Inelastic soil amplification in three sites during the Tokachi-oki MJMA 8.0 earthquake

Earthquake case histories of soil response in the last half century have revealed the nonlinear behavior of soils under strong shaking. Over the years numerous efforts have been made to represent realistically soil behavior in the nonlinear range. In the aftermath of an international benchmark exercise (PRENOLIN) aimed at verifying 23 nonlinear codes and then validating them against the recorded seismic responses of two soil sites, this paper investigates the response of 3 soil sites in Hokkaido, Japan, shaken by the Tokachi-oki 2003 earthquake. Surface and deep (in-borehole) accelerographs recorded the event at each site. The objective of our study is to see if available nonlinear codes, without any after-the-event calibration and with imperfect knowledge of the soil layer properties, can lead to practically acceptable results. Before embarking into the nonlinear analyses, the linear response to an aftershock is first simulated with reasonable success. Then, using three different nonlinear 1D soil-amplification codes, which incorporate different constitutive relations, numerical wave propagation analyses are performed for each site, excited with the motion recorded in each borehole (at the depth of the presumed bedrock). Computed results in the form of acceleration time histories and response spectra for the ground surface are systematically compared with the respective recorded motions. It is found that the numerical codes perform generally quite satisfactorily. Exceptions are discussed in the light of the other research findings.