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An unusual earthquake swarm began in December 2021 between the towns of Elgin and Lugoff in South Carolina, United States. This area is characterized by historically low seismicity, but by April 2024 it has experienced 97 small earthquakes listed in the U.S. Geological Survey (USGS) catalog, presenting a unique opportunity to investigate the dynamics of earthquake swarms in stable continental regions. These events are located in a north–south diffuse trend, cross cutting the East Piedmont fault system, a Late Paleozoic dextral strike-slip fault; however, the location uncertainties were too large to reveal any obvious structure. Starting from October 2022, we deployed 86 Smartsolo 5-Hz three-component seismic nodes for four months in the direct vicinity of the Elgin swarm. Using a combination of deep learning and match filter techniques for event detection, and double-difference relocation method for precise earthquake locations, we obtain up to 100 high-resolution microearthquake locations, as compared with four events listed in the USGS catalog for the deployment period. In our improved catalog, we report significantly smaller magnitudes in comparison to those listed in the USGS catalog, with a local magnitude ranging from −2.17 to 2.54 and achieving a magnitude of completeness at −0.20. The relocated catalog outlined a single-fault plane of nearly north–south strike and west dipping, inconsistent with either known fault strikes or the magnetic anomalies in this region. We also determine focal mechanism solutions for selected events in this swarm sequence, which shows mainly strike-slip faulting with nodal planes aligning with the north–south-striking seismic cluster. Our relocated catalog can be used to constrain the location of other swarm events outside the nodal recording period and provide a robust benchmark data set for further analysis of the swarm sequence. 

Abstract On August 9, 2020, anM_w5.1 earthquake ruptured the uppermost crust near the town of Sparta, North Carolina, creating the first co-seismic faulting surface rupture documented in the Eastern United States. Combining deep learning and matched filter earthquake detection, with differential-travel times relocation, we obtain a catalog of 1761 earthquakes, about 5.8 times the number of events listed in the standard USGS/NEIC catalog. The relocated seismicity revealed a complex fault structure with distinct planar alignments, supported by a moment tensor inversion with significant non-double-couple component. The Sparta mainshock with a centroid depth of 1.3 km is interpreted to have nucleated near the intersection of two main fault strands. The mainshock likely ruptured a blind strike-slip fault and a reverse fault associated with the identified surface rupture, both possibly part of a flower structure-like diffuse fault zone. Our observations highlight a complex behavior of extremely shallow earthquakes in stable continental regions.