SURFACE RUPTURE FROM THE 9 AUGUST 2020, MW 5.1 SPARTA, NORTH CAROLINA EARTHQUAKE AND POSSIBLE PALEOSEISMICITY: EVIDENCE FROM DETAILED BEDROCK AND SURFICIAL GEOLOGIC MAPPING

The 9 August 2020, Mw 5.1 Sparta, North Carolina earthquake produced the first documented coseismic surface rupture in the eastern U.S. The earthquake epicenter was located in polydeformed crystalline rocks of the Ashe and Alligator Back metamorphic suites in the eastern Blue Ridge. Our research involves detailed geologic mapping and surficial studies, facilitated by new post-earthquake QL0 lidar coverage. Geologic mapping and lidar analysis document a coseismic surface rupture, named the Little River fault, that is traced for over 3 km. Individual fault strands occur in an en echelon pattern within a 12 m-wide zone. The fault scarp is marked by a 5–50 cm high ridge with consistent southwest-side up and reverse (thrust) kinematics. Detailed bedrock geologic mapping identified several strands of brittle faults oriented similar to the coseismic rupture and fault (110°–120°/45–70°). The brittle faults are characterized by deformation zones 1 to 25 cm wide, often with several brittle faults localized within several meters in outcrops (<5 m). The Paleozoic foliation is fractured and rotated into the fault, and microbreccia and Mn-coated slickenside clay seams occur along some fault fractures. Transpressional kinematics are documented on several faults, but slip estimates are difficult to determine. The brittle faults are mapped for an additional length of 3 km to northwest of the Little River fault along Bledsoe Creek valley. Numerous other manganese-coated slickensided faults with variable orientations are mapped in saprolitic bedrock exposures adjacent to the Little River fault, and in the bottom of a major topographic lineament. Surficial mapping, carried out as traverses along rivers and larger streams both in kayak and on foot, document soft-sediment deformation in alluvium at five locations, which occur along sections of other NW-trending lineaments. In addition to the first coseismic rupture in the eastern U.S., these results collectively highlight the reactivation of an older brittle fault in the Blue Ridge and possible paleoseismicity. Further, the combined length of Little River fault, including its unruptured older segments, increase the total length of rupture and unruptured fault to ~6 km and suggest a much larger earthquake could be possible on this structure.