THE POSSIBILITY OF OBSERVING NONLINEAR PATH EFFECT IN EARTHQUAKE-INDUCED SEISMIC-WAVE PROPAGATION

Observations of the dependence of elastic-wave velocities on stress in the lithosphere, ultrasonic modeling, and field experiments using con trollable sources reveal that significant nonlinear elastic effects ma y occur in seismic wave propagation. Theoretical modeling involving no nlinear wave equation shows that the most pronounced and practically o bservable implication of the nonlinear elasticity in broadband-signal propagation is the gradual enrichment of the spectra in high-frequency components. We check the significance of nonlinear path effects using the strong ground motion data from two accelerograph arrays in Taiwan . The data cover the motions with peak ground accelerations from 1 to 161 Gal (cm/sec(2)) and peak surface strains from 10(-4) to 10(-7). Th e differences between the average spectra of seismic waves recorded by groups of stations separated by distances of 20 to 40 km are examined to identify the possible nonlinear path effect. Mixed results have be en obtained. In the SMART2 array, the increase in high-frequency energ y is detected in compliance with the theory, which corroborates elasti c nonlinearity. On the other0 hand, in the NCCU array, no symptoms of nonlinear wave propagation are found. Poor signal-to-noise conditions and restrictions imposed by the frequency band of standard instrumenta tion might be accountable for the negative result.