In September 2023 the global network of seismic recorders detected a sequence of low-strength earth movements. It resembled the reverberation of a church bell albeit one that lasted for 9 days. rising and falling in strength every 90 seconds. For months this strange event on seismograms baffled geophysicists. All they could tell was that the signals did not show signs of having been generated by earthquakes; they were too regular. It was, however, possible to triangulate the position of the source of each individual event. There turned out to be only a single location for the seismic ‘campanology’ – at about 73° N on the eastern coast of Greenland, in Dickson Fjord and isolated branch of the enormous Kong Oscar Fjord system. Greenland is not noted for volcanic activity, ruling out the rumblings of a magma chamber that sometimes presages major eruptions. Whatever the cause, there were no human witnesses at the time. The only real clue lay at the start of the signal: the very long-period (VLP) signal was preceded by a sharp, high energy signal that could be matched with some kind of landslide.
On 16 September 2023 the military base for the famous Sirius Dog Sled Patrol on Ella Island was smashed by a tsunami – fortunately it had been closed for the coming winter. When the Danish Navy patrolled Dickson Fjord some days later they found clear signs that the shores opposite the site of a recent colossal rock and ice slide (see images) had been scoured to a height of 200 m. For 5 km either side shoreline scouring averaged 60 m. The initial tsunami was gigantic, yet the fjord was able to contain its worst effects because the outlet to the rest of the system was at right angles to its trend. Some energy obviously was released to reach Ella Island near the mouth of the system to destroy the Danish Army post. The bizarre seismic signal was probably a result of the displaced water sloshing around in the fjord to dissipate the enormous energy released by the collapse of a mountain peak and a substantial amount of a valley glacier. Such behaviour is known as a seiche. Topographic analysis of Dickson Fjord enabled the researchers to calculate its resonant frequency: at 11 millihertz it matched that of the fluctuating seismic signal. (Svennevig, K. and 67 others 2024. A rockslide-generated tsunami in a Greenland fjord rang Earth for 9 days. Science, v. 385, p. 1196-1205; DOI: 10.1126/science.adm9247).
Valley glaciers in Greenland bolster their rocky flanks against collapse. With climatic warming being much faster there than for the rest of the world, its almost innumerable valley glaciers are shrinking. Yet they have been eroding the crust for tens of thousand years. The fjords that they occupied at the height of the last glacial maximum have very steep sides. Likewise, the remaining glaciers have carved U-shaped valleys. So when the glaciers retreat their exposed flanks become gravitationally unstable. Despite the fact that much of Greenland is underpinned by very hard crystalline rocks, that presents a major hazard for water craft. East Greenland’s spectacular scenery draws many tourist cruisers and Innuit fishing boats each summer. Moreover, removal of the ice load allows elastic strain that had built up in the upper crust to be released along joint systems that further weaken resistance to collapse.
A great deal of publicity has been given to the rapid melting of the huge ice sheet that covers most of Greenland. That is currently the biggest contributor to sea-level rise: a few millimetres per year. The Dickson Fjord event highlights the potential deadly threat of deglaciation, although the extremely complex nature of most of its fjord systems may prevent regional tsunamis from escaping their damping effect. Bu there are increasing dangers from the largest, more open fjords, such as Scoresby Sund, which conceivably might blurt catastrophic tsunamis towards Iceland, Svalbard and the west coast of Norway. Even small ones could wreak havoc on wildlife, such as seal and walrus nurseries.
See also: Carrillo-Ponce, A. et al. 2024. The 16 September 2023 Greenland Megatsunami: Analysis and Modeling of the Source and a Week‐Long, Monochromatic Seismic Signal. The Seismic Record, v. 4, p. 172-183; DOI: 10.1785/0320240013; Le Page, M. 2024. Greenland landslide caused freak wave that shook Earth for nine days. New Scientist 12 September 2024
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