Imma Perfetto
Cosmos science journalist
On September 16, 2023, an unusual seismic signal shook the world. It appeared like clockwork every 90 seconds and lasted for 9 consecutive days.
A month later, a similar signal reappeared and lasted for another week before disappearing.
Scientists have since determined that these seismic anomalies were triggered by 2 massive landslides which ripped through a remote fjord in east Greenland. The rock-ice avalanche generated mega-tsunamis which reached 200m high at points and even damaged an empty military base.
Researchers believed these massive waves triggered a “seiche” (pronounced saish) or standing wave, which sloshed back and forth within the ford beating on either side like a drum to produce the very long signals.
But a Danish military vessel surveying the Dickson fjord just 3 days into the first seismic event never saw such a wave. In fact, until now the only evidence came from analytical and numerical models.
But next generation satellite observations have now confirmed the existence of these standing waves, which elevated and decreased the water level in the fjord by up to 2m.
A study presenting the findings has been published in the journal Nature Communications.
The landslides were caused by thinning of glacial ice within the valley brought on by climate change, which according to the study’s lead author Thomas Monahan, a PhD student in the Department of Engineering Science at the University of Oxford in the UK, is giving rise to new, unseen extremes.
“These extremes are changing the fastest in remote areas, such as the Arctic, where our ability to measure them using physical sensors is limited,” says Monahan.
“This study shows how we can leverage the next generation of satellite Earth observation technologies to study these processes.”
The researchers used data collected by the Surface Water Ocean Topography (SWOT) satellite to make the discovery.
“SWOT is a game changer for studying oceanic processes in regions such as fjords which previous satellites struggled to see into,” says Monahan.
Satellite altimetry measures the height of the Earth’s surface (including the ocean) by recording how long it takes for a radar pulse to travel from a satellite to the surface and back again.
Conventional satellite altimeters aren’t sensitive enough to spot the standing waves. But SWOT’s cutting-edge Ka-band Radar Interferometer instrument allowed it to measure ocean and surface water levels with unprecedented resolution – every 2.5m along a 50km-wide area.
The team then constructed sea surface elevation maps of various time points and revealed cross-channel slopes in which sea surface height increased or decreased by up to 2m. These slopes occurred in opposite directions, showing that water moved backwards and forwards across the channel.
Co-author Dr Thomas Adcock, a professor of engineering science at Oxford, says: “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,” he says.
“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.”
