Calculate the standard deviation as a function of G/G_max.

        Equation 7.29 from Darendeli (2001).

        Parameters
        ----------
        mod_reduc : array_like
            Modulus reduction values.

        Returns
        -------
        std : :class:`numpy.ndarray`
            Standard deviation.

def calc_std_mod_reduc(mod_reduc):
        """Calculate the standard deviation as a function of G/G_max.

        Equation 7.29 from Darendeli (2001).

        Parameters
        ----------
        mod_reduc : array_like
            Modulus reduction values.

        Returns
        -------
        std : :class:`numpy.ndarray`
            Standard deviation.
        """
        mod_reduc = np.asarray(mod_reduc).astype(float)
        std = (np.exp(-4.23) + np.sqrt(0.25 / np.exp(3.62) - (mod_reduc - 0.5)
                                       ** 2 / np.exp(3.62)))
        return std