For 9 Days, Earth Was Sending Out Mysterious Signals. Now We Know What They Were.
Here's what you'll learn when you read this story:
Strange signals coming from the Arctic in 2023 were assumed to be a seiche (trapped water with waves sloshing back and forth), but this was never confirmed.
Previous instruments used to measure seismic weather phenomena were not able to pick up enough information, but NASA's SWOT satellite eventually found that the signal actually was from a seiche caused by a landslide.
Reconstructions of what the weather was like during the days SWOT picked up the signal also show that it couldn't have been anything but a seiche.
As fascinating as bizarre signals from other planets can be—teaching us about earthquakes on Mars or auroras in the skies of Jupiter—sometimes even weirder signals come from weather extremes happening right here on Earth.
For nine days in 2023, an unknown seismic pulse was generated by the Earth every 90 seconds. It first appeared that September, vanished, and then returned in October. The signals began after a landslide triggered by a megatsunami in Dickson Fjord, Greenland, and was thought to have been produced by a seiche, or standing wave. This wave had probably been stirred up by the tsunami and then trapped by ice in the fjord—but there was no way to prove it.
Satellite observations were able to document avalanches and the tsunamis they caused, and scientists collected further data in a research station. There was just one problem—the hypothesized seiche was eluding detection. It remained a mystery, even though studies at the time found seismic data that seemed to align with the sloshing motions of standing waves. So, researcher Thomas Monahan of Oxford University decided to take a closer look.
Using data from the KaRIn (Ka-band Radar Interferometer) instrument on board NASA's Surface Water Ocean Topography (SWOT) satellite—an international collaboration capable of high-resolution measurements that extended into Dickson fjord—Monahan and his team finally found evidence for a seiche whose waves were slowly losing intensity.
'Based on the seismic attribution, and systematic ruling out of other dynamic phenomena, we conclude that the observed variability in the SWOT data is consistent with that of a slowly decaying seiche,' the team wrote in a study recently published in Nature Communications.
Seiches can occur in lakes and other enclosed (or partially enclosed) bodies of water. The tsunami unleashed in Dickson Fjord had enough strength to leave powerful winds and sudden atmospheric pressure shifts in its wake, pushing water from one end of the enclosure to the other. The water then sloshed back and forth, oscillating for anywhere from hours to days after winds ceased. Tsunamis are often seismic phenomena, and the very long period (VLP) seismic signal that came from the fjord was the aftermath of a tsunamigenic landslide.
Previous attempts at recording evidence for this particular seiche had been thwarted by the limitations of satellite altimeters, which did not pick up data during extended gaps between observations. They were also not able to record the differences in the height of waves beyond the area directly under the satellite.
They were, however, able to get an especially accurate read on the water below. The landslides in Dickson Fjord happened right when SWOT was transitioning to its Science phase, during which it would orbit and survey most of the planet's surface from an altitude of 890 km (553 miles) for 21 days. This orbit was purposely out of sync with the Sun to lower the chances of misidentifying signal frequencies.
The researchers went through the data from every pass the satellite made over the region for the weeks in September and October and used this data to create maps of the fjord, modeling it how would have behaved during different times after the landslide and the height differences between waves (which reached up to two meters, or about 6.5 feet). Reconstructions of weather conditions ruled out all other possible causes behind the signal, and convinced scientists that it could only have been caused by a seiche.
'This study shows how we can leverage the next generation of satellite earth observation technologies to study these processes,' Monahan said in a recent press release. 'SWOT is a game changer for studying oceanic processes in regions such as fjords which previous satellites struggled to see into.'
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