Ray R

This study proposes to use a method of nonlinear, nonstationary data processing and analysis, i.e., the Hilbert-Huang transform (HHT), to explore and characterize the signatures of soil nonlinearity and/or liquefaction from earthquake ground recordings. The paper first summarizes symptoms of soil nonlinearity and liquefaction shown in earthquake ground motion recordings. It then reviews the Fourier-based approach to characterizing the features in the recordings and demonstrates the deficiencies. In contrast, the paper subsequently introduces the HHT method and offers the justifications for the HHT in addressing the nonlinearity issues. With the use of hypothetic data and recordings from the 1964 Niigata and 2001 Nisqually earthquake and of Fourier-based approach as a reference, this study shows that the HHT-based approach is effective in characterizing soil nonlinearity and liquefaction and quantifying the influences in seismic ground responses. Selected results are shown below. 1. The first HHT-based component and Hilbert amplitude spectra can identify abnormal highfrequency spikes in the recording at site where strong soil nonlinearity and/or liquefaction occurs, which can help, at the first glance, detect the nonlinear and/or liquefaction sites. 2. The HHT-based site amplification is defined as the ratio of marginal Hilbert amplitude spectra, similar to the Fourier-based one, i.e., the ratio of Fourier amplitude spectra. The HHT-based site amplification is effective in quantifying soil nonlinearity or liquefaction in terms of frequency downshift in the low-frequency range and amplification downshift in intermediate-frequency range. 3. Marginal and Hilbert damping spectra are identified in ways similar to marginal and Hilbert amplitude spectra. Consequently, the HHT-based soil damping is found as the difference of marginal Hilbert damping spectra, which can be extracted from the HHT-based site