code: def _compute_base_term(self, C, rup, dists):
          
          c1 = self.CONSTS['c1']
          R = np.sqrt(dists.rrup ** 2 + self.CONSTS['c4'] ** 2)
  
          base_term = (C['a1'] +
                       C['a8'] * ((8.5 - rup.mag) ** 2) +
                       (C['a2'] + self.CONSTS['a3'] * (rup.mag - c1)) *
                       np.log(R))
  
          if rup.mag <= c1:
              return base_term + self.CONSTS['a4'] * (rup.mag - c1)
          else:
              return base_term + self.CONSTS['a5'] * (rup.mag - c1)
docs: Compute and return base model term, that is the first term in equation
          1, page 74. The calculation of this term is explained in paragraph
          'Base Model', page 75.