A RECORDING-BASED APPROACH FOR IDENTIFYING SEISMIC SITE LIQUEFACTION AND NONLINEARITY VIA HHT DATA ANALYSIS

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This study proposes an approach to identify earthquake-induced site liquefaction and/or nonlinearity from Hilbert–Huang–Transformation- or HHT-based data analysis of seismic motion recordings. The proposed approach fully utilizes unique features of the HHT method in characterizing instantaneous frequency and damping as well as temporal-frequency motion from the recordings, so as to single out and quantify liquefaction- and/or nonlinear-soil-related nonlinear phenomena shown in the recordings. With post-earthquake, site investigation as a reference, this study shows that the proposed approach is effective in characterizing site nonlinearity, quantifying nonlinear influences in site amplification, and diagnosing site liquefaction. Major results from this study are listed below.1. Predominant instantaneous frequency of earthquake motion is defined as the frequency with the largest amplitude in Hilbert amplitude spectra of the motion at a time instant, and subsequently used together with other motion features for identifying site liquefaction conditions. Analysis of 29 sets of seismic recordings with different liquefaction conditions shows that the proposed approach is more effective in detecting site liquefaction than other Fourier-based methods.2. HHT-based site amplification is defined as the ratio of marginal Hilbert amplitude spectra, similar to the Fourier-based one that is the ratio of Fourier amplitude spectra. The HHT-based site amplification can be used for effectively quantifying site nonlinearity in terms of frequency downshift in the low-frequency range and amplification-reduction factor in intermediate-frequency range in comparison with the Fourier-based one.3. Instantaneous damping, and Hilbert and marginal Hilbert damping spectra are defined in way similar to instantaneous frequency, and Hilbert and marginal Hilbert amplitude spectra, respectively. Consequently, the HHT-based site damping is found as the difference of marginal Hilbert damping spectra, which can be used as an alternative, complementary index to measure influences of site nonlinearity in seismic ground responses.