def _compute_distance(self, rup, dists, C):
        """
        equation 3 pag 1960:

        ``c31 * logR + c32 * (R-Rref)``
        """
        rref = 1.0
        c31 = -1.7
        return (c31 * np.log10(dists.rhypo) + C['c32'] * (dists.rhypo - rref))