EFFECTS OF SOIL NONLINEARITY ON GROUND RESPONSE IN 3 D SIMULATIONS — AN APPLICATION TO THE SALT LAKE CITY BASIN

Forty percent of the population in Utah lives in the Salt Lake Basin, in the vicinity of the Wasatch Front. This front, formed by the Wasatch fault, poses a significant seismic hazard. Recently, there has been an increased effort to understand the levels of excitation and ground response expected in the Salt Lake Basin. The development of the Wasatch Front Community Velocity Model has made it possible for the first time to analyze, through simulations, the relative importance of factors such as the depth of the sedimentary deposits, edge effects, and focusing, that influence ground shaking in this region. Another factor that has been observed to influence strong ground motion is the extent of inelastic deformation that occurs during an earthquake. We present initial results from a set of full three-dimensional (3D) simulations incorporating nonlinear soil behavior in the soft-soil deposits (Vs ≤ 500 m/s) in the basin, under a Mw 6.8 scenario earthquake. Simulations are performed using Hercules, the finite-element octree-based parallel earthquake simulator developed by the Quake Group at Carnegie Mellon University. Hercules incorporates a rate-dependent approach to simulate the elasto-visco-plastic behavior of soil materials. Our results are qualitatively consistent with observations from past earthquakes. They indicate that nonlinear soil behavior greatly affects the spatial variability of the ground motion, causes permanent displacements, and reduces peak ground velocities and accelerations. They also indicate that nonlinearity is influenced by 3D effects that cannot be reproduced by alternative hybrid or pseudo-nonlinear approaches commonly used in seismic hazard analysis.