Glassy hillslopes and quiescent sandpiles: rendering sub-yield granular creep with Diffusing Wave Spectroscopy

Hills and mountainsides are wrapped in a blanket of soil: an amorphous granular packing, constrained by geologic boundaries and pulled by gravity. At first glance, soil-mantled hillslopes beneath the angle of repose appear jammed and remain as such until a yield threshold is reached. Here we render creep dynamics in a sub-threshold granular heap with Diffusing Wave Spectroscopy in the absence of excitations or driving. The system is isolated from vibrations and is for all practical purposes quiescent. Nonetheless, we observe persistent creeping strains (10^-6). The exponential decay of the relaxation dynamics is governed by a single time scale, which is sufficient to collapse the data into a single master curve. We also observe the growth of dynamical heterogeneities and a quadrapolar shape of the spatial correlations, signatures of glassy behavior which are consistent with the phenomenology of supercooled liquids and amorphous solids. Further, depth-averaged strain profiles are consistent with decades-long records of soil deformation, which hint that these dynamics are at play in natural hillslopes. This opens the possibility for the re-interpretation of field data through the lens of soft matter and for future work examining the role of weak mechanical disturbances.