1 CHARACTERIZING AND QUANTIFYING EARTHQUAKE-INDUCED SITE NONLINEARITY

This study proposes to use a method of nonlinear, nonstationary data processing and analysis, i.e., the Hilbert-Huang transform or HHT, to characterize and quantify the influences of nonlinear phenomena of soil layer(s) near the ground surface or simply site nonlinearity (exemplified as soil nonlinearity and/or liquefaction) in earthquake ground recordings. The paper first reviews the Fourier-based approach to characterizing the nonstationary features in the recordings and demonstrates the deficiencies for the nonstationarity caused by site nonlinearity. The paper then introduces the HHT method and offers the justifications for the HHT in addressing the issues of nonlinearity-related nonstationarity. 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 site nonlinearity and quantifying the influences in seismic ground responses. Major results from this study are shown below. 1. The 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 is effective in quantifying site nonlinearity in terms of frequency downshift in the low-frequency range and amplification-downshift factor in intermediate-frequency range. 2. Instantaneous damping, and Hilbert and marginal Hilbert damping spectra are defined in ways 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 the influences of site nonlinearity in seismic ground responses.