The paper presents a new formulation for the use of pseudo-dynamic method to compute the passive earth pressure coefficients of bridge abutment gravity retaining walls by using composite (combination of log-spiral and planar) failure mechanism in the framework of limit equilibrium method when subjected to seismic loads. Numerical optimization of passive earth pressure coefficients is performed using the "improved Nelder-Mead simplex method" which is a direct search algorithm in the optimization of nonlinear functions and also accounts for variable bounds. Predictions by the present method are compared with those given by other authors. It is shown that the pseudo-static methods overestimate the passive earth pressure coefficients due to non-consideration of the effect of time and phase difference due to finite shear wave and primary wave velocities. Copyright ASCE 2008.