SPECFEM2D what is the propagation direction of the source wave

[snahn1]

hello. I am a student studying SPECFEM for seismic wave propagation in the ground

When using a ricker wavelet for the source in which direction is the source propagation direcetion? Does one ricker wavelet propagate in both the xx (horizontal) and xz (vertical) directions?

If I check the acceleration time series of the station, in the XZ direction station appear ricker wavelet shape, but a completely unknown waveform is recorded in the XX direction.
I think the XX direction simulates the S wave, but which direction should I check when considering seismic waves?

Thank you!