Earthquakes in South Carolina
Earthquakes are probably the most frightening naturally occurring hazard encountered. Why? Earthquakes typically occur with little or no warning.There is no escape from an earthquake! While South Carolina is usually not known for earthquakes, ten to twenty earthquakes are recorded annually and two to five earthquakes are felt each year. These earthquakes tend to be less than magnitude 3.0 on the magnitude scale and cause little damage.
An earthquake is the violent shaking of the earth caused by a sudden movement of rock beneath its surface.
Although earthquakes can occur anywhere on earth, the majority of earthquakes worldwide occur at plate boundaries. These earthquakes are known as interplate earthquakes. In contrast, South Carolina is located within the interior of the North American plate, far from any plate boundary. Earthquakes occuring within a plate are intraplate earthquakes.
Little is known as to why intraplate earthquakes occur. The most widely accepted model is that several geologically old fault systems of varying orientation within the subsurface are being reactivated while being subjected to stress.This stress buildup may be due to the Plate Tectonic Theory. For hundreds of millions of years, the forces of continental drift have reshaped the Earth. Continental drift is based on the concept 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.
South Carolina's Fault System
Most of South Carolina’s earthquakes occur in the Coastal Plain where the underlying rocks are very faulted or broken from the break-up of the plates. These cracks in the deep rocks mean that this area of the plate is weak. If pressure is exerted on the edge of the plate, some of these faults/breaks will allow the rocks to move.
Faults in South Carolina have been mapped and estimated. Fault rupture is not the only cause of earthquakes. Small earthquakes may also occur near dams from water pressure and near the Appalachian Mountains.
Threat Level For South Carolina
Currently, there is no reliable method for predicting the time, place, and size of an earthquake. Several areas of South Carolina regularly experience earthquakes and have experienced strong earthquakes in the past. Approximately 70% of all earthquakes in the state occur in the Coastal Plain with most clustered around three areas of the State:
Ravenel-Adams Run-Hollywood, Middleton-Place-Summerville, and Bowman. There is a consensus among seismologists that where earthquakes have occurred before, they can again.
Major Earthquakes In South Carolina's History
The two most significant historical earthquakes in South Carolina were the 1886 Charleston earthquake and the 1913 Union County earthquake. The August 31, 1886 earthquake which struck in the Summerville/Charleston area is the largest event to have occurred in the southeastern U.S. and the most destructive, killing 60 people.
On January 1, 1913, Union County experienced an earthquake that by today’s standards would probably be measured as a M 4.1 on the Magnitude scale. Not much is known about the cause of the Union County earthquake because of the lack of technology at the time.
Few, if any, (masonry) structures remain standing; bridges destroyed; rails bent greatly. Damage total; lines of sight and level are distorted; objects thrown into the air.
Major to Great
IV: Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sounds. Vibrations felt are similar to a heavy truck striking a building.
V: Felt by nearly everyone; many awakened; some dishes, windows broken and unstable objects overturned; pendulum clocks may stop.
Swarms and Background Seismicity
The majority of the low-magnitude earthquakes in South Carolina can be described as normal, background seismicity for our region. However, clusters, or swarms, of earthquakes can occur in one particular area, alarming nearby residents who report feeling these successive, low-magnitude tremors. In 2022, Kershaw County experienced more than 80 microearthquakes centered near Elgin, South Carolina. Part of a lengthy swarm that began with a 3.9m quake on December 27, 2021, the Elgin earthquakes have not been intense enough to cause damage, and the state’s seismologists continue to research these occurrences.
In late 2021, USGS confirmed seven low-magnitude earthquakes in the vicinity of the Monticello Reservoir in Fairfield County. Seismologists believe these were normal, background activity and not indicators of larger earthquakes to come. A much larger swarm of microearthquakes occurred as the reservoir was first filled starting in December 1977. Thousands of earthquakes, none larger than magnitude 2.9, occurred in the years that followed. Earthquake activity declined in the late 1980’s through the mid-1990’s, but then picked up again in late 1996. Between December 1996 and mid-1999 several more earthquake swarms occurred, with nearly 1,000 earthquakes occurring at that time, with the largest being a magnitude 2.5. (Source: Dr. Steven Juame, College of Charleston)