code: def _compute_term_3(self, C, rhypo):
          
          d = rhypo
          d[d <= 15.0] = 15.0
  
          return C['a3'] * np.log10(d)
docs: Compute term 3 in equation 2 page 462.
          Distances are clipped at 15 km (as per Ezio Faccioli's personal
          communication.)