Lithologic Controls on the 2020 Mw 5.1 Sparta, North Carolina (USA), Earthquake Illuminated by a Gravity Investigation of the Little River Fault and Epicentral Zone

Abstract The 2020 Mw 5.1 Sparta, North Carolina (United States), mainshock remains the largest earthquake in the central and eastern United States since 2011. Shallow sinistral-reverse slip on the east-southeast-striking Little River fault (LRF) caused the first observed surface rupture in the eastern United States, providing a unique opportunity to study an active intraplate fault. Crystalline bedrock is at or near the surface in the epicentral region; in such situations, geophysical imaging can supplement geologic mapping to constrain subsurface geology and illuminate connections between fault-zone structure and seismogenesis. To image at scales ranging from ∼2 km to tens of meters, a microGal-precision relative gravity survey collected 722 readings at 631 unique locations. The LRF footwall comprises east-northeast-trending, alternating ±1 mGal anomalies 0.5–1 km wide. Gravity highs align with mapped amphibolite bands in the Ashe Metamorphic Suite, the most prominent of which intersects—and obliquely underthrusts—the LRF up-dip from the hypocenter, near where peak coseismic slip occurred in the subsurface. The surface rupture propagated through the flanking low-density material to the east and west but arrested against the next high-gravity zones. The gravity high is interpreted as a comparatively strong lithology that resists underthrusting, locking the LRF interseismically and nucleating the Mw 5.1 mainshock. This interpretation suggests that the Sparta earthquake may exemplify how footwall lithology influences strain partitioning across the seismic cycle. This effect may be especially important in low-strain-rate continental interiors, where old, cold rocks can be strong compared to those in plate-boundary fault zones—perhaps strong enough to shape rupture patterns and fault segmentation.