Comparison between seismograms computed with the elliptic wave equation and with the isotropic wave equation in a stretched coordinate system depending on δ. On the right-hand graphs, the seismograms computed with the elliptic wave equations are plotted in the background in red and blue, and the seismograms computed with the isotropic wave equation in black wiggles. The two seismograms are in phase when the blue loops match the black wiggles. On the left-hand graphs, the red traces correspond to the seismograms computed with the elliptic wave equation and the dashed black traces to the ones computed with the isotropic wave equation.