USGS NEIC: Earthquake Bulletin: SOUTH CAROLINA

PREVIOUS EARTHQUAKES

Charleston and its surroundings were devastated by a very large earthquake in 1886 (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, damaging earthquakes much smaller than that of 1886 have occurred every decade or two, most recently in 1995. Typically, smaller earthquakes are felt each year or two. East of the Rockies, an earthquake shakes the ground over an area up to ten times the area typically shaken by a similar magnitude earthquake on the west coast. For example, an eastern magnitude 4.0 earthquake typically can be felt at many locations as far as 100 km (60 mi) from where it occurred, and it might or might not cause damage near its source. An eastern magnitude 5.5 earthquake usually can be felt out to 500 km (300 mi) in most directions and can cause damage out to 40 km (25 milies).

GEOLOGY

Earthquakes occur deep within bedrock. 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. Today the Charleston area is far from the nearest plate boundary, which is in the Caribbean Sea.

FAULTS

At plate boundaries, the relation of earthquakes to faults is comparatively well understood. In contrast, in the Charleston area, as in most other parts of the U.S. east of the Rockies, the relation is enigmatic. Bedrock in the area is laced with faults that date mainly from the formation of the Appalachians and the birth of the Atlantic. However, in the Charleston area bedrock and its faults are buried beneath sand, silt, clay, and soft sedimentary rocks that may be as thick as 3 km (2 miles). Accordingly, few of the areas earthquakes can be linked to known faults. As in most other regions east of the Rockies, the best guide to earthquake hazards is the earthquakes themselves.