A dense microearthquake catalog reveals the complex rupture of the extremely shallow M5.1 Sparta, North Carolina Earthquake

Abstract On August 9, 2020, an M w 5.1 earthquake ruptured the uppermost crust near the town of Sparta, North Carolina. The earthquake generated a ~2 km-long surface rupture, the first co-seismic faulting rupture identified in the Eastern United States. Combining deep learning earthquake phase picking, and matched filter detection, with differential-travel times relocation, we obtain a catalog of 1761 earthquakes with high-resolution locations, about 5.8 times the number of events listed in the standard NEIC catalog. Our results show the Sparta mainshock has a centroid depth of 1.3 km and nucleated near the intersection point of two fault strands, a blind strike-slip fault where the rupture possibly initiated, and a reverse fault associated with the identified surface rupture, possibly part of a flower structure-like diffuse fault zone. We also observe that earthquake stress drops during the Sparta sequence are on the lower end of estimates for similar magnitude Eastern North American earthquakes.