ANALYSIS OF NETWORK AND NODAL SEISMIC DATA TO CHARACTERIZE THE ELGIN, SOUTH CAROLINA EARTHQUAKE SWARM

The South Carolina midlands experienced an extremely rare earthquake swarm beginning in December 2021 continuing throughout 2022. The ongoing swarm of small (magnitude <=3.6) shallow (depth <10 km) earthquakes in Elgin, South Carolina, has prompted excitement and concern in an area unaccustomed to seismicity. This swarm consists of over 80 events >1.2M that have been located, but many more smaller unlocated events exist. Currently, the earthquakes are distributed in a diffuse zone elongated N-S and coincident with the NE-SW trending Eastern Piedmont Fault System. It has been suggested that the earthquakes are occurring at a transpressional bend. Determining the cause of this swarm and the fault geometry will depend upon accurate event locations and focal mechanisms.

In response to the swarm, our colleagues at Georgia Institute of Technology with the assistance from the University of South Carolina deployed a nodal array and a broadband seismometer in October 2022 directly over the earthquake swarm to bolster the regional ANSS stations. These data will allow us to better constrain earthquake locations and focal mechanisms for smaller magnitude events. Currently only 4 moment tensor solutions are available for this swarm, all consistent with dextral oblique reverse slip motion.

Here we present our analysis of the properties of the swarm events using the existing network and new nodal data. We will show relocations, first motion analyses, and preliminary efforts to construct a local velocity model and analysis of deeper crustal and mantle structure beneath South Carolina. Moreover, we can leverage this unusually located collection of seismic sources to increase resolution of the local crustal and lithospheric structure. Our analyses will contribute to efforts to understand the broader tectonic setting of the Southeast US, and passive margin seismicity in general.