Oddly Enough / Episode 02
The Nine-Day Wave
A strange seismic signal. A rhythm repeating about every 92 seconds, detectable for up to nine days. What could possibly keep the Earth ringing?
September 2023. Instruments around the world pick up a persistent beat. The explanation leads to a remote fjord in Greenland.
Investigate the signal ↓The beat kept coming.
In September 2023, seismometers recorded a signal dominated by one slow rhythm. It remained detectable internationally for up to nine days.
Its persistence was the puzzle. The instruments revealed movement in the ground; they did not immediately reveal what could keep it going.
Researchers combined records from different places with reports and imagery from Greenland. The trail led to Dickson Fjord, on the island’s eastern side.
A mountainside had fallen into the water.
On 16 September 2023, a huge avalanche of rock and ice had plunged into the fjord. The rapid displacement of water generated a destructive tsunami.
That explained a violent opening event. The longer mystery lay in what the water did afterwards.
The water was still rocking.
Think of bathwater moving after a push. One side rises as the other falls, then the positions reverse. A basin can keep a back-and-forth rhythm after the original disturbance has ended.
The fjord’s shape restricted how quickly that motion could escape. The resulting standing oscillation is called a seiche, pronounced “saysh”.
The initial tsunami reached a local run-up of approximately 200 metres: the height water reached on shore. The later seiche was much smaller, modelled at roughly 7 metres.
A 200-metre wave did not continue for nine days. It was the later sloshing that persisted.
A fjord was ringing the Earth.
As the mass of water moved back and forth, it exerted alternating forces on the fjord. Those forces coupled into the ground, producing a signal instruments could detect far away.
The 2024 investigation connected the collapse, the trapped oscillation and the seismic record. The long-lasting rhythm had a mechanism: water losing its motion slowly in a confined place.
A mystery recorded around the world led back to one mountainside, one fjord and the motion left behind.
Svennevig and colleagues, Science (2024); read the publication abstract.
A little further into the story
What do the different sizes measure?
Approximately 25 million cubic metres describes the volume of rock and ice that collapsed. It is not a water height.
The roughly 200-metre figure is the initial tsunami’s local run-up: its highest reach on shore above normal sea level. It is different from a crest travelling through open water.
The approximately 7-metre seiche comes from the study’s reconstruction of the later standing oscillation. These quantities describe different parts of the event, rather than one enormous wave lasting for days.
Did the water model match the 92-second signal exactly?
No. The observed seismic signal had a dominant period of approximately 92 seconds. The 2024 numerical water model gave about 87 seconds: close, but not identical.
The numerical reconstruction helps explain the mechanism. It does not turn model outputs into measurements.
The event is described as a freely decaying seiche, continuing after the initial disturbance. Resonance, in which repeated well-timed pushes reinforce motion, is a related idea; this explanation does not require fresh pushes for nine days.
What did satellites add later?
A 2025 study used SWOT observations of the water’s surface slope to provide independent support for the seiche explanation.
The satellite passes were snapshots, not a continuous nine-day film. They added another kind of evidence to the seismic records and numerical reconstruction.
Monahan and colleagues, Nature Communications (2025); read the paper.
What made the mountainside collapse?
The 2024 investigation linked the slope’s failure to the thinning of a glacier that had supported it.
The event connected changes in ice and rock to water motion and then to the solid Earth. Its explanation drew on several disciplines because the process crossed those boundaries.
Follow the evidence
- Svennevig et al.: A rockslide-generated tsunami in a Greenland fjord rang Earth for 9 daysScience, 12 September 2024. The original multidisciplinary investigation and numerical reconstruction.
- USGS publication record and abstractThe event volume, initial tsunami, modelled seiche and recorded seismic signal in one primary publication summary.
- Monahan et al.: satellite evidence for the seicheNature Communications, 3 June 2025. SWOT surface-slope observations provide independent support.
- NOAA: what is a seiche?An everyday introduction to standing oscillations and their nodes.
- Read the 2025 satellite paperObservations and inference from satellite passes, rather than continuous observation of every oscillation.
All drawings are original schematics. No research image or recorded seismic trace has been reproduced.