina Department of Insurance encourages property owners to review their coverage and consider purchasing separate earthquake insurance. Emergency management officials also recommend assembling a basic emergency kit that includes food, water, and first-aid supplies.