The Reduction of Friction in Long Runout Landslides as an Emergent Phenomenon.

Abstract:

Long runout landslides are one of the most remarkable and enigmatic geologic processes. During these events, large masses of rock fall a height, H,from a mountainside and runout extraordinary distances, L, along relatively flat surfaces. For rock masses with volumes exceeding 10⁹ m³, these landslides regularly runout more than 10 times longer than the height they fall from (H/L<0.1). Even at seemingly safe distances from mountainsides, these landslides are devastating to life and property. Understanding this puzzling process can aid in hazard mitigation and may even help us understand the apparent reduction of friction observed in other geologic processes such as earthquakes. Campbell et al. [1995, doi:10.1029/94JB00937] model long runout landslides as granular flows using a soft-particle code. Their model largely simulates landslides from first principles and, although no fluid or obvious mechanism for reduction of friction is included, the model successfully reproduced many observations of long runout landslides (eg. the preservation of source stratigraphy in the final slide mass and decreasing H/L with increasing slide volume). We extend the work of Campbell et al. [1995] with a focus on the mechanism that reduces friction in these slides. We find sliding preferentially occurs when the overburden is relieved by pressure variations in the slide, similar to the pore pressure fluctuations observed in debris flows [Iverson and LaHusen, 1989 doi:10.1126/science.246.4931.796], but in this case without interstitial fluid. Although this is the hallmark of the acoustic fluidization hypothesis, we find that low frequency pressure variations are responsible for the relief of overburden instead of the kilohertz frequencies suggested by Melosh [1979, doi:10.1029/JB084iB13p07513].