A ‘swarm’ of 85,000 earthquakes in Antarctica that lasted about six months in 2020 was triggered by magma from an underwater volcano, a new study says.
The swarm occurred at Orca Seamount, a deep-sea volcano near King George Island in Antarctica, in the Bransfield Strait, which has been inactive for ‘a long time’.
Researchers have used seismometers and remote sensing techniques to determine how long the swarm lasted, and what caused it.
Swarm quakes mainly occur in volcanically active regions, so the movement of magma in the Earth’s crust is therefore suspected as the cause.
During the swarm, ground on neighbouring King George Island moved 4.3 inches (11cm) – suggesting a ‘finger’ of magma almost reached the surface, the scientists report in their new study.
The Carlini base on King George Island, hosting the seismometer located closest to the seismic region, and the Bransfield Strait
Orca Seamount is located in the Bransfield Strait, an ocean channel between the Antarctic Peninsula and the South Shetland Islands, southwest of the southern tip of Argentina
What is an earthquake swarm?
Earthquake swarms occur when many earthquakes take place over several weeks or months, with no clear sequence.
Traditional earthquakes feature a main event, followed by a series of aftershocks.
Seismic activity could be a sign of an impending eruption of the supervolcano, although this is hard to predict.
Earthquake swarms are common in Yellowstone in the western United States and, on average, comprise about 50 per cent of the total activity in the Yellowstone region.
The so-called ‘swarm quake’ reached proportions not previously observed for this region, the experts say.
‘In the past, seismicity in this region was moderate,’ said lead study author Dr Simone Cesca from the German Research Centre for Geosciences (GFZ) Potsdam.
‘However, in August 2020, an intense seismic swarm began there, with more than 85,000 earthquakes within half a year.
‘It represents the largest seismic unrest ever recorded there.’
It’s well-known that earthquakes are caused in subduction zones, when two tectonic plates that are sliding in opposite directions stick and then slip suddenly.
Severe earthquakes normally occur over fault lines where tectonic plates meet, although minor tremors can happen in the middle of these plates.
Meanwhile, earthquake swarms occur when many earthquakes take place over several weeks or months, with no clear sequence.
In comparison, traditional earthquakes feature a main event, followed by a series of aftershocks.
Most earthquakes are caused by the interaction of the plates; however, some are related to volcanoes and the movement of magma.
Most earthquakes directly beneath a volcano are caused by the movement of magma, which exerts pressure on the rocks until it cracks the rock. Then the magma squirts into the crack and starts building pressure again.
Every time the rock cracks, it makes a small earthquake. These earthquakes are usually too weak to be felt but can be detected and recorded by sensitive instruments called seismometers.
Orca Seamount is a large submarine shield volcano with a height of about 900 metres (nearly 3,000 feet) above the sea floor and a base diameter of about 6.8 miles (11km). It is located in the Bransfield Strait, an ocean channel between the Antarctic Peninsula and the South Shetland Islands, southwest of the southern tip of Argentina
Rumblings of the 2020 swarm were originally detected by scientists at research stations on King George Island.
What is the Orca Seamount?
Orca Seamount is a large submarine shield volcano with a height of about 900 metres (nearly 3,000 feet) above the sea floor and a base diameter of about 6.8 miles (11km).
It is located in the Bransfield Strait, an ocean channel between the Antarctic Peninsula and the South Shetland Islands, southwest of the southern tip of Argentina.
Researchers say an earthquake swarm occurred at Orca Seamount in 2020, caused by magma transfer below the seafloor.
However, there are few conventional seismological instruments in the remote area – namely only two seismic and two GNSS stations (ground stations of the Global Navigation Satellite System which measure ground displacement).
Because of this, it was thought more could be revealed about the nature of the rumblings and how long they lasted with a wider analysis.
So the team also analysed data from farther seismic stations and data from InSAR satellites, which use radar interferometry to measure ground displacements.
They used a number of geophysical methods to model the events, in order to make sure they were interpreting the data correctly.
The researchers backdated the start of the unrest to August 10, 2020 and managed to extend the original global seismic catalogue, containing only 128 earthquakes, to more than 85,000 events.
The swarm peaked with two large earthquakes on October 2, 2020 (measuring 5.9 on the moment magnitude scale, or Mw) and November 6 (Mw 6.0) 2020 before decreasing abruptly in mid November after about three months of sustained activity.
By February 2021, seismic activity had decreased significantly.
Researchers say the swarm was caused by a rapid transfer of magma from the Earth’s mantle near the crust-mantle boundary to almost the surface.
The crust-mantle boundary is the point where Earth’s crust, the outermost shell, meets the mantle (a layer of silicate rock)
According to the scientists, the swarm marks ‘the largest magmatic unrest ever geophysically monitored in Antarctica.
A magma intrusion (the migration of a larger volume or ‘finger’ of magma) was identified as the main cause of the swarm quake, because seismic processes alone cannot explain the observed land surface deformation on King George Island.
The study shows that such events can be studied and described in great detail even in such remote and therefore poorly instrumented areas.
‘Our study represents a new successful investigation of a seismo-volcanic unrest at a remote location on Earth, where the combined application of seismology, geodesy and remote sensing techniques are used to understand earthquake processes and magma transport in poorly instrumented areas,’ Dr Cesca said.
‘This is one of the few cases where we can use geophysical tools to observe intrusion of magma from the upper mantle or crust-mantle boundary into the shallow crust – a rapid transfer of magma from the mantle to almost the surface that takes only a few days.’
The study, which involved researchers from Germany, Italy, Poland and the US, has been published in the journal Communications Earth and Environment.
EARTHQUAKES ARE CAUSED WHEN TWO TECTONIC PLATES SLIDE IN OPPOSITE DIRECTIONS
Catastrophic earthquakes are caused when two tectonic plates that are sliding in opposite directions stick and then slip suddenly.
Tectonic plates are composed of Earth’s crust and the uppermost portion of the mantle.
Below is the asthenosphere: the warm, viscous conveyor belt of rock on which tectonic plates ride.
They do not all not move in the same direction and often clash. This builds up a huge amount of pressure between the two plates.
Eventually, this pressure causes one plate to jolt either under or over the other.
This releases a huge amount of energy, creating tremors and destruction to any property or infrastructure nearby.
Severe earthquakes normally occur over fault lines where tectonic plates meet, but minor tremors – which still register on the Richter sale – can happen in the middle of these plates.
The Earth has fifteen tectonic plates (pictured) that together have molded the shape of the landscape we see around us today
These are called intraplate earthquakes.
These remain widely misunderstood but are believed to occur along minor faults on the plate itself or when ancient faults or rifts far below the surface reactivate.
These areas are relatively weak compared to the surrounding plate, and can easily slip and cause an earthquake.
Earthquakes are detected by tracking the size, or magnitude, and intensity of the shock waves they produce, known as seismic waves.
The magnitude of an earthquake differs from its intensity.
The magnitude of an earthquake refers to the measurement of energy released where the earthquake originated.
Earthquakes originate below the surface of the earth in a region called the hypocenter.
During an earthquake, one part of a seismograph remains stationary and one part moves with the earth’s surface.
The earthquake is then measured by the difference in the positions of the still and moving parts of the seismograph.