Structural Geology seminars

Abyssal marine turbidites provide some of the longest and most spatially extensive records of subduction zone earthquake recurrence globally; however, correlation of these deposits over long distances and interpretation of synchronous emplacement requires both an understanding of the turbidite generating systems and precise dating. Here, we present an integrated suite of high-resolution bathymetry, subbottom profiles, and sediment cores from combined autonomous underwater vehicle, remotely operated vehicle, and ship-based studies at a key paleoseismic site in the southern Cascadia subduction zone. We demonstrate how widespread, earthquake-triggered landslides on the lower slope deposit discrete, proximal mass transport deposits (MTDs) that grade offshore into complex, interfingered abyssal turbidites, which correspond to records of megathrust earthquake history. We propose accretion and oversteepening of thrust folds on the lower slope both preconditions the slope to fail and provides a perpetual source of unstable material to fail during every earthquake cycle. Furthermore, we suggest the periodic and pervasive landsliding indicates coseismic deformation of the outer accretionary wedge during megathrust rupture.

This presentation introduces the first range-scale Deep-Seated Gravitational Slope Deformation (DSGSD) inventory in North America, focusing on the Sawatch Range of central Colorado. Through geomorphic mapping, field campaigns, and InSAR displacement analysis, we identified and characterized 39 previously unreported DSGSD. We analyzed this inventory to reveal the governing mechanisms that condition slopes for DSGSD development and to compare it with the global DSGSD inventory. Site-specific surveys of the South Mt. Massive and Mt. Jones DSGSDs result in interpretive cross sections that relate surface morphology to subsurface geological parameters and highlight how Sawatch DSGSDs form and evolve in unique ways. Furthermore, we discuss the implications of this work to the broader context of Rocky Mountain DSGSD. Widespread Deep-Seated Gravitational Slope Deformations Across the Sawatch Range, CO, Evans (2026), USGS Landslide Hazard Seminar, 09 September 2026.

Hurricane Helene devastated western North Carolina and parts of surrounding Tennessee and Virginia. A stalled front over the Blue Ridge prior to landfall of the hurricane resulted in cumulative rainfall totals of 20–30 inches from Sept. 25–27, 2024. This triggered numerous landslides and devastating flooding, and has changed the landscape of the Blue Ridge and mountain communities. This mapping and assessment of the landslides was conducted as part of the USGS Landslide Assessments, Situational Awareness and Event Response Research (LASER) and in cooperation with the North Carolina Geological Survey Landslide Hazard Program. Mapping efforts focused on the northernmost area heavily impacted from the storm: parts of Watauga, Avery, and Ashe County, NC, and parts of Johnson County, TN. Initial observational descriptions of the landslides are presented. Landslides mapped are separated into three type occurrences, hyperconcentrated earth flows, debris flows, and modified slope failures. Earth flows occurred on steep grassed slopes and consisted of soil with a mixture of vegetated clumps, saprolite, and rock. The landslide tracks were 10–338 m long, and scarp widths of 10–30 m that often cut down 1–2 m to bedrock. Earth flows often occurred in clusters, and some merged down slope. Debris flows had longer tracks, 96–1085 m, and consisted of a mixture of soil, rock, trees, vegetation, and other debris. Head scarps were 10–114 m wide and cut down through 1–3 m of colluvium to bedrock. Bedrock structures controlling the failures included joints, fractures, and foliation. Seven debris flows on Howards Creek initiated on steep slopes with aspect and slope close to the orientation of the foliation in bedrock (330/20–40° NE). Modified slope failures included landslides that initiated on road cuts, road embankments, steep cut slopes, and other slopes modified by construction (e.g., failed drainages or septic fields). Although the slope modification was important to initiate these slides, some localized along preexisting bedrock (saprolite) structures. Collectively, the impact of landslides from Hurricane Helene highlights the importance of basic geologic mapping and landslide assessment to build robust datasets to better understand and model the occurrence of landslides and strengthen our planning and rebuilding of mountain communities.