Climate change is creating new invisible extremes. These extremes are changing rapidly in remote places like the arctic, where we can hardly determine them with physical sensors. This research demonstrates the way can be used to utilize the next generation of satellite earth observation technologies in the study of these processes. Thomas Monahan, Engineering Science, University of Oxford.
A slender fjord had a momentary planetary effect in East Greenland: a body of water which was so exact a timekeeper that it was noticed vibrating by seismometers in other parts of the world over a period of over a week. The signature was observed as a constant beat-after 92 seconds-recurring in nine days, when the first shock had settled in the water.
It started with the chain reaction way above Dickson Fjord, when rock and glacier ice loosened up on the steep side of Hvide Stovhorn. This was the primary slide, which had 25 million cubic meters of material, and it plummeted downslope at a speed quick enough enough to hit the fjord at about 47 m/s. The preconditioning glacier melting and loss of ice support provided, the collapse provided one, violently delivered transfer of momentum into the confined water, exactly the environment in which a displacement wave can become tall before it can dissipate and spread.
Geometry amplified a good deal. The almost parallel walls and deep cross-section of Dickson Fjord reduced the impact to a mega-tsunami with a local run-up close to 200 meters. Later on, near Ella Island, which is approximately 72 kilometers downstream of the source, the wave still reached heights of approximately 4 meters destroying research installations and washing away traces of vulnerable shorelines. The more peculiar effect was, however, not the height of the initial wave; it was the effect when energy was no longer able to find an easy escape. The movement did not spread out to open sea, but instead rearranged itself into a standing wave, a seiche, to and fro on both sides of the sides of the fjord. The September 2023 event modeling showed a stabilized oscillation with an amplitude of about 7.4 meters with a resonant period of just under 90-92 seconds. The weight of the water, on each swinging, pushed the fjord floor to the side, and the estimated cyclic force was 5 x 1011 newtons, or sufficient to send out a pure, low-pitched seismic signal.
That was unique: a monochromatic very-long-period signal centred around 10.88 mHz, coherent with a very slow decay of nine days. Directional information also had the waves types. The Rayleigh-wave energy was observed to be strongest perpendicular to the fjord, with the Love-wave energy being alignment to its axis, as would be expected of a side-to-side “piston” forcing source and not a typical earthquake source.
The hard part has been to confirm the water movement in such a fjord in the remote arctic. Traditional satellite altimeters take measurements of the sea surface height as the ground track is very narrow, and it usually overlooks the extremes that occur in the short run and tells very little about cross-channel structure. That was altered with wide-swath mapping. Ka-band Radar Interferometer KA-RIN device on SWOT measures two-dimensional snapshots of water-surface in a wide field of view, extending to coastlines and fjords. Following the Greenland events, cross channel slopes of up to two meters of height disparity in passes were observed tilting on one side in one pass then the other way in the next – consistent with a basin-wide slosh and not with tides or wind-driven setup.
Such satellite “snapshots” could not, alone, tell at what point in the cycle of oscillation the fjord was at each overpass. The mapped slopes were thus related by researchers to the filtered ground motion in distant seismic stations and through travel time and phase differences, related water tilt to crustal motion. Further deposition separated seiche signal and confounding factors by comparing tidal structure and wind conditions across the fjord, and reduced the remaining explanation to the standing wave itself.
The case of Greenland is no exception, and it lies in an increasing list of slope failures on the periphery of glaciers, which create the most extreme local waves. In 2015, a slope failure at the end of the Tyndall Glacier in the Taiga Bay at Alaska generated a maximum runup of 193 meters in a tsunami and was sensed hours later by automated seismological methods that have since been verified by remote sensing. These benchmarks are important since they relate the mechanics which are ice retreat, slope deformation, and depth of water, and shape of basin to observables which can be measured at a distance.
The lesson that is longest lasting in Greenland is instrumental. Only the combination of global seismology and wide-swath altimetry made a fjord seiche legible, making a distant hazard a factual system. The same coupling can provide an effective channel into future observation of steep, glacier-cut coasts with sparse direct sensors and with the most significant movement persisting long after the initial wave has arrived on the shore.
