Mega tsunamis in Greenland fjord confirmed as source of nine-day global seismic signal

Seismic sensors around the world picked up a persistent, ultra-low frequency vibration at 10.88 millihertz in September 2023 that lasted for 9 days and came back a month later. These very-long-period (VLP) seismic signals were eventually traced back to a remote fjord in East Greenland, where two massive landslides had caused tsunamis.

The real source of the vibration was confirmed to be a seiche bouncing back and forth inside the fjord, an inference based on satellite observations and seismic correlation.

First direct evidence of a fjord seiche

For the first time, scientists have obtained direct evidence of a fjord seiche using satellite observations from NASA’s Surface Water and Ocean Topography (SWOT) mission. The research team from the University of Oxford combined satellite data, seismic records, and Bayesian machine learning to verify a natural event that had previously only been suggested by models or indirect evidence.

A seiche is a standing wave that can form in enclosed or semi-enclosed basins like lakes, harbors, or fjords.

Seiches are usually short-lived and localized, but the one in this case study persisted for more than nine days and produced a seismic signal strong enough to be detected worldwide.

Seiche characterization via SWOT and seismic correlation

Following the landslide on September 16, 2023, the SWOT satellite made several passes over Dickson Fjord, including one about 12 hours after the event. Its Ka-band Radar Interferometer (KaRIn) captured detailed measurements of the water’s surface, revealing gentle but consistent tilts characteristic of a standing wave.

The research team used a Bayesian regression model to estimate the maximum cross-channel slope at approximately 1.83 ± 0.59 m per km (9.68 to 3.11 feet per mile), an independent estimate that aligned with earlier analytical and numerical predictions.

Researchers matched the SWOT satellite data with filtered seismic signals from the International Institute seismic station at Alert, Canada (II.ALE), which sits about 1300 km (808 miles) away from the fjord. The timing and size of the vertical ground movement in the seismic data lined up with the surface slopes captured by SWOT. This allowed them to estimate the initial amplitude of the seiche at approximately 7.9 m (26 feet), inferred from a combination of satellite and seismic data.

A few weeks later, in October, a second landslide hit the same area, though this one was smaller and produced a weaker signal. SWOT passed over the fjord again within approximately half a day, giving the team another chance to measure the wave. This time, with more accurate timing, they were able to pin down the maximum cross-channel slope more precisely at about 1.37 ± 0.13 m per km (7.25 – 0.69 feet per mile).

The result from the second time around lined up well with their earlier estimate and helped further solidify the initial findings.

To make sure the wave wasn’t just the result of tides or wind, the team dug into tidal models and checked data from a local weather station, but the patterns didn’t match. Tides and wind-driven currents like Ekman transport were ruled out based on tidal modeling and local weather data, which didn’t match the observed cross-channel slopes. The spatial pattern and timing of the water’s motion lined up too well with the concept of a seiche for it to be anything else.

Expanding the role of satellite altimetry

Another great finding of this research was how satellite altimetry, which is traditionally used for slow-moving ocean trends, can now resolve fast, local events in coastal environments.

“This study is an example of how the next generation of satellite data can resolve phenomena that has remained a mystery in the past. We will be able to get new insights into ocean extremes such as tsunamis, storm surges, and freak waves. However, to get the most out of these data we will need to innovate and use both machine learning and our knowledge of ocean physics to interpret our new results,” commented Professor Thomas Adcock, one of the study’s co-authors from the University of Oxford’s Department of Engineering Science.

References:

¹ Observations of the seiche that shook the world – Thomas Monahan, Tianning Tang, Stephen Roberts and Thomas A. A. Adcock – Nature Communications. – June 3, 2025 – DOI https://www.nature.com/articles/s41467-025-59851-7 – OPEN ACCESS

² First direct observation of the trapped waves that shook the world – University of Oxford – June 3, 2025