Kerguelen and lithospheric uplift
Physicists have noted that seismic activity in the Kerguelen archipelago could be linked to the retreat of the Cook glacier since 1999.
At an altitude of 1,049 metres and a latitude of 49 degrees south, the air is cool, especially at the top of the Cook Glacier, a vast 400 square kilometre mountain located to the west of the Kerguelen archipelago in the southern Indian Ocean.
At the foot of the colossus, the ice is melting and retreating at one of the fastest rates in the world (20% of its surface in 40 years). And as physicists from the Institut Terre et Environnement in Strasbourg reported in the journal Seismica last month, this could well shake the Earth.
The archipelago’s only permanent seismograph continuously records ground vibrations slightly away from Port-aux-Français so as not to be disturbed too much by the station’s activity. It is the only instrument within a radius of several thousand kilometres, and the only one that can detect earthquakes with a magnitude of less than four on the Richter scale.
“The idea was to analyse 20 years of seismograph data using artificial intelligence, a convolutional neural network,” explains Olivier Lengliné, seismologist and principal author of the study. “We were surprised to see that there was a huge amount of seismicity so far from the plate boundaries.”
On the western flank of the Cook Glacier, records show regular earthquakes since 1999. In the centre of the archipelago, however, tremors are more occasional.
Are the earth tremors linked to the retreat of the ice, and therefore to the reduction in the weight exerted on the earth’s crust? Or, despite the slow decline in volcanic activity on the island since its last eruption 24,000 years ago, is it rather the pressure of the magma that is triggering these fractures in the crust?
“It’s more likely that what we’re seeing in the west has more to do with the loss of load caused by melting ice. And, on the contrary, the events that are more concentrated in time would be linked to deep magmatism.” says Olivier Lengliné.
To validate the hypothesis that the lithosphere is being lifted by the retreating ice, additional seismographs will be placed in other parts of the archipelago by the French Polar Institute.
Researchers will then be able to locate the epicentres more precisely, understand the depth at which they are triggered and deduce the exact causes. “We need to find other evidence, such as the movement of the epicentres, to confirm the hypothesis,” explains the researcher.
The phenomenon is well known, and in Iceland it has even been proven that the lightening of an ice cap above a pressurised magma chamber can trigger volcanic eruptions.
Other physicists, working with ESA’s Sentinel-1 satellite, observed a magnitude 4.5 earthquake on the surface on 6 October 2017 to the west of the glacier, an element that supports the seismic observations.
They also observed a continuous “uplift” of a few millimetres per year. “This can be interpreted, at least in part, as an elastic rebound induced by the surface discharge caused by the melting of the ice cap,” explains Raphaël Grandin from the Institut de Physique du Globe de Paris.
Camille Lin, PolarJournal
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