Estimation of Nonlinear Soil Behavior During the 1999 Chi-Chi, Taiwan, Earthquake

The 1999 Chi-Chi, Taiwan, earthquake (M_w = 7.6) was one of the strongest earthquakes in recent years recorded by a large number of strong-motion devices. Though only surface records are available, the obtained strong-motion database indicates the variety of ground responses in the near-fault zones. In this study, accelerograms of the Chi-Chi earthquake were simulated at rock and soil sites, and models of soil behavior were constructed at seven soil sites (TCU065, TCU072, TCU138, CHY026, CHY104, CHY074, and CHY015), for which parameters of the soil profiles are known down to depths of at least ~70 m and at 24 other soil sites, for which parameters of the soil profiles are known down to 30–40 m; all the sites were located within ~50 km from the fault. For reconstructing stresses and strains in the soil layers, we used a method similar to that developed for the estimation of soil behavior based on vertical array records. As input for the soil layers, acceleration time histories simulated by stochastic finite-fault modelling with a prescribed slip distribution over the fault plane were taken. In spite of the largeness of the earthquake’s magnitude and the proximity of the studied soil sites to the fault plane, the soil behavior at these sites was relatively simple, i.e., a fairly good agreement between the spectra of the observed and simulated accelerograms and between their waveforms was obtained even in cases where a single stress-strain relation was used to describe the behavior of whole soil thickness down to ~70–80 m during strong motion. Obviously, this is due to homogeneity in the characteristics of soil layers in depth. At all the studied sites, resonant phenomena in soil layers (down to ~40–60 m) and nonlinearity of soil response were the main factors defining soil behavior. At TCU065, TCU110, TCU115, CHY101, CHY036, and CHY039 liquefaction phenomena occurred in the upper soil layers, estimated strains achieved ~0.6–0.8%; at other stations, maximum strains in the soil layers were as high as 0.1–0.4%, according to our estimates. Thus, valuable data on the in situ soil behavior during the Chi-Chi earthquake was obtained. Similarity in the behavior of similar soils during the 1995 Kobe, 2000 Tottori (Japan), and Chi-Chi (Taiwan) earthquakes was found, indicating the possibility of forecasting soil behavior in future earthquakes. In the near-fault zones of the three earthquakes, “hard-type” soil behavior and resonant phenomena in the upper surface layers prevail, both leading to high acceleration amplitudes on the surface.