def _compute_mean_on_rock(self, C, mag, rrup, rvol, hypo_depth, CN, CR,
                              f4HW):
        """
        Compute mean value on site class A/B (equation 2 on page 22)
        """

        # Define subduction flag (page 23)
        # SI=1 for subduction interface, 0 otherwise
        # DS=1 for subduction intraslab, 0 otherwise
        SI = 0
        DS = 1

        lnSA_AB = (
            # line 1 and 2 of equation 2
            C['c11'] + (C['c12y'] + (C['c15'] - C['c17']) * C['c19y']) *
            (mag - 6) +
            # line 3
            C['c13y'] * (10 - mag) ** 3 +
            # line 4
            C['c17'] * np.log(rrup + C['c18y'] * np.exp(C['c19y'] * mag)) +
            # line 5
            C['c20'] * hypo_depth + C['c24'] * SI +
            # line 6
            C['c46'] * rvol * (1 - DS)
        )

        return lnSA_AB
Compute mean value on site class A/B (equation 2 on page 22)