code: def _compute_partial_derivative_site_amp(self, C, pga1100, vs30):
          
          delta_amp = np.zeros_like(vs30)
          vlin = C['VLIN']
          c = self.CONSTS['c']
          b = C['b']
          n = self.CONSTS['n']
  
          idx = vs30 < vlin
          delta_amp[idx] = (- b * pga1100[idx] / (pga1100[idx] + c) +
                            b * pga1100[idx] / (pga1100[idx] + c *
                            ((vs30[idx] / vlin) ** n)))
  
          return delta_amp
docs: Partial derivative of site amplification term with respect to
          PGA on rock (equation 26), as described in the errata and not
          in the original paper.