def _compute_standard_dev(self, rup, imt, C):
        """
        Compute the the standard deviation in terms of magnitude
        described on p. 1866, eq. 6
        """
        sigma_mean = 0.
        if rup.mag <= 7.0:
            sigma_mean = (C['c12'] * rup.mag) + C['c13']
        elif rup.mag > 7.0:
            sigma_mean = (-0.00695 * rup.mag) + C['c14']
        return sigma_mean
Compute the the standard deviation in terms of magnitude
        described on p. 1866, eq. 6