This swarm, which began in August 2020 and subsided by November of that year, is the strongest earthquake ever recorded in the region. The quakes were most likely caused by a “finger” of hot magma poking into the Earth‘s crust, a new study says.
“There have been similar intrusions elsewhere on Earth, but we’ve seen them here for the first time,” study co-author Simon Cheska, a seismologist at the German Geoscience Research Center GFZ in Potsdam.
“Typically, such processes occur on geological time scales,” not over the course of a human lifetime, Cheska said. “So in a way we were lucky to see that.”
The swarm originated around Mount Orka, an inactive volcano that rises 2,950 feet (900 meters) above the seabed in Bransfield Sound, a narrow passage between the South Shetland Islands and the northwestern tip of Antarctica.
In this region, the Phoenix tectonic plate is subducting under the continental Antarctic plate, creating a network of fault zones, stretching some sections of the crust and opening faults elsewhere, according to a 2018 study in the journal Polar Science.
Scientists at research stations on King George Island, one of the South Shetland Islands, were the first to feel the rumble of small earthquakes.
This soon became known to Cheska and his colleagues around the world, some of whom collaborated on separate projects with researchers on the island.
The team wanted to understand what was going on, but King George Island is far away and there are only two seismic stations nearby, Cheska said.
Therefore, the researchers used data from these seismic stations, as well as data from two ground stations of the global satellite navigation system to measure ground displacement.
They also studied data from more distant seismic stations and from satellites circling the Earth that use radar to measure ground-level displacement, the authors of the study reported April 11 in the journal Communications Earth & Environment.
Nearby stations are quite simple, but they are good at detecting the smallest earthquakes. More distant stations use more sophisticated equipment and can provide a more detailed picture of large earthquakes.
By putting this data together, Chesky said, the team was able to get an idea of the geologic basis that caused this massive earthquake swarm.
The two largest earthquakes in this series were a magnitude 5.9 earthquake in October 2020 and a magnitude 6.0 earthquake in November.
After the November earthquake, seismic activity began to decline. According to the results of the study, the earthquakes caused the ground to move on King George Island by about 4.3 inches (11 centimeters).
Only 4 percent of this shift can be directly explained by the earthquake; scientists suspect that the abrupt shift of the ground is mainly due to the movement of magma in the earth’s crust.
“We think the magnitude 6 quake somehow created faults and relieved pressure on the magma dam,” Cheska said.
If there was an underwater eruption on the seamount, then most likely it happened at that time, Cheska added.
But so far there is no direct evidence of an eruption; to confirm that a massive shield volcano has blasted its summit, scientists will have to send a mission to the strait to measure the bathymetry, or depth of the seabed, and compare it to historical maps, he said.
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