What kind of Earth process can tap out the same beat, every 90 seconds, for more than a week? At the end of 2023, sensitive devices worldwide started detecting a strange vibration: a single, unbroken note, which lasted nine days. This signal, as opposed to an earthquake record, busy with a great many frequencies, resembled more a metronome, coming and going and coming. The cause was deep up in remote Greenland, in the Dickson Fjord, where the water was shaken violently by the sudden fall of rock and ice in a manner never observed before by seismologists on a global level.
Detectives finally led the hum to a cascade of systems coupled: to a displaced mountain, to a side fjord and to a standing wave, which refused to perish speedily. Studies associated the onset of the signal with the collapse of a mountain peak into the Dickson Fjord that caused a huge wave to traverse the narrow channel. In that geometry, which is long and narrow and bent, the water did not simply run out and found rest. It started to rise and fall in seiche, throwing a great bulk of water here and there. The impulse of each swing was a push to the adjacent rock, and transmitted a constant very-long-periodic vibration into the crust which was sensed by instruments as far south as the Arctic and as far north as the Antarctic.
Engineers familiar with the work around enclosed basins, harbors, and reservoirs, understand seiches as the reaction of the wind or pressure variations to the establishment of oscillations which continue beyond the original perturbation. Dickson Fjord introduced two peculiar ingredients into it, the magnitude of the initial push, and the confinement of the basin. One reconstruction estimated that some 25 million m3 of rock and ice fell into the fjord, sending a surge that was reported to be as high as 200 meters in the immediate area, and then recycling the standing wave a number of times over. The height was as nothing to the persistence. The strict periodicity of the vibration, which corresponded to the natural slosh period of the fjord, ensured that what had taken place was not so much a shock as a continuing load on the planet, repetitive and repetitive, until the motion gradually dissipated.
It was difficult to prove that the water in fact moved as the seismic data was suggesting.
Conventional satellite altimeters tend to overlook short or spatially complicated features in fjords since they are infrequently sampled and tend to be sampling along one line only under the spacecraft. The consequence of that restriction was that the early interpretations were stagnant: the seiche solution was consistent with the seismic “tone” but the actual measurement of the water surface within the fjord could not be made. The turning point was as a result of the Surface Water Ocean Topography (SWOT) satellite whose Ka-band radar interferometry is able to solve two-dimensional water-height patterns in a swath. With such observations, Thomas Monahan and others were able to map the fjord surface over repeated overpasses and discovered that the difference in wave heights on the surface was as much as 2 metres -which is in keeping with a slowly fading standing wave, not a wind-driven chop or a fresh landslide. According to the seismic attribution, and systematic elimination of other dynamic processes, the observed variability in the SWOT data is attributed to a slowly decaying seiche by the team, and “SWOT is a game changer for studying oceanic processes in regions such as fjords which previous satellites struggled to see into.”
The Greenland environment is not an incident. The ice sheets seasonal cycle has long-term records indicating that summer defeats prevail in the ice sheet with up to 61 percent to 96 percent of the yearly ice loss in summer, which affects the glacier depth and integrity along the shores. At Dickson Fjord, the scientists associated the collapse to base of slope glacial thinning, which is a destabilization pathway involving warming, ice retreat and rock failure.
The greater engineering lesson is on observability. Even a relatively small, localized slope event could cause a global geophysical signal, but only a combination of higher density on-ground sensors and the next generation satellite mapping would completely determine the mechanism. In narrow waterways (and those where the cryosphere undergoes significant change): the interface between ocean processes and solid-Earth signals is no longer just academic; it is quantifiable, and it can take days.
