Nonlinear soil behavior during the Chi-Chi (Taiwan) earthquake of 1999 from data of the Dahan vertical array

Records of the Chi-Chi (Taiwan) earthquake of 1999 (M _w = 7.7) obtained at the Dahan vertical array (a part of the SMART-2 array) in NE Taiwan are analyzed. The Dahan array was located at a distance of ～80 km from the fault plane of the earthquake. Using the method previously applied to the analysis of soil behavior during the Japan earthquakes in Kobe (1995) and Tottori (2000), nonlinear stress-strain relations are estimated in soil layers at depths of 0–200 m at the Dahan site for successive time intervals. The resulting soil behavior model is used for estimating variations in shear moduli and for the identification of nonlinear behavior of soil layers during an earthquake. According to the model estimates, the earthquake-related decrease in shear moduli did not exceed ～5% and the soil response was nearly linear (the amount of its nonlinear components was also no more than ～5%). The stress-strain relations describing the soil responses to the earthquakes in Kobe (1995), Tottori (2000), and Chi-Chi (1999) are similar, implying that, in principle, it is possible to describe the behavior of soils of the same type by the same stress-strain relations and predict the soil response to a future earthquake.