The interaction of a seismic source device with the free surface of the earth changes the radiation characteristic to be different from that of the equivalent body point source. Usual seismic sources are built to give enough energy to penetrate a few kilometers in the earth. It is not guaranteed that when the source is activated the surface material behavior stays within a region that can be appropriately described using infinitesimal stress strain relationships. Finite differences can easily model radiation characteristics and wave propagation when the equations of motion are no longer linear. I compare two nonlinear elastic wave equation to model such source behavior.