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On 16 September 2023, a cascade of events occurred in East Greenland, involving a large tsunami that hit a military unit. Here, we use seismic waveform data recorded on regional to global scales and compare to high-resolution satellite images to learn about the cascade of events. We find two distinct seismic signals and develop a conceptual and physical model explaining the observations: initially, the high-energy seismic signals (0.02–0.06 Hz) occurred, followed by an over one-week-long monochromatic signal (0.0109 Hz) recorded even at 5000 km distance. Our single force (SF) inversions characterize both an initial rockslide and the one-week-long seiche oscillation processes. The rockslide signal is well reproduced by west and downward SF, with an orientation consistent with observations on satellite imagery. The amplitude decay of the week-long oscillation, stacked at three teleseismic arrays, is fitted with a damped oscillator model. Using a simple analytical model of water seiching in a narrow fjord, we can explain the force direction and frequency consistent with the results from SF inversion.