def _compute_mean(self, C, mag, dists, rake):
        """
        Compute and return mean value without site conditions,
        that is equations (1a) and (1b), p.2981-2982.
        """
        mean = (
            C['a1'] +
            self._compute_linear_magnitude_term(C, mag) +
            self._compute_quadratic_magnitude_term(C, mag) +
            self._compute_logarithmic_distance_term(C, mag, dists) +
            self._compute_faulting_style_term(C, rake)
        )

        return mean
Compute and return mean value without site conditions,
        that is equations (1a) and (1b), p.2981-2982.