Collaborative Research: High-resolution imaging of the Elgin-Lugoff earthquake swarm sequence and subsurface structures in South Carolina using a dense seismic nodal array

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A prolonged earthquake swarm sequence started on December 27, 2021, beginning with a magnitude 3.3 earthquake between Elgin and Lugoff in South Carolina, a region with very low prior seismic activity. More than 90 small earthquakes have been reported in this region since then, with the largest magnitude of 3.6 occurring on June 29, 2022. These earthquakes have continued until at least summer of 2023, making this highly unusual for the region. This swarm has had no clear main shock and has no obvious tectonic or anthropogenic causes. So far, standard earthquake locations do not reveal a clear relationship with the pre-existing Eastern Piedmont Fault System. This project will analyze seismic data from a set of instruments deployed by the researchers in October 2022 to better image the fault structure hosting the swarm and the underlying driving force. This project will naturally provide opportunities for public engagement with the impacted community in Elgin-Lugoff, and the broader midlands of South Carolina, where many residents have felt earthquakes for the first time in their lives. This project also provides inclusion and STEM workforce development opportunities, by funding research by undergraduate students from the local South Carolina community to perform research in their own state. Students on this grant will participate in outreach to build STEM literacy through knowledge transfer and further broaden participation in geoscience. This project will also provide funding to support summer internships at both institutions.In October 2022, 85 seismic nodes were deployed for 4 months by the research team over the Elgin-Lugoff swarm to record further earthquakes. These instruments were complemented by one broadband station and one nearby strong motion station operated by the South Carolina Seismic Network. The rich dataset can be used to identify additional smaller earthquakes, which will better image fault structures hosting this swarm sequence and elucidate the driving forces. The work will attempt to reveal small earthquakes during the nodal deployment period, below the detection threshold before this deployment, by using both machine learning and template matching techniques. The team will then relocate these newly detected events with waveform-based techniques, followed by focal mechanism determinations. This information will be used to construct a new model of the seismic velocities beneath Elgin. Using both the more complete catalog and the improved velocity model, the earthquakes earlier in this swarm can be relocated relative to the later ones thus improving the accuracy of the whole catalogue. Results from this project will answer the following questions: 1) does the swarm occur in a diffuse or well-defined region, 2) how the swarm evolved in space and time, and 3) what is the likely process that drives this swarm. The information gained from the analysis will inform on the ongoing tectonic processes in South Carolina, and on East coast seismicity and tectonics generally.This project is jointly funded by the Geophysics program and the Established Program to Stimulate Competitive Research (EPSCoR).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.