M 3.6 - 5 km N of Sangaree, South Carolina

Earthquakes in the Charleston, South Carolina Area

Charleston and its surroundings were devastated in 1886 by a very large earthquake (magnitude 7.3). Aftershocks, some of them large enough to be damaging by themselves, continued for years. Prehistoric earthquakes of similar size to the 1886 shock have occurred in coastal South Carolina at intervals of several centuries to several thousands of years. In recent decades, smaller earthquakes that cause little or no damage have been felt roughly once a year in coastal South Carolina and a small part of adjacent Georgia.

Earthquakes in the central and eastern U.S., although less frequent than in the western U.S., are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 km (60 mi) from where it occurred, and it infrequently causes damage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 500 km (300 mi) from where it occurred, and sometimes causes damage as far away as 40 km (25 mi).

Faults

Earthquakes everywhere occur on faults within bedrock, usually miles deep. Most bedrock beneath the Charleston area was assembled as continents collided to form a supercontinent about 500-300 million years ago, raising the Appalachian Mountains. Most of the rest of the bedrock formed when the supercontinent rifted apart about 200 million years ago to form what are now the southeastern U.S., the Atlantic Ocean, and Africa.

At well-studied plate boundaries like the San Andreas fault system in California, often scientists can determine the name of the specific fault that is responsible for an earthquake. In contrast, east of the Rocky Mountains this is rarely the case. The Charleston area is far from the nearest plate boundaries, which are in the center of the Atlantic Ocean and in the Caribbean Sea. Bedrock and its faults are buried beneath sand, silt, clay, and sedimentary rocks that may be as thick as 1-3 km (1-2 mi). Accordingly, few earthquakes in the Charleston area can be linked to named faults. It is difficult to determine if most known faults are still active and could slip and cause an earthquake. As in most other areas east of the Rockies, the best guide to earthquake hazards in the Charleston area is the earthquakes themselves.