code: def _compute_distance_term(self, C, rup, rjb):
          
          return (C['c4'] + C['c5'] * rup.mag) * np.log(
              np.sqrt(rjb ** 2. + C['c6'] ** 2.)) + C['c7'] * rjb
docs: This computes the term f2 equation 8 Drouet & Cotton (2015)