def _compute_distance(self, rup, dists, C):
        """
        Compute the distance function, equation (9):
        """
        mref = 3.6
        rref = 1.0
        rval = np.sqrt(dists.rhypo ** 2 + C['h'] ** 2)
        return (C['c1'] + C['c2'] * (rup.mag - mref)) *\
            np.log10(rval / rref) + C['c3'] * (rval - rref)
Compute the distance function, equation (9):