RELICT FEATURES AND ACTIVE INTRAPLATE STRUCTURES IN THE 2020 M5.1 SPARTA, NC EPICENTRAL REGION

The 2020 M_w5.1 Sparta, NC earthquake caused the first observed surface rupture in the eastern United States, resulting in some $30 million of damage. The remarkably shallow (<1 km) mainshock focal mechanism and geodetic monuments record dextral oblique slip on the ESE-striking, SSW-dipping Little River Fault, yet surface deformation comprises NE-vergent dip-slip on low-angle reverse faults, and the majority of aftershocks fall on near-vertical E-W and NNE–SSW fault networks even shallower than the mainshock. To investigate this apparent slip partitioning and the relationship of the mainshock to regional stress and structures, we present stress inversions and associated fault slip potential modeling, aftershock sequence statistics, and analysis of satellite gravity data. The inherited faults illuminated by aftershocks are near optimal orientations and likely decomposed hypocentral motion into nearly pure dip-slip and strike-slip elements in the near-surface. Aftershock decay is typical of CEUS events, also consistent with stress transfer onto regionally well oriented faults as the main triggering mechanism. Seismicity is confined to a high-gravity wedge-shaped structural block. The WNW and ESE ends of the densest activity may be influenced by lithologic variations within the Alligator Back and Ashe Metamorphic Suites, but the belt of seismicity cuts across the (fault?) contact between the two. Gravity torsion (third horizontal derivative) is a sensitive indicator of structural grain, and the Little River Fault lies within a ~75-km WNW-ESE alignment of torsion-trend truncations consistent with Cenozoic–Recent faulting that cross-cuts major ancestral structures. A topographic lineament along this same band was previously identified in LiDAR data; these observations suggest that the Little River Fault is part of a longer Cenozoic-modern structure. Notably, a similar alignment of torsion truncations and NE-facing topographic scarp bound the WSW end of the seismically active wedge, suggesting the presence of at least one other seismically capable feature in the region. Future work to gather and analyze terrestrial relative gravity data could reveal details of hypocentral structure, the aftershock fault network, and the seismogenic potential of the Little River Fault and others in the region.