def _compute_site_scaling(self, vs30, mean):
        """
        Scales the ground motions by increasing 40 % on NEHRP class D/E sites,
        and decreasing by 40 % on NEHRP class A/B sites
        """
        site_factor = np.ones(len(vs30), dtype=float)
        idx = vs30 <= 360.
        site_factor[idx] = 1.4
        idx = vs30 > 760.0
        site_factor[idx] = 0.6
        return np.log(np.exp(mean) * site_factor)