Crevasse on the Canwell glacier created by the earthquake that struck near Denali National Park in Alaska in November 2002
PARIS — Between 1900 and 1950, the Earth recorded an average of 3.4 earthquakes per year with a magnitude greater than 6.5. That figured doubled to 6.7 a year until the early 1970s, and was almost five times that in the 2000s.
Their intensity would also have increased with more than 25 major earthquakes per year, double the previous periods. This is according to the EM-DAT emergency events database, which compiled the occurrence and effects of 22,000 mass disasters worldwide in the 20th century.
Can we conclude that there is a causal relationship with the rise of human activities, as some experts suggest? The idea was first suggested in 2011 by an Australian research team led by geology professor Giampiero Iaffaldano. At the time, it reported that it had found that the intensification of the monsoon in India had accelerated the movement of the Indian tectonic plate by 20% over the past 10 million years.
Iaffaldano wrote in Earth and Planetary Science Letters that the closing and opening of ocean basins, or the emergence of high mountains like the Andes or Tibet, are geological processes that affect climate. "We have shown for the first time that the reverse is true, that the evolution of the climate can affect in return the movement of tectonic plates."
Pressure released
Since this study, no team has been able to demonstrate a convincing link between short-term climate change and the increase in the number of earthquakes in the world. However, scientists are wondering about the impact of glacial melt on the Earth's mantle.
"The ice is holding back large parts of the Earth's crust locally," says geophysicist Andrea Hampel. Together with her colleagues from the Ruhr University in Bochum in Germany, she has discovered that the weight of the large glaciers on the earth slows down the movement of the continental plates.
As proof: she demonstrated with the help of computer simulations that the seismic rebound that Scandinavia experienced temporarily 9,000 years ago coincided with the rapid melting of the Fino-Scandinavian ice cap that covered the entire region at the time.
According to this simulation, the vertical pressure exerted by the ice could have prevented the spontaneous sliding of continental plates along geological faults. But like a spring, the mechanical tensions due to the movements of the earth's crust continued to accumulate and were released when the ice melted, causing more frequent and intense earthquakes in Northern Europe.
Consequences of a natural disaster that hit Jacksonville, Florida, USA, in December 2019.
Elastic effect
Can the same phenomenon occur more widely today with global warming? Some researchers say so and speculate that this mechanism has already begun to take place, notably in Alaska. It is there that we find the largest glaciers in the world extending over several hundred square kilometers and hundreds of meters thick.
The degradation of the Greenland and Antarctic ice sheets will lead to an increase in the frequency of earthquakes.
In 200 years, they have lost more than 5,000 km2 of ice. "The earth rises consecutively, and this uplift occurs in two stages," explains Chris Rollins, a researcher at the University of Fairbanks in Alaska. "First, it's instantaneous due to the elastic effect of the loss of ice mass, and then it's slower as the mantle re-positions."
Southern Alaska is a red zone in the Earth's crust where many earthquakes occur due to the overlap of the North American continental plate and the Pacific plate. "As the ice melts, the faults sometimes reach their stress limit," he explains. That's what probably happened in 1958 when a magnitude 7.8 earthquake triggered a phenomenal landslide.
"Based on our models, we predict that the degradation of the Greenland and Antarctic ice sheets during global warming will lead to an increase in the frequency of earthquakes in these regions," says Andrea Hampel, who took over this work.
Consequences of a natural disaster, USA, February 2020.
Mega-tsunamis
That's not all. According to several vulcanologists, first and foremost Bill McGuire, who teaches at the University College of London, the melting of the Greenland ice cap could release the pressure that kept the volcanic vents closed and cause gigantic landslides on the seabed off the coast of this territory attached to Denmark.
"These climate change events have occurred several times in Earth's history and produce devastating mega-tsunamis," he recently warned in the magazine New Scientist.
More landslides are likely as mountain glaciers continue to shrink and alpine permafrost thaws.
Scientists have found geological traces. 100,000 years ago, the collapse of the Hawaiian volcano Mauna Loa would have caused a cataclysmic tidal wave of over 300 meters high. More recently, on Oct. 17, 2015, the melting of Alaska's Tyndall Glacier precipitated 180 million tons of rock into the waters of the deserted Taan Fjord, raising a giant 193-meter wave that flooded 20 square kilometers of land before subsiding offshore.
"More such landslides are likely to occur as mountain glaciers continue to shrink and alpine permafrost thaws," writes American geologist Bretwood Higman, lead author of a well-researched study of the cataclysm published in Nature.
Vicious cycle
A vicious cycle could be hidden in these upheavals: these earthquakes would accelerate global warming, says a team of Russian scientists. To come to this conclusion, the researchers observed two periods of sudden increases in temperature in the Arctic, in the 1930s and 1980s.
"These two periods were preceded by major earthquakes in a region 2,000 km away," explain the authors. But it took time for the tectonic waves to reach the ice shelf from the Aleutian Islands in southwest Alaska: between 15 and 20 years, at a rate of about 100 km per year.
If this hypothesis is confirmed, it would mean that large quantities of methane, the most powerful greenhouse gas, will be released by the tremors that have occurred since then.
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