This study extends the linear spectral‐element method (SEM) program SPECFEM3D to include nonlinear constitutive relationships. The iterations for solving the nonlinear equations are bypassed considering that the explicit integration used in the code requires a very small time increment; as a result, the high efficiency of the original linear procedure is preserved. This nonlinear code allows us to perform a deterministic seismic‐hazard analysis that considers the nonlinear behaviors of the propagation path and local site for an engineering site. After checking its accuracy against the commercial finite‐element method program ABAQUS, the extended nonlinear SEM is used to investigate the effect of noncausative faults on the seismic ground motion at the Dagangshan dam site in Southwest China. The analysis of the study case shows that the isolation effect of noncausative faults is dependent on the fault‐source relative position, and the nonlinear responses of the faults are sensitive to earthquake magnitude.