Tales of compacting sand to anticipate strain budget of rupture processes

Prior to earthquake ruptures and slope failures, accelerated surface deformations can sometimes be observed. To anticipate rupture processes, these deformations are interpreted in terms of a strain budget and its stressors. If the budget exceeds an assumed critical value, rupture happens. But not all components of the budget can readily be inferred from the bulk deformation. For example, elastic strain build-up and other ‘silent’ contributions challenge the predictability of these potential natural hazards. We present preliminary experimental results, focussing on deformation by compaction. We report an analogous experiment of loading and unloading to constrain compaction behaviour, elastic strain-build up, and release to understand their ‘silent’ contributions to the strain budget. As analogue material, we use sand to assess emergent bulk behaviour. Using natural quartz crystals allows to apply in-situ neutron diffraction to measure elastic strain during loading and unloading stages. We find that while compaction and remnant compaction scale linearly with load magnitudes, elastic strain build-up seems to be independent of stresses ≥60 MPa. In addition to the in-situ neutron diffraction experiments, we conducted mechanical compaction tests at ramped load stages and analysed the post-compaction changes of the grain size distribution. With increased loading, the mean grain size decreased, leading to increased bulk density in the compacted portion. Based on these observations, we reason that the linear elastic bulk compaction of our samples is due to non-linear local brittle deformation. There is only limited elastic strain built up during the compaction, which is likely released due to local crushing. Localized failure produces a denser material in which strain can build up more homogeneously, causing rupture at its bulk elastic limit. Our experiments show that deducing or simply converting loading and displacement to stress-strain relationships to establish a strain budget may be inadequate. Silent components that are likely due to non-linear and emergent processes can in the short term lead to local elastic strain energy release or bulk dynamic ruptures. Conceptually, to especially anticipate the timing of slope failures and the magnitude of earthquake ruptures, the hidden costs, e.g. due to localized failure, and internal changes, concerning density or elastic properties, are crucial components that need to be constrained while compiling a strain or energy budget of these processes.