FRICTIONAL PROPERTIES OF THE BRIAN HEAD FORMATION AND CONSTRAINTS ON CONDITIONS LEADING TO NUCLEATION AND FAILURE OF THE MARKAGUNT AND SEVIER GRAVITY SLIDES

Subaerial long-runout landslides are among Earth’s most catastrophic natural hazards due to the volume of landslide material and transport distance, yet the factors that lead to the initiation of runaway unstable long-runout landslides are unclear. The Oligocene-Miocene Marysvale Gravity Slide Complex (MGSC) in southwestern Utah is currently the largest known subaerial long-runout landslide complex on Earth, comprising three distinct gravity slides structurally defined by a breakaway zone, a bedding plane detachment surface, a ramp cutting up from the bedding plane segment to the surface, and a runout area covering the former land surface. For deep-seated rockslides, initiation is governed by the frictional resistance of the detachment surface, which occurred within the clay-rich, structurally weak Eocene-Oligocene Brian Head Formation for the MGSC. This study aims to constrain the conditions leading to initiation of the MGSC slides by understanding frictional properties of the Brian Head Formation as a function of lithology, stratigraphic position, and pore fluid pressure. Samples were collected from undeformed, autochthonous outcrops proximal to the distal toe of the easternmost Sevier Gravity Slide at key stratigraphic intervals. Frictional resistance in sliding is a dynamic process governed by periods of stability and instability described as rate-and-state dependent friction. We tested the rate-and-state frictional behavior using a direct-shear L-block configuration in a triaxial apparatus under dry and saturated conditions. Most samples are neutral to weakly rate strengthening, but two horizons have steady state friction coefficients approaching zero under saturated conditions. We explore implications of the frictional behavior of these horizons on the ultimate size of the slides by considering the effect of the (A) steady state friction coefficient in Boundary Element Method models of the detachment surface under the southern slope of the Marysvale volcanic pile, and (2) rate-dependence of friction using rate-and-state friction theory.