Shear surface undulations modulate gouge strength and contribute to divergent landslide acceleration

Landslides display a broad spectrum of speeds for incompletely known reasons. Sliding occurs along slickensided undulatory shear surfaces within boundary shear gouge comprising clay mineral and non-clay (granular) particle mixtures, similar to tectonic fault gouge. Laboratory tests reveal that gouge shear resistance increases, decreases, or is unchanged with shear rate, yet understanding these diverse behaviors remains elusive. Such strength variability can result in accelerating or decelerating failure, or both behaviors under variable stressing conditions. We performed advanced laboratory shear-resistance testing to evaluate failure behavior of a landslide near Oso, WA, USA. Many of our observations are poorly explained by existing theory. We hypothesize that undulations along shear surfaces impart shear resistance, but may also cause dilatant weakening and stress modification that reduces particle-scale shear resistance. We tested our hypotheses in the laboratory and through finite element modeling of granular-clay mixtures. Our results revealed that relatively rapid failure along undulatory shear surfaces involves strength degradation with finite displacement, persistent pore-pressure elevation, and persistent dilation-induced weakening. Overall effects differed depending on material composition, with granular-rich material losing significant resistance and clay-rich material losing little or gaining resistance during relatively rapid failure. Testing by others support these observations and reinforce our finding that landslides with clay-rich gouge are less likely and landslides with granular-rich gouge are more likely to move catastrophically during relatively rapid stress changes; gouge with little clay may not develop persistent undulatory surfaces and thus show invariant shear resistance. Our findings reveal previously unknown mechanisms that may explain divergent landslide behavior and improve understanding of shear gouge strength variability for any intensely shearing environment, including tectonic fault rupture.

Schulz (2022) Shear surface undulations modulate gouge strength and contribute to divergent landslide acceleration. USGS Landslide Hazards Seminar, 22 June 2022.