ELGIN, SOUTH CAROLINA EARTHQUAKE SWARM, DECEMBER 2022 TO PRESENT

Beginning in December 2021 and continuing to the present day, a swarm of more than 81 earthquakes have been recorded northeast of Columbia, SC, between the towns of Elgin and Lugoff. This earthquake sequence initiated with a M3.3 and was followed by 4 more widely-felt earthquakes with magnitudes greater than 3. Each >M3 earthquake was followed by a mini-swarm lasting a few days. Intensity ranges from moderate (V) to weak (III), with higher intensities reported near the epicenters of M3+ earthquakes. No structural damage has been reported. High-resolution airborne geophysical surveys and relations from geologic maps provide insight toward the potential origin of the Elgin earthquake swarm.

The Elgin earthquakes are occurring along the Eastern Piedmont Fault System (EPFS), a Late Paleozoic dextral strike-slip fault network that strikes NE-SW from GA to VA. The EPFS has a history of multiple fault styles, inherited structures, and fault reactivation. The EPFS formed initially as a ductile shear zone at depth and was subsequently exhumed during uplift and erosion of the Appalachian Mountains.

The Elgin earthquakes define a NNE trending seismogenic zone that is located between two major strands of the EPFS. Current location information indicates several fault planes involved in the swarm; however, multiple fault planes might be the result of imprecise hypocenters. Focal mechanism studies of the larger (>M3) Elgin earthquakes indicate one solution on oblique dextral reverse motion along faults oriented north-northeast, which suggests a short fault(s) in between the extensive northeast-striking EPFS faults. Geologic mapping adjacent to the swarm and across the eastern Piedmont has recognized this structural relation. The focal mechanisms of the Elgin earthquakes also fit the general pattern of mainly reverse faulting earthquakes in SC and nearby states; notably, reverse faulting was also seen in the 2014 Edgefield earthquake, a 2021 M 3.3 earthquake under Summerville, SC, and a June 2022 M3.9 earthquake in southeastern GA.

In the southeastern United States, the principal stress direction is horizontal and oriented approximately ENE–WSW. The structures in the EPFS exist in this stress regime. Structures at an angle to this stress orientation, such as the Elgin faults, are favorable for reactivation.