code: def _compute_mean(self, C, S, mag, rrup, rvol, hypo_depth, CN, CR, f4HW,
                        delta_C, delta_D):
          
  
          # Stage 1: compute PGA_ABCD and PGA'_ABCD which are then used in
          # equation 6
          # Equation 1 PGA unprimed version
          lnSA_AB = self._compute_mean_on_rock(C, mag, rrup, rvol, hypo_depth,
                                               CN, CR, f4HW)
  
          # Equation 4 PGA unprimed version
          lnSA_ABCD = lnSA_AB + S *\
              self._compute_nonlinear_soil_term(C, lnSA_AB, delta_C, delta_D)
  
          return lnSA_ABCD
docs: Compute mean value on site class A,B,C,D (equation 4)
          returns lnSA_ABCD