Three Dimensional Nonlinear Soil and Site-City Effects in Earthquake Simulations

We present our recent developments in ground motion modeling for including full three-dimensional nonlinear soil behavior and the presence of the built environment in large-scale earthquake simulations in highly heterogeneous basins, and illustrate their effects with a scenario earthquake. These additions to our simulation framework are implemented in Hercules, the parallel octree-based finite-element earthquake simulator developed by the Quake Group at Carnegie Mellon. Our new implementation is applicable to both long- and shorter-period earthquake ground motions. It helps make a direct connection to subjects of interest in earthquake engineering and seismology, such as seismic hazard assessment in large urban areas, path and site effects, and the analysis and design of sensitive infrastructure and tall buildings. Nonlinear soil behavior is incorporated in ground motion simulations employing a rate-dependent plasticity approach to predict the nonlinear state of the material explicitly at every time step. At this initial stage, the soil is modeled as a perfectly elastoplastic material. The presence of urban structures is modeled by representing buildings as homogeneous blocks made up of the same type of hexahedral elements used in the finite-element mesh. We test our extended simulation capabilities for soil nonlinearity and site-city effects under realistic earthquake conditions in a heterogeneous geological structure. In the case of nonlinear ground motion modeling, results indicate that soil nonlinearities greatly modify the ground response, confirming previous observations of deamplification effects and spatial variability, and evidencing three-dimensional basin effects not previously visible through idealized one- and two-dimensional approximations. In turn, the presence of building clusters causes multiple soil-structure interaction phenomena that change both the near ground motion and the individual behavior of the buildings themselves. This substantiates the argument that in soft-soil basins it may no longer be valid to ignore the presence of neighboring structures. The analysis drawn from the applications presented here confirms aspects known from previous, though limited studies, and broadens our knowledge of the effects of nonlinear soil and the built environment on the ground motion due to earthquakes, at a regional level explored here for the first time.