Earthshaking waves from a landslide observed by Swot in a fjord

A 9-day-long, global seismic signal was detected in September 2023. The signal reappeared again in October 2023. Scientific investigations traced the signals to the same fjord in East Greenland and noted that they began after two landslides and consequent tsunamis. These investigations suggested that the signal was produced by a standing-wave sloshing back in forth in the fjord, but no direct observation was made of the phenomenon. Fascinatingly, despite the wave shaking the entire Earth, the wave was not observed by the Danish military during their expedition into the fjord on September 19th (4 days after the original event). This is a consequence of the long period and the low amplitude. Such standing waves in enclosed bodies of water are called seiches. They are commonly observed in lakes, harbors and bays and can be produced by seismic waves, winds, or atmospheric pressure oscillations. Notably, a seiche persisting in an enclosed basin for this duration has never been observed before.

It happened that Swot overflew the area half a day after the first landslide on September 16th, 2023, and then 1.5 and 4.8 days after. It also observed passed over the October 11th event just half a day later.  The satellite is acquiring data in high-resolution (hydrology) mode over Greenland, including its fjords. Thus, sea surface height variations were possible to retrieve from the measurements over the waters of the fjord, estimated to a maximum of 7.9 m with a cross-fjord slope. Comparisons with tidal estimates derived from the Swot data using a specialized empirical approach, and with in-situ wind strength and direction measurements ruled out those as the origin of the Swot observation.

Using such observations in seiche modelling can help in better understanding these phenomena and forecasting their propagation in extreme cases such as this one. The possibility of retrieving sea surface height in a fjord from Swot data is also an interesting perspective, especially since these locations are often where the effects of climate change are felt the strongest due to increasing ice-melt or landslides, and in-situ measurements are extremely sparse.