The article describes local microplastic inelasticity in loam during propagation of P-wave with a frequency of 240-1000 Hz between two shallow wells, one holding piezoelectric radiator and the other containing piezoelectric receiver. The seismic record displays microplasticity phenomenon as the step-wise changes of the signal amplitude with time and the flat sections in the curve from microsecond to dozens microseconds long. These plateaus create short-terms breaks of the process-the time lags that depend on the signal amplitude. As a result, the first arrival time of the wave is changed, and the wave is transformed. It is presumable that microplasticity is conditioned by stress concentration at different defects. The research findings can be used in solving applied problems in mining engineering and seismology.