def _compute_sigma_b(self, C, mag, vs30measured):
        """
        Equation 23, page 81.
        """
        sigma_0 = self._compute_sigma_0(C, mag, vs30measured)
        sigma_amp = self.CONSTS['sigma_amp']

        return np.sqrt(sigma_0 ** 2 - sigma_amp ** 2)
Equation 23, page 81.