The study of atypical manifestations of rock inelasticity improves understanding of the physical mechanisms of seismic wave propagation and attenuation in real environments. In the field experiments, the propagation of longitudinal wave at frequency of 240-1000 Hz between two shallow boreholes in low speed zone was investigated. The measurements were performed using a piezoelectric pulse emitter and similar receiver tools positioned in the boreholes. "Stress-time" σ(t) digital responses were recorded by the open channel with microsecond temporal resolution. The unusual short-period variations of amplitude in the form of sharp flattening wave front, stress drop, or plateau of different width (tens of microseconds) were detected in the wave profile. These low-amplitude variations in the waveform were regarded as manifestations of hopping intermittent inelasticity. This inelastic process was assumed to affect the waveform transformation. The contribution of hopping inelasticity depends on the applied stress magnitude, i.e. in this case, the seismic response amplitude. The mechanism of hopping inelasticity at small strains may be explained by microplasticity of rocks. The findings obtained represent a new step in understanding of physics of seismic and acoustic wave propagation in rocks and can be useful for handling of applied problems in geophysics and mining.
