code: def _get_site_amplification(self, C_AMP, vs30, pga_rock):
          
          # Get nonlinear term
          bnl = self._get_bnl(C_AMP, vs30)
          #
          f_nl_coeff = np.log(60.0 / 100.0) * np.ones_like(vs30)
          idx = pga_rock > 60.0
          f_nl_coeff[idx] = np.log(pga_rock[idx] / 100.0)
          return np.log(np.exp(
              C_AMP["blin"] * np.log(vs30 / self.CONSTS["Vref"]) +
              bnl * f_nl_coeff))
docs: Gets the site amplification term based on equations 7 and 8 of
          Atkinson & Boore (2006)