code: def _get_phi(self, imt, mag):
          
          phi = get_phi_ss(imt, mag, self.PHI_SS)
          if self.ergodic:
              C = self.PHI_S2SS[imt]
              phi = np.sqrt(phi ** 2. + C["phi_s2ss"] ** 2.)
          return phi
docs: Returns the within-event standard deviation (phi)