OBSERVATION AND IMPLICATIONS OF NONLINEAR ELASTIC-WAVE RESPONSE IN ROCK

Experiments in rock show a large nonlinear elastic wave response, far greater than that of gases, liquids and most other solids. The large r esponse is attributed to structural discontinuities in rock such as mi crocracks and grain boundaries. The magnitude of the harmonics created by nonlinear interactions grows linearly with propagation distance in one-dimensional systems. In the earth, a large nonlinear response may be responsible for significant spectral alteration of a seismic wave at amplitudes and distances currently considered to be within the line ar elastic regime. We argue, based on observations at ultrasonic frequ encies, that the effect of nonlinear elasticity on seismic wave propag ation may be large, and should be considered in modelling.