The nonlinear dynamic theory and the chaotic time series analysis method were adopted to examine the nonlinear characteristics of strong earthquake ground motions in this paper.Based on the power spectrum analysis

principal component analysis

and Cao’s method

the acceleration time series of ground motions have been determined qualitatively being of the chaotic property.Then

the chaotic time series analysis method is applied to calculate quantitatively the three nonlinear characteristic parameters of 30 records of ground acceleration.Numerical results show that the correlation dimension of these ground motions is a fractal dimension between 2.0 and 4.0.Their Kolmogorov entropy K2is a finite value larger than zero

and their maximal Lyapunov exponent is in the interval 0-1.0.The result has demonstrated that the strong ground motions possess the chaotic character

and the severe irregularity and complexity of ground motions are the reflection of high nonlinearity of earthquake process.