Site response analysis for deep and soft sedimentary deposits of Dhaka City, Bangladesh

The ground motion intensity of an earthquake is significantly changed when seismic waves propagate from the bedrock to the near-surface soft geological materials. The ground where the shear wave velocity (Vs) exists greater than 760 m/s is generally considered as bedrock. As a common practice in the last three decades, the surface ground motion of a soil site is estimated by multiplying the bedrock motion with the site coefficient that is empirically determined from the time-averaged shear wave velocity in the top 30 m (Vs30) of the site. The site coefficient is defined as the ratio of the ground motion intensity at the ground surface to that of the bedrock. If the bedrock of a site exists at a depth of greater than 30 m, the site effect from the depth of 30 m to the bedrock is not accounted in the Vs30-based site coefficient. In Dhaka City, the minimum depth of the bedrock is approximately 175 m. Therefore, the use of the Vs30-based site coefficient to estimate the surface ground motion is not appropriate for the soft and deep sedimentary deposits of this city. In this study, site response analysis using the Vs30-based site coefficient, linear, equivalent-linear, and nonlinear approaches has been performed to estimate the surface ground motion at different sites of Dhaka City and to compare the results of different approaches. It is observed that the surface ground motion is decreased with increasing the depth of the bedrock due to low shear strain and viscous damping in the soft sedimentary deposits.