NONLINEAR SOIL RESPONSE - A REALITY

Geotechnical models consistently indicate that the stress-strain relat ionship of soils is nonlinear and hysteretic, especially at shear stra ins larger than similar to 10(-5) to 10(-4). Nonlinear effects, such a s an increase in damping and reduction in shear-wave velocity as excit ation strength increases, are commonly recognized in the dynamic loadi ng of soils. On the other hand, these effects are usually ignored in s eismological models of ground-motion prediction because of the lack of compelling corroborative evidence from strong-motion observations. Th e situation is being changes by recently obtained data. Explicit evide nce of strong-motion deamplification, accompanied by changes in resona nt frequencies, are found in the data from the 1985 Michoacan, Mexico, and the 1989 Loma Prieta, California earthquakes, the events recorded by the vertical and surface accelerograph arrays in Taiwan, as well a s a number of other events throughout the world. Evidence of nonlinear behavior becomes apparent beyond a threshold acceleration of similar to 100 to 200 gal. Nonlinearity is considerable in cohesionless soil b ut may be negligible in stiff soils. The findings of recent years indi cate that nonlinear site effects are more common than previously recog nized in strong-motion seismology.