This thesis project has set out to investigate the response in fore-aft direction of a monopile wind foundation supporting an NREL 5MW turbine, offshore the coast of Taiwan, which is affected by weak to strong earthquake ground motion in combination with normal sea state conditions. The soil at the location of the monopile is layered, with sand overlying clay, mud-, sandstone and gravel. Liquefaction, although of great concern in seismic design of piled structures, is not investigated. Two soil-structure interaction models’ responses are compared in this study, one denoted as an elastic and one as an inelastic beam on a nonlinear Winkler spring foundation. The goal of using two different soil definitions was threefold: 1. quantify the effect of inelastic soil-structure behavior on the system’s response, by comparing a nonlinear elastic SSI model with a nonlinear inelastic series hysteretic-viscous damping SSI model, 2. analyze the earthquake intensity level at which nonlinear inelastic effects become dominant, 3. verify whether the elastic response assumption made in design standards is valid and leads to a safe design. Incremental dynamic time-series analysis of the response of a Timoshenko beam finite-element model was conducted, where this model is exposed to co-directional steady wind, regular waves and an as-recorded earthquake signal of increasing intensity. This earthquake record is scaled to different magnitudes, including the Extreme Level Earthquake and Abnormal Level Earthquake as defined by ISO19901-2.