I found an interesting phenomenon while modelling an explosion source (m_xx=m_zz=1, m_xz=0) with Specfem2D with a Dirac source-time function.
With a filter of 0.3-3Hz amplitudes appear in the seismogram after the P-wave. Their time difference would suggest, that they are related to S-waves, which I didn’t expect for an explosion source. With a lower filtering and by using a different source-time function (e.g. Ricker) those amplitudes disappear for the same model.
I was wondering whether this is a common behavior of modelling with Specfem2D, that in higher frequencies especially S-waves pattern appear due to the Dirac source time function?
Do you have a suggestion, how to choose the filter in general in order to supress non-physical patterns related to the Dirac source-time function?
Below I provided a figure with a representation of the model I used, (homogeneous, v_p=3000m/s, v_p/v_s=1.76) and the resulting seismogram for a simulation with different source models (explosion vs. horizontal fracture) and source-time functions (Dirac vs. Ricker).