Stress Constraints on Fault Activity in the 2020 Mw 5.1 Sparta, North Carolina, Earthquake and Aftershock Sequence

Abstract During the 2020 Mw 5.1 Sparta earthquake, sinistral-reverse slip reactivated the pre-existing Little River fault (LRF), creating the first observed surface rupture in the eastern United States and an unprecedented opportunity to investigate intraplate fault reactivation under modern stress. This study examines the relationships between stress and faulting for the mainshock and 16 fore/aftershocks using stress inversions and geomechanical calculations. Sparta’s N60°E–N68°E maximum compression direction and strike slip to transpressive regime (stress ratio: Φ=0.17±0.05) are both consistent with the broader eastern North America stress field. The LRF is well aligned for slip (normalized instability I = 0.94 ± 0.02), as are 14 of the 16 fore/aftershocks (I ≳ 0.9). Constraints on active focal planes are then used to develop a kinematic model of the seismic cycle. This model suggests that interseismic strain accumulation was asymmetric about the seismogenic asperity, leading to asymmetric rupture with more coseismic offset east of the hypocenter than west. Following the mainshock, aftershocks were bimodal: The primary population on the higher-slip east side accommodated minor additional slip along the LRF, as is typical. Aftershocks on the low-slip west side, however, delineated the trailing edge of the coseismic slip patch, which parallels the dominant regional joint orientation but is poorly oriented for slip (I = 0.24 ± 0.08). The west-side aftershocks provide a cautionary example that aligned aftershocks need not delineate an active fault. Although epicenters coalesce parallel to regional jointing, the focal planes are highly oblique to that trend. These are interpreted as en echelon faults that accommodate deformation on well-oriented planes within a pre-fractured belt. This interpretation contrasts with the typical assumption that active fault orientations can be inferred from planes fit to microseismic hypocenters. The Sparta sequence exemplifies reactivation of favorably oriented ancestral faults and highlights the importance of understanding crustal stress in identifying inherited faults prone to reactivation in the modern stress field.