GRAVITY, STRESS ANALYSES, AND AFTERSHOCKS OF THE 2020 M5.1 SPARTA, NC EARTHQUAKE REVEAL REACTIVATED SECONDARY FAULT SYSTEMS AT MULTIPLE SCALES

The 2020 M_w5.1 Sparta, NC mainshock resulted from remarkably shallow (hypocenter < 1.5 km) oblique-reverse slip on the ESE-striking Little River Fault (LRF), causing the first observed surface rupture in the eastern U.S. Aftershocks exhibited typical spatiotemporal behavior for CEUS sequences, nearly all occurring within 10 km of the mainshock at rates that decayed approximately as time^-0.9. Fault slip potential modeling suggests that the LRF was well oriented relative to local stress; most aftershocks were as well, consistent with typical triggering by stress transfer. Regionally, satellite gravity data show that the mainshock and aftershock sequence are confined to a high-gravity wedge-shaped feature. Nevertheless, seismicity cut across primary NE-striking Paleozoic faults. In fact, the LRF is embedded in previously-identified topographic lineament and a ~75-km-long WNW–ESE-trend of gravity torsion (third horizontal derivative) truncations. Torsion aligns parallel to structural grain, making truncations a sensitive indicator of secondary faulting. The LRF appears to be part of a longer post-Paleozoic structure or deformation zone that cross-cuts primary ancestral faults. The seismically active wedge is bounded to the SW by a parallel trend of torsion truncations and topographic lineament, perhaps indicating a second seismically capable fault system in the region. A microGal-precision terrestrial gravity survey scheduled for September 2023 will provide high-resolution imaging of hypocentral structure, shallow features that accommodated surface displacements, and surrounding ancestral faults. These new data will constrain the seismogenic potential of the LRF and other faults in the region and may shed light on the processes or structures responsible for the anomalously shallow moment release.