code: def _compute_linear_magnitude_term(self, C, mag):
          
          if mag <= self.c1:
              # this is the second term in eq. (1a), p. 2981
              return C['a2'] * (mag - self.c1)
          else:
              # this is the second term in eq. (1b), p. 2982
              return C['a3'] * (mag - self.c1)
docs: Compute and return second term in equations (1a)
          and (1b), pages 2981 and 2982, respectively.