Microplasticity effects in loam caused by seismic wave of frequency about 1000 Hz are detected in the borehole-to-borehole measurements. Microplasticity manifestations on seismic record are presented as the ladder-like stepwise changes in amplitude course. The step (plateau) on seismic trace is time delay, its duration depends on the strain-amplitude value. Time delay changes the frequency characteristic of stress pulse, nonlinearly transforms wave front, and shifts the amplitude maximum along time axis. The microplastic process occurs owing to the anomalous realignment of the internal stresses on the microstructural defects in the area of small deformations. This is the useful contribution to wave propagation physics. The received results can also be used in solving the applied problems in material science, seismic prospecting, diagnostics, etc.