code: def _compute_mean_on_rock(self, C, mag, rrup, rvol, hypo_depth, CN, CR,
                                f4HW):
          
  
          # Define subduction flag (page 23)
          # SI=1 for subduction interface, 0 otherwise
          # DS=1 for subduction intraslab, 0 otherwise
          SI = 0
          DS = 1
  
          lnSA_AB = (
              # line 1 and 2 of equation 2
              C['c11'] + (C['c12y'] + (C['c15'] - C['c17']) * C['c19y']) *
              (mag - 6) +
              # line 3
              C['c13y'] * (10 - mag) ** 3 +
              # line 4
              C['c17'] * np.log(rrup + C['c18y'] * np.exp(C['c19y'] * mag)) +
              # line 5
              C['c20'] * hypo_depth + C['c24'] * SI +
              # line 6
              C['c46'] * rvol * (1 - DS)
          )
  
          return lnSA_AB
docs: Compute mean value on site class A/B (equation 2 on page 22)