Acoustic fluidization and the extraordinary mobility of sturzstroms

Abstract

[1] Sturzstroms are a rare category of rock avalanche that travel vast horizontal distances with only a comparatively small vertical drop in height. Their extraordinary mobility appears to be a consequence of sustained fluid-like behavior during motion, which persists even for driving stresses well below those normally associated with granular flows. One mechanism that may explain this temporary increase in the mobility of rock debris is acoustic fluidization; where transient, high-frequency pressure fluctuations, generated during the initial collapse and subsequent flow of a mass of rock debris, may locally relieve overburden stresses in the rock mass and thus reduce the frictional resistance to slip between fragments. In this paper we develop the acoustic fluidization model for the mechanics of sturzstroms and discuss the conditions under which this process may sustain fluid-like flow of large rock avalanches at low driving stresses.

The formation of peak-ring basins: Working hypotheses and path forward in using observations to constrain models of impact-basin formation

“…It is likely, however, that the cratering process itself, and in particular the dramatic rise and collapse of the central uplift, can generate an additional component of vibrational energy that is localized in the region of most extreme deformation. Theoretical models of acoustic fluidization of faults and long run-out landslides have both highlighted the importance of regeneration of vibrational energy (Melosh, 1996;Collins and Melosh, 2003) and a preliminary implementation of vibrational-energy regeneration in impact simulations suggests its role could be significant (Hay et al, 2014). In the context of cratering, regeneration of vibrational energy may provide a mechanism for the necessary threshold in central-peak and peak-ring formation by prolonging mobilization of the flanks of the central uplift in large craters where shear strains are largest.…”

Radar Albedo Feature

“…Several theories have been proposed to explain the low apparent coefficient of friction exhibited by large rock avalanches (e.g., Bagnold 1956;Collins and Melosh 2003;Davies and McSaveney 2009). All of these explain the apparent reduction in the coefficient of friction by a temporary lowering of the normal stress between fragments, but none is universally accepted.…”

Rock‐and‐soil avalanches: Theory and simulation

“…These theories may be classified into two groups, depending on the physical mechanism that is put forward. In the first group, the long runout is explained in terms of physical processes occurring within granular rock material in the bulk, such as mechanical fluidization, acoustic fluidization, or dynamic fragmentation [e.g., Campbell, 1989;Collins and Melosh, 2003;McSaveney and Davies, 2006]. In this case, the avalanche may propagate over a rough surface, without a need to invoke lowered basal friction.…”

“…[52] The acoustic fluidization theory for long runout of rock avalanches is also based on two hypotheses concerning the kinematics and the dynamics of granular flows [Collins and Melosh, 2003]. The hypothesis on kinematics assumes that acoustic waves with wavelengths comparable to avalanche size may propagate through the granular material, generating random vibrations of groups of particles organized into waves; acoustic waves may result from large, high-frequency pressure fluctuations generated during the initial collapse and subsequent flow of a mass of rock debris.…”