Anne Witt | Geology Reports

USGS2025

Insights into widespread landsliding in southern Appalachia from Hurricane Helene

Between 23 and 27 September 2024, antecedent rain followed by Hurricane Helene produced one of the most damaging weather events in southern Appalachia history. The back-to-back storm events resulted in a maximum cumulative rainfall of 848 mm and hurricane-force wind gusts over 170 km/h in western North Carolina, eastern Tennessee, and southwestern Virginia. The resulting regional flooding, landslides, and tree blowdown caused over 100 fatalities, damaged or destroyed critical infrastructure and thousands of structures, and severed connectivity across the region. Over the next several weeks, a multi-agency landslide response produced a rapid hazard assessment and mapped 2217 landslides, 55% of which damaged infrastructure or property. Orographic uplift enhanced rainfall, resulting in concentrated landsliding along the ~250 km swath of the Blue Ridge escarpment in western North Carolina. Landslides initiated predominantly on windward-facing (southeast-facing) slopes, and localized clustering of initiation points indicated a strong influence of hillslope-scale meteorological and geomorphic factors. Many shallow landslides mobilized into larger, highly mobile, and damaging debris flows that graded into floods. Here, we put our preliminary observations in the context of historical storm-driven landslide events and open new avenues for investigating the nature and extent of landslides and their effects in southern Appalachia and similar environments.

Georgia, North Carolina, South Carolina, Tennessee↗

USGS2022

The Mw 5.1 9 August 2020, Sparta earthquake, North Carolina: The first documented seismic surface rupture in the eastern United States

At 8:07 a.m. EDT on 9 Aug. 2020 a Mw 5.1 earthquake located ~3 km south of Sparta, North Carolina, USA, shook much of the eastern United States, producing the first documented surface rupture due to faulting east of the New Madrid seismic zone. The co-seismic surface rupture was identified along a 2-km-long traceable zone of predominantly reverse displacement, with folding and flexure generating a scarp averaging 8–10-cm-high with a maximum observed height of ~25 cm. Widespread deformation south of the main surface rupture includes cm-dm–long and mm-cm–wide fissures. Two trenches excavated across the surface rupture reveal that this earthquake propagated to the surface along a preexisting structure in the shallow bedrock, which had not been previously identified as an active fault. Surface ruptures by faulting are rarely reported for M <6 earthquakes, and hence the Sparta earthquake provides an opportunity to improve seismic hazard knowledge associated with these moderate events. Furthermore, this earthquake occurred in a very low strain rate intraplate setting, where earthquake surface deformation, regardless of magnitude, is sparse in time and rare to observe and characterize.

North Carolina↗