code: def _compute_mean_on_rock(self, C, mag, rrup, F, HW):
          
          f1 = self._compute_f1(C, mag, rrup)
          f3 = self._compute_f3(C, mag)
          f4 = self._compute_f4(C, mag, rrup)
  
          return f1 + F * f3 + HW * f4
docs: Compute mean value on rock (that is eq.1, page 105 with S = 0)