"The Earth Rang Like a Bell" - Nine Days in a Row
An unexplained hum puzzled seismologists around the world – who joined forces to solve the mystery.
Climate change was the triggering factor, according to the study.
A year ago, a seismic wave was recorded that lasted nine days. The signal, a vibration in the earth, was unusual in two ways: that it lasted so long and that it stayed at the same frequency, like a long monotonous hum.
Seismologists around the world were puzzled.
– It was a seismic signal we had never seen before. It was very exciting and gave rise to all sorts of wild suggestions about what it could be,’ says Björn Lund, a seismologist at Uppsala University.
Tsunami
Suspicion soon focused on a tsunami in a Greenland fjord on 16 September, around the time the mysterious signal was detected.
This prompted 68 scientists from institutions in 15 countries to collaborate to get to the bottom of the mystery using a variety of data, field measurements, satellite images and simulations. And now the mystery is solved, according to the study published in the journal Science.
A mountain peak collapsed into the Dickson Fjord in eastern Greenland, triggering a huge tsunami. The wave surged back and forth in the narrow fjord, giving rise to the vibrations that were recorded by gauges around the world, according to the researchers.
– A standing wave has formed in the fjord that has been beating back and forth for nine days and has not slowed down much in the meantime, says Lund, who participated in the study.
– It made the earth ring like a church bell for nine days, in a single tone.
Climate effect
The triggering factor was most likely climate change, the researchers say – because the melting glacier mass no longer helps stabilise the mountainside, which eventually collapsed into the sea.
– We have seen seismic waves in the past that could last for hours, but not for this long. Global warming may also cause phenomena that we have never seen before, says Lund.
No people were in the vicinity, but there is a cruise ship route at the mouth of the fjord. The researchers say that more monitoring may be needed in places previously thought to be relatively safe.
– This is the first time we’ve seen a major tsunami in East Greenland, says Mr Lund. It shows the dangers of global warming in ice-covered areas, that it not only causes the ice to calve and disappear but also destabilises the mountainsides. We are likely to see more landslides and tsunamis in the fjords of Greenland