Arctic can seem to a distance still ice, rock and water in the familiar mould. The East Greenland Dickson Fjord was an example of how rapidly this stability may reverse when a mountain slope, a thinning glacier and a narrow basin align in an incorrect manner.
A huge rock and ice avalanche in September 2023 caused a mega-tsunami to go through the fjord and then harnessed the energy in a long-run slosh. What this produced was more than a great wave in an extreme inlet, it was a clean, regular seismic pulse that could be traced by an instrument all the way around the world over the course of days.
1. A mountainside failure big enough to redraw the shoreline
A trigger was an avalanche of rock-ice, falling down some 1,200 m above the fjord. Approximately 25 million cubic meters of metamorphic rock and glacier ice peeled off and fell down a deep gully, scraping more ice on their path. That trailing ice decreased friction and enabled the mass to retain its speed till impact. On-site observations and satellite images showed a new breakage in the mountain and new debris patterns at the waterfall, linking the collapse to a particular slope of the source and not a remote cause in the offshore disturbance.
2. A fjord built to amplify a wave
The impact energy could not be distributed as Dickson Fjord could not spread it out in geometry. On one side the water was enclosed in a deep basin (up to 540 meters where the walls were nearly parallel) and forced upward and outwards by the first wave, like a piston chamber. The run up of the waves was approximately 200 meters (650 feet) near the impact zone. The wave reached all the way down the fjord system with force enough to cause property damage in Ella Island, approximately 72 kilometers distant. The same effect can give much greater local extremes than in an open coast, in narrow waterways such as this.
3. The mega-tsunami that refused to end: seiche formation
Since the initial violent waves the fjord did not calm down into normal rippling decay. The water body instead formed a regular beat of seiche, a standing wave-like oscillation formed by masses of water sloshing a wall to wall. It was modeled at a resonant period of approximately 92 seconds and the oscillations lasted much longer than a normal tsunami pulse. Physically, every swing moved the mass of water in the fjords back and forth filling the seafloor and fjord sidewalls. That is why the event was significant not only on the fjord: the ocean surface was a repeatable forcing mechanism on the solid Earth.
4. A planet-scale “tone” in seismic data
The seismometers around the world registered an unusually clean, monochromatic very-long-period signal with the centre frequency of 10.88 mHz equal to the 92-second rhythm of the seiche. This trace, in contrast to geodetic waveforms, which are complex and broadband, had smooth repeated peaks that gradually diminished over about nine days. As analyses stated, there was a combination of the Rayleigh and the Love waves and the behavior of the decay was not similar to the rapid and exponential decay commonly observed in many natural seismic sources as this was according to the direction of the fjord. The resultant effect was a global signature of a process that started with a localized water impact.
5. SWOT satellite altimetry turned a hidden slosh into a measurable slope
Sensitivity of fjord ice Rapid oscillations within a distant ice-influenced fjord can be hard to measure directly with traditional sensors alone. Another type of evidence, offered by the SWOT pixel-cloud dataset (SWOT_L2 HR PIXC 2.0) consisted of snapshots of sea-surface height over a broad swath as opposed to one narrow altimeter track. Scientists also subjected output repeat passes of the Science phase orbit of the mission to standard geophysical corrections, removing spurious heights and other anomalies as well as taking into consideration factors like tidal correction limits and sea-ice contamination. The critical point to note was the cross-channel slope, which was water being detected to be significantly greater on one side of the fjord than the other, which was in line with a continuing seiche but not a single wave.
6. Supercomputer modeling stitched rock, water, and crust into one storyline
No single tool was able to record the full range of the slope collapse to world shakes, thus blends of methods were used. Granular-flow and wave models replicated how the avalanche penetrated the fjord, how the first tsunami was generated, and how energy was confined as a standing oscillation. Bathymetry Fine-grid bathymetry and bottom-friction assumptions regulated the rate at which the system dissipated energy, and contributed to the persistence of the event over more open basins. According to Alice Gabriel of the Scripps Institution of Oceanography at UC San Diego, it proved to be a huge challenge to do an accurate computer simulation of such a long lasting, sloshing tsunami.
7. Glacier “debuttressing” connected climate-driven thinning to slope failure
The ice support formed terrain on which the collapse had taken place. Since glaciers melt away and recede, they are able to strip the stabilizing pressure on the base of steep rock walls a process termed as debuttressing. The situation of Dickson Fjord was also useful in that the ability to thin over a long-period of time assisted to establish an unstable slope on top of the fjord so that a huge fall was more probable once the conditions had surpassed a limit. The climate change is alleviating what is common on the earth and can cause the uncusual occurrences into motion, said Gabriel. The larger engineering inference is not a monolithic dramatic wave, but a progressive array of steep recently-deglaciated slope, which stands above constrained waters in which effects are effectively converted into extreme waves.
8. The seismic archive became a new hazard sensor for remote fjords
As soon as scientists understood the fingerprint of a long period seiche, particularly a continuing rhythm of 92 seconds, they obtained a new search tool. The strategy transforms world-wide seismic systems to a passive monitoring layer of fjords that are difficult to instrument, and hopes to supplement satellites and small in-situ gauges. These similar identifications can also underlie expedient observational ideas that combine the real-time seismic observation with water-level measurements and satellite purposes and enhance the situational awareness of shipping routes around narrow, landslide-sensitive inlets.
The lesson of Dickson Fjord is mechanical just as much as it is geological: As long as the energy of the impact is contained in the shape of the basin, water can continue to act on the rock even after the initial wave has passed. The enduring contribution of the event could also be methodological demonstrating how satellites, seismology and high resolution modeling can recreate rare extremes in locations where conventional sensors cannot easily be sustained.
