An Extraordinary Tsunamigenic Rockslide Into a Greenland Fjord Rang the Earth for 9 Days

Climate change is preconditioning large landslides in polar regions through several mechanisms including hillslope debruttressing from glacial retreat, permafrost degradation, and changes in precipitation. Tsunamigenic landslides have occurred recently in Greenland (Kalaallit Nunaat), but there have been no previously documented occurrences on the east coast of the island. On 16 September 2023, we observed an unprecedented monochromatic very-long period (VLP) seismic signal at 10.88 mHz that was recorded on some seismic stations for 9 days following the event and was found to originate from East Greenland. We demonstrate how this event started with a 25 M m3 glacial thinning-induced rockslide impacting within Dickson Fjord which triggered a 200 m high tsunami that caused extensive damage to the Danish research base at Ella Island, ~65 km down fjord. Simulations show the tsunami stabilized into a 5 m high transverse seiche within Dickson Fjord, oscillating at a frequency similar to the VLP surface waves. However, the observed ultra-long seismic duration may reflect an elusive component of the source. Due in part to the event occurring after the summer travel and field season, no fatalities were associated with this event. Analysis of historical seismic data revealed four previously unknown VLP events of similar period; a subsequent event also occurred on 11 October 2023. The four most recent events have occurred since 2016 and, based on satellite altimetry, are correlated with landslides in Dickson Fjord. Ongoing warming will increase the societal risk posed by tsunamigenic landslides in polar regions including Greenland and Alaska.