def _compute_intra_event_std(self, C, vs30, pga1100, sigma_pga):
        """
        Returns the intra-event standard deviation at the site, as defined in
        equation 15, page 147
        """
        # Get intra-event standard deviation at the base of the site profile
        sig_lnyb = np.sqrt(C['s_lny'] ** 2. - C['s_lnAF'] ** 2.)
        sig_lnab = np.sqrt(sigma_pga ** 2. - C['s_lnAF'] ** 2.)
        # Get linearised relationship between f_site and ln PGA
        alpha = self._compute_intra_event_alpha(C, vs30, pga1100)

        return np.sqrt(
            (sig_lnyb ** 2.) +
            (C['s_lnAF'] ** 2.) +
            ((alpha ** 2.) * (sig_lnab ** 2.)) +
            (2.0 * alpha * C['rho'] * sig_lnyb * sig_lnab))
Returns the intra-event standard deviation at the site, as defined in
        equation 15, page 147