LINEAR AND NONLINEAR BEHAVIORS ON A SOIL SITE USING LOTUNG DOWNHOLE ARRAY IN TAIWAN

摘要

We study the linear and nonlinear behaviors of soft soil layers using DHB downhole recordings at LSST site, Lotung, Taiwan. This array includes five triaxial accelerometers deployed at depths of 0m, 6m, 11m, 17m and 47m. During a 6-year operation from 1985 to 1990, 30 earthquakes (4.0≤ M_L ≤ 6.5) triggered this array. The maximum PGA value is 257 gals recorded at the surface station. Spectral analyses show that the strong motion causes the peaks of ratio to shift to lower predominant frequencies. The averaged spectral ratio of 15 well-recorded weak motion records are selected as a reference, the shift of the maximum predominant frequency can reach 20%. Comparing to the weak motions, the strong motions also decrease the amplification factor. The maximum reduction of the amplification can reach 50%. Besides, the results of waveform simulation show that the linear model based on the Haskell method can well predict the weak motions (PGA < 60 gals) at various depths. However, this linear model does not work for the strong motion data (PGA > 150 gals). Here the nonlinear numerical scheme, such as DESRA-2, is required and can significantly improve the simulation results although the PGA value at the surface station is still underestimated. Overall, it is feasible to predict the strong motions for a horizontal layered structure.