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

Abstract

Observations of the dependence of elastic-wave velocities on stress in the lithosphere, ultrasonic modeling, and field experiments using controllable sources reveal that significant nonlinear elastic effects may occur in seismic wave propaga- tion. Theoretical modeling involving nonlinear wave equation shows that the most pronounced and practically observable implication of the nonlinear elasticity in broadband-signal propagation is the gradual enrichment of the spectra in high-fre- quency 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 . The 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 been obtained. In the SMART2 array, the increase in high-frequency energy is detected in compliance with the theory, which corroborates elastic nonlinearity. On the other 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 instrumentation might be accountable for the negative result.