Based on BA18 fourier empirical model.
Linear is only based on vs30 and constants
Non Linear is dependant on vs30, Vref and PGA (EASref)
PGA is taken from the HF waveform rather than using the EAS relationship.
Uses the same application function as Campbell and Borzogina (CB14) and also uses the same bandpass function. As with prior implementation we do not do site amplification for LF.
The interpolation function (to fourier frequencies) from CB14 produced overestimated interpolations so this has been updated to a linear numpy interpolation (interp) function.
Transfer functions for a few station as below.
