The low interface shear strength of many geosynthetic interfaces offers the potential for cost effective seismic hazard mitigation by frictional base isolation. To facilitate engineering evaluation of performance, previous investigators have suggested values of equivalent linear stiffness and damping for use in frequency-domain seismic response analyses of geosynthetic base isolation systems. However, time-domain finite difference analyses using an elastic-perfectly plastic interface model indicate that frequency-domain analyses conducted using these equivalent linear parameters may significantly underestimate the seismic response of a frictional base isolated mass subject to earthquake-generated strong ground motions. Underestimation of the response is attributed to the use of damping values from uniform sinusoidal loading tests for the duration of the earthquake time history. Additional analyses are required to develop practical recommendations for use of frequency domain analysis in design of geosynthetic base isolation systems.