In August 2023, Søren Rysgaard of Aarhus University in Denmark travelled to Dickson Fjord in northeastern Greenland. Fjorded between steep cliffs over a kilometre high, Dixon Fjord forms a complex system of winding fjords along with other fjords, large and small.
图片来源:Stephen Hicks; Kristian Svennevig; Alexis Marbeouf
Rysgaard came here to install a series of monitoring equipment in the mountains and underwater to complement the real-time subglacial marine environmental monitoring network in northeastern Greenland. But just a few weeks after they left, on September 16, a massive landslide broke the calm sea. The equipment installed by Rysgaard happened to be monitoring unusual fluctuations in sea level – a tsunami had occurred here. Immediately afterward, with eastern Greenland as the center, an earthquake signal spread throughout the globe. And for the next 9 days, this seismic wave continued to reverberate under the feet of everyone around the world.
滑坡发生前后的卫星图像（图片来源：Copernicus, Sentinel-2, EO browser）
Mystic seismic waves
Kristian Svennevig 是丹麦和格陵兰地质调查局（GEUS）的研究员，他最初注意到这个信号，是应丹麦北极联合司令部（Greenland and Danish Joint Arctic Command）的要求，调查这场滑坡和海啸事件。
From the site and satellite photos, it is clear what is happening: a mountaintop that rises about 1,200 meters above the fjord has collapsed, triggering a landslide. The falling mountain slid all the way down the glacier, carrying more ice and sediment with it and into the fjord. The volume of the collapsed mountain rock and ice reached 25 million cubic meters, enough to fill 10,000 Olympic-sized swimming pools.
Image source: Original paper
The sudden shock sparked a violent tsunami in Dixon Sound, with the first wave reaching 200 meters high – well above the wave height of the 2004 tsunami in Indonesia and the 2011 tsunami that hit the Fukushima nuclear power plant in Japan. Just 70 kilometres east of the fjord is the monitoring station Ella ø. The waves are still up to 4 metres high, and the water spreads inland 80 metres from the shoreline. Fortunately, there were no people working or living at the station at this time, and the tsunami only washed away a large amount of equipment here, but there were no casualties.
Mountains and glaciers from the fjord before (left) and after (right) before and after landslides and tsunamis, with falling rocks in yellow and tsunami-affected areas in red. (Image source: Søren Rysgaard, Danish Army)
Around the same time, an online community gathered a large group of seismologists from all over the world. What caught their attention was the same mysterious seismic signal: a strange seismic wave was detected by highly sensitive seismic sensors from the Arctic to the South Pole in the wake of the landslide and tsunami in Dixon Sound.
One of the reasons why it's strange is that it's a very simple, single-frequency seismic wave. If you convert earthquake signals into audible sounds, the earthquakes we are familiar with sound sound like an orchestral ensemble – it is made up of many waves of different frequencies together. However, the signal from Greenland is very monotonous, like a steady beep that lasts for about 90 seconds (a frequency that is so slow that humans cannot feel it directly). Another strange thing about it is that this signal maintains a certain strength for up to 9 days. So much so that researchers initially referred to this signal as an "Unidentified Seismic Object" (USO)
Soon, Svennevig's team joined forces with scientists in the online community to form a multidisciplinary team of 68 scientists from 40 institutions in 15 countries. Just one year after the landslide, their findings appeared in the journal Science, revealing how a tsunami in Greenland triggered a global earthquake that lasted nine days.
Lake Whistling Mountain crashes
To connect the tsunami to this long earthquake, the clues are hidden in the earthquake signal. Unlike the body waves that propagate within the Earth (that is, the transverse and longitudinal waves that we are familiar with), the surface waves that propagate on the surface have some special properties. According to the mode and direction of vibration, surface waves can be mainly divided into horizontal vibration Love wave, and Rayleigh wave (which can be regarded as a particle for circumferential rolling). Focusing on East Greenland, the research team mapped surface wave signals detected by seismic stations around the world. It was found that the Loeh-Lev wave with the maximum amplitude was in the southwest to northeast direction, while the Rayleigh wave with the maximum amplitude was in the northwest to southeast direction. This angle is exactly parallel to the long and minor axes of Dixonfjord.
"We therefore speculate that this can only be caused by the presence of a force at 90° to the long axis of Dixonfjord, and that the oscillation of this force back and forth can cause such a surface wave." Paula Koelemeijer of the University of Oxford, United Kingdom, explains. It is the bodies of water in Dixonfjord that do this.
The red color on the left represents the Rayleigh Wave, the green color represents the Loew Wave, and the right picture shows the Dixon Sound (Image source: original paper)
The researchers found that the main oscillation period of seismic waves is about 90 seconds, which coincides with the resonance frequency of the fjord, considering that Dixon Sound is about 3 kilometers wide and 540 meters deep. In other words, what started as a violent tsunami evolved into a "seiche" (also known as a tsunami) along a minor axis in the Dixonfjord. As the body of water in the fjord rocks back and forth every 90 seconds, the kinetic energy of the water is transferred to the crust on both sides of the fjord. "You could say that this lake earthquake was like the heartbeat of Dixon Sound," says Stephen Hicks of University College London in United Kingdom, and the seismic waves then spread across the globe.
In fact, lake earthquakes are a fairly common phenomenon that usually occurs in weather such as strong winds, occurring in lakes or other enclosed or semi-enclosed waters. However, no previous study has found that lake quakes can last up to nine days.
The final piece of the puzzle is filled with numerical simulations. In order to accurately reconstruct the ongoing evolution of the lakequake, the research team built a very detailed model to try to explain why the tremor lasted for nine days. Eventually, they locked in the unique topography of Dixon Sound.
Sandwiched between thousands of kilometres of cliffs, the Dixon Sound has no access to the west and a sharp turn of nearly 90° on the east, which the researchers suggest allows the energy of the water to swing steadily within the fjord like a pendulum rather than easily escaping.
At the same time, as the landslide accelerates over the glacier and heads straight to the side of the fjord, most of the energy is concentrated on the shoreline parallel to the long axis of the fjord, which may also help build up enough impact.
Climate change underfoot
Such an earthquake that shook the world may seem like a random thing, but from the perspective of climate change, everything has been foreshadowed for a long time. As part of a complex fjord system in eastern Greenland, the cliffs on both sides of Dixonfjord are filled with ravines filled with ancient glaciers that support the rocks higher up.
However, researchers found that from 1987 to 2018, the glacier beneath the summit where the landslide occurred had thinned by 30 meters. "We believe that [from 2018] to the end of the year the glacier has thinned even more, causing it to no longer be able to support the weight of the rocks at the summit, and eventually a landslide occurred." "This is the first time in history that landslides and tsunamis have been observed in eastern Greenland, showing that climate change is already having a significant impact here." ”
At the same time, it is certain that this will not be the last megatsunami caused by a landslide here. Climate change is weakening permafrost and glaciers. Not only in Dixon Fjord and eastern Greenland, but similar events will occur more frequently and on a larger scale in the polar regions and in the mountains of the world. As with this 9-day earthquake signal, more unthinkable extremes that were previously unimaginable are becoming a reality.
"This is perhaps the first time we've seen the effects of climate change from our feet," says Hicks, "and the vibrations in Dixonfjord have reached everyone's feet, and no area has been spared." ”
Planning and production
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