A single slope failure in a remote Greenland fjord created the strangely extended wave: a massive wave, a trapped wave, which would not dissipate, a worldwide seismic beat that was not like the normal earth tremor but like a metronome, slow and steady.
It is not only that the water was highest at its peak but that the lesson that impressed me the most as an engineer was how a small basin is able to contain and discharged energy making the local geography an international signal registered by instruments all over the world.
1. A mountain collapse big enough to redraw the shoreline
The cause of the trigger was identified by investigators as a rock and ice fall off high terrain above Dickson Fjord in north-eastern Greenland. Approximately 25 million cubic metres of material shedding material and rushing downslope, the mass of material progressing gathering more glacier ice as it squeezed through a steep gully. Mixed ice has the potential to reduce friction, as well as to keep a slide fast, which explains the choice of the impact speed being tens of metres per second in modelling and reconstructions. Later satellite images revealed a new scar on the mountainside that was indicative of a massive block slide and a runout/out flow that deposited the debris into restricted water.
2. A “mega-tsunami” shaped by a fjord’s natural walls
As the avalanche hit the fjord, displaced water rose up and out in the local run-up of approximately 200 metres around the impact zone and reported wave heights as high as 110 metres later on in the channel as the disturbance travelled through it. The steep sides of Dickson Fjord and its comparatively small corpus is important: the steep sides diminish lateral propagation and can force energy into a narrow area, which caused the first wave to be significantly bigger than the normal coastal tsunami heights. The effects were still felt miles away and one of the damaging waves was felt on infrastructure on the Ella Island, tens of kilometres away.
3. The hidden main act: a seiche that kept sloshing for days
The first dramatic wave did not accommodate the story. The energy was still held up in the fjord as a standing wave a seiche swilling water to and fro across the conflicting shoreline. An oscillation period of about 9092 seconds was described as modelling, and the amplitude of the oscillation was estimated in different research groups since the exact shape of the basin and the losses due to friction are difficult to determine in such a distant position. The important thing was perseverance: the movement did not cool off within hours, but lasted over a week, acting like a gigantic horizontal piston filling the fjord bottom with each stroke.
4. A planet-wide “bell” tone at very long period
Seismic stations in the world felt a very clean signal repeating at approximately 10.88 mHz, the same frequency as the slush of the fjord. The researchers split two parts in the data a short and energetic burst of the first slide and a very-long-period (VLP) tone that was much longer lasting and was related to the sustained water oscillation. Angela Carrillo-Ponce of German Research Centre of Geoscience described it as follows: The fact that the signal of a rockslide induced sloshing wave in a distant part of Greenland could be felt globally and more than a week is a thrilling fact. The strength of the signal, and its radiation pattern, assisted in connecting the ringing with the geometry of the fjord, instead of the rupture of tectonic faults.
5. Why older satellites struggled and what changed with SWOT
Previously radar altimeters treated oceans as an extremely thin layer under a spacecraft and tended to overlook important spatial structure within the convoluted coastal basins. The said limitation was significant in this case as a seiche is not merely a crest but it is a tilting water surface along a channel that varies with time. Surface Water and Ocean Topography (SWOT) mission was another one which introduced a different method swath mapping with enough width to solve the cross-fjord height variation. SWOT analyses of Dickson Fjord registered water standing with a noticeable elevation on one side compared to the other a observation that is in harmony with an oscillation which is gradually fading and not with an instantaneous rush or burst of pressure.
6. The modelling toolchain that turned a mystery pulse into a mechanism
As it was rebuilt, it needed the cooperation of several models: granular-flow simulations of the rock-ice avalanche, wave models of dispersive propagation and reflection in confined water, and shallow-water treatments of run-up and inundation. Fine bathymetry grids, bottom-friction sensitivity tests, and comparisons to seismic radiation patterns were used to reduce the range of possible fjord motions that might potentially cause the observed global VLP signal. One difficulty was duration: to maintain an oscillation with a coherent amplitude it is necessary to understand that small losses are a realistic effect in models, since small errors grow exponentially with numbers of oscillations.
7. Glacier “debuttressing” as an engineering red flag for steep coastal slopes
Long-term destabilisation processes that are in line with warming in the Arctic, such as loss of supportive glacier ice at the base of slopes, commonly referred to as glacier debuttressing, and the overall weakening process of thawing permafrost, were described by researchers.
The Dickson Fjord example was another case that was following a growing trend: once the ice support reduces, the marginally stable slopes may enter failures. The threat is not exclusive to Greenland; steep-walled and high relief fjords as well as lakes of the same kind are present across high latitudes, and infrastructure, research facilities and sea routes intertwine with changing cryosphere conditions.
8. A new early-warning concept: pair seismometers with water-surface mapping
The useful lesson that can be learnt is the usefulness of integrated monitoring in ground that is difficult to instrument. Seismic networks have the capability to record the unique long period requirements within a short period of time, even without any human being who can observe a wave. High-resolution satellite altimetry can be useful in proving that this is not an event of tectonic but water oscillation. Real time seismic feeds, along with specific satellite monitoring and in-fjord gauges may assist in fast hazard evaluation of vessels and coastal structures in narrow basins where trapped waves may linger long after the initial surge.
The episode by Dickson Fjord brought out the fact that only a single collapse will bind rock, ice, water, and the solid Earth together as one system whereby the Signal that the whole world could see was not the one at the peak but rather the one that followed the wave. Having a greater understanding of the mechanism, scientists can now seek seismic archives with other slow, monochromatic pulses that could indicate previously unknown fjord scale oscillations of other distant areas.
