GEOLOGIC AND PALEOLIQUEFACTION CONSTRAINTS ON PAST SEISMICITY IN THE EPICENTRAL AREA OF THE 9 AUGUST 2020 M5.1 SPARTA, NORTH CAROLINA, EARTHQUAKE

The M5.1 Sparta, NC, earthquake produced the first documented co-seismic surface rupture in the eastern US. Following the quake, the USGS and collaborators began a multiyear geologic mapping and paleoliquefaction survey effort in northwestern North Carolina and southwestern Virginia. Initially, approximately 4 km of surface rupture was mapped during ground surveys, which utilized lidar-derived hillshade, percent slope and raster subtraction models. The rupture is a reverse fault (Little River fault – LRF) oriented 110°-115°/45° with top-to-NE displacement. Locally, the LRF rotates Paleozoic foliation in saprolitized bedrock and is marked by cm-thick zones of breccia, clayey fault gouge, and mm-thick manganese-coated slickenlined fractures. The LRF reactivated a portion of an 8-km-long x 20-m-wide similarly-oriented brittle deformation zone in the eastern Blue Ridge bedrock (Ashe and Alligator Back Metamorphic Suites).

Subsequent geologic mapping extends the unruptured length of the LRF for an additional 4 km to the NW and SE. One strand of the unruptured LRF underlies an undeformed terrace deposit that dates the last movement on the strand to ⪆500 Ka. A fluvial terrace near Jefferson, NC consists of blocks of saprolitized bedrock that could not have been transported as such; a burial age at that locality indicates bedrock saprolitization younger than ~ 1 Ma and may provide a maximum age limit on brecciation and slickenlined fractures.

Paleoliquefaction surveys were conducted along 93 km of the New River, Little River, and Brush Creek. We found a likely paleoseismogenic dike, sills, and numerous soft sediment deformation features at 4 sites, including one locality on Bledsoe Creek in the immediate epicentral area of the Sparta earthquake, and 3 localities on the New River. The New River sites occur northwest of the 2020 Sparta earthquake but are on strike with the LRF. No liquefaction features were found on the Little River or Brush Creek, off-strike from the sites that did preserve paleoliquefaction features. OSL ages are pending, however an age of 150 ± 30 BP (using ¹⁴C) of an undisturbed unit, suggests no liquefaction-producing events in this region in historic time.