Underground structures exist in many active earthquake regions, but empirical data of the seismic behavior of these structures is very limited. This research works toward filling that empirical data gap with scale model shake table testing. Underground seismic soil-structure interaction (USSSI) effects were investigated for a stiff tunnel embedded within soft clay. Young Bay mud was used as a prototype soil for developing a scale model soil mixture. A Bay Area Rapid Transit (BART) cut-and-cover subway tunnel was used as the prototype for the one-tenth scale model subway cross section. The flexible wall test container in these tests allowed the soil to deform in simple shear. Similitude analysis scaled critical variables to properly capture prototype behavior at the model scale subjected to 1-g shake table testing. The measured horizontal “racking” distortions of the model structure compared to numerical analyses suggest that current simplified design methods may overestimate distortions in soft-soil/stiff-structure situations.