  A better understanding and anticipation of natural processes such as
landsliding or seismic fault activity requires detailed theoretical and
experimental analysis of rock mechanics and geomaterial dynamics. These last
decades, considerable progress has been made towards understanding deformation
and fracture process in laboratory experiment on granular rock materials, as
the well-known shear banding experiment. One of the reasons for this progress
is the continuous improvement in the instrumental techniques of observation.
But the lack of real time methods does not allow the detection of indicators of
the upcoming fracture process and thus to anticipate the phenomenon. Here, we
have performed uniaxial compression experiments to analyse the response of a
granular rock material sample to different shocks. We use a novel
interferometric laser sensor based on the nonlinear self-mixing interferometry
technique to observe in real time the deformations of the sample and assess its
usefulness as a diagnostic tool for the analysis of geomaterial dynamics. Due
to the high spatial and temporal resolution of this approach, we observe both
vibrations processes in response to a dynamic loading and the onset of failure.
The latter is preceded by a continuous variation of vibration period of the
material. After several shocks, the material response is no longer reversible
and we detect a progressive accumulation of irreversible deformation leading to
the fracture process. We demonstrate that material failure is anticipated by
the critical slowing down of the surface vibrational motion, which may
therefore be envisioned as an early warning signal or predictor to the
macroscopic failure of the sample. The nonlinear self-mixing interferometry
technique is readily extensible to fault propagation measurements. As such, it
opens a new window of observation for the study of geomaterial deformation and
failure.
