INVESTIGATIONS OF SURFACE RUPTURE CAUSED BY THE 2020 MW 5.1 SPARTA, NORTH CAROLINA, USA EARTHQUAKE AND ASSOCIATED PALEOSEISMOLOGY OF THE LITTLE RIVER FAULT

In August 2020, a shallow Mw 5.1 earthquake occurred near Sparta, North Carolina, on the Little River fault. This previously unknown fault strikes obliquely to regional Paleozoic structures. The earthquake generated the first documented intraplate surface rupture in the eastern United States. The surface rupture is expressed by topographic steps trending ~N110°, ranging from a 2/3- to ~30-cm-high reverse scarps and folds of the southern hanging wall, initially recognized for an along-fault length of ~2 km. These topographic features are consistent with an oblique-reverse focal mechanism on a SW-dipping WNW-ESE trending structure. Geodetic monument surveys corroborate an off-fault left-lateral displacement not apparent along the rupture trace. A 2020 November lidar survey confirmed an additional ~2.5 km surface rupture length, for a total of ~4.5 km. Initial excavations across the rupture exposed the seismogenic fault as a cm-thick cataclastic zone interpreted to result from the reactivation of pre-existent discontinuities in poly-deformed bedrock. These excavations lacked Quaternary markers, thus limiting interpretations of paleoseismic activity. Ground-penetrating radar and electrical tomography resistivity surveys were conducted at favorable locations across the zone of surface rupture, highlighting a ~10-15 m wide fault zone and south-dipping reflectors in the GPR. Paleoseismologic trenches at the same locations exposed a sequence of Quaternary soil and colluvium overlaying the weathered bedrock, altogether deformed by WNW to NW strands in a complex 15-m wide fault zone. The 2020 rupture was accommodated by three reverse strands, with a total along-fault dip-slip displacement of ~50 cm, associated with a ~30 cm scarp. A Bt soil horizon of probable Late Pleistocene age (luminescence age pending) has a cumulative displacement of ~1m, indicating that the Little River fault has been active during the Quaternary and generated surface ruptures before the 2020 earthquake. The Little River fault is an active fault in the Eastern U.S., with subtle geomorphic expression and absence of instrumental seismicity; nonetheless, it has the capability to produce surface ruptures, as demonstrated by the Mw 5.1 Sparta event and interpretation of paleoseismology evidence.