The effects of torsion and motion coupling in site response estimation

Abstract

SUMMARYSoil ampliÿcation characteristics are investigated using data from the Chibaken-Toho-Oki earthquake and its aftershocks recorded at Chiba dense array in Japan. The frequency-dependent ampliÿcation function of soil is calculated using uphole-to-downhole spectral ratio analysis, considering the horizontal components of shear wave. The identiÿed spectral ratios consistently demonstrate the splitting of peaks in their resonance frequencies and low ampliÿcation values in comparison with a 1D model. The torsional behaviour and horizontal ground motion coupling are clariÿed as the reasons for these phenomena at the site. To prove the hypothesis, the torsional motion is directly evaluated using the data of the horizontal dense array in di erent depths at the site. The comparison between Fourier spectra of torsional motion and identiÿed transfer functions reveals the peaks at the same frequencies. The wave equation including torsion and horizontal motion coupling is introduced and solved for the layered media by applying wave propagation theory. Using the developed model, the e ects of torsional motion with horizontal motion coupling on soil transfer function are numerically examined. Splitting and low ampliÿcation at resonance frequencies are conÿrmed by the results of numerical analysis. Furthermore, the ground motion in two horizontal directions at the site is simulated using site geotechnical speciÿcation and optimizing the model parameters. The simulated and recorded motions demonstrate good agreement that is used to validate the hypothesis. In addition, the spectral density of torsional ground motions are compared with the calculated one and found to be well predicted by the model. Finally, the results are used to explain the overestimation of damping in back-calculation of dynamic soil properties using vertical array data in small strain level.