Distribution of eigenvalues in the complex plane for m = 0 and E = 10^-5 with a spectral resolution of N_r = 60 and L = 150. The top and bottom rows are for thick and thin shells (η = 0.35 and η = 0.71, respectively), while the left and right columns are for ε = 0.30 and ε = 0.60. In all panels, the vertical blue (resp. red) line depicts the transition between the frequency ranges of D and DT modes (resp. DT and non-existant modes).