161-12 EVOLUTION OF MONITORING APPROACHES AND CONSEQUENT UNDERSTANDING OF LANDSLIDING MECHANISMS AT THE SLUMGULLION LANDSLIDE, COLORADO, USA

Session: Dynamics of Natural and Built Environments

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Abstract:

Landslides present significant hazards and are incompletely understood. Monitoring landslides and related conditions has enabled improved understanding of landsliding dynamics and mechanisms. The Slumgullion landslide, a continuously moving ~20x10⁶ m³ translational debris slide, was monitored by U.S. Geological Survey scientists and colleagues since 1958 using a variety of evolving approaches, evolving our understanding of landsliding mechanisms. This talk describes some of those studies.

Surface displacement was monitored using stake lines, tide gauges, time-lapse photography, leveling, aerial photographs, cable extensometers, Global Positioning System (GPS), manual and robotic total stations, lidar, unoccupied aerial systems, and terrestrial, aerial, and satellite interferometric synthetic aperture radar (InSAR) providing line-of-sight to inverted three-dimensional displacements. Subsurface displacement was monitored using a borehole inclinometer array. Subsurface hydrology was monitored using electronic piezometers and tensiometers, and surface hydrological features were monitored using GPS. Seismometer arrays monitored landslide seismicity. Most in situ sensor measurements were nearly continuously recorded by solar- and battery-powered dataloggers.

Earliest monitoring found that slide movement was block-like along narrow bounding shear zones, whose shear mode and location were later found to temporally vary using GPS, resulting in periods of dilative shear. Lidar and InSAR reinforced block-like movement and were utilized to infer landslide basal geometry; GPS monitoring revealed tens-of-m-scale undulations along the landslide base that modify sliding stresses. Photogrammetric and GPS-based kinematic element mapping was advanced by InSAR. Initially believed steady movement thought to result from steady hydrological conditions was later found to vary nearly continuously. GPS and extensometers revealed variable speed ranges along the slide’s length and ~10x speed variation on daily-seasonal scales. GPS revealed that the slide head preferentially slowed, apparently in response to drying conditions. Terrestrial InSAR revealed mm-scale displacement propagation along the slide length on hourly bases. Extensometers showed acceleration during low atmospheric tides when landslide seismicity was greatest. Earliest groundwater monitoring revealed response to snowmelt and rainfall within a few weeks. Later, deeper monitoring showed responses within hours and that pore-pressure drop along a bounding shear zone accompanied acceleration; this and observations of shear-zone dilation suggested that dilative strengthening restrained landslide motion, possibly resulting in its ~300 y movement absent catastrophic failure. As monitoring approaches continue to evolve, I expect additional evolution of the understanding of mechanisms controlling Slumgullion and other landslides.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-4470

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