def _compute_mean(self, C, mag, rrup):
        """
        Compute mean value according to equation 30, page 1021.
        """
        mean = (C['c1'] +
                self._compute_term1(C, mag) +
                self._compute_term2(C, mag, rrup) +
                self._compute_term3(C, rrup))
        return mean
Compute mean value according to equation 30, page 1021.