By using a soil-water coupled finite element method, deformation/failure behavior during and after the occurrence of an earthquake in a clay foundation-embankment system was numerically examined. The elasto-plastic constitutive model named Super/subloading Yield Surface Cam-clay model (SYS Cam-clay model in short) is applied for soil skeleton. The embankment was modeled as a saturated soil. The results showed that the embankment maintained stability without any significant large deformation during the earthquake, while slip failure penetrating to the crest was suddenly generated a few hours after the earthquake.