Complex rupture dynamics of the extremely shallow August 2020 M5.1 Sparta, North Carolina earthquake

On August 9, 2020, an M_w 5.1 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.