Time evolution of normalized Fourier amplitudes, A_m/A₀, for m = 1 to m = 4 in simulation g106. Vertical dotted lines mark the first three pericentric passages of the last massive gas-rich merger, followed by a rapid sequence of additional close passages and complete disruption by lbt ∼ 6.7 Gyr. The shaded region to the left in each panel highlights the influence of two dry mergers (mass ratio of ∼1:50), with pericenters between lbt ∼ 6.5 and 4.9 Gyr. The right shaded region indicates a more chaotic phase, driven by a sequence of minor interactions (mass ratio ≳1:100). Bursts of multi-mode activity are seen near these events. Radial bins are color-coded as in Fig. 4. The timing of mode amplification closely mirrors SFR peaks, suggesting a causal link between external perturbations and disk structure.