code: def _get_mean(self, C, rup, dists, sites):
          
          return (self._get_magnitude_scaling_term(C, rup.mag) +
                  self._get_distance_scaling_term(C, dists.rjb, rup.mag) +
                  self._get_site_amplification_term(C, sites.vs30))
docs: Returns the mean value of ground motion - noting that in this case
          the style-of-faulting term is neglected