At high magnetic fields and low temperatures, 0.02% YLiF4:Er3+ is observed to show strongly non-exponential two photon echo decay which becomes independent of the field above 27 kG. We describe the optical dephasing using a computer simulation which takes into account the mutual spin flips of 3000 F host nuclei. The highly nonexponential echo decay results from the "frozen core" of nuclear spins near the large Er3+ magnetic moment, whose spin flip rates are greatly slowed down compared to the unperturbed spins. The simulations predict the echo decay shape and provide a means to estimate the nuclear spin flip rates. The simulation results are compared with theories for the electron spin and photon echo decay. © 1992.