Inelastic horizontal sediment deformation from the MS 7.1, 17 October 1989 Loma Prieta earthquake has been estimated to be on the order of 20 cm at a site in Gilroy, California, about 32 km east of the epicenter. This estimate was based on combined deterministic‐stochastic simulations using ground motion data and sediment properties measured from in situ seismic tests and laboratory tests on cored sediment samples of up to 170 m deep. The calculated deformation is comparable to measured horizontal coseismic displacements at various stations in the Santa Cruz network, suggesting that previously back‐figured fault rupture mechanisms may have been influenced by the coseismic sediment deformation.