UNKNOWN SEISMIC SOURCES IN EASTERN USA - THE 2020 MW 5.1 SPARTA, NORTH CAROLINA EARTHQUAKE AND EVIDENCE OF RECURRENT QUATERNARY DEFORMATION

A Mw 5.1 earthquake shook North Carolina and surrounding states on August 9, 2020. At its epicenter, near the town of Sparta, the Modified Mercalli intensity was VI causing damage and economical loss of >US$2 million. A previously unknown fault ruptured the ground surface making it the first historically recognized surface rupture in the eastern USA. Field surveys, initiated immediately identified a rupture trending ~N 110°, with scarps with reverse displacement and folding/flexure of the hanging wall and vertical displacements of 8–20 cm. These are consistent with an oblique-reverse focal mechanism in a WNW-ESE southwest dipping structure. The rupture was initially mapped for ~2km, with limited observations due to the feature's subtlety and vegetation cover (woods). High-resolution Lidar (~30 pulse/m²) acquired by NCALM a few months after the earthquake revealed a surface rupture length of ~4 km. Initial observations in two small excavations across the rupture suggested a re-activation of pre-existent discontinuities, whose tectonic activity was poorly understood. To investigate the sub-surface geometry of the structure that ruptured the surface, ground-penetration radar and electrical tomography resistivity profiles were acquired normal to the rupture trace, confirming the existence of a m-wide south-dipping fault zone. To further investigate the unknown fault and evidence of previous deformation, a ~25-m-long and ~2-m-deep paleoseismologic trench was excavated across the fault zone. The trench exposed a complex fault zone ~15 m wide, with multiple WNW-ESE to NW-SE strands (some with oblique striations), where a low grade metamorphosed greywacke is thrusting amphibolite, both from the Ashe Metamorphic Suite. The exposure also revealed that the 2020 surface rupture was accommodated mainly by 3 reverse fault strands. A sequence of middle to late Pleistocene soil and colluvium overlays the weathered bedrock. This sequence, in the hanging wall is locally disturbed in close association with fault strands, sealed by younger soil horizons, which is interpreted as likely related to previous deformation seismically induced. All these observations indicate that the structure that ruptured during the Mw 5.1 Sparta earthquake is an active Quaternary WNW-ESE to NW-SE striking and predominantly reverse fault.