This paper investigates long-period ground motion characteristic of the 1999 Jiji (Chi-Chi), Taiwan, mainshock and aftershocks on the basis of lots of high quality digital strong motion records. The study attaches the importance to the variation of strength of the long-period ground motion with the magnitude, distance, and site condition. In the meantime, the near-fault long-period ground motion characteristic is analyzed. The result shows that the shape of the long-period response spectrum is mainly controlled by site condition and magnitude (the spectrum of class D+E is wider than that of class B+C, and the spectrum of larger magnitude is wider than that of smaller magni- tude), and the effect of fault distance on the shape is not evident. And near-fault long-period ground motion char- acteristic depends on fault activity apparently, that is to say, the long-term ground motion in the hanger is stronger than that in the footwall, and the long-term ground motion in the north is stronger than that in the south.
In this paper, a method, which synthesizes the artificial ground motion compatible with the specified peak velocity as well as the target acceleration response spectrum, was proposed. In this method, firstly, an initial acceleration time history a g(0)(t), which satisfies the prescribed peak ground acceleration, the target spectral acceleration ST(ω, ζ ), and the specified intensity envelope, is generated by the traditional method that generates the re- sponse-spectrum-compatible artificial ground motion by modifying the Fourier amplitude spectrum in the fre- quency domain; secondly, a g(0)(t) is further modulated by superimposing narrow-band time histories upon it in the time domain to make its peak velocity, approach the target peak ground velocity, and at the same time to improve its fitting precision to the target spectrum. Numerical examples show that this algorithm boasts high calculation precisions.
