Abstract:
Time-frequency analysis of earthquake records using Fourier transform is a fundamental, reliable and useful tool in earthquake engineering and engineering seismology. It will be however no longer functional if the frequency variation is analysed in time domain. Short time Fourier transform is utilized for the same purpose but this has also its own limitations and restrictions. In this research, Stockwell transform (S-transform), is evaluated in investigating frequency content of signal in time domain. First, the effectiveness of S-transform was tested by a non-stationary synthetic signal series with a sum of various instantaneous time varying frequency functions. Then, transform was employed to three different earthquake waveforms of Iwate-Miyagi Nairiku earthquake (M-w 6.9, 2008); recorded in near, moderate and far sites. Finally, an application was demonstrated for determining dynamic response of three-story frame structure by using El Centro earthquake compiled with harmonic motion. Unlike widely used continuous wavelet transform, which provides temporal and spectral information simultaneously, S-transform is very straightforward and easy to manipulate for interpretations. All cases considered in this research showed that S- transform can be implemented for further research activities related with frequency-dependent strong motion analysis by practitioners and engineers. S-transform can distinguish abrupt frequency changes in structures effectively and accurately.
