A new method, the Hilbert-Huang Technique (HHT), is being developed to perform analysis of highly transient time series from nonlinear, dispersive, and nonstationary systems. Such data as typical of records on natural disasters for which the conventional Fourier and Fourier-akin (such as the various wavelet schemes) have severe drawbacks. This new method consists of two parts: the Intrinsic-Mode Decomposition (IMP) scheme, and the Hilbert Spectral Analysis (HSA) technique. The IMD generates the Intrinsic Base Functions from the raw data; and the HSA then concerts the IBF into an energy-frequency-time distribution through the Hilbert transform. The result yields the Hilbert Spectrum depending on both time and time varying-frequency as two spectral independent variables. This project will conduct the following research: First, a thorough understanding of the physical characteristics of the HHT in seismic data analysis will be established. For this purpose, both rigorous theoretical analyses and controlled laboratory tests will conducted. Second, the HHT method will be applied to seismic data and explore the potential applications in the specific take of determining site-specific spectra. Data collected during the Northridge earthquake (1994) and those collected from Taiwan including the SMART-l and SMART-2 network will be used. Specific tasks include: (a) To derive accurate information on strong-motion and weak-motion evolutions due to the geophysical effects along the paths of propagation, and the site effects on seismic waves. (b) To determine the energy-frequency-time history of the seismic activities and their response spectra; to evaluate the nonlinear wave evolution and surface wave dispersion due to complex earth structure. (c) To clarify the role of the suspected excessive vertical motion by comparison with the known earthquake damages. (d) To establish a methodology for evaluating the site-specific spectra based on the HHT and the data analyzed above. The data will be classified by their sources and grouped by the seismic strengths to isolate the propagation effects through earthquakes of all sizes. Specific attention will be on the identification of propagation path and site-specific geo-engineering properties of the site.
