Fault Intersections Control the Extremely Shallow 2020 Mw 5.1 Sparta, North Carolina, Earthquake Sequence

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Abstract
Some earthquakes in stable continental regions are very shallow, and their seismogenesis mechanism remains unresolved. Here, we focus on the extremely shallow 2020 Mw 5.1 Sparta earthquake sequence in North Carolina. Joint analysis of short-period surface waves from shallow earthquakes and ambient noise-derived empirical Green’s functions reveals a low-velocity zone with strong velocity gradient in the top 3 km of the Sparta-1D velocity model. Furthermore, we validate focal depths in the aftershock catalog with the Rayleigh-wave amplitude spectra method, confirming ∼90% of aftershocks occurred within the shallow low-velocity zone, with event rates decreasing at depths at which shear velocity increases. Geological field studies imply that this feature originates from a shallow weak layer containing intersecting fracture network. The velocity structure, aftershock distribution, and mapped near-surface structures in the Sparta region suggest that fault intersections within a shallow weak layer may provide mechanically favorable conditions for extremely shallow earthquakes.

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