Their final words:

Our previous work shows that permanent damage to the grains themselves is negligible and therefore cannot be the origin of the behaviours observed. Moreover, acoustical studies in three-dimensional glass bead packs under similar wave strain amplitudes, and under (smaller) static stresses of 0.02–0.1 MPa, show no evidence for grain rearrangement; however, the material exhibits very small, irreversible compaction as well as nonlinear-induced modulus softening and slow dynamics. Hertz–Mindlin contact mechanics describe these observations. The compaction we measure in our experiments without vibration is small and does not lead to instability. The addition of vibration shows additional compaction but it is extremely small. Taken together, the observations suggest that minute compaction plays a part in what we observe, but there is no clear evidence suggesting that it is the cause. Our data do not rule out the possibility that instability is abetted, or initiated, by localized compaction (for example, within a shear band in the layer), which would be invisible to our measurements. Local compaction within a granular material would reduce normal stress at contact junctions, which could lead to stick–slip instability. For the moment, the origin of what we observe when stick–slip is combined with vibration remains unknown.

The origin of dynamic earthquake triggering by transient seismic waves is a complex problem. Our results show that granular-friction processes are consistent with two as-yet-unexplained observations in earthquake seismology: (1) small-amplitude waves can trigger both immediate failure and delayed failure relative to the strain transient, and (2) earthquake recurrence patterns are complex. Understanding the role of vibration-induced disruption of earthquake recurrence could have significant implications for seismic hazard assessment and reliable forecasting of earthquakes.